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Add new function mbedtls_asn1_write_named_bitstring()

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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.
This commit is contained in:
Andres Amaya Garcia 2018-09-26 10:48:24 +01:00 committed by Simon Butcher
parent 86016a03a1
commit 6e95914f0e
4 changed files with 109 additions and 24 deletions
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20
include/mbedtls/asn1write.h
View file

@ -282,6 +282,26 @@ int mbedtls_asn1_write_ia5_string( unsigned char **p, unsigned char *start,
int mbedtls_asn1_write_bitstring( unsigned char **p, unsigned char *start,
const unsigned char *buf, size_t bits );
/**
* \brief Write a named bitstring tag (MBEDTLS_ASN1_BIT_STRING) and
* value in ASN.1 format
* Note: function works backwards in data buffer
*
* As stated in RFC5280 Appending B, trailing zeroes are
* omitted when encoding named bitstrings in DER.
*
* \param p Reference to current position pointer.
* \param start Start of the buffer (for bounds-checking).
* \param buf The bitstring.
* \param bits The total number of bits in the bitstring.
*
* \return The length written or a negative error code.
*/
int mbedtls_asn1_write_named_bitstring( unsigned char **p,
unsigned char *start,
const unsigned char *buf,
size_t bits );
/**
* \brief Write an octet string tag (#MBEDTLS_ASN1_OCTET_STRING)
* and value in ASN.1 format.

71
library/asn1write.c
View file

@ -290,26 +290,75 @@ int mbedtls_asn1_write_ia5_string( unsigned char **p, unsigned char *start,
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 ) );

32
library/x509write_crt.c
View file

@ -221,23 +221,36 @@ int mbedtls_x509write_crt_set_authority_key_identifier( mbedtls_x509write_cert *
int mbedtls_x509write_crt_set_key_usage( mbedtls_x509write_cert *ctx,
unsigned int key_usage )
{
unsigned char buf[4], ku;
unsigned char buf[5], ku[2];
unsigned char *c;
int ret;
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 |
MBEDTLS_X509_KU_ENCIPHER_ONLY |
MBEDTLS_X509_KU_DECIPHER_ONLY;
/* 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;
c = buf + 5;
ku[0] = (unsigned char)( key_usage );
ku[1] = (unsigned char)( key_usage >> 8 );
ret = mbedtls_asn1_write_named_bitstring( &c, buf, ku, 9 );
if( ( ret = mbedtls_asn1_write_bitstring( &c, buf, &ku, 7 ) ) != 4 )
if( ret < 0 )
return( ret );
else if( ret < 3 || ret > 5 )
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 );
@ -253,12 +266,13 @@ int mbedtls_x509write_crt_set_ns_cert_type( mbedtls_x509write_cert *ctx,
c = buf + 4;
if( ( ret = mbedtls_asn1_write_bitstring( &c, buf, &ns_cert_type, 8 ) ) != 4 )
ret = mbedtls_asn1_write_named_bitstring( &c, buf, &ns_cert_type, 8 );
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 );

10
library/x509write_csr.c
View file

@ -94,12 +94,13 @@ int mbedtls_x509write_csr_set_key_usage( mbedtls_x509write_csr *ctx, unsigned ch
c = buf + 4;
if( ( ret = mbedtls_asn1_write_bitstring( &c, buf, &key_usage, 7 ) ) != 4 )
ret = mbedtls_asn1_write_named_bitstring( &c, buf, &key_usage, 8 );
if( ret < 3 || ret > 4 )
return( ret );
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 );
@ -115,12 +116,13 @@ int mbedtls_x509write_csr_set_ns_cert_type( mbedtls_x509write_csr *ctx,
c = buf + 4;
if( ( ret = mbedtls_asn1_write_bitstring( &c, buf, &ns_cert_type, 8 ) ) != 4 )
ret = mbedtls_asn1_write_named_bitstring( &c, buf, &ns_cert_type, 8 );
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 );