TLS_AES_256_GCM_SHA384

What each component means

A cipher suite names the algorithms a client and server agree on in one handshake. TLS_AES_256_GCM_SHA384 combines the AES-256-GCM AEAD with the SHA384 hash. Each part is explained below with its known weaknesses; for the bigger picture see how cipher suites work.

TLS 1.3 suites only describe the symmetric part. The TLS 1.3 specification (RFC 9846) says its suites specify only the symmetric ciphers and cannot be used with TLS 1.2. Key exchange is negotiated separately through the supported_groups and key_share extensions (for example X25519, P-256 or the hybrid X25519MLKEM768) and is always ephemeral (EC)DHE unless a client resumes in the PSK-only psk_ke mode, so a full handshake always has forward secrecy. The certificate signature algorithm (for example RSA-PSS, ECDSA or Ed25519) is negotiated with signature_algorithms. In TLS_AES_256_GCM_SHA384, SHA384 is the hash behind the HKDF key schedule and the handshake transcript hash.

Encryption: AES with a 256-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. In TLS 1.3 the nonce is the record sequence number XORed with a per-direction IV, so nothing extra travels with each record and nonces cannot repeat.

Why this rating

ssl.sh rating: Recommended. The reasons:

• A TLS 1.3 suite marked "Recommended" by IANA; TLS 1.3 always uses ephemeral key exchange and AEAD.

How ssl.sh rates cipher suites

Which TLS libraries support it

Support for TLS_AES_256_GCM_SHA384 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.

Enabled by default in 7 of the 9 libraries tracked here: BoringSSL, Go crypto/tls, NSS 3.133, Firefox, Java 21 (SunJSSE), Windows Schannel (Windows 11 / Server 2022) and rustls 0.23.46. 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_AES_256_GCM_SHA384 (0x13,0x02) is registered in the IANA TLS Cipher Suites registry with RFC 9846, “The Transport Layer Security (TLS) Protocol Version 1.3” (July 2026, Standards Track) as its specification. TLS 1.3 was first published as RFC 8446 (2018) and republished as RFC 9846 (2026), which obsoletes RFC 8446. Section 9.1 of the specification makes TLS_AES_128_GCM_SHA256 mandatory to implement and says implementations SHOULD also support TLS_AES_256_GCM_SHA384 and TLS_CHACHA20_POLY1305_SHA256. The registry marks it Recommended (Y).

Enable or disable it in Nginx and Apache

Enable

Both servers use OpenSSL names. The snippet keeps a modern list and adds this suite; TLS 1.3 suites are configured separately from the TLS 1.2 list.

# Nginx
ssl_protocols TLSv1.2 TLSv1.3;
# nginx 1.19.4+ with OpenSSL 1.1.1+
ssl_conf_command Ciphersuites TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SHA256;

# Apache httpd (mod_ssl)
SSLProtocol -all +TLSv1.2 +TLSv1.3
SSLCipherSuite TLSv1.3 TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SHA256

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_conf_command Ciphersuites TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SHA256;

# Apache httpd (mod_ssl)
SSLCipherSuite TLSv1.3 TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SHA256

How to test whether a server accepts it

Offer only TLS_AES_256_GCM_SHA384 with openssl s_client; if the handshake completes, the server accepts it:

openssl s_client -connect example.com:443 -tls1_3 -ciphersuites TLS_AES_256_GCM_SHA384 </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_AES_256_GCM_SHA384 in its output:

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

If your nmap build doesn't print a TLS 1.3 section, rely on the openssl command instead.

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.