Clarify why we don't want short hashes

This is supposed to supersede https://github.com/sipa/bips/pull/158.
I tried to say this carefully. I don't think that multiparty signing is in general broken with short hashes. For example the attack in #158 could be avoided by letting everybody not only commit to the nonce but also to the message. It's just that using a collision-resistant hash just eliminates the problem entirely...

1 files changed, 3 insertions, 3 deletions

diff --git a/bip-schnorr.mediawiki b/bip-schnorr.mediawiki
index 4cfe6b4..a5f7c59 100644
--- a/bip-schnorr.mediawiki
+++ b/bip-schnorr.mediawiki
@@ -49,12 +49,12 @@ encodings and operations.
 === Design ===
 
 '''Schnorr signature variant''' Elliptic Curve Schnorr signatures for message ''m'' and public key ''P'' generally involve a point ''R'', integers ''e'' and ''s'' picked by the signer, and the base point ''G'' which satisfy ''e = hash(R || m)'' and ''s⋅G = R + e⋅P''. Two formulations exist, depending on whether the signer reveals ''e'' or ''R'':
-# Signatures are ''(e, s)'' that satisfy ''e = hash(s⋅G - e⋅P || m)''. This supports more compact signatures, since [http://www.neven.org/papers/schnorr.pdf the hash ''e'' can be made as small as 16 bytes without sacrificing security], whereas an encoding of ''R'' inherently needs about 32 bytes. Moreover, this variant avoids minor complexity introduced by the encoding of the point ''R'' in the signature (see paragraphs "Encoding R and public key point P" and "Implicit Y coordinates" further below in this subsection).
-# Signatures are ''(R, s)'' that satisfy ''s⋅G = R + hash(R || m)⋅P''. This supports batch verification, as there are no elliptic curve operations inside the hashes. Batch verification enables significant speedups.
+# Signatures are pairs ''(e, s)'' that satisfy ''e = hash(s⋅G - e⋅P || m)''. This variant avoids minor complexity introduced by the encoding of the point ''R'' in the signature (see paragraphs "Encoding R and public key point P" and "Implicit Y coordinates" further below in this subsection). Moreover, revealing ''e'' instead of ''R'' allows for potentially shorter signatures: Whereas an encoding of ''R'' inherently needs about 32 bytes, the hash ''e'' can be tuned to be shorter than 32 bytes, and [http://www.neven.org/papers/schnorr.pdf a short hash of only 16 bytes suffices to provide SUF-CMA security at the target security level of 128 bits]. However, a major drawback of this optimization is that finding collisions in a short hash function is easy. This complicates the implementation of secure signing protocols in scenarios in which a group of mutually distrusting signers work together to produce a single joint signature (see Applications below). In these scenarios, which are not captured by the SUF-CMA model due its assumption of a single honest signer, a promising attack strategy for malicious co-signers is to find a collision in the hash function in order to obtain a valid signature on a message that an honest co-signer did not intent to sign.
+# Signatures are pairs ''(R, s)'' that satisfy ''s⋅G = R + hash(R || m)⋅P''. This supports batch verification, as there are no elliptic curve operations inside the hashes. Batch verification enables significant speedups.
 
 [[File:bip-schnorr/speedup-batch.png|center|frame|This graph shows the ratio between the time it takes to verify ''n'' signatures individually and to verify a batch of ''n'' signatures. This ratio goes up logarithmically with the number of signatures, or in other words: the total time to verify ''n'' signatures grows with ''O(n / log n)''.]]
 
-We choose the ''R''-option to support batch verification.
+Since we would like to avoid the fragility that comes with short hashes, the ''e'' variant does not provide significant advantages. We choose the ''R''-option, which supports batch verification. 
 
 '''Key prefixing''' Using the verification rule above directly makes Schnorr signatures vulnerable to "related-key attacks" in which a third party can convert a signature ''(R, s)'' for public key ''P'' into a signature ''(R, s + a⋅hash(R || m))'' for public key ''P + a⋅G'' and the same message ''m'', for any given additive tweak ''a'' to the signing key. This would render signatures insecure when keys are generated using [https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki#public-parent-key--public-child-key BIP32's unhardened derivation] and other methods that rely on additive tweaks to existing keys such as Taproot.