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Make mp_unsafe_mod_integer not be unsafe.

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I've moved it from mpunsafe.c into the main mpint.c, and renamed it
mp_mod_known_integer, because now it manages to avoid leaking
information about the mp_int you give it.

It can still potentially leak information about the small _modulus_
integer - hence the word 'known' in the new function name. This won't
be a problem in any existing use of the function, because it's used
during prime generation to check divisibility by all the small primes,
and optionally also check for residue 1 mod the RSA public exponent.
But all those values are well known and not secret.

This removes one source of side-channel leakage from prime generation.
This commit is contained in:
Simon Tatham
2021-08-27 17:43:40 +01:00
parent 22fab78376
commit 59409d0947
5 changed files with 90 additions and 20 deletions
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7
keygen/mpunsafe.h
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@ -36,11 +36,4 @@
mp_int *mp_unsafe_shrink(mp_int *m);
mp_int *mp_unsafe_copy(mp_int *m);
/*
* Compute the residue of x mod m. This is implemented in the most
* obvious way using the C % operator, which won't be constant-time on
* many C implementations.
*/
uint32_t mp_unsafe_mod_integer(mp_int *x, uint32_t m);
#endif /* PUTTY_MPINT_UNSAFE_H */