How to decrypt TLS 1.3 PSK sent by Zabbix?
Hi, I'm trying to decrypt TLS 1.3 traffic, generated by Zabbix. This is configured with pre-shared key as described in the documentation in https://www.zabbix.com/documentation/... .
I have configured Wireshark 3.2.1 (or tshark 3.2.1) to use the configured TLS PSK. I'm not able to see the traffic unencrypted however. What am I missing here?
The example capture file is in https://my.syncplicity.com/share/daz6.... The PSK is "dac86d7afb589557e876f16b59b8b5b4". PSK Identity is "psk" in Zabbix but I don't think that's relevant for TLS decryption in Wireshark. (It is shown plaintext in frame 4 in tls.handshake.extensions.psk.identity.identity anyway)
For example:
tshark -r zabbix-tls-agent.pcap -Y "frame.number==28" -V -O tls -o tls.psk:dac86d7afb589557e876f16b59b8b5b4
Frame 28: 90 bytes on wire (720 bits), 90 bytes captured (720 bits)
Ethernet II, Src: VMware_fa:68:50 (00:0c:29:fa:68:50), Dst: VMware_3e:95:31 (00:0c:29:3e:95:31)
Internet Protocol Version 4, Src: 192.168.7.10, Dst: 192.168.7.12
Transmission Control Protocol, Src Port: 60130, Dst Port: 10050, Seq: 433, Ack: 407, Len: 24
Transport Layer Security
TLSv1.3 Record Layer: Application Data Protocol: Application Data
Opaque Type: Application Data (23)
Version: TLS 1.2 (0x0303)
Length: 19
Encrypted Application Data: 6e493a8a471ddd55eb82d095f02d88e037db62
What puzzles me as well is the output of "tshark -G userprefs":
# Pre-Shared-Key as HEX string. Should be 0 to 16 bytes. # A string #tls.psk:
Where is that 16-byte limit coming from?
Update: TLS debug file (-o tls.debug_file:debug.txt) says:
dissect_ssl enter frame #4 (first time) packet_from_server: is from server - FALSE conversation = 000002F97BB5FA40, ssl_session = 000002F97BB605E0 record: offset = 0, reported_length_remaining = 312 dissect_ssl3_record: content_type 22 Handshake decrypt_ssl3_record: app_data len 307, ssl state 0x00 packet_from_server: is from server - FALSE decrypt_ssl3_record: using client decoder decrypt_ssl3_record: no decoder available dissect_ssl3_handshake iteration 1 type 1 offset 5 length 303 bytes Calculating hash with offset 5 307 ssl_dissect_hnd_hello_common found CLIENT RANDOM -> state 0x01 dissect_ssl enter frame #6 (first time) packet_from_server: is from server - TRUE conversation = 000002F97BB5FA40, ssl_session = 000002F97BB605E0 record: offset = 0, reported_length_remaining = 258 ssl_try_set_version found version 0x0303 -> state 0x91 dissect_ssl3_record: content_type 22 Handshake decrypt_ssl3_record: app_data len 161, ssl state 0x91 packet_from_server: is from server - TRUE decrypt_ssl3_record: using server decoder decrypt_ssl3_record: no decoder available dissect_ssl3_handshake iteration 1 type 2 offset 5 length 157 bytes ssl_try_set_version found version 0x0304 -> state 0x91 ssl_dissect_hnd_hello_common found SERVER RANDOM -> state 0x93 ssl_set_cipher found CIPHER 0x1303 TLS_CHACHA20_POLY1305_SHA256 -> state 0x97 ssl_load_keyfile dtls/tls.keylog_file is not configured! tls13_load_secret transitioning to new key, old state 0x97 tls13_load_secret Cannot find CLIENT_HANDSHAKE_TRAFFIC_SECRET, decryption impossible tls13_load_secret transitioning to new key, old state 0x97 tls13_load_secret Cannot find SERVER_HANDSHAKE_TRAFFIC_SECRET, decryption impossible record: offset = 166, reported_length_remaining = 92 dissect_ssl3_record: content_type 20 Change Cipher Spec record: offset = 172, reported_length_remaining = 86 dissect_ssl3_record: content_type 23 Application Data decrypt_ssl3_record: app_data len 23, ssl state 0x97 packet_from_server: is from server - TRUE decrypt_ssl3_record: using server decoder decrypt_ssl3_record: no decoder available record: offset = 200, reported_length_remaining = 58 dissect_ssl3_record: content_type 23 Application ...
TLS 1.3 doesn't require a PSK cipher (TLS_CHACHA20_POLY1305_SHA256 in this pcap). The pre-shared key is used to transfer the session keys which Wireshark needs to decrypt.
"If you plan to use pre-shared keys (PSK) consider using GnuTLS or mbed TLS libraries in Zabbix components using PSKs."
GnuTLS supports SSLKEYLOGFILE to get session keys:
https://gnutls.org/manual/html_node/D...
Sharkfest presentation on using secrets to decrypt TLS:
https://sharkfestasia.wireshark.org/a...
Page 34 shows the various keys:
https://owasp.org/www-chapter-london/...
Thanks Chuck for the information. I hoped that the PSK option in Wireshark/tshark would enable the decrypt. So I take it that since Zabbix pre-packaged components are built with openssl (which doesn't support saving the session keys), there is no way to decrypt the data, without distributing custom-built executables.
Any insight if the PSK option in Wireshark/tshark is useful in earlier TLS versions?
Yes. There are test pcaps here:
https://code.wireshark.org/review/git...
Just ran test with
tls12-aes128ccm.pcapusing PSKca19e028a8a372ad2d325f950fcaceed.This is the test case (https://code.wireshark.org/review/git...):
def test_tls12_psk_aes128ccm(self, cmd_tshark, capture_file): '''TLS 1.2 with PSK, AES-128-CCM''' self.assertRun((cmd_tshark, '-r', capture_file('tls12-aes128ccm.pcap'), '-o', 'tls.psk:ca19e028a8a372ad2d325f950fcaceed', '-q', '-z', 'follow,tls,ascii,0', )) self.assertTrue(self.grepOutput('http://www.gnu.org/software/gnutls')