Avoid using out-of-bounds field elements (in impossible cases) #289
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secp256k1_fe_set_b32_limit says that when it returns 0, one is not allowed to use the resulting output value. This refactors the code so that 0 is returned early (indicating failure) in cases where sha256 would output an out-of-bounds hash value. This makes secp256k1_generator_generate_internal variable-time in its "t" argument, but this not a problem because this value is public in applications. Note: This situation is cryptographically impossible to occur. Co-authored-by: Russell O'Connor <roconnor@blockstream.io>
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Seems okay to me if you folks are okay with the variable time in these (impossible) cases. |
AFAIU, we should be okay even if the cases were more likely. The derivation of generators should only be a public computation (except maybe blinding), but it will be nice if @apoelstra can confirm that I haven't overlooked anything in this argument. |
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I agree with you, except that the code seems to have been clearly and deliberately written to be constant time for some reason. |
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secp256k1_fe_set_b32_limit says that when it returns 0, one is not allowed to use the resulting output value.
This refactors the code so that 0 is returned early (indicating failure) in cases where sha256 would output an out-of-bounds hash value. This makes secp256k1_generator_generate_internal variable-time in its "t" argument, but this not a problem because this value is public in applications.
Note: This situation is cryptographically impossible to occur.
Alternative to #282. cc @roconnor-blockstream What do you think?