Add new function mbedtls_asn1_write_named_bitstring()
Add a new function mbedtls_asn1_write_named_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 a4d23f6..b54e26b 100644
--- a/library/asn1write.c
+++ b/library/asn1write.c
@@ -290,26 +290,75 @@
return( mbedtls_asn1_write_tagged_string(p, start, MBEDTLS_ASN1_IA5_STRING, text, text_len) );
}
+int mbedtls_asn1_write_named_bitstring( unsigned char **p,
+ unsigned char *start,
+ const unsigned char *buf,
+ size_t bits )
+{
+ size_t unused_bits, byte_len;
+ const unsigned char *cur_byte;
+ unsigned char cur_byte_shifted;
+ unsigned char bit;
+
+ byte_len = ( bits + 7 ) / 8;
+ unused_bits = ( byte_len * 8 ) - bits;
+
+ /*
+ * Named bitstrings require that trailing 0s are excluded in the encoding
+ * of the bitstring. Trailing 0s are considered part of the 'unused' bits
+ * when encoding this value in the first content octet
+ */
+ if( bits != 0 )
+ {
+ cur_byte = buf + byte_len - 1;
+ cur_byte_shifted = *cur_byte >> unused_bits;
+
+ for( ; ; )
+ {
+ bit = cur_byte_shifted & 0x1;
+ cur_byte_shifted >>= 1;
+
+ if( bit != 0 )
+ break;
+
+ bits--;
+ if( bits == 0 )
+ break;
+
+ if( bits % 8 == 0 )
+ cur_byte_shifted = *--cur_byte;
+ }
+ }
+
+ return( mbedtls_asn1_write_bitstring( p, start, buf, bits ) );
+}
+
int mbedtls_asn1_write_bitstring( unsigned char **p, unsigned char *start,
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 ) );
