jamtis.md

JAMTIS

This document describes a new addressing scheme for Monero.

Chapters 1-2 are intended for general audience.

Chapters 3-7 contain technical specifications.

Table of Contents

• 1. Introduction
  • 1.1 Why a new address format?
  • 1.2 Current Monero addresses
  • 1.3 Jamtis
• 2. Features
  • 2.1 Address format
    • 2.1.1 Recipient IDs
  • 2.2 Light wallet scanning
    • 2.2.1 Wallet component
    • 2.2.2 Third party services
  • 2.3 Wallet tiers for merchants
    • 2.3.1 Address generator
    • 2.3.2 Payment validator
  • 2.4 Full view-only wallets
  • 2.5 Janus attack mitigation
  • 2.6 Robust output detection
• 3. Notation
  • 3.1 Serialization functions
  • 3.2 Hash function
  • 3.3 Elliptic curves
    • 3.3.1 Curve25519
    • 3.3.2 Ed25519
  • 3.4 Block cipher
  • 3.5 Base32 encoding
• 4. Wallets
  • 4.1 Wallet parameters
  • 4.2 New wallets
    • 4.2.1 Standard wallets
    • 4.2.2 Multisignature wallets
  • 4.3 Migration of legacy wallets
    • 4.3.1 Standard wallets
    • 4.3.2 Multisignature wallets
  • 4.4 Additional keys
  • 4.5 Key hierarchy
  • 4.6 Wallet access tiers
    • 4.6.1 Address generator (AddrGen)
    • 4.6.2 Output scanning wallet (FindReceived)
    • 4.6.3 Payment validator (ViewReceived)
    • 4.6.4 View-balance wallet (ViewAll)
    • 4.6.5 Master wallet (Master)
  • 4.7 Wallet public keys
• 5. Addresses
  • 5.1 Address generation
    • 5.1.1 Address keys
    • 5.1.2 Address tag
  • 5.2 Sending to an address
  • 5.3 Receiving an output
  • 5.4 Change and self-spends
  • 5.5 Transaction size
    • 5.5.1 Transactions with 2 outputs
    • 5.5.2 Transactions with 3 or more outputs
• 6. Address encoding
  • 6.1 Address structure
  • 6.2 Address header
    • 6.2.1 Version character
    • 6.2.2 Network type
  • 6.3 Checksum
  • 6.4 Binary-to-text encoding
    • 6.4.1 QR Codes
  • 6.5 Recipient authentication
• 7. Test vectors
• References
• Appendix A: Checksum

1. Introduction

1.1 Why a new address format?

Sometime in 2024, Monero plans to adopt a new transaction protocol called Seraphis [1], which enables much larger ring sizes than the current RingCT protocol. However, due to a different key image construction, Seraphis is not compatible with CryptoNote addresses. This means that each user will need to generate a new set of addresses from their existing private keys. This provides a unique opportunity to vastly improve the addressing scheme used by Monero.

1.2 Current Monero addresses

The CryptoNote-based addressing scheme [2] currently used by Monero has several issues:

1. Addresses are not suitable as human-readable identifiers because they are long and case-sensitive.
2. Too much information about the wallet is leaked when scanning is delegated to a third party.
3. Generating subaddresses requires view access to the wallet. This is why many merchants prefer integrated addresses [3].
4. View-only wallets need key images to be imported to detect spent outputs [4].
5. Subaddresses that belong to the same wallet can be linked via the Janus attack [5].
6. The detection of outputs received to subaddresses is based on a lookup table, which can sometimes cause the wallet to miss outputs [6].

1.3 Jamtis

Jamtis is a new addressing scheme that was developed specifically for Seraphis and tackles all of the shortcomings of CryptoNote addresses that were mentioned above. Additionally, Jamtis incorporates two other changes related to addresses to take advantage of this large upgrade opportunity:

• A new 16-word mnemonic scheme called Polyseed [7] that will replace the legacy 25-word seed for new wallets.
• The removal of integrated addresses and payment IDs [8].

2. Features

2.1 Address format

Jamtis addresses, when encoded as a string, start with the prefix xmra and consist of 196 characters. Example of an address: xmra1mj0b1977bw3ympyh2yxd7hjymrw8crc9kin0dkm8d3wdu8jdhf3fkdpmgxfkbywbb9mdwkhkya4jtfn0d5h7s49bfyji1936w19tyf3906ypj09n64runqjrxwp6k2s3phxwm6wrb5c0b6c1ntrg2muge0cwdgnnr7u7bgknya9arksrj0re7whkckh51ik

There is no "main address" anymore - all Jamtis addresses are equivalent to a subaddress.

2.1.1 Recipient IDs

Jamtis introduces a short recipient identifier (RID) that can be calculated for every address. RID consists of 25 alphanumeric characters that are separated by underscores for better readability. The RID for the above address is regne_hwbna_u21gh_b54n0_8x36q. Instead of comparing long addresses, users can compare the much shorter RID. RIDs are also suitable to be communicated via phone calls, text messages or handwriting to confirm a recipient's address. This allows the address itself to be transferred via an insecure channel.

2.2 Light wallet scanning

Jamtis introduces new wallet tiers below view-only wallet. One of the new wallet tiers called "FindReceived" is intended for wallet-scanning and only has the ability to calculate view tags [9]. It cannot generate wallet addresses or decode output amounts.

View tags can be used to eliminate 99.6% of outputs that don't belong to the wallet. If provided with a list of wallet addresses, this tier can also link outputs to those addresses. Possible use cases are:

2.2.1 Wallet component

A wallet can have a "FindReceived" component that stays connected to the network at all times and filters out outputs in the blockchain. The full wallet can thus be synchronized at least 256x faster when it comes online (it only needs to check outputs with a matching view tag).

2.2.2 Third party services

If the "FindReceived" private key is provided to a 3rd party, it can preprocess the blockchain and provide a list of potential outputs. This reduces the amount of data that a light wallet has to download by a factor of at least 256. The third party will not learn which outputs actually belong to the wallet and will not see output amounts.

2.3 Wallet tiers for merchants

Jamtis introduces new wallet tiers that are useful for merchants.

2.3.1 Address generator

This tier is intended for merchant point-of-sale terminals. It can generate addresses on demand, but otherwise has no access to the wallet (i.e. it cannot recognize any payments in the blockchain).

2.3.2 Payment validator

This wallet tier combines the Address generator tier with the ability to also view received payments (including amounts). It is intended for validating paid orders. It cannot see outgoing payments and received change.

2.4 Full view-only wallets

Jamtis supports full view-only wallets that can identify spent outputs (unlike legacy view-only wallets), so they can display the correct wallet balance and list all incoming and outgoing transactions.

2.5 Janus attack mitigation

Janus attack is a targeted attack that aims to determine if two addresses A, B belong to the same wallet. Janus outputs are crafted in such a way that they appear to the recipient as being received to the wallet address B, while secretly using a key from address A. If the recipient confirms the receipt of the payment, the sender learns that they own both addresses A and B.

Jamtis prevents this attack by allowing the recipient to recognize a Janus output.

2.6 Robust output detection

Jamtis addresses and outputs contain an encrypted address tag which enables a more robust output detection mechanism that does not need a lookup table and can reliably detect outputs sent to arbitrary wallet addresses.

3. Notation

3.1 Serialization functions

1. The function BytesToInt256(x) deserializes a 256-bit little-endian integer from a 32-byte input.
2. The function Int256ToBytes(x) serialized a 256-bit integer to a 32-byte little-endian output.

3.2 Hash function

The function Hb(k, x) with parameters b, k, refers to the Blake2b hash function [10] initialized as follows:

• The output length is set to b bytes.
• Hashing is done in sequential mode.
• The Personalization string is set to the ASCII value "Monero", padded with zero bytes.
• If the key k is not null, the hash function is initialized using the key k (maximum 64 bytes).
• The input x is hashed.

The function SecretDerive is defined as:

SecretDerive(k, x) = H32(k, x)

3.3 Elliptic curves

Two elliptic curves are used in this specification:

1. Curve25519 - a Montgomery curve. Points on this curve include a cyclic subgroup 𝔾1.
2. Ed25519 - a twisted Edwards curve. Points on this curve include a cyclic subgroup 𝔾2.

Both curves are birationally equivalent, so the subgroups 𝔾1 and 𝔾2 have the same prime order ℓ = 2252 + 27742317777372353535851937790883648493. The total number of points on each curve is 8ℓ.

3.3.1 Curve25519

Curve25519 is used exclusively for the Diffie-Hellman key exchange [11].

Only a single generator point B is used:

Point	Derivation	Serialized (hex)
B	generator of 𝔾1	0900000000000000000000000000000000000000000000000000000000000000

Private keys for Curve25519 are 32-byte integers denoted by a lowercase letter d. They are generated using the following KeyDerive1(k, x) function:

1. d = H32(k, x)
2. d[31] &= 0x7f (clear the most significant bit)
3. d[0] &= 0xf8 (clear the least significant 3 bits)
4. return d

All Curve25519 private keys are therefore multiples of the cofactor 8, which ensures that all public keys are in the prime-order subgroup. The multiplicative inverse modulo ℓ is calculated as d-1 = 8*(8*d)-1 to preserve the aforementioned property.

Public keys (elements of 𝔾1) are denoted by the capital letter D and are serialized as the x-coordinate of the corresponding Curve25519 point. Scalar multiplication is denoted by a space, e.g. D = d B.

3.3.2 Ed25519

The Edwards curve is used for signatures and more complex cryptographic protocols [12]. The following three generators are used:

Point	Derivation	Serialized (hex)
G	generator of 𝔾2	5866666666666666666666666666666666666666666666666666666666666666
U	Hp("seraphis U")	126582dfc357b10ecb0ce0f12c26359f53c64d4900b7696c2c4b3f7dcab7f730
X	Hp("seraphis X")	4017a126181c34b0774d590523a08346be4f42348eddd50eb7a441b571b2b613

Here Hp refers to an unspecified hash-to-point function.

Private keys for Ed25519 are 32-byte integers denoted by a lowercase letter k. They are generated using the following function:

KeyDerive2(k, x) = H64(k, x) mod ℓ

Public keys (elements of 𝔾2) are denoted by the capital letter K and are serialized as 256-bit integers, with the lower 255 bits being the y-coordinate of the corresponding Ed25519 point and the most significant bit being the parity of the x-coordinate. Scalar multiplication is denoted by a space, e.g. K = k G.

3.4 Block cipher

The function BlockEnc(s, x) refers to the application of the Twofish [13] permutation using the secret key s on the 16-byte input x. The function BlockDec(s, x) refers to the application of the inverse permutation using the key s.

3.5 Base32 encoding

"Base32" in this specification referes to a binary-to-text encoding using the alphabet xmrbase32cdfghijknpqtuwy01456789. This alphabet was selected for the following reasons:

1. The order of the characters has a unique prefix that distinguishes the encoding from other variants of "base32".
2. The alphabet contains all digits 0-9, which allows numeric values to be encoded in a human readable form.
3. Excludes the letters o, l, v and z for the same reasons as the z-base-32 encoding [14].

4. Wallets

4.1 Wallet parameters

Each wallet consists of two main private keys and a timestamp:

Field	Type	Description
km	private key	wallet master key
kvb	private key	view-balance key
birthday	timestamp	date when the wallet was created

The master key km is required to spend money in the wallet and the view-balance key kvb provides full view-only access.

The birthday timestamp is important when restoring a wallet and determines the blockchain height where scanning for owned outputs should begin.

4.2 New wallets

4.2.1 Standard wallets

Standard Jamtis wallets are generated as a 16-word Polyseed mnemonic [7], which contains a secret seed value used to derive the wallet master key and also encodes the date when the wallet was created. The key kvb is derived from the master key.

Field	Derivation
km	BytesToInt256(polyseed_key) mod ℓ
kvb	kvb = KeyDerive1(km, "jamtis_view_balance_key")
birthday	from Polyseed

4.2.2 Multisignature wallets

Multisignature wallets are generated in a setup ceremony, where all the signers collectively generate the wallet master key km and the view-balance key kvb.

Field	Derivation
km	setup ceremony
kvb	setup ceremony
birthday	setup ceremony

4.3 Migration of legacy wallets

Legacy pre-Seraphis wallets define two private keys:

• private spend key ks
• private view-key kv

4.3.1 Standard wallets

Legacy standard wallets can be migrated to the new scheme based on the following table:

Field	Derivation
km	km = ks
kvb	kvb = KeyDerive1(km, "jamtis_view_balance_key")
birthday	entered manually

Legacy wallets cannot be migrated to Polyseed and will keep using the legacy 25-word seed.

4.3.2 Multisignature wallets

Legacy multisignature wallets can be migrated to the new scheme based on the following table:

Field	Derivation
km	km = ks
kvb	kvb = kv
birthday	entered manually

4.4 Additional keys

There are additional keys derived from kvb:

Key	Name	Derivation	Used to
dfr	find-received key	kfr = KeyDerive1(kvb, "jamtis_find_received_key")	scan for received outputs
dua	unlock-amounts key	kid = KeyDerive1(kvb, "jamtis_unlock_amounts_key")	decrypt output amounts
sga	generate-address secret	sga = SecretDerive(kvb, "jamtis_generate_address_secret")	generate addresses
sct	cipher-tag secret	ket = SecretDerive(sga, "jamtis_cipher_tag_secret")	encrypt address tags

The key dfr provides the ability to calculate the sender-receiver shared secret when scanning for received outputs. The key dua can be used to create a secondary shared secret and is used to decrypt output amounts.

The key sga is used to generate public addresses. It has an additional child key sct, which is used to encrypt the address tag.

4.5 Key hierarchy

The following figure shows the overall hierarchy of wallet keys. Note that the relationship between km and kvb only applies to standard (non-multisignature) wallets.

4.6 Wallet access tiers

Tier	Knowledge	Off-chain capabilities	On-chain capabilities
AddrGen	sga	generate public addresses	none
FindReceived	dfr	recognize all public wallet addresses	eliminate 99.6% of non-owned outputs (up to § 5.3.5), link output to an address (except of change and self-spends)
ViewReceived	dfr, dua, sga	all	view all received except of change and self-spends (up to § 5.3.14)
ViewAll	kvb	all	view all
Master	km	all	all

4.6.1 Address generator (AddrGen)

This wallet tier can generate public addresses for the wallet. It doesn't provide any blockchain access.

4.6.2 Output scanning wallet (FindReceived)

Thanks to view tags, this tier can eliminate 99.6% of outputs that don't belong to the wallet. If provided with a list of wallet addresses, it can also link outputs to those addresses (but it cannot generate addresses on its own). This tier should provide a noticeable UX improvement with a limited impact on privacy. Possible use cases are:

1. An always-online wallet component that filters out outputs in the blockchain. A higher-tier wallet can thus be synchronized 256x faster when it comes online.
2. Third party scanning services. The service can preprocess the blockchain and provide a list of potential outputs with pre-calculated spend keys (up to § 5.2.4). This reduces the amount of data that a light wallet has to download by a factor of at least 256.

4.6.3 Payment validator (ViewReceived)

This level combines the tiers AddrGen and FindReceived and provides the wallet with the ability to see all incoming payments to the wallet, but cannot see any outgoing payments and change outputs. It can be used for payment processing or auditing purposes.

4.6.4 View-balance wallet (ViewAll)

This is a full view-only wallet than can see all incoming and outgoing payments (and thus can calculate the correct wallet balance).

4.6.5 Master wallet (Master)

This tier has full control of the wallet.

4.7 Wallet public keys

There are 3 global wallet public keys. These keys are not usually published, but are needed by lower wallet tiers.

Key	Name	Value
Ks	wallet spend key	Ks = kvb X + km U
Dua	unlock-amounts key	Dua = dua B
Dfr	find-received key	Dfr = dfr Dua

5. Addresses

5.1 Address generation

Jamtis wallets can generate up to 2¹²⁸ different addresses. Each address is constructed from a 128-bit index j. The size of the index space allows stateless generation of new addresses without collisions, for example by constructing j as a UUID [15].

Each Jamtis address encodes the tuple (K1j, D2j, D3j, tj). The first three values are public keys, while tj is the "address tag" that contains the encrypted value of j.

5.1.1 Address keys

The three public keys are constructed as:

• K1j = Ks + kuj U + kxj X + kgj G
• D2j = daj Dfr
• D3j = daj Dua

The private keys kuj, kxj, kgj and daj are derived as follows:

Keys	Name	Derivation
kuj	spend key extensions	kuj = KeyDerive2(sga, "jamtis_spendkey_extension_u" || j)
kxj	spend key extensions	kxj = KeyDerive2(sga, "jamtis_spendkey_extension_x" || j)
kgj	spend key extensions	kgj = KeyDerive2(sga, "jamtis_spendkey_extension_g" || j)
daj	address keys	daj = KeyDerive1(sga, "jamtis_address_privkey" || j)

5.1.2 Address tag

Each address additionally includes an 18-byte tag tj = (j', hj'), which consists of the encrypted value of j:

• j' = BlockEnc(sct, j)

and a 2-byte "tag hint", which can be used to quickly recognize owned addresses:

• hj' = H2(sct, "jamtis_address_tag_hint" || j')

5.2 Sending to an address

TODO

5.3 Receiving an output

TODO

5.4 Change and self-spends

TODO

5.5 Transaction size

Jamtis has a small impact on transaction size.

5.5.1 Transactions with 2 outputs

The size of 2-output transactions is increased by 28 bytes. The encrypted payment ID is removed, but the transaction needs two encrypted address tags t~ (one for the recipient and one for the change). Both outputs can use the same value of De.

5.5.2 Transactions with 3 or more outputs

Since there are no "main" addresses anymore, the TX_EXTRA_TAG_PUBKEY field can be removed from transactions with 3 or more outputs.

Instead, all transactions with 3 or more outputs will require one 50-byte tuple (De, t~) per output.

6. Address encoding

6.1 Address structure

An address has the following overall structure:

Field	Size (bits)	Description
Header	30*	human-readable address header (§ 6.2)
K1	256	address key 1
D2	255	address key 2
D3	255	address key 3
t	144	address tag
Checksum	40*	(§ 6.3)

* The header and the checksum are already in base32 format

6.2 Address header

The address starts with a human-readable header, which has the following format consisting of 6 alphanumeric characters:

"xmra" <version char> <network type char>

Unlike the rest of the address, the header is never encoded and is the same for both the binary and textual representations. The string is not null terminated.

The software decoding an address shall abort if the first 4 bytes are not 0x78 0x6d 0x72 0x61 ("xmra").

The "xmra" prefix serves as a disambiguation from legacy addresses that start with "4" or "8". Additionally, base58 strings that start with the character x are invalid due to overflow [16], so legacy Monero software can never accidentally decode a Jamtis address.

6.2.1 Version character

The version character is "1". The software decoding an address shall abort if a different character is encountered.

6.2.2 Network type

network char	network type
"t"	testnet
"s"	stagenet
"m"	mainnet

The software decoding an address shall abort if an invalid network character is encountered.

6.3 Checksum

The purpose of the checksum is to detect accidental corruption of the address. The checksum consists of 8 characters and is calculated with a cyclic code over GF(32) using the polynomial:

x8 + 3x7 + 11x6 + 18x5 + 5x4 + 25x3 + 21x2 + 12x + 1

The checksum can detect all errors affecting 5 or fewer characters. Arbitrary corruption of the address has a chance of less than 1 in 10¹² of not being detected. The reference code how to calculate the checksum is in Appendix A.

6.4 Binary-to-text encoding

An address can be encoded into a string as follows:

address_string = header + base32(data) + checksum

where header is the 6-character human-readable header string (already in base32), data refers to the address tuple (K1, D2, D3, t), encoded in 910 bits, and the checksum is the 8-character checksum (already in base32). The total length of the encoded address 196 characters (=6+182+8).

6.4.1 QR Codes

While the canonical form of an address is lower case, when encoding an address into a QR code, the address should be converted to upper case to take advantage of the more efficient alphanumeric encoding mode.

6.5 Recipient authentication

TODO

7. Test vectors

TODO

References

1. https://github.com/UkoeHB/Seraphis
2. https://github.com/monero-project/research-lab/blob/master/whitepaper/whitepaper.pdf
3. monero-project/meta#299 (comment)
4. https://www.getmonero.org/resources/user-guides/view_only.html
5. https://web.getmonero.org/2019/10/18/subaddress-janus.html
6. monero-project/monero#8138
7. https://github.com/tevador/polyseed
8. monero-project/monero#7889
9. monero-project/research-lab#73
10. https://eprint.iacr.org/2013/322.pdf
11. https://cr.yp.to/ecdh/curve25519-20060209.pdf
12. https://ed25519.cr.yp.to/ed25519-20110926.pdf
13. https://www.schneier.com/wp-content/uploads/2016/02/paper-twofish-paper.pdf
14. http://philzimmermann.com/docs/human-oriented-base-32-encoding.txt
15. https://en.wikipedia.org/wiki/Universally_unique_identifier
16. https://github.com/monero-project/monero/blob/319b831e65437f1c8e5ff4b4cb9be03f091f6fc6/src/common/base58.cpp#L157

Appendix A: Checksum

# Jamtis address checksum algorithm

# cyclic code based on the generator 3BI5PLC1
# can detect 5 errors up to the length of 994 characters
GEN=[0x1ae45cd581, 0x359aad8f02, 0x61754f9b24, 0xc2ba1bb368, 0xcd2623e3f0]

M = 0xffffffffff

def jamtis_polymod(data):
    c = 1
    for v in data:
        b = (c >> 35)
        c = ((c & 0x07ffffffff) << 5) ^ v
        for i in range(5):
            c ^= GEN[i] if ((b >> i) & 1) else 0
    return c

def jamtis_verify_checksum(data):
    return jamtis_polymod(data) == M

def jamtis_create_checksum(data):
    polymod = jamtis_polymod(data + [0,0,0,0,0,0,0,0]) ^ M
    return [(polymod >> 5 * (7 - i)) & 31 for i in range(8)]

# test/example

CHARSET = "xmrbase32cdfghijknpqtuwy01456789"

addr_test = (
    "xmra1mj0b1977bw3ympyh2yxd7hjymrw8crc9kin0dkm8d3"
    "wdu8jdhf3fkdpmgxfkbywbb9mdwkhkya4jtfn0d5h7s49bf"
    "yji1936w19tyf3906ypj09n64runqjrxwp6k2s3phxwm6wr"
    "b5c0b6c1ntrg2muge0cwdgnnr7u7bgknya9arksrj0re7wh")

addr_data = [CHARSET.find(x) for x in addr_test]
addr_enc = addr_data + jamtis_create_checksum(addr_data)
addr = "".join([CHARSET[x] for x in addr_enc])

print(addr)
print("len =", len(addr))
print("valid =", jamtis_verify_checksum(addr_enc))

Here are some additions to my proposal:

View tag filter target

The filter target should be 480 enotes/day. Because the view tag filter rate must be a power of 2, this will actually result in a range from 480 to 960 enotes per day depending on the tx volume. If we "average the averages" over all possible values of tx volume, this will give a mean of 720 enote matches per day, or roughly 1 match per block, which is what was suggested by @jeffro256. I think this is close to the upper limit of what is acceptable for light wallet clients (~200 KB/day) and should provide a good number of false positives even if there was a short term drop in tx volume.

The fomula to calculate the view tag size in bits is:

tag_size = trunc(log2(3 * num_outputs_100k / 200000))

where num_outputs_100k is the total number of outputs in the last 100 000 blocks. The trunc(log2(x)) function can be easily calculated using only integer operations (it's basically the position of the most significant bit).

As an example, the value of num_outputs_100k is currently about 7.9 million, which results in a view tag size of 6 bits when plugged into the formula. With around 56000 daily outputs, there will be about 880 matches per day. If the long-term daily volume increases to about 62000 ouputs, the view tag size will be increased to 7 bits and the number of matches will drop to 480 per day.

View tag size encoding

The view tag size must be encoded explicitly to avoid UX issues with missed transactions at times when the view tag size changes. This can be done with a 1-byte field per transaction (all outputs will use the same tag size).

I'm proposing a range of valid values for the tag size between 1 and 16 bits (instead of the previously proposed 5-20 bits).

A 1-bit view tag requires num_outputs_100k > 133333. Since there are always at least 100k coinbase outputs, the 1-bit view tag would be "too large" only if there were fewer than 120 transactions per day, which hasn't happened on mainnet except for a few weeks shortly after launch in 2014.

A 17-bit view tag that would overflow the supported range would require num_outputs_100k > 8738133333, an increase of more than 1000x over the current tx volume. If this somehow happened, the number of false positives would exceed 960 per day, which would only have performance implications for light wallets, but would not cause any privacy problems.

So the proposed range of 1-16 bits is sufficient.

Complementary view tag

Regardless of the tag_size, the view tag is always encoded in 2 bytes as a 16-bit integer per enote. The remaining bits are filled with a "complementary" view tag calculated from s^sr_1, which needs a different private key.

For example, with tag_size = 6, the 16 bits would be CCCCCCCCCCTTTTTT, where T is a view tag bit and C is a complementary view tag bit. This construction ensures that only a few K_o recomputations are needed per 65536 enotes.

Wallet type	For each enote	For ~720 enotes/day	For 1/65536 enotes
Full wallet (ViewAll)	1x DH	1x DH	3x recompute K_o
Full wallet (ViewReceived)	1x DH	1x DH	1x recompute K_o
Light wallet (ViewAll)	-	1x DH	3x recompute K_o
Light wallet (ViewReceived)	-	1x DH	1x recompute K_o

A 3rd party scanner would need to be provided with the view-received key d_vr in order to calculate the full 16-bit view tag for normal enotes. There are 3 deterrents against such usage:

1. Complete loss of privacy for received payments (everything is leaked including amounts).
2. Self-send enotes are not detected this way.
3. The CPU savings for the light client are small (~100 ms/day at best).

Btw, in order to properly support self-sends, I think the self-send shared secrets need to include the output index in the hash, otherwise a 2-out transaction with two self-sends (1 self-spend and 1 change) would have the same shared secrets (and view tags) for both outputs. I'm not sure how it's handled in the current Seraphis library.

I'll answer myself here: this is currently solved by including K_o in the view tag hash, which is actually a better solution than just using the output index.

I don't have bandwidth to respond to everything, but wanted to clarify this:

For the current Jamtis code in seraphis_lib with PLAIN, DUMMY, CHANGE, & SELFSPEND enote types

What is the reasoning for these types?

PLAIN = normal enote
DUMMY = self-send with zero amount inserted to ensure a tx has at least one self-send (a requirement added so that a remote scanner only needs to transmit key images from txs with view tag matches instead of all txs)
CHANGE = change
SELFSPEND = non-change/dummy self-send (e.g. churn), which is differentiated from change to aid bookkeeping

DUMMY = self-send with zero amount inserted to ensure a tx has at least one self-send (a requirement added so that a remote scanner only needs to transmit key images from txs with view tag matches instead of all txs)

I don't think this one is needed. It can be a CHANGE with zero amount. My calculations above assume that only 3 K_o recomputations are needed (1x PLAIN, 1x CHANGE, 1x SELFSPEND).

Wallet Tiers

Tier	Knowledge	Off-chain capabilities	On-chain capabilities
GenAddr	s_ga	generate public addresses	none
FilterInvolved	d_fi	recognize all public wallet addresses	calculate view tags
ViewReceived	d_vr, d_fi, s_ga	all	view all received enotes (w/o amounts) except for change and self-spends
ViewSent	s_vs, d_fi, s_ga	all	view all change and self-spends enotes (w/o amounts)
HotWallet	s_vs, d_vr, d_fi, s_ga,	all	view all received, change, and self-spends enotes (w/o amounts)
PaymentValidator	d_fi, d_vr, d_ua, s_ga,	all	view all received enotes with amounts
ViewBalance	k_vb	all	view all enotes, calculate key images
Master	k_m	all	all

I think there is an infinite number of ways how the protocol can be made more complex with more features that we think might be useful. However, we should avoid unnecessarily bloating the specs and overloading users with choices.

Let's have a look at the features of Jamtis that have clear evidence of popular demand:

GenAddr wallet tier

It has been voiced many times by merchants that providing the ability to generate addresses without view access to the wallet is important. It came up for example during the discussion about deprecating integrated addresses: monero-project/meta#299 (comment)

FilterReceived wallet tier and dynamic view tags

The poplar demand for wallets that scan the blockchain on behalf of users is clear from the existence of such services, e.g. mymonero.com. The FilterReceived tier together with the dynamic view tag size provides a solution that preserves privacy to certain extent and only has a small fixed cost over providing full view access to the wallet.

Full view access tier

There is plenty of evidence that view-only wallets that cannot recognize spent outputs and thus display incorrect balance are bad for UX.

monero-project/monero#8613
monero-project/monero#7365
https://old.reddit.com/r/Monero/comments/4ce5ui/what_is_the_use_of_view_only_wallet_when_its/

Robust output recognition

Again, there is plenty of evidence that the lookup-table based approch for recognizing owned outputs is problematic for UX and sometimes causes the wallet to miss payments.

monero-project/monero#8138
https://monero.stackexchange.com/questions/10704/accounts-got-deleted-from-the-wallet
https://monero.stackexchange.com/questions/10184/funds-received-from-subwallet-are-not-showing

Janus attack protection

The attack was described in an official advisory here: https://web.getmonero.org/2019/10/18/subaddress-janus.html

The proposed mitigation is quite problematic for UX:

Use separate wallets instead of separate subaddresses if you need to keep two different addresses completely unlinkable. Alternatively, do not notify any sender of the receipt of funds to your wallet.

Users not aware of the existence of this attack might get exposed, so I think there is a sufficiently strong case to mitigate it, even if the cost is 51 extra characters in every address.

Payment validator (ViewReceived) wallet tier

I think this feature does not have such a strong case as the above features. Its functionality can be achieved with the ViewBalance tier, with only a small loss of privacy compared to current CryptoNote view-only access (explained by @UkoeHB here). I coudn't find any evidence of merchants requesting more privacy for view-only wallets.

However, since we are on the track to implement full-chain membership proofs, this might tip the balance in favor of a wallet tier that cannot strongly identify outgoing payments.

Such a wallet would not be compatible with other wallet software if it was using different derivation paths and additional private keys. Yes, you can't prevent someone from inventing custom wallets that allow users to lose privacy, but it should not be supported by the official software.

I guess we should clarify what we're actually trying to do here: are we trying to describe an address protocol or a specific wallet design? Assuming that we use some deterministic seed phrase and/or we allow the view-balance key to be exported, people will be able to make worse wallet implementations that offer more efficient scanning. If we're trying to mitigate this issue at the addressing protocol level using incentives, we need to make this actually part of the protocol. Dynamic view tags are protocol level, and so is normal enote DH exchanges against addresses. Self-send secrets and account secret derivation really aren't. If the protections we're providing are based on telling users to pretty please not do certain actions with their keys, we can have the "official" wallet software not do these things, but we're back to square one with @UkoeHB's key offloading initial observation. Enough tempting, and users might move from the "official" wallet software to a worse implementation, and then our wallet design means nothing.

I don't see exactly how it's useful for hot wallets. We already have the "PaymentValidator" tier that is intended as a (view-only) hot wallet. If you are using a hardware wallet, presumably you have a "ViewBalance" tier and the hardware wallet only stores the master key. Without seeing amounts, you can't prepare a transaction to be signed with the hardware wallet.

How cold/hot wallet setups work today is that the hot wallet is connected to the internet and has the view key, while the cold wallet is air-gapped and has the spend key. The hot wallet first scans for incoming transactions, and then sends those to the cold wallet. The cold wallet calculates key images and sends the key images back to the hot wallet to scan for outgoing transactions. When the user wants to spent, the hot wallet collects the output distribution, sends it to the cold wallet, the cold wallet signs, then sends the signed transaction back to the hot wallet wallet, which submits to the network.

In the proposed cold/hard wallet setup, the hot wallet would only scan all involved transactions without knowing amounts and send public transaction info to the cold wallet. When a user wants to spend funds, they do input selection and ownership proofs on the cold wallet, then send the signed transaction to the hot wallet, which finishes the transaction by completing membership proofs and submitting to the network. This type of setup is orthogonal from PaymentValidator, because it can be used without a PaymentValidator wallet, but if someone wants to spend funds that are collected from a PaymentValidator, they have to have some wallet somewhere with signing capabilities. And with the "HotWallet" tier, you can export outgoing txs to a signing wallet without knowing amounts, which is especially beneficial for merchants who frequently have large amounts of income flow to turn over.

I think there is an infinite number of ways how the protocol can be made more complex with more features that we think might be useful. However, we should avoid unnecessarily bloating the specs and overloading users with choices.

This is a UX problem, and IMO out of the scope of Jamtis. Which options are given to the users is down to the exact wallet implementation, because not all wallets have to support all features. It is my opinion that we should work as hard as possible to make Jamtis as flexible as possible for all sorts of users, while minimizing risk at the protocol level by using incentives to make users have fewer reasons to bomb their privacy for the sake of efficiency. I want to reiterate that an extremely simple "solution" to scanning inefficiency is giving away the view-balance key. That is always a choice for the user, so we have to always be competing with that. Making the protocol less flexible for users will squeeze users into worse privacy tiers. I agree that if there is some "official" implementation, their shouldn't be complete freedom on the part of the users to expose certain secrets, but again, that's outside of the scope of Jamtis IMO.

Derive s^sr_2 to decrypt the amount and the 1-bit flag. The flag will tell you if this self-send should be displayed in the transaction history (because the user actively sent funds to their own address) or rather be subtracted from the spent amount (because it's a change enote).

Okay, this makes a lot of sense actually. I like this idea.

I guess we should clarify what we're actually trying to do here: are we trying to describe an address protocol or a specific wallet design?

This is a UX problem, and IMO out of the scope of Jamtis.

Jamtis specifies part of the Seraphis wallet design, which includes:

1. The format of the mnemonic seed.
2. How private keys are derived from the mnemonic seed.
3. How public addresses are generated from private keys.
4. How enotes are constructed based on public addresses.
5. How owned enotes are recognized based on wallet private keys.

All of this must be part of the specs to avoid fragmentation of the Monero ecosystem. This way, a user can restore their mnemonic seed into any compliant wallet implementation and will see the correct balance.

For example, the original CryptoNote protocol didn't specify how the private spend key and view key are derived and as a result, we got at least two incompatible wallet designs that produce different view keys:

https://xmr.llcoins.net/addresstests.html

You may have noticed a critical difference between this style and the Electrum Style: MyMonero's Private View Key derivation is done by hashing random integer a, while Electrum Style derivation is done by hashing the Private Spend Key. This means that 13 and 25 word seeds are not compatible – it is not possible to create an Electrum Style seed (and account) that matches a MyMonero Style seed (and account) or vice versa; the view keypair will always be different.

How cold/hot wallet setups work today is that the hot wallet is connected to the internet and has the view key, while the cold wallet is air-gapped and has the spend key. The hot wallet first scans for incoming transactions, and then sends those to the cold wallet. The cold wallet calculates key images and sends the key images back to the hot wallet to scan for outgoing transactions. When the user wants to spent, the hot wallet collects the output distribution, sends it to the cold wallet, the cold wallet signs, then sends the signed transaction back to the hot wallet wallet, which submits to the network.

Current Seraphis/Jamtis design improves this flow significantly. The hot wallet is a ViewBalance tier that can see everything and allows the user to safely prepare an unsigned transaction, including the exact list of enotes to spend and all outputs. Then there is one interaction with the cold wallet, which only displays the most basic information for confirmation and then sends the ownership proofs to the hot wallet. The hot wallet then performs decoy selection, completes the membership proof and submits the transaction to the network.

In the proposed cold/hard wallet setup [...]

I don't see a strong case to support the additional tiers in the official specs.

input selection [...] on the cold wallet

Hardware wallets have a tiny screen and a few buttons, which are enough to display and confirm the intent, but are completely inadequate for a full wallet interface. The setup with a ViewBalance main wallet and a signing hardware wallet has a much more appealing UX and achieves the main goal why users purchase hardware wallets: theft protection.

Current Seraphis/Jamtis design improves this flow significantly. The hot wallet is a ViewBalance tier that can see everything and allows the user to safely prepare an unsigned transaction, including the exact list of enotes to spend and all outputs. Then there is one interaction with the cold wallet, which only displays the most basic information for confirmation and then sends the ownership proofs to the hot wallet. The hot wallet then performs decoy selection, completes the membership proof and submits the transaction to the network.

This new flow would still be possible with almost exactly the same operations if you separate one-time address recovery and amount recovery with different keys.

I don't see a strong case to support the additional tiers in the official specs.

I think this might be tied to the following argument you make later:

The setup with a ViewBalance main wallet and a signing hardware wallet has a much more appealing UX and achieves the main goal why users purchase hardware wallets: theft protection.

For the same reason why one would want theft protection, one would want to hide amounts: mitigating compromised internet-connected computing devices. If you are working hard to mitigate this threat, I don't see why it would be such a leap to assume that some wouldn't also want to also hide the balances from their assumed-to-be compromised devices, given the option.

Hardware wallets have a tiny screen and a few buttons, which are enough to display and confirm the intent, but are completely inadequate for a full wallet interface

They wouldn't have to implement any interfacing, just the transaction input selection algorithm, which is neither or a compute heavy task nor memory heavy. At any rate, this also wouldn't be a problem for cold wallets using a normal laptop/desktop/smartphone.

All of this must be part of the specs to avoid fragmentation of the Monero ecosystem. This way, a user can restore their mnemonic seed into any compliant wallet implementation and will see the correct balance.

The key word here is compliant. Thus far we've been trying to mitigate people moving off of a compliant wallet design to something more suitable. The only way we can do this is by using incentives, which is why it's my opinion that its fruitless to base the privacy off of doing optionally doing something with your keys, given that you can interact with the ecosystem all the same. As such, telling users to not multiply by some d_ua factor is merely a recommendation, and we should not expect the ecosystem to evolve in that manner. And since, IMO, separating the one time address recovery and amount recovery in self-send enotes provides a tangible benefit for a real use case without affecting the performance of others, we should move forward with that feature.

Regardless of the tag_size, the view tag is always encoded in 2 bytes as a 16-bit integer per enote. The remaining bits are filled with a "complementary" view tag calculated from s^sr_1, which needs a different private key.

I like the complimentary view tag as long as s^sr_1 depends upon DH_2 only, and not DH_1. The only difference I would make here is that since the length of the view tag needs to be encoded anyways with 4 bits, we can extend the complimentary view tag by 4 bits to squeeze extra performance out of the complimentary view tag without increasing transaction sizes. As for the table you provided, if we use the technique of encoding the enote type into the blinding factor, we only have to recalculate K_o once for any enote.

one would want to hide amounts

Ideally, one would want to hide everything.

Users can just share d_fr with the hot "wallet" and scan the remaining 200 KB/day on the cold wallet. Assuming a USB 2.0 interface between the hot and the cold wallet, you can easily download several months of matches per second. This way, the hot wallet doesn't even learn which outputs you own. The bottleneck would be the DH calculation in the hardware wallet, but even tiny Cortex M4 can do a few hundred per second (1-2 minutes per month of enotes, bearable for this level of privacy).

They wouldn't have to implement any interfacing, just the transaction input selection algorithm

Most wallets nowadays allow for manual input selection, which needs an interface that lists all unspent inputs and their amounts.

The key word here is compliant.

I'm pretty sure that most wallet implementations will follow the specs. "Non-compliant" is not something you want to explain to your users.

As such, telling users to not multiply by some d_ua factor is merely a recommendation, and we should not expect the ecosystem to evolve in that manner.

IMO, "users might do it anyways" is not a valid reason to put something in the specs.

The original reason why d_ua was added is explained in this comment:

In the current version of Jamtis, a find-received service and generate-address hub can combine to create a payment validator. This seems suboptimal, so I'd like to add an internal private key to the key structure.

This issue no longer applies to the proposed scheme, so I don't see a reason to keep it. Having a separate wallet tier that can identify all owned enotes without amounts facilitates the existence of scanning services that harm users' privacy. Remote scanners that don't have to deal with decoy enotes have much smaller storage and bandwidth requirements, so I see this wallet tier as the most likely one to be used. Unfortunately, it also makes the dynamic tag feature and the extra pubkey in each address redundant.

The only difference I would make here is that since the length of the view tag needs to be encoded anyways with 4 bits, we can extend the complimentary view tag by 4 bits to squeeze extra performance out of the complimentary view tag without increasing transaction sizes.

It would actually increase transaction size a tiny bit because you would need an extra byte per output, while the current proposal only needs a byte for the whole tx. I'm also not sure if allowing different view tag sizes for every input is a good idea for tx uniformity.

As for the table you provided, if we use the technique of encoding the enote type into the blinding factor, we only have to recalculate K_o once for any enote.

At least twice, because self-sends have a different shared secret.

As for the view tag filter target, when you say "target" here, do you mean in the sense that it is a relay-enforced minimum amount of filtering to do?

The filter target should be 480 enotes/day. Because the view tag filter rate must be a power of 2, this will actually result in a range from 480 to 960 enotes per day depending on the tx volume. If we "average the averages" over all possible values of tx volume, this will give a mean of 720 enote matches per day, or roughly 1 match per block, which is what was suggested by @jeffro256. I think this is close to the upper limit of what is acceptable for light wallet clients (~200 KB/day) and should provide a good number of false positives even if there was a short term drop in tx volume.

The fomula to calculate the view tag size in bits is:

tag_size = trunc(log2(3 * num_outputs_100k / 200000))

I think we should also enforce a maximum size as well for uniformity purposes, make the minimum not so lax, while also allowing wiggle room for fluctuation between signing and propagating. Let's say that we want to plan for a fluctuation of 25% in the value of num_outputs_100k between signing and submitting. Nodes could do the following:

min_relay_tag_size = round(log2(num_outputs_100k * 3 / 4 / 100000))
max_relay_tag_size = round(log2(num_outputs_100k * 5 / 4 / 100000)) 

The wallets would do the following:

wallet_tag_size = round(log2(num_outputs_100k / 100000))

When it came time for transaction verification, nodes would check that min_relay_tag_size <= wallet_tag_size <= max_relay_tag_size.
The worst case scenario for a wallet is that between signing and propagating, the enote volume over the last 100k blocks goes up or down more than 25%, and the transaction does not enter the mempool and has to be edited. Also, round(log2()) can be implemented with integer instructions portably as well, but its a little more complicated.

As for the view tag filter target, when you say "target" here, do you mean in the sense that it is a relay-enforced minimum amount of filtering to do?

No, I mean this is the target we plug into the formula. You can see that the bitsize is calculated for 2/3 of matches per block. If we could have fractional bits, the target of 480 enotes/day would be exact. The range 480-960 is an artifact of rounding down.

wallet_tag_size = round(log2(num_outputs_100k / 100000))

Since round(log2(num_outputs_100k / 100000)) = trunc(log2(num_outputs_100k / 100000)+0.5) = trunc(log2(sqrt(2) * num_outputs_100k / 100000)), you'll effectively get a range of from 720/sqrt(2) to 720*sqrt(2), which is about 509-1018. Is there a benefit to this compared to the range 480-960 I'm proposing?

I think we should also enforce a maximum size as well for uniformity purposes

My idea was that nodes would use the same formula, but would apply the formula at each of the last let's say 10k blocks and build a range based on that. Effectively, this would give you all possible view tag sizes from the last 10k blocks as valid values. Most often, this would just be a single value. This means transactions would never be invalidated unless they took more than 10k blocks between signing and propagation.

It would actually increase transaction size a tiny bit because you would need an extra byte per output, while the current proposal only needs a byte for the whole tx. I'm also not sure if allowing different view tag sizes for every input is a good idea for tx uniformity.

Okay yes, this is a much better idea... I was assuming that view tag sizes were going to be per-enote,

At least twice, because self-sends have a different shared secret.

True, my bad

Ideally, one would want to hide everything.

Users can just share d_fr with the hot "wallet" and scan the remaining 200 KB/day on the cold wallet. Assuming a USB 2.0 interface between the hot and the cold wallet, you can easily download several months of matches per second. This way, the hot wallet doesn't even learn which outputs you own.

This is a really excellent reason to not include d_ua: the ones who are paranoid enough to do this type of setup would probably be willing a couple extra minutes per month of DH calculations. Okay, I'm sold on keeping out d_ua for now.

My idea was that nodes would use the same formula, but would apply the formula at each of the last let's say 10k blocks and build a range based on that. Effectively, this would give you all possible view tag sizes from the last 10k blocks as valid values. Most often, this would just be a single value. This means transactions would never be invalidated unless they took more than 10k blocks between signing and propagation.

This covers almost everything except for slight future changes. There's three edge case scenarios in which honestly built transactions might fail to propagate on the network:

1. The wallet's node is ahead of its other connected nodes, and its current tag_size value is a previously unseen value because of a slightly different num_output_100k. The tx will be accepted on this node, but not propagated to other mempools (depending on the exact p2p rules, this issue might be mitigated b/c nodes won't share mempool information until they are synced to the same chain height).
2. Same as scenario #1, but the inconsistency of this node with the network is due to a reorg
3. The node gives a wallet a num_output_100k value, then reorgs, then the wallet submits a tx with an invalid num_output_100k. The tx will not enter any mempool, but at least this time, the user will get an error message.

In addition, this solution doesn't require storing a history of allowed values (although you could make the history very small with O(1) access using a map of values -> number of instances of that value).

you'll effectively get a range of from 720/sqrt(2) to 720*sqrt(2), which is about 509-1018. Is there a benefit to this compared to the range 480-960 I'm proposing?

Not necessarily, although the range is biased towards a slightly higher degree of privacy.

The wallet's node is ahead of its other connected nodes, and its current tag_size value is a previously unseen value

I don't think we need to handle this case. It can already happen with decoys, which might still be invalid (locked) for other nodes that are behind. I don't know how it's handled currently.

reorg

This can be solved by shifting the calculation 10 blocks to the past, i.e. we will use the range [chain_tip-100009, chain_tip-10] instead of [chain_tip-99999, chain_tip] to calculate the view tag size. If there was a reorg deeper than 10 blocks, transactions could be invalidated anyways due to invalid decoys.

the range is biased towards a slightly higher degree of privacy

Actually, it is biased towards a lower degree of privacy.

The two formulas can be approximated as follows:

trunclog2(3 * x / 200000) ~ trunclog2(x / 66667)
roundlog2(x / 100000)     ~ trunclog2(x / 70711)

These approximations are accurate to within 0.01%, which is more than enough for the intended use case. trunclog2 is a very simple function that returns the index of the most significant one-bit. It can be calculated by repeated shifting and some CPUs even have a dedicated instruction for it (x86 BSR).

I ran the numbers for the two distributions and here are the results (converted to the number of false-positive matches per day):

formula	min	max	mean	st. dev.	median
trunclog2(x / 66667)	480.0	960.0	720.0	138.6	720.0
trunclog2(x / 70711)	509.1	1018.2	720.0	144.8	689.1

The second formula has median lower than the mean, which means it's skewed towards smaller values. It makes sense if you think about it: both distributions have the same mean of 720 but the second one has a higher min and max, so it must be skewed.

If we can accept a worst case filtering deviation of 2x, versus 1.5x or 1.41x with our current ranges, then we can get a best-case aggregate deviation of 0x with user cooperation using the following scheme. For any given value of num_output_100k, consensus rules allow wallets to choose between two values of view tag filter: t_1 = trunclog2(num_output_100k / 1000000) and t_2 = t_1 + 1 (your stored history method can be used here or not). The wallet gets to choose between these two values, and the general idea is that they will pick between these two values in a ratio that makes the aggregate filtering rate close to 720/day, no matter what the value of num_output_100k is. This is how it is done:

Our aggregate filtering rate is can be defined as follows, where v is enote volume per block (this is to make calculations simpler, practically we would set v = num_outputs_100k / 100000 or some other way of smoothing this value):

F(v) = v / (w(v) * p(v) + (1 - w(v)) * 2p(v))

where p(x) = 2 ^ trunclog2(x) [the greatest power of 2 less than or equal to x]
     and v = enote volume per block
     and w(v) is a weight function with values [0, 1] between choice of tag size t_1 and t_2 for a given v

Ideally, we want F(v) = 1 (aggregate filtering rate is 1 enote per block). If we set F(v)=1 and solve for w(v), we get:

w(v) = 2 - v/p(v)

So to pick between between t_1 and t_2, wallets will generate a random value c in range [0, 1]. If c <= w(v), then wallets will pick t_1, else they will pick t_2.

This will get us close to the ideal filtering rate, 720 enotes/day, no matter what num_outputs_100k is, assuming most people cooperate. If we don't assume that, the aggregate filtering rate can swing anywhere from 360 to 1440 enotes/day.

We could optionally make the choice c deterministic as a function of num_output_100k and input_context, which would maybe increase uniformity while having the same effect for the aggregate filtering target. The downside is transactions would be a couple bytes bigger because we would need to encode num_output_100k instead of t_1 or t_2. Also, if a user can choose multiple values for input_context (e.g. by changing their ring member set), then they could brute-force c. However, that's a lot of work to encode 1 bit worth of information; there's already much, much more efficient ways to encode arbitrary data. Overall, probably not worth making c deterministic, but that's always an option.

Here is a better method that follows the target of 720 enotes/day and doesn't rely on user cooperation.

Global parameters

There is a single global parameter:

filter = max(1, 6553600000 / num_outputs_100k)

It's an integer between 1 and 65536 and should be specified in every transaction. This needs 2 bytes per tx, but it still has a slightly smaller blockchain footprint than the current Jamtis spec:

# of tx outputs	current spec	proposal
2	6 bytes (2x view tag, 2x tag hint)	6 bytes (1x filter, 2x view tag)
16	48 bytes (16x view tag, 16x tag hint)	34 bytes (1x filter, 16x view tag)

Enote parameters

Every enote has two "fingerprints":

1. fingerprint1 = H("jamtis_fingerprint1" || DH_1 || K_o) % 2^16
2. fingerprint2 = H("jamtis_fingerprint2" || s^sr_1) % 2^64

fingerprint1 is a 16-bit integer and fingerprint2 is a 64-bit integer.

View tag derivation

The 16-bit view_tag is calculated as follows:

view_tag = (fingerprint2 % filter - fingerprint1) % 2^16

Checking for a match

The view tag is checked with the following condition:

(fingerprint1 + view_tag) % 2^16 < filter

For non-owned enotes, (fingerprint1 + view_tag) % 2^16 is a uniformly distributed random number, so it will match for filter/65536 enotes on average, which simplifies to 1 enote per block if we substitute the definition of filter.

For owned enotes, (fingerprint1 + view_tag) % 2^16 equals to fingerprint2 % filter by construction, which is always less than filter. Additionally, if we know fingerprint2, we can eliminate all but 1/filter false matches by checking:

(fingerprint1 + view_tag) % 2^16 ?= fingerprint2 % filter

This gives an overall false positive rate of 1/65536 for wallets that are able to calculate both fingerprints.

I was interested to see how the dynamic view tags work in practice, so I took real blockchain data (the number of RingCT outputs in every block) from about March 2018 to August 2023 (blocks 1519796-2959795). It's a total of 1440000 blocks, interpreted as 2000 days times 720 blocks per day (in reality it was about 2004 days).

Method 1

The first method uses the "discrete" view tags based on the formula:

view_tag_size = trunclog2(num_outputs_100k / 66667)

We can see that the view tag size nicely follows the long-term trend of growth, growing from 4 bits in 2018 to 6 bits, with a few periods of 7 bits during high transaction volume in 2022. Around day 1700, fluctuations in the view tag size occur for about 4 days, which is the time when the explicitly encoded size would come handy.

The number of false positive matches roughly follows the target of 720/day, with some significant fluctuations. Around day 400, the daily matches shot to over 2000/day, while the lowest number of matches recorded is 231 on day 1624. The average over the whole period is 780 matches/day.

Method 2

The second method uses the "smooth" view tag method described in the previous comment.

Here the filter rate tracks the long-term trends much more precisely.

However, if we look at the number of false positive matches, there is not a huge qualitative difference from the first method. It follows the target of 720/day slightly more closely, but short-term tx volume fluctuations still make it deviate quite far. Around day 400, the daily matches also exceed 2000. The lowest number of matches is 301 on day 960. The average over the whole period is 762 matches/day.

Conclusion

The "smoother" methods of following the daily target are not much better than the simple discrete view tag due to short-term tx volume fluctuations. I therefore think that we should adopt method 1, which is simpler and better for tx uniformity.

view_tag = (fingerprint2 % filter - fingerprint1) % 2^16

Damn, that's clever! I don't know if it was intentional, but here's a cool feature of doing the view tags this way: you can't check against the view tag using only knowledge of fingerprint2; fingerprint1 acts as a random "mask" to the fingerprint2 % 2^16 value. What this means for scanning setups is that the incentive to only send d_vr instead of d_fi/d_fr is destroyed! If you only send d_vr to a light wallet server, they only thing they can do is compute s^sr_1 and nominal address tags, but they'd have to send you that information for every single enote; they can't actually weed any out. If they also had s_ct, they could 100% identify all incoming enotes, but it would require huge amounts of processing for the server (since they can't use view tags), and it still wouldn't cover self-sends.

If you were calculating fingerprint1 and fingerprint2 separately (i.e. light wallet), the server would need to send fingerprint1 unless the client wanted to do 2x DH ops instead of 1x DH ops per each filter enote, but since that's only 2 bytes, it's still much much smaller than the nominal address tag (16 bytes), which is what it would replace.

For this reason alone, I think this is the best way thus far.

Here the filter rate tracks the long-term trends much more precisely.

The "smoother" methods of following the daily target are not much better than the simple discrete view tag due to short-term tx volume fluctuations

I think the long term rate of filtering is more important anyways. When a user is scanning a small amount of volume, it won't matter much from a UX perspective if the small volume is a little bigger. It's when the scanning process would otherwise take many minutes or even hours (poor souls), that the long-term filtering rate would make a difference performance-wise.

I therefore think that we should adopt method 1, which is simpler and better for tx uniformity.

Since we ostensibly have to choose filter from a common, public, deterministic list of values, what would this non-uniformity tell us exactly? It would (maybe) reveal the time we constructed the transaction to the granularity of block-time (2 minutes). In most cases, this is already known by the nature of how transactions propagate in nodes' mempools. Discretized fees and ring member selection also leak this information. And in the future, if/when we go for FCMPs, a hash of the root of the curve tree for a given block will need to be included to verify the transaction, which would further cement for external observers when in time a transaction was constructed.

you can't check against the view tag using only knowledge of fingerprint2; fingerprint1 acts as a random "mask" to the fingerprint2 % 2^16 value

Thinking about it now, this is orthogonal to the smoothness of the view tag filtering rate, we could always include a 2-byte residue of DH_1 in the calculation of the complementary view tag.

With that in mind, I'd agree that doing tag_size = trunclog2(3 * num_outputs_100k / 200000) is probably best. Also, thanks for doing those simulations, that was actually really insightful!

It would (maybe) reveal the time we constructed the transaction to the granularity of block-time (2 minutes). In most cases, this is already known by the nature of how transactions propagate in nodes' mempools. Discretized fees and ring member selection also leak this information. And in the future, if/when we go for FCMPs, a hash of the root of the curve tree for a given block will need to be included to verify the transaction, which would further cement for external observers when in time a transaction was constructed.

It would leak the approximate time when the transaction was signed, which might be a long time before it's actually submitted to the network (e.g. multisig or offline-signed transactions). This might be considered to be a regression because Seraphis already allows for the membership proof to be added just prior to publishing the transaction, which removes the leaks caused by the member selection and only leaves fees as a possible leak.

It could be solved by removing view tags from the signed data and sign them later with the membership proof, but that might be problematic.

After working on implementing the new changes, I think the complementary view tag should be bound to DHE_2 instead of s^sr_1 (as well as a residue of the primary view tag calculation). The reason for this is that, if we make the complementary view tag a function of s^sr_1, in most cases, we will need to do the DH operation anyways, but now we're also doing 4 hash operations (derive plain s^sr_1, hash plain s^sr_1 -> complementary view tag, derive self-send s^sr_1, hash self-send s^sr_1 -> complementary view tag) instead of 1 (hash DHE_2 -> complementary view tag) for each 720 enotes/day. So in summary, this is how I think the view tag computations should go:

npbits = the number of primary view tag bits, explicitly mentioned in the transaction
ncbits = the total size of the view tag in bits - npbits
primary_view_tag || primary_view_tag_residue = H(DHE_1, K_o)
complementary_view_tag = H(DHE_2 || primary_view_tag_residue)
view_tag = primary_view_tag[first npbits] || complementary_view_tag[first ncbits]

The only downside I can think of when complementary_view_tag binds DHE_2 instead of s^sr_1 is the DLP solvers can check both view tags on self-sends transactions (versus just the primary view tag on self-sends) if they know your public address. However, they will not know the types of self-sends nor what the outgoing amounts are.

I think the complementary view tag should be bound to DHE_2 instead of s^sr_1

This has an undesirable side effect of revealing¹ self-spends to anyone in possession of d_vr, e.g. a PaymentValidator tier, which would only be able to calculate primary_view_tag otherwise. Because PaymentValidator is likely to be a hot wallet with an increased risk of key compromise, I think this side effect should be avoided and the complementary_view_tag should be calculated differently for self-sends. The cost of this can be just 1 extra hash, which is negligible compared to the 2 DHE calculations that precede it.

¹ Technically, it reduces the false-positive rate to 1/65536, which is about 6 false matches per week with current tx volume.

Ah shoot you're right, I wasn't thinking about that tier... nevermind.

What is the reasoning for these types? AFAICS we only need 2 types to tell the wallet if the enote should be displayed in history or not (this could also be achieved with a 1-bit flag encrypted with s^sr_2, so only 1 extra K_o recomputation is needed).

We can't do this because if you have a 2-output tx (using a shared xK_e), and one of your outputs is a self-spend and the other is a change output, Ko will be shared between the enotes which will 1) reveal that this is a tx where a user is trying to churn and 2) burn funds for one of the enotes.

So we will need to do 2x extra Ko re-computations (3 total) for each enote that matches both view tags.

We can't do this because if you have a 2-output tx (using a shared xK_e), and one of your outputs is a self-spend and the other is a change output, Ko will be shared between the enotes

This can be fixed by including the output index in the shared secret calculation. That would make both outputs have unique K_o.

In fact, the same problem applies to normal enotes. If someone sends a 2-out tx where both outputs go to the same address, both outputs will have the same K_o. This is non-standard, but AFAIK the protocol allows it. This shows that the input_context that only consists of key images is insufficient to ensure the uniqueness of K_o and fails to prevent "the burning bug".

In fact, the same problem applies to normal enotes. If someone sends a 2-out tx where both outputs go to the same address, both outputs will have the same K_o. This is non-standard, but AFAIK the protocol allows it.

The protocol doesn't allow it though. One of the rules of Jamtis is that every transaction contains at least one self-send output (for this reason, as well as allowing third-party light wallet servers to trim the key image set and give the clients access to their outgoing transactions). If you have 2 normal outputs to the same destination, and need at least one self-send, that means you wouldn't be doing the shared xK_e optimization.

Also, having K_o bound to the tx output index would be really annoying (AKA involve brute-forcing private ephemeral keys) since IIRC the enotes in Seraphis are ordered by one-time addresses.

The protocol doesn't allow it though. One of the rules of Jamtis is that every transaction contains at least one self-send output

How is this rule enforced?

Imagine the following scenario:

Mallory registers at an exchange and is provided with a deposit address. She crafts a 2-output transaction without change, sending both outputs to the deposit address, each output worth 1000 XMR. In order to do this, she needs to provide inputs with a total sum of exactly 2000 XMR + fee, but that should not be hard to do.

Unless the exchange has a wallet that is aware of the burning bug, Mallory will be credited with 2000 XMR and can proceed to withdraw the funds back to her custody. However, the exchange will later realize that only one of the 1000 XMR outputs can be spent. This scam can be repeated until the wallet of the exchange is completely drained. It only costs some tx fees.

Relying on all wallet implementations to be able to detect this bug is not going to work, so there are basically two solutions:

1. Mandating unique K_o within each transaction as a consensus rule.
2. Including the output index when deriving K_o.

IIRC the enotes in Seraphis are ordered by one-time addresses

Is this a consensus rule or just a recommendation for tx builders?

How is this rule enforced?

That rule specifically is not enforced at a consensus level, it's just a Jamtis rule-of-thumb that is derived from the Seraphis protocol consensus rule that enote outputs within a transaction should be ordered and unique by one-time address. See this code for details and implementation: https://github.com/UkoeHB/monero/blob/eeca802ccee217d26acd8bc89ee69bbd3c47e254/src/seraphis_main/tx_validators.cpp#L365-L367.

In this way, and assuming that input_context differs from transaction to transaction, all cases of the burning bug should be covered.

consensus rule that enote outputs within a transaction should be ordered and unique by one-time address

Cool. It's the first time I hear about this rule. Maybe it's worth adding it to the Seraphis specs?

The current "Implementing Seraphis" paper says the following:

To further ensure uniqueness within a transaction, transaction verifiers must mandate that all values K_e in a transaction are unique.

Uniqueness of K_e is not sufficient to prevent the burning bug as shown above.

Uniqueness of K_e is not sufficient to prevent the burning bug as shown above.

That's true and a good thing to point out more explicitly in the spec. I can open an issue on that repo to clarify that passage.

This brings me to an interesting privacy hiccup when distributing xk_fr to a third-party, under both the new and old schemes: depending on the size of the previous view tag / primary view tag, a third-party will see that outgoing transaction enotes are exponentially more likely to to be owned by a user the more self-send enotes there are. This affects both light wallets and people using the payment validator tier.

We can reason that the number of successful view tag checks within a transaction unrelated to you follows a binomial distribution. Each view tag check is a Bernoulli trial, so we can expect the number of successful view tag checks X for a transaction with n outputs to follow the distribution X ~ B(n, VTFP), where VTFP is the view tag false positive rate. The probability mass function for getting k view tag matches can be written as P(X = k) = (n choose k) * VTFP^k * (1 - VTFP)^(n-k). As an extreme example, someone may implement PocketChange-like feature which breaks up outputs to help users work around the 10-block lock. Let's say they create 16 self-send outputs and the false positive rate is 1/256. All 16 outputs will be matched by view tag, which should normally only have a probability of 2.939 x 10^-39 (the same chance as randomly guessing someone's AES key). This can also happen with 2-output transactions with one self-spend and one change, although not as severe: the probability should be 1/65536.

We need a way to have third-parties scan the information they need without this privacy downside. I propose that we split up the self-send types into three self-send types: SELF_SPEND, PLAIN_CHANGE, & AUXILIARY_CHANGE. When doing an outgoing transaction, enote types SELF_SPEND XOR PLAIN_CHANGE (one or the other, not both) will always be present. For these enotes, primary view tags will calculated as normal. For any additional desired self-sends, we set the primary view tag to random bits and the self send type to AUXILLIARY_CHANGE, but do everything else the same (meaning binding the self-send type to s^sr_1). When it comes time to scan, we also scan all enotes in transactions in which any of the view tags matched even if their view tag did not match (hence "auxiliary"), but only scan them for type AUXILIARY_CHANGE. Unfortunately, this change will more than double the bandwidth required for light wallet clients, but only marginally affect compute time as no extra DH ops are required, and depending on the complementary view tag size, most enotes won't have to have K_o recomputed.

This wouldn't have to slow down non-auxiliary enote scanning at all (besides an extra amount commitment recomputation on an already confirmed owned enote) due to the following reason: since we assume that exactly one of SELF_SPEND or PLAIN_CHANGE is present in a transaction, they can share the same s^sr_1 derivation, and only have s^sr_2 derivation differ (this avoids the problem of sharing xK_e leading to the same K_o). The s^sr_1 derivation for AUXILIARY_CHANGE would differ, which leaves us with the same number of K_o re-computations that we have to do: 1x for plain check and 2x for self-send check.

For any additional desired self-sends, we set the primary view tag to random bits...

To speed up auxiliary enote scanning, we could actually fill the primary view tag bits up with all complementary view tag bits, since we don't care about it matching anyways, but we're also going to check the complementary view tag.

Responding here to a reddit comment.

My first major issue with them is that they break one of the big improvements promised by Seraphis, that being the ability to sign a transaction, let it sit, and then broadcast it later by creating the membership proof just before being shipped off.

Dynamic view tags behave exactly like dynamic fees in this regard. When signing a tx, you already commit to the chain state by selecting a fee amount. If the dynamic fees adjust upwards before the tx is submitted, you might have to mine the tx yourself as it won't be relayed. The same applies to dynamic view tags, with the minor difference that the tag size can adjust both upwards and downwards. The dynamic tag size will adjust very infrequently in typical situations. With current chain history, the last adjustment would have been about a year ago (chart). You can mitigate the risk for pre-signed transactions by signing two versions with the 2 most likely view tag sizes.

Second is that it makes Monero even more complex.

That's a non-argument. All new features make Monero even more complex. The question is if the complexity is worth the benefits it brings. For dynamic view tags, I think the answer is yes if we're already introducing an extra public key in every address just to support 3rd party scanning. If we don't adopt dynamic view tags, I think we should revert back to the original Jamtis design with 3 public keys as it seems like a better compromise between complexity and privacy with 3rd party scanning.

@tevador I can't personally support dynamic view tags if dynamic view tags are part of the signed blob and requires re-signing to adjust the size of them. It's very different from creating a TX, saving a 2-byte view tag, publishing it as 1-byte, then rebroadcasting it as 2-byte if necessary, than re-signing entirely.

If they're not part of the signed blob, then they lose their integrity, as anyone can frontrun a TX with invalid view tags in the mempool.

Accordingly, I'm unable to voice support for this complexity due to the practical issues it'd cause (not just complexity which may cause practical issues).

I will also note that while I'm not up to date on JAMTIS, I leaned towards adding an extra key per @jeffro256. What I'd most like however is not to decide on whether or not to have an extra key, yet to have a complete spec document considered up-to-date (not with hundreds of errata comments) and final barring:

1. Incredibly minor tweaks (DST choices, round counts)
2. Major issues found. I would not call any issues in view tag tiers major unless they fundamentally invalidate the tier.

Though I'm sure this desire to be finite is well shared, meaning my statement of it may not contribute.

I want to reiterate my view that the proposed change to extend Jamtis addresses to 4 public keys to improve 3rd party scanning might be wasteful without dynamic view tags.

The static 8-bit view tag works well with a "medium" transaction volume, which is a range from mid tens of thousands to mid hundreds of thousands of enotes per day. We're presently near the bottom of this range. Within this range, the 8-bit view tag provides both good filtering and sufficient anonymity.

However, if a bear market hit and the tx volume plummeted for some reason, then there would be nearly no privacy advantage compared to the 3-key variant of Jamtis. Similarly, if Monero is successful and the tx volume goes up by 2 orders of magnitude, light wallets might be forced to switch to a less private wallet tier to reduce the bandwidth and computation costs. This would also remove any privacy advantage of the 4-key variant.

In both of these scenarios, 3rd party scanning will suffer a privacy loss and we'll be stuck with longer addresses and bloated specs.

Note that even the 3-key variant of Jamtis significantly improves 3rd party scanning. Currently, light wallet clients have to give up their private view key and leak practically all of their transaction history to the scanning server. With 3-key Jamtis, light wallet clients would only give up their "find-received" private key, which will reveal only some incoming transactions (e.g. recurring payments to the same address) without amounts to the scanning server.

A major advantage of 3-key Jamtis is that the 3rd party scanning improvements come "for free" because they are simply a byproduct of Janus attack protection provided by the 3rd public key, so even if 3rd party scanning doesn't catch on, we won't be wasting anything.

👍

I can't react to comments, apparently, so I'm forced to leave a new post. I hear you, that all sounds sane, and I have no further comments to contribute at this time.

I think what would solve all these issues is a arbitrary-size-by-concensus (with a reasonable limit, e.g. 24 bits) fixed-size-by-relay view tag. It's just as simple to implement because it does not depend on chain data. A cold signed tx won't temporally be invalidated unless you hold it so long that relay rules change (which is already an issue for fees). The view tag can be part of the signed blob without the need for multiple signings. We can adjust for really low and/or current tx volume if the anonymity set gets dangerously small. Conversely, if there is a large outcry from users that the bandwidth/computational requirements are unmanageable, we can manually increase the size (this really shouldn't happen more than a once every several years if at all, since we can assume that most user's machines / network connections will get at least slightly better year over year). Attackers cannot affect the view tag size by spamming the chain. All in all, we would reap the benefit of fixed-size view tags' linear increase in privacy with transaction volume and general robustness, but we could have a community handbrake if things got bad.

I think that we're all trying really hard to look ahead into the future and predict what tech trends will be like and what user's reactions to them will be and we could sit here all day postulating different user's different rationales for doing things, and create a decent solution for that specific use-case. Ideally, we want something that is both flexible and simple, and I think that making the view tags arbitrary size by consensus, but fixed size by relay is the best way to do that.

When signing a tx, you already commit to the chain state by selecting a fee amount

Sure, but that's not quite the same. You can always just set an aggressively high fee rate which practically guarantees that it'll work, whereas the view tag size would have a specific range enforced by consensus (I assume). Unless we take a different approach like @jeffro256 's proposal.

You can mitigate the risk for pre-signed transactions by signing two versions with the 2 most likely view tag sizes.

That seems like an incomplete solution, and isn't guaranteed to work in case of a large surge in volume, whether malicious or not.

The other option would be to have the tag itself signed normally, but its "accuracy level" bundled with the membership proof. That way you can overshoot the number of bits while constructing the transaction, but set its "claimed" accuracy immediately before broadcasting. The downside would be potentially allowing a 3rd party scanner to filter more accurately.

But it might not make much of a difference depending on how many extra bits you fill in: even 4 extras would be expected in 1/16 matches just by random chance, which isn't really that bad, considering how many matches there would already be per day. Especially since not all transactions will use this method.

Maybe this has been brought up before, and if so, then... whoops my bad.

view tag size would have a specific range enforced by consensus

No, the dynamic tag size would be a relay rule, just like fees.

isn't guaranteed to work in case of a large surge in volume

The view tag size is calculated based on the last 100 000 blocks. Even a large surge in tx volume will take weeks to affect the tag size. Note that during hard forks, we give old transactions only 24 hours to be confirmed before they are permanently invalidated (v9, v11, v14 and v16). The view tag adjustment would never permanently invalidate transactions, it would only make it harder, but not impossible, for them to be mined.

I'd be fine with a relay rule so long as on-change, the prior and new value are both valid for a period of 24 hours.

No, the dynamic tag size would be a relay rule

Okay, so not a consensus rule. However it still would, effectively for most users, make it an unusable transaction.

we give old transactions only 24 hours to be confirmed

The 24 hour grace period isn't really an issue since it only happens once every few years and is known in advance, compared to (potentially) within weeks. I think this is a pretty significant tradeoff, potentially not worth the benefits.

Signing multiple different versions of the transaction does work, but it's very much a bandaid-type solution which we should try to avoid.

I'd be fine with a relay rule so long as on-change, the prior and new value are both valid for a period of 24 hours.

The current proposal has the following:

1. 10-block delay between the calculation and the view tag size taking effect. This prevents short reorgs from reverting view tag size changes. Longer reorgs will already invalidate transactions due to decoys.
2. 10000-block (2-week) grace period after each change when both the previous and the current tag sizes are relayed.

However it still would, effectively for most users, make it an unusable transaction.

There is a big difference between invalidated-by-consensus and invalidated-by-view-tag presigned transactions. The former one is useless as you can never hope for the old consensus rules to be restored. The latter can be mined in two situations:

1. If tx volume changes back and the view tag size again matches the one used in the tx.
2. The user solomines the tx themself. Due to the existence of hashpower rental services, this option is available to anyone and probably worth it if the presigned tx is valuable enough.

The 24 hour grace period isn't really an issue since it only happens once every few years and is known in advance

If a 24-hour grace period known 1-2 months in advance (e.g. the v11 fork date was finalized 25 days prior to the fork) to permanently invalidate presigned transactions is acceptable, then I think a grace period of a few weeks to temporarily invalidate presigned transactions is also fine.

Here's my attempt at distilling weeks of conversations into a nice table of different contentious Jamtis proposals crossed against properties, so others don't have to read all those comments.

Abbreviation Table

Abbreviation	Term
XXK	Extra Exchange Key
VT	View tag
ATH	Address Tag Hint
LW	Light Wallet
LWS	Light Wallet Server
BCR	Bandwidth & Computation Requirements

Quick Summary Table of Contentious Diffie-Hellman-Related Jamtis Proposals

	Current	XXK	XXK + 2 fixed-size VTs - ATH	XXK + "dynamic" VT - ATH	XXK + "flexible" VT - ATH
Public Address Size	196	247	244	244	244
Fixes Nominal Address Tag Privacy Issues?	❌	✅	✅	✅	✅
Can do Delegated Public Address Generation?	❌	✅	✅	✅	✅
Transaction Size Change	None	None	None	+1 byte/tx	+1 byte/tx
Doesn't Need Recent Chaindata for VT constr.?	✅	✅	✅	❌	✅
Balances LW BCR Automatically?	❌	❌	❌	✅	❌
Community can change LW BCR w/o fork?	❌	❌	❌	✅	✅
LW Anon Set (Against LWS) Increases w/ Volume?	✅	✅	✅	❌	✅
Scan Speed Change (w/ 8-bit primary VT)	0%	-0.4%	-0.4%	-0.4%	-0.4%
Post-Primary VT CPU Time Required For Scanning	1x	100x	100x	100x	100x

What the Rows Mean

• Public Address Size - The character count for the human-readable Jamtis address you give to sender
• Fixes Nominal Address Tag Privacy Issues? - Under this scheme are you protected against:
  • A light wallet server identifying incoming enotes with 100% accuracy if they know your public address
  • A light wallet server identifying incoming enotes with 100% accuracy if the public address is sent to more than once
• Can do Delegated Public Address Generation? - Can a third-party generate addresses on your behalf without any additional loss of privacy?
• Transaction Size Change - self-explanatory
• Doesn't Need Recent Chaindata for VT constr.? - Are transaction constructors free from needing up-to-date chain info specifically for constructing view tags?
• Balances LW BCR Automatically - Does the amount of enotes matched by a LWS stay relatively constant over long time periods so LW BCRs don't increase over time?
• Community can change LW BCR w/o fork? - Can the community manually change the view tag match rate to meet current user demands?
• LW Anon Set (Against LWS) Increases w/ Volume? - Does the set of transactions per time period that the LWS knows your wallet is limited to increase with transaction volume?
• Post-Primary VT CPU Time Required For Scanning - Compared to current Jamtis, the CPU time required to scan for incoming enotes after performing the primary view tag check is 100 times more. This is because each Twofish decipher op is replaced by a x25519 scalar multiplication op. This means that light wallet clients will do ~100 times more CPU work than before, but the bandwidth needed remains the same.

Summary of "Auxiliary" Enotes

Refer to this comment for the auxiliary enote proposal. This change can apply to any of the above proposals, including current Jamtis. Basically, when a transaction has too many self-send enotes, that fingerprints that tx as owned by a LWS. The proposal would allow LW users to churn and to create pocket change without any additional loss of privacy. The downsides is that if any enote matches a primary view tag, all other enotes in the transaction must be attempted to be scanned against the AUXILIARY_CHANGE enote type. For full wallets, this only entails a 1+ extra hash operation slowdown every time an enote matches a primary view tag, but for light wallets, these extra enotes must ALSO be sent "over the wire", also increasing bandwidth requirements.

Difference between "Dynamic" and "Flexible" View Tags

On-chain, both will be serialized as a fixed size buffer of 3 (optionally 2?) bytes per enote. Additionally, there is one integer per transaction, called npbits, constrained to range 0-24 (16 if view tag buffer only 2 bytes wide), which encodes the number of bits from the front of the buffer used to match the primary view tag. Likewise, ncbits is the number of bits from the back of the buffer used to match the complementary view tag, and it is calculated as ncbits = 24 - npbits. The difference between "dynamic" and "flexible" is how the value npbits is enforced. Under the "flexible" scheme, the value npbits is set to a constant value, enforced by relay rule. Under the "dynamic" scheme, the value npbits is enforced to be a function of the on-chain transaction volume. See this comment for the exact proposed formula. Neither of these schemes cause uniformity issues because at transaction construction time, there is only one correct value to choose for npbits. What's nice about the flexible vs dynamic debate is that, as long as npbits is enforced only by relay rule, the community can switch back and forth between the two proposals as it sees fit without forking.

"Less-Contentious" Shared XXK Tweaks

Here's some extra details about tweaks accumulated through discussion that are shared for all XXK proposals and aren't hotly debated, but might be still worth a mention:

• Suggested by @tevador
  • Get rid of unlock-amounts key in order to help thwart identify-involved fake tier
  • Make one DH privkey depend upon other in secrets derivation tree for same reason
• Suggested by @jeffro256
  • Bind second/complementary view tag to "residue" of primary DHE in order to help thwart identify-involved fake tier
  • Bind the amount baked key to the first Diffie-Helman exchange to prevent probabilistic Janus attack

@tevador anything I'm missing?

so does column XXK still have a -0.4% scan speed change, even though the column name doesn't include VT?

Yes, because every time the primary view tag matches (about 1/256 enotes using a 1-byte view tag), we have to do an extra Diffie-Hellman exchange.  Oct 31, 2023 at 21:33 by ***@***.***:

…

@Gingeropolous> commented on this gist. so does column XXK still have a -0.4% scan speed change, even though the column name doesn't include VT? — Reply to this email directly, > view it on GitHub <https://gist.github.com/tevador/50160d160d24cfc6c52ae02eb3d17024#gistcomment-4745498>> or > unsubscribe <https://github.com/notifications/unsubscribe-auth/ACSWLOQOB57VF3IZASBAQFDYCGYG5BFKMF2HI4TJMJ2XIZLTSKBKK5TBNR2WLJDHNFZXJJDOMFWWLK3UNBZGKYLEL52HS4DFQKSXMYLMOVS2I5DSOVS2I3TBNVS3W5DIOJSWCZC7OBQXE5DJMNUXAYLOORPWCY3UNF3GS5DZVRZXKYTKMVRXIX3UPFYGLK2HNFZXIQ3PNVWWK3TUUZ2G64DJMNZZDAVEOR4XAZNEM5UXG5FFOZQWY5LFVEYTCMZWGA3TSNRZU52HE2LHM5SXFJTDOJSWC5DF>> . You are receiving this email because you commented on the thread. Triage notifications on the go with GitHub Mobile for > iOS <https://apps.apple.com/app/apple-store/id1477376905?ct=notification-email&mt=8&pt=524675>> or > Android <https://play.google.com/store/apps/details?id=com.github.android&referrer=utm_campaign%3Dnotification-email%26utm_medium%3Demail%26utm_source%3Dgithub>> .                                                            

So then what is the point of viewtags? With the whole auxiliary enotes thing, it seems that viewtags have a critical flaw that is being hacked around by auxiliary enotes, which seems like added complexity for no gain if the XXK column has the same -0.4% scan speed change without VT.

View tags

I think a flexible view tag is a reasonable option.

• Relayers start by enforcing a 1 byte view tag.
• Consensus allows dynamic view tags.
• If volume starts to increase significantly and sustainably AND light wallets are widely used AND the UX for light wallets starts to degrade AND the use case of submitting presigned txs many weeks in advance ends up an edge case that is outweighed by the light wallet use case, then relayers start enforcing a dynamic view tag.

A major advantage of 3-key Jamtis is that the 3rd party scanning improvements come "for free" because they are simply a byproduct of Janus attack protection provided by the 3rd public key, so even if 3rd party scanning doesn't catch on, we won't be wasting anything.

I think adding a 4th key to the address is still worth it, even if a dynamic view tag is never implemented, because I would rather users be in a situation where an extra key ends up wasted as opposed to a situation where their privacy is worse than it could otherwise be.

On auxiliary enotes

I think the benefits of this are worth it especially with full chain membership proofs. Pocket change is a use case people seem to clearly want (even despite its privacy issues today), to the point where I can see it being a reasonable default wallet behavior with full chain membership proofs.

So then what is the point of viewtags?

@gingeropouls in the current Jamtis spec (and in this matrix of proposals), its primary value-add is offloading the bulk of scanning to a server while preventing the server from being able to know all enotes the user received and spent with cryptographic certainty. It also speeds up full wallet scanning basically the same as view tags do today (i.e. when you don't give up a key to the server).

As currently proposed, if you send yourself e.g. 9 enotes in a tx (e.g. if you were to use a pocket change-like feature), and you give your "find-received" key to a server that can only identify view tag matches of txs which may belong to you, then the server could see "hey, this user had 9 view tag matches in this tx which is statistically very unlikely, therefore the user almost certainly received all 9 enotes." The auxiliary enote proposal above is strictly to ensure even pocket change-like txs would only have a single view tag match among all 9 enotes, so the server would still identify the tx as one that the user needs to scan all enotes for.

Two points about Janus attacks under the new proposed scheme:

1. It is possible to do a Janus attack on any address when the attacker knows any one address private key. Let's say the attacker knows address private key k^j_a and the Jamtis address corresponding to that address private key. This address private key may be revealed, for example, during an address index proof. The attacker generates ephemeral pubkey K^e = r Kⁱ_base (i is index of a new dest to be attacked). The attacker uses the ephemeral key based on an address index i, but actually encrypts the addr_tag^j from the old Jamtis address j. We then do the rest of enote building from the address j. When it comes to make the amount baked key, the receiver will calculate k^j_a * K^e. The attacker will know what this value is supposed to be since he knows k^j_a. He can then calculate the "correct" amount baked key, by doing k^j_a * K^e instead of r * G, and therefore, calculate the "correct" s^sr₂. We can fix this by including a factor of our account secrets in our base key (e.g. the view-received key). This is the reason this attack doesn't work on Jamtis currently: K^j₃ contains a factor of k_ua, which the attacker wouldn't know.
2. Including K^d_af in the hash of the amount baked key doesn't actually do anything to prevent Janus attacks, since the attacker knows what it should be. Also it isn't needed as long as we can prove that the ephemeral key is "bound" to a certain address index j, since otherwise, we won't reach the same shared secret.

This might seem like a trivial change, but I suggest that we remove the 'a' character from the address header. Two reasons: 1) we make addresses one character shorter (duh), but second is more important: 2) this prefix scheme falls in line with Bech32, litecoin, etc where you have characters in the ticker followed by a version number. I can see confusion possibly arising when people generate an address with the letter 'a' inside the prefix: "Huh? What is XMRA? I don't want XMRA, I want XMR!" initiate frustration

I suggest that we remove the 'a' character from the address header.

After some searching I found out what that "a" stands for in the first place, see this comment: The letter there has two possible values, "a" for "anonymous address" and "c" for "certified address". There was quite some discussion here over many comments whether such certified addresses are a good idea, ok but overkill, or even a bad idea; I did not go through it all. But I am sure the decision to remove that "a" amounts to deciding whether we support this address distinction in the proposed form, or at least will in the future, with more extensive tooling.

If I was to decide alone I would probably let that stand without much further ado and research ...

I don't believe it'd be an issue to have xmr1 vs xmrcert1 and prefer xmr alone.

I also don't believe we should have multiple distinct address in general (though I'd have to double check the certified address discussion). If there's no active plans to support certified addresses now, I'd remove "a".

The Jamtis invoice encoding starts with the prefix xmri. The Jamtis Wallet keys encoding starts with the prefix xmrw. Other encodings are possible in the future.

The address prefix xmra clearly distinguishes addresses from the other encodings. It might be possible to have just xmr if we avoid using i and w as the version character (which would be beyond version 10, so unlikely to be needed anyways).

no active plans to support certified addresses

Certified addresses have been superseded by certified invoices.