TLS_PSK_WITH_AES_128_GCM_SHA256

What each component means

A cipher suite names the algorithms a client and server agree on in one handshake. TLS_PSK_WITH_AES_128_GCM_SHA256 combines PSK key exchange, PSK authentication, AES-128-GCM encryption and AEAD integrity protection. Each part is explained below with its known weaknesses; for the bigger picture see how cipher suites work.

Key exchange and authentication: pre-shared key (PSK), defined in RFC 4279 (2005). Both sides already hold the same symmetric secret, provisioned out of band, and the handshake only names which key to use (the PSK identity). No certificates and no public-key operations are needed, which is why PSK suites appear in constrained devices and closed systems rather than on the public web. Plain PSK has no forward secrecy: whoever learns the key can decrypt every session made with it.

Encryption: AES with a 128-bit key, the 128-bit block cipher of FIPS 197 (2001). Most server and phone CPUs have AES instructions (AES-NI on x86, the ARMv8 cryptography extensions), which make it fast and avoid the cache-timing leaks of table-based software implementations.

Mode: GCM (Galois/Counter Mode), an AEAD: counter-mode encryption plus a GHASH authenticator with a 16-byte tag, checked in one step, so the CBC padding attacks don't apply. In TLS 1.2 the 12-byte nonce is a 4-byte implicit salt from the key block plus an 8-byte explicit part sent with every record, as first specified for AES-GCM in RFC 5288 (2008); a nonce must never repeat under the same key.

Integrity: there is no separate MAC because the AEAD tag authenticates every record. The SHA256 at the end of the name selects the TLS 1.2 PRF hash used to derive keys and compute the Finished messages.

Why this rating

ssl.sh rating: Weak. The reasons:

• No forward secrecy: the session keys are protected by the server's long-term key (static RSA/DH/ECDH, PSK or Kerberos), so a leaked key decrypts recorded traffic.

How ssl.sh rates cipher suites

Which TLS libraries support it

Support for TLS_PSK_WITH_AES_128_GCM_SHA256 as read from each library's own source code or official documentation on 2026-11-10. “Enabled by default” means the library negotiates it with no configuration; “off by default” means it is implemented but has to be switched on.

None of the 9 libraries tracked here enables it by default. Supported but off by default in: Windows Schannel (Windows 11 / Server 2022). Also implemented by: OpenSSL 3.5 and GnuTLS.

NSS shows the library's own defaults; Firefox switches every NSS suite off at start-up and re-enables only those with a security.ssl3.* or security.tls13.* preference. Java defaults are derived from the jdk.tls.disabledAlgorithms value Oracle documents for JDK 21, which later updates may extend. The OpenSSL and GnuTLS rows only show whether the suite is implemented.

History and standards status

TLS_PSK_WITH_AES_128_GCM_SHA256 (0x00,0xA8) is registered in the IANA TLS Cipher Suites registry with RFC 5487, “Pre-Shared Key Cipher Suites for TLS with SHA-256/384 and AES Galois Counter Mode” (March 2009, Standards Track) as its specification. The registry marks it N, which per RFC 8447 (2018) means it either hasn't been through the IETF consensus process, has limited applicability or is meant only for specific use cases, not necessarily that it is flawed.

Enable or disable it in Nginx and Apache

Disable

Either leave the suite out of your list or exclude it explicitly with a leading "!". Reload the server afterwards (nginx -s reload, apachectl graceful).

# Nginx
ssl_ciphers 'ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:!PSK-AES128-GCM-SHA256';

# Apache httpd (mod_ssl)
SSLCipherSuite ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:!PSK-AES128-GCM-SHA256

How to test whether a server accepts it

Offer only TLS_PSK_WITH_AES_128_GCM_SHA256 with openssl s_client; if the handshake completes, the server accepts it:

openssl s_client -connect example.com:443 -tls1_2 -cipher PSK-AES128-GCM-SHA256 </dev/null

If the handshake succeeds, the server accepts the suite; "no cipher match" or a handshake failure means it doesn't.

nmap's ssl-enum-ciphers script connects repeatedly, trying one suite at a time, and lists every suite the server accepts with a grade from A to F. Look for TLS_PSK_WITH_AES_128_GCM_SHA256 in its output:

nmap --script ssl-enum-ciphers -p 443 example.com

To decode a name or hex code, use the cipher suite lookup; the SSL checker shows which protocol versions a live server accepts and the cipher suite it negotiates.