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Don't send key_share for PSK-only key exchange

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TLS 1.3 allows for the "psk_ke" and "psk_dhe_ke" key-exchange modes.
Only the latter mode introduces a new ephemeral (Diffie-Hellman)
key exchange, with the PSK being the only key material used in the
former case.

It's a compliance requirement of RFC 8446 that the server MUST NOT
send a KeyShareEntry when using the "psk_ke" mode, but prior to
this commit we would send a key-share based solely on whether the
client sent one.  This bug goes unnoticed in our internal test suite
since openssl communicating with openssl can never negotiate the
PSK-only key-exchange mode.  However, we should still be compliant
with the spec, so check whether the DHE mode was offered and don't
send a key-share if it wasn't.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14749)
This commit is contained in:
Benjamin Kaduk 2021-03-29 21:27:49 -07:00
parent f84ab284e9
commit e776858bce
1 changed files with 7 additions and 0 deletions
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7
ssl/statem/extensions_srvr.c
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@ -1614,6 +1614,13 @@ EXT_RETURN tls_construct_stoc_key_share(SSL *s, WPACKET *pkt,
}
return EXT_RETURN_NOT_SENT;
}
if (s->hit && (s->ext.psk_kex_mode & TLSEXT_KEX_MODE_FLAG_KE_DHE) == 0) {
/*
* PSK ('hit') and explicitly not doing DHE (if the client sent the
* DHE option we always take it); don't send key share.
*/
return EXT_RETURN_NOT_SENT;
}
if (!WPACKET_put_bytes_u16(pkt, TLSEXT_TYPE_key_share)
|| !WPACKET_start_sub_packet_u16(pkt)