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/include/mbedtls/asn1write.h b/include/mbedtls/asn1write.h
index dc6947b..d62853e 100644
--- a/include/mbedtls/asn1write.h
+++ b/include/mbedtls/asn1write.h
@@ -204,9 +204,10 @@
const unsigned char *buf, size_t bits );
/**
- * \brief Write an octet string tag (MBEDTLS_ASN1_OCTET_STRING) and
- * value in ASN.1 format
- * Note: function works backwards in data buffer
+ * \brief Write an octet string tag (#MBEDTLS_ASN1_OCTET_STRING)
+ * and value in ASN.1 format.
+ *
+ * \note This function works backwards in data buffer.
*
* \param p reference to current position pointer
* \param start start of the buffer (for bounds-checking)
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 ) );
diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index 41dfe87..4cdb941 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -222,26 +222,51 @@
}
#endif /* MBEDTLS_SHA1_C */
+static size_t crt_get_unused_bits_for_named_bitstring( unsigned char bitstring,
+ size_t bit_offset )
+{
+ size_t unused_bits;
+
+ /* Count the unused bits removing trailing 0s */
+ for( unused_bits = bit_offset; unused_bits < 8; unused_bits++ )
+ if( ( ( bitstring >> unused_bits ) & 0x1 ) != 0 )
+ break;
+
+ return( unused_bits );
+}
+
int mbedtls_x509write_crt_set_key_usage( mbedtls_x509write_cert *ctx,
unsigned int key_usage )
{
unsigned char buf[4], ku;
unsigned char *c;
int ret;
+ size_t unused_bits;
+ const unsigned int allowed_bits = MBEDTLS_X509_KU_DIGITAL_SIGNATURE |
+ MBEDTLS_X509_KU_NON_REPUDIATION |
+ MBEDTLS_X509_KU_KEY_ENCIPHERMENT |
+ MBEDTLS_X509_KU_DATA_ENCIPHERMENT |
+ MBEDTLS_X509_KU_KEY_AGREEMENT |
+ MBEDTLS_X509_KU_KEY_CERT_SIGN |
+ MBEDTLS_X509_KU_CRL_SIGN;
- /* We currently only support 7 bits, from 0x80 to 0x02 */
- if( ( key_usage & ~0xfe ) != 0 )
+ /* Check that nothing other than the allowed flags is set */
+ if( ( key_usage & ~allowed_bits ) != 0 )
return( MBEDTLS_ERR_X509_FEATURE_UNAVAILABLE );
c = buf + 4;
- ku = (unsigned char) key_usage;
+ ku = (unsigned char)key_usage;
+ unused_bits = crt_get_unused_bits_for_named_bitstring( ku, 1 );
+ ret = mbedtls_asn1_write_bitstring( &c, buf, &ku, 8 - unused_bits );
- if( ( ret = mbedtls_asn1_write_bitstring( &c, buf, &ku, 7 ) ) != 4 )
+ if( ret < 0 )
return( ret );
+ else if( ret < 3 || ret > 4 )
+ return( MBEDTLS_ERR_X509_INVALID_FORMAT );
ret = mbedtls_x509write_crt_set_extension( ctx, MBEDTLS_OID_KEY_USAGE,
MBEDTLS_OID_SIZE( MBEDTLS_OID_KEY_USAGE ),
- 1, buf, 4 );
+ 1, c, (size_t)ret );
if( ret != 0 )
return( ret );
@@ -253,16 +278,22 @@
{
unsigned char buf[4];
unsigned char *c;
+ size_t unused_bits;
int ret;
c = buf + 4;
- if( ( ret = mbedtls_asn1_write_bitstring( &c, buf, &ns_cert_type, 8 ) ) != 4 )
+ unused_bits = crt_get_unused_bits_for_named_bitstring( ns_cert_type, 0 );
+ ret = mbedtls_asn1_write_bitstring( &c,
+ buf,
+ &ns_cert_type,
+ 8 - unused_bits );
+ if( ret < 3 || ret > 4 )
return( ret );
ret = mbedtls_x509write_crt_set_extension( ctx, MBEDTLS_OID_NS_CERT_TYPE,
MBEDTLS_OID_SIZE( MBEDTLS_OID_NS_CERT_TYPE ),
- 0, buf, 4 );
+ 0, c, (size_t)ret );
if( ret != 0 )
return( ret );
diff --git a/library/x509write_csr.c b/library/x509write_csr.c
index e800538..d59354d 100644
--- a/library/x509write_csr.c
+++ b/library/x509write_csr.c
@@ -85,20 +85,39 @@
0, val, val_len );
}
+static size_t csr_get_unused_bits_for_named_bitstring( unsigned char bitstring,
+ size_t bit_offset )
+{
+ size_t unused_bits;
+
+ /* Count the unused bits removing trailing 0s */
+ for( unused_bits = bit_offset; unused_bits < 8; unused_bits++ )
+ if( ( ( bitstring >> unused_bits ) & 0x1 ) != 0 )
+ break;
+
+ return( unused_bits );
+}
+
int mbedtls_x509write_csr_set_key_usage( mbedtls_x509write_csr *ctx, unsigned char key_usage )
{
unsigned char buf[4];
unsigned char *c;
+ size_t unused_bits;
int ret;
c = buf + 4;
- if( ( ret = mbedtls_asn1_write_bitstring( &c, buf, &key_usage, 7 ) ) != 4 )
+ unused_bits = csr_get_unused_bits_for_named_bitstring( key_usage, 0 );
+ ret = mbedtls_asn1_write_bitstring( &c, buf, &key_usage, 8 - unused_bits );
+
+ if( ret < 0 )
return( ret );
+ else if( ret < 3 || ret > 4 )
+ return( MBEDTLS_ERR_X509_INVALID_FORMAT );
ret = mbedtls_x509write_csr_set_extension( ctx, MBEDTLS_OID_KEY_USAGE,
MBEDTLS_OID_SIZE( MBEDTLS_OID_KEY_USAGE ),
- buf, 4 );
+ c, (size_t)ret );
if( ret != 0 )
return( ret );
@@ -110,16 +129,25 @@
{
unsigned char buf[4];
unsigned char *c;
+ size_t unused_bits;
int ret;
c = buf + 4;
- if( ( ret = mbedtls_asn1_write_bitstring( &c, buf, &ns_cert_type, 8 ) ) != 4 )
+ unused_bits = csr_get_unused_bits_for_named_bitstring( ns_cert_type, 0 );
+ ret = mbedtls_asn1_write_bitstring( &c,
+ buf,
+ &ns_cert_type,
+ 8 - unused_bits );
+
+ if( ret < 0 )
+ return( ret );
+ else if( ret < 3 || ret > 4 )
return( ret );
ret = mbedtls_x509write_csr_set_extension( ctx, MBEDTLS_OID_NS_CERT_TYPE,
MBEDTLS_OID_SIZE( MBEDTLS_OID_NS_CERT_TYPE ),
- buf, 4 );
+ c, (size_t)ret );
if( ret != 0 )
return( ret );