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tls13: server: Fix spurious HRR

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If the server during a TLS 1.3 handshake selects
the PSK key exchange mode, it does not matter
if it did not find in the key share extension
a key share for a group it supports. Such a
key share is used and necessary only in the
case of the ephemeral or PSK ephemeral key
exchange mode. This is a possible scenario in
the case of a server that supports only the PSK
key exchange mode and a client that also
supports a key exchange mode with ephemeral keys.

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
This commit is contained in:
Ronald Cron 2023-06-14 17:59:29 +02:00
parent 3d0c8255aa
commit 8a74f07c2a
2 changed files with 21 additions and 2 deletions
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10
library/ssl_tls13_server.c
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@ -1262,6 +1262,7 @@ static int ssl_tls13_parse_client_hello(mbedtls_ssl_context *ssl,
const unsigned char *supported_versions_data_end;
mbedtls_ssl_handshake_params *handshake = ssl->handshake;
int hrr_required = 0;
int no_usable_share_for_key_agreement = 0;
#if defined(MBEDTLS_SSL_TLS1_3_KEY_EXCHANGE_MODE_SOME_PSK_ENABLED)
const unsigned char *pre_shared_key_ext = NULL;
@ -1577,8 +1578,8 @@ static int ssl_tls13_parse_client_hello(mbedtls_ssl_context *ssl,
ret = ssl_tls13_parse_key_shares_ext(
ssl, p, extension_data_end);
if (ret == SSL_TLS1_3_PARSE_KEY_SHARES_EXT_NO_MATCH) {
MBEDTLS_SSL_DEBUG_MSG(2, ("HRR needed "));
hrr_required = 1;
MBEDTLS_SSL_DEBUG_MSG(2, ("No usable share for key agreement."));
no_usable_share_for_key_agreement = 1;
}
if (ret < 0) {
@ -1736,6 +1737,11 @@ static int ssl_tls13_parse_client_hello(mbedtls_ssl_context *ssl,
return ret;
}
if (ssl->handshake->key_exchange_mode !=
MBEDTLS_SSL_TLS1_3_KEY_EXCHANGE_MODE_PSK) {
hrr_required = (no_usable_share_for_key_agreement != 0);
}
mbedtls_ssl_optimize_checksum(ssl, handshake->ciphersuite_info);
return hrr_required ? SSL_CLIENT_HELLO_HRR_REQUIRED : SSL_CLIENT_HELLO_OK;