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Implement optional CA list suppression in Certificate Request

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According to RFC5246 the server can indicate the known Certificate
Authorities or can constrain the aurhorisation space by sending a
certificate list. This part of the message is optional and if omitted,
the client may send any certificate in the response.

The previous behaviour of mbed TLS was to always send the name of all the
CAs that are configured as root CAs. In certain cases this might cause
usability and privacy issues for example:
- If the list of the CA names is longer than the peers input buffer then
  the handshake will fail
- If the configured CAs belong to third parties, this message gives away
  information on the relations to these third parties

Therefore we introduce an option to suppress the CA list in the
Certificate Request message.

Providing this feature as a runtime option comes with a little cost in
code size and advantages in maintenance and flexibility.
This commit is contained in:
Janos Follath 2017-04-10 12:42:31 +01:00 • committed by Simon Butcher
parent 75fdf631fd
commit 088ce43ffe
3 changed files with 61 additions and 23 deletions
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51
library/ssl_srv.c
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@ -2588,35 +2588,40 @@ static int ssl_write_certificate_request( mbedtls_ssl_context *ssl )
* opaque DistinguishedName<1..2^16-1>;
*/
p += 2;
#if defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)
if( ssl->handshake->sni_ca_chain != NULL )
crt = ssl->handshake->sni_ca_chain;
else
#endif
crt = ssl->conf->ca_chain;
total_dn_size = 0;
while( crt != NULL && crt->version != 0 )
if( ssl->conf->cert_req_ca_list == MBEDTLS_SSL_CERT_REQ_CA_LIST_ENABLED )
{
dn_size = crt->subject_raw.len;
#if defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)
if( ssl->handshake->sni_ca_chain != NULL )
crt = ssl->handshake->sni_ca_chain;
else
#endif
crt = ssl->conf->ca_chain;
if( end < p ||
(size_t)( end - p ) < dn_size ||
(size_t)( end - p ) < 2 + dn_size )
while( crt != NULL && crt->version != 0 )
{
MBEDTLS_SSL_DEBUG_MSG( 1, ( "skipping CAs: buffer too short" ) );
break;
dn_size = crt->subject_raw.len;
if( end < p ||
(size_t)( end - p ) < dn_size ||
(size_t)( end - p ) < 2 + dn_size )
{
MBEDTLS_SSL_DEBUG_MSG( 1, ( "skipping CAs: buffer too short" ) );
break;
}
*p++ = (unsigned char)( dn_size >> 8 );
*p++ = (unsigned char)( dn_size );
memcpy( p, crt->subject_raw.p, dn_size );
p += dn_size;
MBEDTLS_SSL_DEBUG_BUF( 3, "requested DN", p - dn_size, dn_size );
total_dn_size += 2 + dn_size;
crt = crt->next;
}
*p++ = (unsigned char)( dn_size >> 8 );
*p++ = (unsigned char)( dn_size );
memcpy( p, crt->subject_raw.p, dn_size );
p += dn_size;
MBEDTLS_SSL_DEBUG_BUF( 3, "requested DN", p - dn_size, dn_size );
total_dn_size += 2 + dn_size;
crt = crt->next;
}
ssl->out_msglen = p - buf;