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csquared / fernet.js

Javascript implementation of Fernet symmetric encryption https://github.com/kr/fernet-spec
MIT License
73 stars 29 forks source link

use constant-time hmac comparison #1

Closed kr closed 10 years ago

kr commented 10 years ago

https://github.com/csquared/fernet.js/blob/7bc5801/lib/token.js#L64 appears to be a short-circuit comparison. The spec requires the comparison to be done in constant time, independent of the contents of the inputs.

The go implementation does this with a library function: https://github.com/fernet/fernet-go/blob/54c3c8f/fernet.go#L76 and the ruby implementation does it with a loop: https://github.com/fernet/fernet-rb/blob/9190f48/lib/fernet/verifier.rb#L69

cc @tmaher @hgmnz @will

hgmnz commented 10 years ago

The spec requires the comparison to be done in constant time, independent of the contents of the inputs.

Amen to that; this direct comparison is vulnerable to a timing attack.

kr commented 10 years ago

I suggest reopening this issue because of https://github.com/csquared/fernet.js/pull/7/files#r10286901.

csquared commented 10 years ago

@kr duh. it has to take 64 iterations no matter what...

i think i got it this time: https://github.com/csquared/fernet.js/commit/e99e6ae964289068fd620060a232f71b80f793a6

if that's wrong i'll reopen the issue.

kr commented 10 years ago

I think it still needs work: https://github.com/csquared/fernet.js/commit/e99e6ae964289068fd620060a232f71b80f793a6#commitcomment-5577669. :)