BIP 340: Recommend synthetic nonces and verifying signing output #886

 165 | @@ -166,7 +166,9 @@ The algorithm ''Sign(sk, m)'' is defined as:
 166 |  
 167 |  It should be noted that various alternative signing algorithms can be used to produce equally valid signatures. The algorithm in the previous section is deterministic, i.e., it will always produce the same signature for a given message and secret key. This method does not need a random number generator (RNG) at signing time and is thus trivially robust against failures of RNGs. Alternatively 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.'''
 168 |  
 169 | -'''Synthetic nonces''' For instance when a RNG is available, 32 bytes of RNG output can be appended to the input to ''hash<sub>BIPSchnorrDerive</sub>''. This will change the corresponding line in the signing algorithm to ''rand = hash<sub>BIPSchnorrDerive</sub>(bytes(d) || m || get_32_bytes_from_rng())'', where ''get_32_bytes_from_rng()'' is the call to the RNG. Adding RNG output may improve protection against [https://moderncrypto.org/mail-archive/curves/2017/000925.html fault injection attacks and side-channel attacks], and it is safe to add the output of a low-entropy RNG.
 170 | +'''Synthetic nonces''' For instance when a RNG is available, 32 bytes of RNG output can be appended to the input to ''hash<sub>BIPSchnorrDerive</sub>''. This will change the corresponding line in the signing algorithm to ''rand = hash<sub>BIPSchnorrDerive</sub>(bytes(d) || m || get_32_bytes_from_rng())'', where ''get_32_bytes_from_rng()'' is the call to the RNG. It is safe to add the output of a low-entropy RNG. Adding high-entropy RNG output may improve protection against [https://moderncrypto.org/mail-archive/curves/2017/000925.html fault injection attacks and side-channel attacks]. Therefore, '''synthetic nonces are recommended in settings where these attacks are a concern''' - in particular on offline signing devices.
 171 | +
 172 | +'''Verifying the signing output''' To prevent random or attacker provoked computation errors, the signature can be verified with the verification algorithm defined below before leaving the signer. This prevents pubslishing invalid signatures which may leak information about the secret key. '''If the additional computational cost is not a concern, it is recommended to verify newly created signatures and reject them in case of failure.'''

real-or-random commented at 11:01 PM on January 28, 2020:

Maybe we can make clearer what we mean by "reject". I think any reasonable implementation should just error out/die when this happens (aka error_callback, assert).