Move mbedtls_mpi_lt_mpi_ct function to the constant-time module
Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
diff --git a/library/constant_time.c b/library/constant_time.c
index 65547a9..9ddd21d 100644
--- a/library/constant_time.c
+++ b/library/constant_time.c
@@ -637,4 +637,70 @@
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 */