Add sancus_untag functions and fix sancus_unwrap_with_key
#37
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gianlu33
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Oct 8, 2021
src/sancus_support/sm_support.h
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| // compare MAC with provided reference `tag` | ||
| return constant_time_cmp(tag, computed_tag, SANCUS_TAG_SIZE) == 0; |
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@jovanbulck would this comparison be a security issue as well?
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yes good point,it might, so better to avoid this. Since the function already returns an int, no need to do this comparison to zero.
I changed the order of these functions so that we could call `sancus_untag_with_key` inside sancus_`unwrap_with_key`. To do so, the former needs to be declared before the latter
Normally, `sancus_unwrap_with_key` always fails if cipher_len is zero. This commit solves this issue by leveraging `sancus_untag_with_key`
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jovanbulck
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Oct 20, 2021
jovanbulck
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Merged, thanks @gianlu33 ! 👍 |
gianlu33
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Nov 4, 2021
Add `sancus_untag` functions and fix `sancus_unwrap_with_key`
gianlu33
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Nov 5, 2021
Add `sancus_untag` functions and fix `sancus_unwrap_with_key`
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This implements what discussed in #36 and fixes (in sw of course) the bug mentioned in sancus-tee/sancus-core#26.
About the constant-time comparison function, I basically copied the NaCl function referenced in #31. However, I didn't really understand why they use this logic in the return statement:
Is there a specific reason for this logic (I mean, in terms of security)? Can't this be simplified to something like
return d == 0?I also had to move
sancus_tagandsancus_tag_with_keyup in the file. The reason is because I added a call tosancus_untag_with_keyinsidesancus_unwrap_with_key, therefore the former needed to be declared before the latter.Edit: I just read what @jovanbulck wrote in #31, so that logic in the
returnstatement is to avoid having an if branch. Good!