TheAlgorithms/Java · error · IllegalArgumentException

Encrypted message should be a multiple of 64 characters in l

Error message

Encrypted message should be a multiple of 64 characters in length

What it means

Thrown by DES.decrypt(String) when the ciphertext length is not a multiple of 64. This implementation treats each 64-bit block as a 64-character binary substring and processes them in a loop; a length not divisible by 64 means the input is malformed or truncated.

Source

Thrown at src/main/java/com/thealgorithms/ciphers/DES.java:237

            for (j = 0; j < 8; j++) {
                bitBlock.append(pad(Integer.toBinaryString(bytes[j]), 8));
            }
            encryptedMessage.append(encryptBlock(bitBlock.toString(), subKeys));
        }
        return encryptedMessage.toString();
    }

    /**
     * @param message The encrypted string. Expects it to be a multiple of 64 bits, in binary format
     * @return The decrypted String, in plain English
     */
    public String decrypt(String message) {
        StringBuilder decryptedMessage = new StringBuilder();
        int l = message.length();
        int i;
        int j;
        if (l % 64 != 0) {
            throw new IllegalArgumentException("Encrypted message should be a multiple of 64 characters in length");
        }
        for (i = 0; i < l; i += 64) {
            String block = message.substring(i, i + 64);
            String result = decryptBlock(block, subKeys);
            byte[] res = new byte[8];
            for (j = 0; j < 64; j += 8) {
                res[j / 8] = (byte) Integer.parseInt(result.substring(j, j + 8), 2);
            }
            decryptedMessage.append(new String(res));
        }
        return decryptedMessage.toString().replace("\0", ""); // Get rid of the null bytes used for padding
    }
}

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Ensure the ciphertext is the binary-string output produced by this DES implementation's encrypt method.
2. Verify length % 64 == 0 before calling decrypt.
3. If ciphertext was truncated, re-obtain a complete copy rather than decrypting a partial message.

Example fix

// before
String out = des.decrypt(received);

// after
if (received.length() % 64 != 0 || !received.matches("[01]+")) {
    throw new IllegalArgumentException("Ciphertext must be a binary string, length multiple of 64");
}
String out = des.decrypt(received);

if (ciphertext == null || ciphertext.length() % 64 != 0
        || !ciphertext.matches("[01]+")) {
    throw new IllegalArgumentException("Ciphertext must be a binary string with length % 64 == 0");
}
String out = des.decrypt(ciphertext);

static boolean isDesCiphertext(String s) {
    return s != null && s.length() % 64 == 0 && s.matches("[01]+");
}

try {
    String out = des.decrypt(ciphertext);
} catch (IllegalArgumentException e) {
    // malformed/truncated ciphertext; re-obtain a complete copy
}

When it happens

Trigger: Passing ciphertext that is not in the library's 64-char-per-block binary-string format, or ciphertext that was truncated/corrupted so its length is no longer a multiple of 64.

Common situations: Feeding hex or Base64 ciphertext instead of the binary-string format; partial transmission; mixing this DES implementation's format with another tool's output.

Related errors

• DES key must be supplied as a 64 character binary string
• Invalid Baconian code: {}
• Base, secret, and prime must be non-null and positive values
• Other public value must be non-null and positive.
• Bit length must be at least {} for security.

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/d375864474a6c087. Report an issue: GitHub.