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

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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

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SwapChannels: hide your atomic swaps using ordinary payment channels

Dr. Mark B Lundeberg, 2018 July 13 bitcoincash:qqy9myvyt7qffgye5a2mn2vn8ry95qm6asy40ptgx2

Earlier this year I posted an advisory on a possible attack on certain cross-cryptocurrency atomic swap contracts.

markblundeberg

Using PGP signatures with bitcoin script OP_CHECKDATASIG

Dr. Mark B. Lundeberg, 2018 August 30 bitcoincash:qqy9myvyt7qffgye5a2mn2vn8ry95qm6asy40ptgx2

Since version 2.1, GnuPG is able to use the very same secp256k1 elliptic curve signature algorithm (ECDSA) as used in bitcoin. Quite soon Bitcoin Cash will add a new script opcode OP_CHECKDATASIG that is able to check signatures not just on the containing transaction, but also on arbitrary data. For fun, let's try to intersect the two signature systems and see what can be done!

Background

OP_CHECKDATASIG signatures

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

Quadratic sighash remains in Segwitv0/BCH/BSV digest algorithms.

Mark Lundeberg 2018 Oct 17

Abstract: Both BCH post-forkday signatures and the BIP143 Segwit signatures are ostensibly designed to remove the 'quadratic hashing problem', however as I will show, they are still vulnerable for long scripts. Back-of-the-envelope calculations show that it will become a serious concern if the existing script limits are relaxed.

Facts

• Every OP_CHECKSIG requires hashing a potentially large amount of data, limited only by the size of scriptCode. The precise length is 159 + len(scriptCode) for scriptCodes longer than 255 bytes, up to 65535 bytes.
• Since many OP_CHECKSIG calls are possible within a given script, this means transactions can be made where the required hashing time is quadratic in the length of script. (though, see the non-push opcod

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

It's quite common to see smart contract constructions like this:

OP_IF
    <clause 1 conditions>
    <pubkey1> OP_CHECKSIG
OP_ELSE
    <clause 2 conditions>
    <pubkey2> OP_CHECKSIG
OP_ENDIF