TheAlgorithms/C-Sharp · error · ArgumentException

Pad block corrupted

Error message

Pad block corrupted

What it means

X932Padding.GetPaddingCount derives the padding count from the last byte of the input and validates it with a constant-time bitwise check: position = input.Length - count must be positive and count - 1 must be non-negative. If either fails (count is 0, or the padding is longer than the block), it throws this ArgumentException indicating a corrupted or non-X9.32 pad block.

Solutions

1. Confirm you are passing decrypted plaintext (not ciphertext) to GetPaddingCount.
2. Check the decryption key/IV; random-looking last bytes indicate a key mismatch.
3. Verify the block passed in is complete and exactly one cipher block long.
4. Catch ArgumentException and treat it as a padding-oracle condition: return a generic decryption-failure to callers.

Example fix

// before
int padLen = padding.GetPaddingCount(decryptedBlock); // throws 'Pad block corrupted'

// after
try
{
    int padLen = padding.GetPaddingCount(decryptedBlock);
}
catch (ArgumentException)
{
    throw new CryptographicException("Decryption failed: invalid padding");
}

int count = input[^1] & 0xFF;
bool valid = count >= 1 && count <= input.Length;

try { int pad = padding.GetPaddingCount(block); }
catch (ArgumentException) { throw new CryptographicException("Decryption failed: corrupted pad block"); }

When it happens

Trigger: Calling GetPaddingCount on a block whose last byte is 0x00 (count = 0), or on a block shorter than the encoded padding count (e.g. last byte 0xFF on a 16-byte block). Any tampered or wrongly decrypted final block.

Common situations: Side-channel-safe padding checks during decryption of data encrypted with the wrong key; ciphertext bit-flipping/tampering; feeding ciphertext bytes instead of decrypted plaintext; blocks assembled in the wrong order.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• Invalid padding length
• Invalid padding length
• Padding block is corrupted
• Invalid padding
• Pad block corrupted

AI-assisted analysis of TheAlgorithms/C-Sharp@96e2905cab (2026-09-13). Data as JSON: /api/errors/00d9b98b18fdbba3. Report an issue: GitHub.

Appendix: source

Thrown at Algorithms/Crypto/Paddings/X932Padding.cs:128

    /// <exception cref="ArgumentException">
    /// Thrown when the input data has a corrupted padding block.
    /// </exception>
    public int GetPaddingCount(byte[] input)
    {
        // Get the last byte of the input data, which is the padding length.
        var count = input[^1] & 0xFF;

        // Calculate the position of the first padding byte.
        var position = input.Length - count;

        // Check if the position and count are valid using bitwise operations.
        // If either of them is negative or zero, the result will be negative.
        var failed = (position | (count - 1)) >> 31;

        // Throw an exception if the result is negative.
        if (failed != 0)
        {
            throw new ArgumentException("Pad block corrupted");
        }

        // Return the padding length.
        return count;
    }
}

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