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Update on Alternative Signing (minor adjustment) #1706

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Update on Alternative Signing (minor adjustment) #1706

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6 changes: 3 additions & 3 deletions bip-0340.mediawiki
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Expand Up @@ -161,14 +161,14 @@ The auxiliary random data should be set to fresh randomness generated at signing

==== Alternative Signing ====

It should be noted that various alternative signing algorithms can be used to produce equally valid signatures. The 32-byte ''rand'' value may be generated in other ways, producing a different but still valid signature (in other words, this is not a ''unique'' signature scheme). '''No matter which method is used to generate the ''rand'' value, the value must be a fresh uniformly random 32-byte string which is not even partially predictable for the attacker.''' For nonces without randomness this implies that the same inputs must not be presented in another context. This can be most reliably accomplished by not reusing the same private key across different signing schemes. For example, if the ''rand'' value was computed as per RFC6979 and the same secret key is used in deterministic ECDSA with RFC6979, the signatures can leak the secret key through nonce reuse.
It should be noted that various alternative signing algorithms can be used to produce equally valid signatures. The 32-byte ''rand'' value may be generated in other ways, producing a different but still valid signature (in other words, this is not a ''unique'' signature scheme). '''No matter which method is used to generate the ''rand'' value, the value must be a fresh uniformly random 32-byte string which is not even partially predictable for the attacker.''' For nonces without randomness, this implies that the same inputs must not be presented in another context. This can be most reliably accomplished by not reusing the same private key across different signing schemes. For example, if the ''rand'' value was computed as per RFC6979 and the same secret key is used in deterministic ECDSA with RFC6979, the signatures can leak the secret key through nonce reuse.

'''Nonce exfiltration protection''' It is possible to strengthen the nonce generation algorithm using a second device. In this case, the second device contributes randomness which the actual signer provably incorporates into its nonce. This prevents certain attacks where the signer device is compromised and intentionally tries to leak the secret key through its nonce selection.
'''Nonce exfiltration protection''' It is possible to strengthen the nonce generation algorithm using a second device. In this case, the second device contributes randomness which the actual signer provably incorporates into its nonce. This prevents certain attacks where the signer's device is compromised and intentionally tries to leak the secret key through its nonce selection.

'''Multisignatures''' This signature scheme is compatible with various types of multisignature and threshold schemes such as [https://eprint.iacr.org/2020/1261.pdf MuSig2], where a single public key requires holders of multiple secret keys to participate in signing (see Applications below).
'''It is important to note that multisignature signing schemes in general are insecure with the ''rand'' generation from the default signing algorithm above (or any other deterministic method).'''

'''Precomputed public key data''' For many uses the compressed 33-byte encoding of the public key corresponding to the secret key may already be known, making it easy to evaluate ''has_even_y(P)'' and ''bytes(P)''. As such, having signers supply this directly may be more efficient than recalculating the public key from the secret key. However, if this optimization is used and additionally the signature verification at the end of the signing algorithm is dropped for increased efficiency, signers must ensure the public key is correctly calculated and not taken from untrusted sources.
'''Precomputed public key data''' For many uses, the compressed 33-byte encoding of the public key corresponding to the secret key may already be known, making it easy to evaluate ''has_even_y(P)'' and ''bytes(P)''. As such, having signers supply this directly may be more efficient than recalculating the public key from the secret key. However, if this optimization is used and additionally the signature verification at the end of the signing algorithm is dropped for increased efficiency, signers must ensure the public key is correctly calculated and not taken from untrusted sources.

==== Verification ====

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