gchq/CyberChef · error · OperationError

Error: Alphabet must be of length 85

Error message

Error: Alphabet must be of length 85

What it means

To Base85 requires an alphabet of exactly 85 unique characters (ASCII85/Z85 variants). The guard enforces both length === 85 and a unique count of 85 before encoding, since Base85 packs 4 bytes into 5 symbols and needs all 85 symbols available.

Source

Thrown at src/core/operations/ToBase85.mjs:57

            }
        ];
    }

    /**
     * @param {ArrayBuffer} input
     * @param {Object[]} args
     * @returns {string}
    */
    run(input, args) {
        input = new Uint8Array(input);
        const alphabet = Utils.expandAlphRange(args[0]).join(""),
            encoding = alphabetName(alphabet),
            includeDelim = args[1];
        let result = "";

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

        if (input.length === 0) return "";

        let block;
        for (let i = 0; i < input.length; i += 4) {
            block = (
                ((input[i])          << 24) +
                ((input[i + 1] || 0) << 16) +
                ((input[i + 2] || 0) << 8)  +
                ((input[i + 3] || 0))
            ) >>> 0;

            if (encoding !== "Standard" || block > 0) {
                let digits = [];
                for (let j = 0; j < 5; j++) {
                    digits.push(block % 85);
                    block = Math.floor(block / 85);

View on GitHub (pinned to 4290ea7539)

Solutions

1. Use the standard ASCII85 alphabet (0-9, A-Z, a-z, plus !-u) or the Z85 set as appropriate.
2. Verify the alphabet has exactly 85 distinct characters.
3. Remove any duplicate or whitespace characters.

Example fix

// before: alphabet = "0-9A-Za-z!-u" mis-typed, <85 unique -> throws
// after:  alphabet = a valid 85-character ASCII85 set            -> passes

const set = new Set([...alphabet]);
if (alphabet.length !== 85 || set.size !== 85) {
  throw new Error("Base85 alphabet must have 85 unique chars");
}

const isValidBase85Alphabet = a => a.length === 85 && new Set(a).size === 85;

try { toBase85(input, [alphabet, includeDelim]); }
catch (e) { if (/length 85/.test(e.message)) { alphabet = DEFAULT_BASE85; } else throw e; }

When it happens

Trigger: Changing the alphabet argument to fewer/more than 85 characters, or including duplicates, so either the length or uniqueness check fails.

Common situations: Swapping between ASCII85 and Z85 alphabets and mistyping one; pasting an alphabet with a stray space or missing character; using an expanded range that does not yield 85 unique symbols.

Related errors

• Error: alphabet must be of length 58
• Invalid Base64 alphabet length (${alphabet.length}): ${alpha
• Error: Base64 alphabet should be 64 characters long, or 65 w
• Byte length must be a positive integer
• Letter ${letter} is not included in LS47

AI-assisted analysis of gchq/CyberChef@4290ea7539 (2026-08-13). Data as JSON: /api/errors/6e4e9bcf90b22d2a. Report an issue: GitHub.