gchq/CyberChef · error · OperationError

Invalid ciphertext length: ${cipherText.length} bytes. Must

Error message

Invalid ciphertext length: ${cipherText.length} bytes. Must be a multiple of 8.

What it means

PRESENT is a 64-bit block cipher (8 bytes). decryptPRESENT requires the ciphertext length to be an exact multiple of BLOCKSIZE so it can slice the input into whole blocks; a non-aligned length means the data is truncated or otherwise invalid before decryption can begin.

Source

Thrown at src/core/lib/Present.mjs:385

    return cipherText;
}

/**
 * Decrypt using PRESENT cipher with specified block mode
 *
 * @param {number[]} cipherText - Ciphertext as byte array
 * @param {number[]} key - Key (10 bytes for 80-bit or 16 bytes for 128-bit)
 * @param {number[]} iv - IV (8 bytes, not used for ECB)
 * @param {string} mode - Block cipher mode ("ECB" or "CBC")
 * @param {string} padding - Padding type ("NO", "PKCS5", "ZERO", "RANDOM", "BIT")
 * @returns {number[]} - Plaintext as byte array
 */
export function decryptPRESENT(cipherText, key, iv, mode = "ECB", padding = "PKCS5") {
    if (cipherText.length === 0) return [];

    if (cipherText.length % BLOCKSIZE !== 0) {
        throw new OperationError(`Invalid ciphertext length: ${cipherText.length} bytes. Must be a multiple of 8.`);
    }

    // Generate round keys based on key length
    const roundKeys = key.length === 10 ?
        generateRoundKeys80(key) :
        generateRoundKeys128(key);

    const plainText = [];

    switch (mode) {
        case "ECB":
            for (let i = 0; i < cipherText.length; i += BLOCKSIZE) {
                const block = bytesToBigInt(cipherText.slice(i, i + BLOCKSIZE));
                const decrypted = decryptBlock(block, roundKeys);
                plainText.push(...bigIntToBytes(decrypted, BLOCKSIZE));
            }
            break;

View on GitHub (pinned to 4290ea7539)

Solutions

Verify the source produced 8-byte-aligned output (PRESENT) and not 16-byte (AES).
Re-acquire the full ciphertext and re-decode any hex/base64 encoding cleanly.
Check for and strip stray delimiters, whitespace, or length prefixes before slicing into bytes.
If truncation is expected, pad the ciphertext with zeros to a multiple of 8 and accept that the last block will decrypt to garbage (wrap in try/catch for padding errors).

Example fix

// before
const pt = decryptPRESENT(bytes, key, iv, 'ECB', 'PKCS5'); // 13 bytes -> error

// after
if (bytes.length % 8 !== 0) {
  // either re-acquire or pad-and-accept
  while (bytes.length % 8 !== 0) bytes.push(0);
}
const pt = decryptPRESENT(bytes, key, iv, 'ECB', 'NO'); // skip PKCS5 since tail is synthetic

Defensive patterns

Strategy: validation

Validate before calling

function isPresentBlockAligned(bytes) {
  return bytes.length % 8 === 0;
}

if (!isPresentBlockAligned(cipherText)) {
  throw new Error('Ciphertext length ' + cipherText.length + ' is not a multiple of 8 bytes');
}
decryptPRESENT(cipherText, key, iv, mode, padding);

Try / catch

try {
  return decryptPRESENT(cipherText, key, iv, mode, 'PKCS5');
} catch (e) {
  if (/multiple of 8/.test(e.message)) {
    // re-acquire or pad-and-accept
    const padded = [...cipherText];
    while (padded.length % 8 !== 0) padded.push(0);
    return decryptPRESENT(padded, key, iv, mode, 'NO');
  }
  throw e;
}

Prevention

Confirm the cipher is PRESENT (8-byte blocks) and not AES (16-byte).
Re-decode hex/base64 cleanly so no bytes are dropped.
Strip delimiters/whitespace before slicing into bytes.
Validate cipherText.length % 8 === 0 before calling decryptPRESENT.

When it happens

Trigger: Calling decryptPRESENT(cipherText, ...) where cipherText.length % 8 !== 0. Happens when bytes were dropped in transit, the input was sliced at the wrong offset, or a non-PRESENT ciphertext (e.g. AES, 16-byte blocks) is fed in.

Common situations: Ciphertext truncated by a transport/storage layer; copy-paste dropped a byte; wrong cipher selected (AES-128 uses 16-byte blocks); hex/base64 decoding produced an odd number of bytes; extra delimiter or whitespace byte appended.

Related errors

AI-assisted analysis of gchq/CyberChef@4290ea7539 (2026-08-13). Data as JSON: /api/errors/1179cc86cfffa095. Report an issue: GitHub.