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* crypto/secp256k1: Fix signature malleability, adopt more efficient encoding This removes signature malleability per ADR 14, and makes secp match the encoding in ADR 15. * (squash this) add lock
64 lines
2.7 KiB
Markdown
64 lines
2.7 KiB
Markdown
# ADR 014: Secp256k1 Signature Malleability
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## Context
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Secp256k1 has two layers of malleability.
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The signer has a random nonce, and thus can produce many different valid signatures.
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This ADR is not concerned with that.
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The second layer of malleability basically allows one who is given a signature
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to produce exactly one more valid signature for the same message from the same public key.
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(They don't even have to know the message!)
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The math behind this will be explained in the subsequent section.
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Note that in many downstream applications, signatures will appear in a transaction, and therefore in the tx hash.
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This means that if someone broadcasts a transaction with secp256k1 signature, the signature can be altered into the other form by anyone in the p2p network.
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Thus the tx hash will change, and this altered tx hash may be committed instead.
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This breaks the assumption that you can broadcast a valid transaction and just wait for its hash to be included on chain.
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One example is if you are broadcasting a tx in cosmos,
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and you wait for it to appear on chain before incrementing your sequence number.
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You may never increment your sequence number if a different tx hash got committed.
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Removing this second layer of signature malleability concerns could ease downstream development.
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### ECDSA context
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Secp256k1 is ECDSA over a particular curve.
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The signature is of the form `(r, s)`, where `s` is a field element.
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(The particular field is the `Z_n`, where the elliptic curve has order `n`)
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However `(r, -s)` is also another valid solution.
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Note that anyone can negate a group element, and therefore can get this second signature.
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## Decision
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We can just distinguish a canonical form for the ECDSA signatures.
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Then we require that all ECDSA signatures be in the form which we defined as canonical.
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We reject signatures in non-canonical form.
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A canonical form is rather easy to define and check.
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It would just be the smaller of the two values for `s`, defined lexicographically.
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This is a simple check, instead of checking if `s < n`, instead check `s <= (n - 1)/2`.
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An example of another cryptosystem using this
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is the parity definition here https://github.com/zkcrypto/pairing/pull/30#issuecomment-372910663.
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This is the same solution Ethereum has chosen for solving secp malleability.
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## Proposed Implementation
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Fork https://github.com/btcsuite/btcd, and just update the [parse sig method](https://github.com/btcsuite/btcd/blob/master/btcec/signature.go#195) and serialize functions to enforce our canonical form.
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## Status
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Implemented
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## Consequences
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### Positive
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- Lets us maintain the ability to expect a tx hash to appear in the blockchain.
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### Negative
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- More work in all future implementations (Though this is a very simple check)
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- Requires us to maintain another fork
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### Neutral
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