Fix theoretical compliance issue in ECDSA
The issue would happen for curves whose bitlength is not a multiple of eight
(the only case is NIST P-521) with hashes that are longer than the bitlength
of the curve: since the wides hash is 512 bits long, this can't happen.
Fixing however as a matter of principle and readability.
diff --git a/library/ecdsa.c b/library/ecdsa.c
index 2072d55..6f09994 100644
--- a/library/ecdsa.c
+++ b/library/ecdsa.c
@@ -43,8 +43,16 @@
static int derive_mpi( const ecp_group *grp, mpi *x,
const unsigned char *buf, size_t blen )
{
+ int ret;
size_t n_size = (grp->nbits + 7) / 8;
- return( mpi_read_binary( x, buf, blen > n_size ? n_size : blen ) );
+ size_t use_size = blen > n_size ? n_size : blen;
+
+ MPI_CHK( mpi_read_binary( x, buf, use_size ) );
+ if( use_size * 8 > grp->nbits )
+ MPI_CHK( mpi_shift_r( x, use_size * 8 - grp->nbits ) );
+
+cleanup:
+ return( ret );
}
/*
