Avoid using out-of-bounds field elements (in impossible cases)#289
Open
real-or-random wants to merge 1 commit intoBlockstreamResearch:masterfrom
Open
Avoid using out-of-bounds field elements (in impossible cases)#289real-or-random wants to merge 1 commit intoBlockstreamResearch:masterfrom
real-or-random wants to merge 1 commit intoBlockstreamResearch:masterfrom
Conversation
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>
Contributor
|
Seems okay to me if you folks are okay with the variable time in these (impossible) cases. |
Member
Author
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. |
Contributor
|
I agree with you, except that the code seems to have been clearly and deliberately written to be constant time for some reason. |
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
2 participants
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
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?