gchq/CyberChef · error · OperationError

Unable to detect input key format.

Error message

Unable to detect input key format.

What it means

When input format is 'Auto', detectKeyFormat() tests the key against a hex regex and a base64 regex. If neither matches, the key data does not resemble either encoding and the format cannot be determined automatically.

Solutions

1. Manually select 'Hex' or 'Base64' as the input format instead of 'Auto'
2. Clean the key data: remove extra whitespace, line breaks, and non-ASCII characters
3. If the key has an OpenSSH new-format prefix not recognized by the parser, strip it manually and provide just the base64 key body

// Pre-check: test if the key matches hex or base64
const HEX_RE = /^(?:[\dA-Fa-f]{2}[ ,;:]?)+$/;
const B64_RE = /^\s*(?:[A-Za-z\d+/]{4})+(?:[A-Za-z\d+/]{2}==|[A-Za-z\d+/]{3}=)?\s*$/;
const keyBody = input.replace(/^(?:ssh|ecdsa-sha2)\S+\s+/, '').trim();
if (!HEX_RE.test(keyBody) && !B64_RE.test(keyBody)) {
  console.warn("Key format could not be auto-detected — specify Hex or Base64 manually");
}

function isHexOrBase64(s) {
  const HEX_RE = /^(?:[\dA-Fa-f]{2}[ ,;:]?)+$/;
  const B64_RE = /^\s*(?:[A-Za-z\d+/]{4})+(?:[A-Za-z\d+/]{2}==|[A-Za-z\d+/]{3}=)?\s*$/;
  return HEX_RE.test(s) || B64_RE.test(s);
}

try {
  const result = chef.parseSSHHostKey(input, ["Auto"]);
} catch (e) {
  if (/detect.*format/i.test(e.message)) {
    // Retry with explicit format
    const result = chef.parseSSHHostKey(input, ["Base64"]);
  }
}

When it happens

Trigger: The key string contains characters invalid in both hex and base64 (e.g., special characters, whitespace in the middle, or non-ASCII bytes). The key was corrupted or partially stripped. The key has a prefix like 'ssh-rsa ' that was not stripped (note: the code does strip 'ssh-/ecdsa-' prefixes, but other prefixes are not handled).

Common situations: User pastes a key with a type prefix that is not matched by the keyPattern regex (e.g., 'sk-ssh-ed25519@openssh.com'). Key has embedded whitespace or line breaks that break both regexes. Key was copy-pasted with formatting artifacts (smart quotes, non-breaking spaces).

Related errors

• Byte length must be a positive integer
• Character at position
• Input private key must be in hex; and should be 32 bytes
• Invalid Base64 alphabet length
• Invalid encoding

AI-assisted analysis of gchq/CyberChef@4290ea7539 (2026-08-13). Data as JSON: /api/errors/d3559560e724a2f3. Report an issue: GitHub.

Appendix: source

Thrown at src/core/operations/ParseSSHHostKey.mjs:127

    }


    /**
     * Detects if the key is base64 or hex encoded
     *
     * @param {string} inputKey
     * @returns {string}
     */
    detectKeyFormat(inputKey) {
        const hexPattern = new RegExp(/^(?:[\dA-Fa-f]{2}[ ,;:]?)+$/);
        const b64Pattern = new RegExp(/^\s*(?:[A-Za-z\d+/]{4})+(?:[A-Za-z\d+/]{2}==|[A-Za-z\d+/]{3}=)?\s*$/);

        if (hexPattern.test(inputKey)) {
            return "Hex";
        } else if (b64Pattern.test(inputKey)) {
            return "Base64";
        } else {
            throw new OperationError("Unable to detect input key format.");
        }
    }


    /**
     * Parses fields from the key
     *
     * @param {byteArray} key
     */
    parseKey(key) {
        const fields = [];
        while (key.length > 0) {
            const lengthField = key.slice(0, 4);
            let decodedLength = 0;
            for (let i = 0; i < lengthField.length; i++) {
                decodedLength += lengthField[i];
                decodedLength = decodedLength << 8;
            }

View on GitHub (pinned to 4290ea7539)