markblundeberg

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Buy your coffee with InstaConf: nearly trustless zero-confirmation guarantees

A proposal by Dr. Mark B Lundeberg, 2018 Feb 21 bitcoincash:qqy9myvyt7qffgye5a2mn2vn8ry95qm6asy40ptgx2

This is a proposal for reducing the risk of accepting unconfirmed (zero-conf) transactions in Bitcoin-like cryptocurrencies. Accepting such transactions is typically safe, however there is always the risk of [double-spending](

markblundeberg

### Keybase proof

I hereby claim:

  * I am markblundeberg on github.
  * I am marklundeberg (https://keybase.io/marklundeberg) on keybase.
  * I have a public key ASDQpnjba7VyKnEj28AwOYk3TzhbWeUP-KJbFs3XesNWlgo

To claim this, I am signing this object:

markblundeberg

This is a brief specification proposing the use of short anyone-can-spend P2SH addresses. This protocol is already in use with BCHMessage as used in OpenSwap, for the 'public bulletin boards' feature, but may be of use in other protocols.

Rationale

Various protocols like on-chain messaging, payment codes, and so on need SPV-wallet-friendly notifications.

• In BIP47, notifications are sent to an address owned by the recipient, which means that notifications create UTXO dust. If the UTXO dust is cleaned up, this causes a privacy concern as its destination can be traced. If not cleaned, then the UTXO set will grow and grow over time. BIP47 v2 tried to solve this by using a 1-of-2 multisig so that the sender can also clean up the dust, however this creates SPV unfriendliness for the recipient.
• In stealth addresses, payments are tagged with an OP_RETURN including a few-byte prefix that can be use

markblundeberg

Currently with OP_CHECKMULTISIG we have the following N-of-M mechanics (legacy mechanics), illustrated by a 2-of-3 example:

• Locking script: OP_2 pubkey_alice pubkey_bob pubkey_carol OP_3 OP_CHECKMULTISIG
• ScriptSig (N+1 pushes): OP_0 sig_alice sig_carol (OP_0 is the dummy element)

This is a rather bad mechanism, where sig_carol needs to be checked against both pubkey_bob and pubkey_carol. As discussed elsewhere, this is a disaster for Schnorr batch verification. So in the May 2019 upgrade, we're going to make it so that the signatures sig_alice and sig_carol are not allowed to be Schnorr signatures. I mean, we could allow them to be Schnorr and just use single-checking, but we have better plans in mind...

Schnorr signature aggregates (with OP_CHECKSIG) are really cool but they aren't a replacement for OP_CHECKMULTISIG. So, we need a new way that at least allows for batch verification.

Our goal is to make it so that all UTXOs can be spent with Schnorr, and that includes multisigs. It is unthi

markblundeberg

(https://github.com/markblundeberg/schnorrfun)



********************
Schnorr multisigger!
********************
Warning: this is for DEMONSTRATION and does not necessarily use safe/secure
techniques. Beware, funds can be easily lost!

markblundeberg

"Pay To Identity" — a proposed use of OP_CHECKDATASIG

Dr. Mark B. Lundeberg, 2018 September 6 bitcoincash:qqy9myvyt7qffgye5a2mn2vn8ry95qm6asy40ptgx2

A mechanism where a Bitcoin Cash payment is made to a personally identifying string (real name, email address, social media handle, etc.) instead of directly to a cryptographic key. The payment can only be claimed by the recipient if they generate a public key and get it certified by an identity verifier. This certification signature is confirmed in script via the new opcode OP_CHECKDATASIG.

Characteristics:

• Pay anyone, right now -- recipient doesn't need to have any cryptographic keys nor do they even need a phone/computer. (They only need these to claim the funds later.)

markblundeberg

Two new features are coming to Bitcoin Cash soon, hopefully in the May 2019 upgrade. I'd like to take the time to explain why I'm so excited about them. In short, we will be able to do:

• Payment channels hidden as ordinary payments.
• Atomic swaps hidden as ordinary payments.
• Lightning-style payment channel networks too, if we want.
• Secure chains of unconfirmed transactions involving multiple parties (layer 2).

That all may sound incredible, and I'm going to explain in this document how it is so.

--- Mark B. Lundeberg, 2019 Jan 29 bitcoincash:qqy9myvyt7qffgye5a2mn2vn8ry95qm6asy40ptgx2

markblundeberg

Basic building: first mkdir build-ninja in repo root, then run cmake -GNinja .. within the new directory, then ninja check-all.

Build with clang:

CC=clang CXX=clang++ cmake -GNinja ..

Build with debug:

CC=clang CXX=clang++ cmake -GNinja .. -DCMAKE_BUILD_TYPE=Debug

markblundeberg

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Advisory: secret size attack on cross-chain hash lock smart contracts

Dr. Mark B Lundeberg, 2018 Feb 15 bitcoincash:qqy9myvyt7qffgye5a2mn2vn8ry95qm6asy40ptgx2

This security advisory notes a vulnerability in the common construction of cross-chain smart contracts (contracts between distinct cryptocurrencies) through hash locking. I focus on the primary use case in [atomic

markblundeberg

Blind Schnorr signatures

Requester has a message-hash m that they want to be signed by pubkey P that is owned by Signer (who knows P = xG for private key x and generator G).

The following is a direct adaptation of Matthew Green's blind scheme adapted for the specific case of bip-schnorr elliptic curve signatures (including the sign-flip factor) and has been used in the wild (for internal protocol message signing, not for transaction signing) since late 2019 in the BCH CashFusion protocol.

Symbols:

• R R' P G are elliptic curve points. Additive convention is used.
• n is the curve order.
• a b e e' i j k s s' x are integers mod n.