TheAlgorithms/C-Sharp · error · ArgumentOutOfRangeException

Invalid block size

Error message

Invalid block size: {blockSize}

What it means

Pkcs7Padding's constructor validates that the block size is between 1 and 255, since PKCS#7 encodes the padding length as a single byte. Passing any other value makes valid PKCS#7 padding impossible, so the library throws ArgumentOutOfRangeException immediately at construction rather than failing later during padding operations.

Solutions

1. Pass a block size in bytes between 1 and 255 (16 for AES, 8 for DES/3DES).
2. If you have a bit-based size, divide by 8 before constructing.
3. Validate configuration values before constructing the padding.
4. If you need larger blocks than 255 bytes, use a different padding scheme or library.

Example fix

// before
var padding = new Pkcs7Padding(128); // bits, throws
// after
var padding = new Pkcs7Padding(128 / 8); // 16 bytes

if (blockSize is < 1 or > 255)
    throw new ArgumentOutOfRangeException(nameof(blockSize), blockSize, "Block size must be 1..255 bytes");
var padding = new Pkcs7Padding(blockSize);

static bool IsValidBlockSize(int size) => size is >= 1 and <= 255;

try { padding = new Pkcs7Padding(cfg.BlockSize); }
catch (ArgumentOutOfRangeException ex) { /* surface config error to operator */ throw new InvalidOperationException("Invalid configured block size", ex); }

When it happens

Trigger: Calling new Pkcs7Padding(n) with n < 1 (e.g. 0 or negative) or n > 255 (e.g. 512, or a bit size like 128 mistaken for a byte size).

Common situations: Developers confusing block size in bits (AES 128-bit) with bytes and passing 128 in bits-based units while intending 16; copying initialization code and forgetting to set the size; reading a config value of 0 when the block size is unset.

Related errors

• Not enough space in input array for padding
• Not enough space in input array for padding
• Input length must be a multiple of block size
• Invalid padding length
• Invalid padding

AI-assisted analysis of TheAlgorithms/C-Sharp@96e2905cab (2026-09-13). Data as JSON: /api/errors/8d529b625375e538. Report an issue: GitHub.

Appendix: source

Thrown at Algorithms/Crypto/Paddings/Pkcs7Padding.cs:29

/// </para>
/// <para>
/// The padding can be easily removed after decryption by looking at the last byte and subtracting that many bytes from the
/// end of the data.
/// </para>
/// <para>
/// This class supports any block size from 1 to 255 bytes, and can be used with any encryption algorithm that requires
/// padding, such as AES.
/// </para>
/// </summary>
public class Pkcs7Padding : IBlockCipherPadding
{
    private readonly int blockSize;

    public Pkcs7Padding(int blockSize)
    {
        if (blockSize is < 1 or > 255)
        {
            throw new ArgumentOutOfRangeException(nameof(blockSize), $"Invalid block size: {blockSize}");
        }

        this.blockSize = blockSize;
    }

    /// <summary>
    /// Adds padding to the end of a byte array according to the PKCS#7 standard.
    /// </summary>
    /// <param name="input">The byte array to be padded.</param>
    /// <param name="inputOffset">The offset from which to start padding.</param>
    /// <returns>The padding value that was added to each byte.</returns>
    /// <exception cref="ArgumentException">
    /// If the input array does not have enough space to add <c>blockSize</c> bytes as padding.
    /// </exception>
    /// <remarks>
    /// The padding value is equal to the number of of bytes that are added to the array.
    /// For example, if the input array has a length of 16 and the input offset is 10,
    /// then 6 bytes with the value 6 will be added to the end of the array.

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