Make mpi_read_binary time constant
This commit modifies mpi_read_binary to always allocate the minimum number of
limbs required to hold the entire buffer provided to the function, regardless of
its content. Previously, leading zero bytes in the input data were detected and
used to reduce memory footprint and time, but this non-constant behavior turned
out to be non-tolerable for the cryptographic applications this function is used
for.
diff --git a/library/bignum.c b/library/bignum.c
index 4829d91..f5130b4 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -678,16 +678,20 @@
int mpi_read_binary( mpi *X, const unsigned char *buf, size_t buflen )
{
int ret;
- size_t i, j, n;
+ size_t i, j;
+ size_t const limbs = CHARS_TO_LIMBS( buflen );
- for( n = 0; n < buflen; n++ )
- if( buf[n] != 0 )
- break;
+ /* Ensure that target MPI has exactly the necessary number of limbs */
+ if( X->n != limbs )
+ {
+ mpi_free( X );
+ mpi_init( X );
+ MPI_CHK( mpi_grow( X, limbs ) );
+ }
- MPI_CHK( mpi_grow( X, CHARS_TO_LIMBS( buflen - n ) ) );
MPI_CHK( mpi_lset( X, 0 ) );
- for( i = buflen, j = 0; i > n; i--, j++ )
+ for( i = buflen, j = 0; i > 0; i--, j++ )
X->p[j / ciL] |= ((t_uint) buf[i - 1]) << ((j % ciL) << 3);
cleanup: