gchq/CyberChef · error · OperationError

Alphabet must be of length 85

Error message

Alphabet must be of length 85

What it means

Thrown by From Base85 when the supplied alphabet, after expandAlphRange expansion, does not have exactly 85 unique characters. Ascii85/Base85 requires an 85-symbol alphabet; both length and uniqueness are verified. This is a configuration error on the Alphabet argument.

Source

Thrown at src/core/operations/FromBase85.mjs:91

                args: ["0-9A-Za-z!#$%&()*+\\-;<=>?@^_`{|}~"],
            },
        ];
    }

    /**
     * @param {string} input
     * @param {Object[]} args
     * @returns {byteArray}
     */
    run(input, args) {
        const alphabet = Utils.expandAlphRange(args[0]).join(""),
            removeNonAlphChars = args[1],
            allZeroGroupChar = typeof args[2] === "string" ? args[2].slice(0, 1) : "",
            result = [];

        if (alphabet.length !== 85 ||
            [].unique.call(alphabet).length !== 85) {
            throw new OperationError("Alphabet must be of length 85");
        }

        if (allZeroGroupChar && alphabet.includes(allZeroGroupChar)) {
            throw new OperationError("The all-zero group char cannot appear in the alphabet");
        }

        // Remove delimiters if present
        const matches = input.match(/^<~(.+?)~>$/);
        if (matches !== null) input = matches[1];

        // Remove non-alphabet characters
        if (removeNonAlphChars) {
            const re = new RegExp("[^~" + allZeroGroupChar +alphabet.replace(/[[\]\\\-^$]/g, "\\$&") + "]", "g");
            input = input.replace(re, "");
            // Remove delimiters again if present (incase of non-alphabet characters in front/behind delimiters)
            const matches = input.match(/^<~(.+?)~>$/);
            if (matches !== null) input = matches[1];
        }

View on GitHub (pinned to 4290ea7539)

Solutions

1. Use a built-in preset (!-u, the IPv6 variant, z85) rather than a hand-typed alphabet.
2. If custom, verify exactly 85 unique characters.
3. Check expandAlphRange output if using A-Z style ranges.

Example fix

// before: 84-char alphabet
fromBase85.run(input, ['!-t']) // expands to 84
// after: standard Ascii85 range (85)
fromBase85.run(input, ['!-u'])

const alphabet = Utils.expandAlphRange(args[0]).join('');
const unique = new Set(alphabet.split(''));
if (alphabet.length !== 85 || unique.size !== 85) {
  // use a built-in preset (!-u, z85, IPv6); do not call fromBase85.run()
}

function isValidBase85Alphabet(a) {
  return a.length === 85 && new Set(a).size === 85;
}

try {
  fromBase85.run(input, args);
} catch (e) {
  if (e.type === 'OperationError' && /Alphabet must be of length 85/.test(e.message)) {
    // reset alphabet to a built-in preset
  } else throw e;
}

When it happens

Trigger: Providing a custom alphabet shorter/longer than 85; an alphabet with duplicate characters; a range like A-Z that expands to fewer than expected; mixing presets.

Common situations: User edits the alphabet field and loses characters; confusion between the standard !-u range and the IPv6/z85 alphabets; copy-paste truncation.

Related errors

• The all-zero group char cannot appear in the alphabet
• Alphabet must be of length 58
• Unrecognised unit
• Error: Alphabet must be of length 85
• Invalid Base64 alphabet length (${alphabet.length}): ${alpha

AI-assisted analysis of gchq/CyberChef@4290ea7539 (2026-08-13). Data as JSON: /api/errors/5530436c62e2dde0. Report an issue: GitHub.