# Reading an F5 Cipher String

> An F5 cipher string is an ordered list of cipher sets separated by colons, where each set combines keywords with a plus sign and a leading operator can exclude, delete, or de-prioritize. Once you can read the grammar, a dense string like ECDHE:RSA:!SSLv3:@STRENGTH becomes a clear set of instructions.

Source: https://ronutz.com/en/learn/f5-cipher-string-syntax  
Updated: 2026-06-29  
Related tools: https://ronutz.com/en/tools/f5-cipher-string-expander

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A BIG-IP cipher string controls which TLS cipher suites an SSL profile will offer and in what order. You will find it as the `cipher` field of an `ltm cipher rule`, or typed directly into an SSL profile. It looks cryptic at first, but it follows a small, consistent grammar.

## Sets, separated and ordered

A cipher string is a list of **sets**, separated by a colon `:`, a comma `,`, or whitespace. Order matters: a server walks its list top to bottom and picks the first suite the client also supports, so the sets near the front are the ones you prefer. `ECDHE:RSA:AES` is three sets, evaluated in that order.

## Keywords combined with plus

Within a set, **keywords** are joined with a plus sign `+` to narrow the selection. Each keyword constrains a different part of a cipher suite: the protocol version (`TLSv1_2`), the key exchange and authentication (`ECDHE`, `RSA`, `ECDHE_ECDSA`), the bulk cipher (`AES-GCM`, `3DES`), and the MAC or hash (`SHA256`). So `TLSv1_2+ECDHE+AES-GCM+SHA384` means exactly the suites that are TLS 1.2, use ECDHE, use AES (Advanced Encryption Standard)-GCM, and use SHA-384. A bare keyword like `AES` with no pluses is broad, matching every AES suite. Keywords are case-insensitive, so `tlsv1_2+ecdhe` and `TLSv1_2+ECDHE` are identical.

## Operators

A set can carry a leading operator that changes what BIG-IP does with it:

- `!` **excludes** the set permanently. `!SSLv3` removes every SSL 3.0 suite, and nothing later in the string can add them back. This is the safe way to forbid weak ciphers.
- `-` **deletes** the set from the list built so far, but a later set may re-add it. It is weaker than `!`.
- A leading `+` **lowers the priority** of the set, moving its suites toward the end of the list rather than removing them. Note this is different from the `+` that joins keywords inside a set.
- `@STRENGTH` is not a set but a directive: it re-sorts everything selected so far by key length, strongest first. `@SPEED` sorts by encryption speed instead.

So `ECDHE:RSA:!SSLv3:!RC4:@STRENGTH` reads as: prefer ECDHE, then RSA, never allow SSL 3.0 or RC4 (Rivest Cipher 4), then sort what remains by strength.

## What the string does not tell you directly

The grammar tells you the instructions, but not the final list of suites those instructions produce. That list depends on which suites a given TMOS version actually supports, so the same string can expand differently across BIG-IP versions. The [F5 cipher-string explainer](https://ronutz.com/en/tools/f5-cipher-string-expander) parses the grammar and explains every keyword and operator; to see the exact ordered suites, run `tmm --clientciphers` on the target box. For which keywords are safe and which are not, see [TLS cipher security keywords](https://ronutz.com/en/learn/tls-cipher-security-keywords); for how rules and groups assemble these strings, see [F5 cipher rules and groups](https://ronutz.com/en/learn/f5-cipher-rules-and-groups).