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Evolve choose_int_from_mask to if_int

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Make the function more robust by taking an arbitrary zero/nonzero
argument instead of insisting on zero/all-bits-one. Update and fix its
documentation.
This commit is contained in:
Gilles Peskine 2018-10-04 18:32:29 +02:00
parent ddffa06501
commit 331d80e162
1 changed files with 15 additions and 11 deletions
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22
library/rsa.c
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@ -1381,9 +1381,9 @@ cleanup:
/** Turn zero-or-nonzero into zero-or-all-bits-one, without branches. /** Turn zero-or-nonzero into zero-or-all-bits-one, without branches.
* *
* \param value The value to analyze. * \param value The value to analyze.
* \return \c 0 if \p value is zero, otherwise \c 0xff. * \return Zero if \p value is zero, otherwise all-bits-one.
*/ */
static unsigned unsigned_all_or_nothing( unsigned value ) static unsigned all_or_nothing_int( unsigned value )
{ {
/* MSVC has a warning about unary minus on unsigned, but this is /* MSVC has a warning about unary minus on unsigned, but this is
* well-defined and precisely what we want to do here */ * well-defined and precisely what we want to do here */
@ -1399,13 +1399,17 @@ static unsigned unsigned_all_or_nothing( unsigned value )
/** Choose between two integer values, without branches. /** Choose between two integer values, without branches.
* *
* \param mask Either \c 0 or \c ~0. * This is equivalent to `cond ? if1 : if0`, but is likely to be compiled
* \param if0 Value to use if \p mask = \c 0. * to code using bitwise operation rather than a branch.
* \param if1 Value to use if \p mask = \c ~0. *
* \return \c if1 if \p value is zero, otherwise \c if0. * \param cond Condition to test.
* \param if1 Value to use if \p cond is nonzero.
* \param if0 Value to use if \p cond is zero.
* \return \c if1 if \p cond is nonzero, otherwise \c if0.
*/ */
static unsigned choose_int_from_mask( unsigned mask, unsigned if1, unsigned if0 ) static unsigned if_int( unsigned cond, unsigned if1, unsigned if0 )
{ {
unsigned mask = all_or_nothing_int( cond );
return( ( mask & if1 ) | (~mask & if0 ) ); return( ( mask & if1 ) | (~mask & if0 ) );
} }
@ -1489,7 +1493,7 @@ int mbedtls_rsa_rsaes_pkcs1_v15_decrypt( mbedtls_rsa_context *ctx,
/* Set bad to zero if the padding is valid and /* Set bad to zero if the padding is valid and
* all-bits-one otherwise. The whole calculation of bad * all-bits-one otherwise. The whole calculation of bad
* is done in such a way to avoid branches. */ * is done in such a way to avoid branches. */
bad = unsigned_all_or_nothing( bad ); bad = all_or_nothing_int( bad );
/* If the padding is valid, set plaintext_size to the number of /* If the padding is valid, set plaintext_size to the number of
* remaining bytes after stripping the padding. If the padding * remaining bytes after stripping the padding. If the padding
@ -1498,7 +1502,7 @@ int mbedtls_rsa_rsaes_pkcs1_v15_decrypt( mbedtls_rsa_context *ctx,
* buffer. Do it without branches to avoid leaking the padding * buffer. Do it without branches to avoid leaking the padding
* validity through timing. RSA keys are small enough that all the * validity through timing. RSA keys are small enough that all the
* size_t values involved fit in unsigned int. */ * size_t values involved fit in unsigned int. */
plaintext_size = choose_int_from_mask( bad, plaintext_size = if_int( bad,
(unsigned) plaintext_max_size, (unsigned) plaintext_max_size,
(unsigned) ( ilen - ( p - buf ) ) ); (unsigned) ( ilen - ( p - buf ) ) );