Fix bug whereby 0 was written as 0200 rather than 020100
0200 is not just non-DER, it's completely invalid, since there has to be a
sign bit.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/asn1write.c b/library/asn1write.c
index 2110052..053dbb6 100644
--- a/library/asn1write.c
+++ b/library/asn1write.c
@@ -133,6 +133,11 @@
//
len = mbedtls_mpi_size( X );
+ /* DER represents 0 with a sign bit (0=nonnegative) and 7 value bits, not
+ * as 0 digits. We need to end up with 020100, not with 0200. */
+ if( len == 0 )
+ len = 1;
+
if( *p < start || (size_t)( *p - start ) < len )
return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );