Fix ASN1 bitstring writing
Refactor the function mbedtls_asn1_write_bitstring() that removes
trailing 0s at the end of DER encoded bitstrings. The function is
implemented according to Hanno Becker's suggestions.
This commit also changes the functions x509write_crt_set_ns_cert_type
and crt_set_key_usage to call the new function as the use named
bitstrings instead of the regular bitstrings.
diff --git a/library/asn1write.c b/library/asn1write.c
index c13e85e..b451887 100644
--- a/library/asn1write.c
+++ b/library/asn1write.c
@@ -294,22 +294,28 @@
const unsigned char *buf, size_t bits )
{
int ret;
- size_t len = 0, size;
+ size_t len = 0;
+ size_t unused_bits, byte_len;
- size = ( bits / 8 ) + ( ( bits % 8 ) ? 1 : 0 );
+ byte_len = ( bits + 7 ) / 8;
+ unused_bits = ( byte_len * 8 ) - bits;
- // Calculate byte length
- //
- if( *p < start || (size_t)( *p - start ) < size + 1 )
+ if( *p < start || (size_t)( *p - start ) < byte_len + 1 )
return( MBEDTLS_ERR_ASN1_BUF_TOO_SMALL );
- len = size + 1;
- (*p) -= size;
- memcpy( *p, buf, size );
+ len = byte_len + 1;
- // Write unused bits
- //
- *--(*p) = (unsigned char) (size * 8 - bits);
+ /* Write the bitstring. Ensure the unused bits are zeroed */
+ if( byte_len > 0 )
+ {
+ byte_len--;
+ *--( *p ) = buf[byte_len] & ~( ( 0x1 << unused_bits ) - 1 );
+ ( *p ) -= byte_len;
+ memcpy( *p, buf, byte_len );
+ }
+
+ /* Write unused bits */
+ *--( *p ) = (unsigned char)unused_bits;
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( p, start, len ) );
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( p, start, MBEDTLS_ASN1_BIT_STRING ) );