From c29a3da5994b0bb7844db84ed32e474b0a4e59f9 Mon Sep 17 00:00:00 2001 From: gabor-mezei-arm Date: Mon, 27 Sep 2021 15:41:30 +0200 Subject: [PATCH] Move mbedtls_mpi_lt_mpi_ct function to the constant-time module Signed-off-by: gabor-mezei-arm --- library/bignum.c | 66 ----------------------------------------- library/constant_time.c | 66 +++++++++++++++++++++++++++++++++++++++++ 2 files changed, 66 insertions(+), 66 deletions(-) diff --git a/library/bignum.c b/library/bignum.c index e0391eca1..ee5f27d70 100644 --- a/library/bignum.c +++ b/library/bignum.c @@ -1091,72 +1091,6 @@ int mbedtls_mpi_cmp_mpi( const mbedtls_mpi *X, const mbedtls_mpi *Y ) return( 0 ); } -/* - * Compare signed values in constant time - */ -int mbedtls_mpi_lt_mpi_ct( const mbedtls_mpi *X, const mbedtls_mpi *Y, - unsigned *ret ) -{ - size_t i; - /* The value of any of these variables is either 0 or 1 at all times. */ - unsigned cond, done, X_is_negative, Y_is_negative; - - MPI_VALIDATE_RET( X != NULL ); - MPI_VALIDATE_RET( Y != NULL ); - MPI_VALIDATE_RET( ret != NULL ); - - if( X->n != Y->n ) - return MBEDTLS_ERR_MPI_BAD_INPUT_DATA; - - /* - * Set sign_N to 1 if N >= 0, 0 if N < 0. - * We know that N->s == 1 if N >= 0 and N->s == -1 if N < 0. - */ - X_is_negative = ( X->s & 2 ) >> 1; - Y_is_negative = ( Y->s & 2 ) >> 1; - - /* - * If the signs are different, then the positive operand is the bigger. - * That is if X is negative (X_is_negative == 1), then X < Y is true and it - * is false if X is positive (X_is_negative == 0). - */ - cond = ( X_is_negative ^ Y_is_negative ); - *ret = cond & X_is_negative; - - /* - * This is a constant-time function. We might have the result, but we still - * need to go through the loop. Record if we have the result already. - */ - done = cond; - - for( i = X->n; i > 0; i-- ) - { - /* - * If Y->p[i - 1] < X->p[i - 1] then X < Y is true if and only if both - * X and Y are negative. - * - * Again even if we can make a decision, we just mark the result and - * the fact that we are done and continue looping. - */ - cond = mbedtls_cf_mpi_uint_lt( Y->p[i - 1], X->p[i - 1] ); - *ret |= cond & ( 1 - done ) & X_is_negative; - done |= cond; - - /* - * If X->p[i - 1] < Y->p[i - 1] then X < Y is true if and only if both - * X and Y are positive. - * - * Again even if we can make a decision, we just mark the result and - * the fact that we are done and continue looping. - */ - cond = mbedtls_cf_mpi_uint_lt( X->p[i - 1], Y->p[i - 1] ); - *ret |= cond & ( 1 - done ) & ( 1 - X_is_negative ); - done |= cond; - } - - return( 0 ); -} - /* * Compare signed values */ diff --git a/library/constant_time.c b/library/constant_time.c index b2203ba9a..8a9c45399 100644 --- a/library/constant_time.c +++ b/library/constant_time.c @@ -637,4 +637,70 @@ cleanup: return( ret ); } +/* + * Compare signed values in constant time + */ +int mbedtls_mpi_lt_mpi_ct( const mbedtls_mpi *X, const mbedtls_mpi *Y, + unsigned *ret ) +{ + size_t i; + /* The value of any of these variables is either 0 or 1 at all times. */ + unsigned cond, done, X_is_negative, Y_is_negative; + + MPI_VALIDATE_RET( X != NULL ); + MPI_VALIDATE_RET( Y != NULL ); + MPI_VALIDATE_RET( ret != NULL ); + + if( X->n != Y->n ) + return MBEDTLS_ERR_MPI_BAD_INPUT_DATA; + + /* + * Set sign_N to 1 if N >= 0, 0 if N < 0. + * We know that N->s == 1 if N >= 0 and N->s == -1 if N < 0. + */ + X_is_negative = ( X->s & 2 ) >> 1; + Y_is_negative = ( Y->s & 2 ) >> 1; + + /* + * If the signs are different, then the positive operand is the bigger. + * That is if X is negative (X_is_negative == 1), then X < Y is true and it + * is false if X is positive (X_is_negative == 0). + */ + cond = ( X_is_negative ^ Y_is_negative ); + *ret = cond & X_is_negative; + + /* + * This is a constant-time function. We might have the result, but we still + * need to go through the loop. Record if we have the result already. + */ + done = cond; + + for( i = X->n; i > 0; i-- ) + { + /* + * If Y->p[i - 1] < X->p[i - 1] then X < Y is true if and only if both + * X and Y are negative. + * + * Again even if we can make a decision, we just mark the result and + * the fact that we are done and continue looping. + */ + cond = mbedtls_cf_mpi_uint_lt( Y->p[i - 1], X->p[i - 1] ); + *ret |= cond & ( 1 - done ) & X_is_negative; + done |= cond; + + /* + * If X->p[i - 1] < Y->p[i - 1] then X < Y is true if and only if both + * X and Y are positive. + * + * Again even if we can make a decision, we just mark the result and + * the fact that we are done and continue looping. + */ + cond = mbedtls_cf_mpi_uint_lt( X->p[i - 1], Y->p[i - 1] ); + *ret |= cond & ( 1 - done ) & ( 1 - X_is_negative ); + done |= cond; + } + + return( 0 ); +} + #endif /* MBEDTLS_BIGNUM_C */