Change short-circuiting logical ORs to bitwise ORs in Ristretto decoding. #243
Conversation
This file contains 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
|
I'm not sure if this is a good change, because the decoding function doesn't operate on secret data, and can't be constant-time anyways because it returns an |
str4d
reviewed
Oct 29, 2019
| @@ -296,7 +296,7 @@ impl CompressedRistretto { | |||
| // t == ((1+as²) sqrt(4s²/(ad(1+as²)² - (1-as²)²)))/(1-as²) | |||
| let t = &x * &y; | |||
|
|
|||
| if ok.unwrap_u8() == 0u8 || t.is_negative().unwrap_u8() == 1u8 || y.is_zero().unwrap_u8() == 1u8 { | |||
| if (ok.unwrap_u8() == 0u8) | (t.is_negative().unwrap_u8() == 1u8) | (y.is_zero().unwrap_u8() == 1u8) { | |||
There was a problem hiding this comment.
Given that Choice implements BitXor and Not, I think this could be written more simply as:
Suggested change
| if (ok.unwrap_u8() == 0u8) | (t.is_negative().unwrap_u8() == 1u8) | (y.is_zero().unwrap_u8() == 1u8) { | |
| if (!ok | t.is_negative() | y.is_zero()).unwrap_u8() == 1u8 { |
str4d
reviewed
Oct 29, 2019
| @@ -266,7 +266,7 @@ impl CompressedRistretto { | |||
| &s_bytes_check[..].ct_eq(self.as_bytes()); | |||
| let s_is_negative = s.is_negative(); | |||
|
|
|||
| if s_encoding_is_canonical.unwrap_u8() == 0u8 || s_is_negative.unwrap_u8() == 1u8 { | |||
| if (s_encoding_is_canonical.unwrap_u8() == 0u8) | (s_is_negative.unwrap_u8() == 1u8) { | |||
There was a problem hiding this comment.
Similarly:
Suggested change
| if (s_encoding_is_canonical.unwrap_u8() == 0u8) | (s_is_negative.unwrap_u8() == 1u8) { | |
| if (!s_encoding_is_canonical | s_is_negative).unwrap_u8() == 1u8 { |
|
If timing is a concern here, the API should probably also be altered to return a CtOption::new(s, s_encoding_is_canonical & !s_is_negative).and_then(|s| {
[...]
CtOption::new(
RistrettoPoint(EdwardsPoint{X: x, Y: y, Z: one, T: t}),
ok & !t.is_negative() & !y.is_zero(),
)
) |
|
Timing isn't a concern here, because point decompression does not operate on secret data. |
pinkforest
pushed a commit
to pinkforest/curve25519-dalek
that referenced
this pull request
Jun 27, 2023
…key-docs-coverage Fix `SigningKey` from/to_bytes docs +coverage
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
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.
This prevents a timing side channel which would allow us to determine which check failed. I can't think of a case where this would create problems in practice, but nonetheless I'd argue we should prevent the leak.
Related to dalek-cryptography/subtle#42