PSK identity hint lengths don't match RFC 4279 (doesn't support identity hint length of 0 from client)

[jerhon]

In RFC 4279, the PSK identity is defined as "opaque psk_identity<0..2^16-1>". RFC 4279 link

However, in the ssl_srv.c, it is indicating that if the PSK identity length is < 1, to return an error of MBEDTLS_ERR_SSL_BAD_HS_CLIENT_KEY_EXCHANGE.

in - ssl_srv.c - ssl_parse_client_psk_identity()

if( n < 1 || n > 65535 || *p + n > end ) { MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad client key exchange message" ) ); return( MBEDTLS_ERR_SSL_BAD_HS_CLIENT_KEY_EXCHANGE ); }

There are a few other places in code where it is checking for existence of an identity hint based on the length of the ssl->conf->psk_identity_len == 0 that may be suspect as well.

For security purposes some implementations utilizing TLS with a PSK based ciphersuite may not want to pass anything as a client identity. However, that is currently not possible with the server side implementation.

[pjbakker]

Thank you for your report. Based on the triage you are correct in marking this as an issue.

[ciarmcom]

ARM Internal Ref: IOTSSL-986