How Does PKCS7 Padding Work with AES Encryption?

PKCS7 Padding for AES Encryption

When encrypting data using 128-bit AES encryption in ECB mode, PKCS7 padding may be necessary to ensure the data is a multiple of the block size. This padding adds a variable number of bytes to the end of the data, where the value of each byte represents the number of padding bytes added.

Adding PKCS7 Padding

To add PKCS7 padding to a plaintext string:

• Calculate the block size of the encryption algorithm (16 bytes for AES)
• Determine the number of padding bytes needed by calculating the remainder of the plaintext length divided by the block size.
• Append the plaintext with the character representing the padding length, repeated the required number of times.

Example in PHP (Mcrypt)

<?php
$block_size = mcrypt_get_block_size('rijndael_128', 'ecb'); // Block size for AES
$padding_size = $block_size - (strlen($plaintext) % $block_size);
$plaintext .= str_repeat(chr($padding_size), $padding_size);
?>

Removing PKCS7 Padding

To remove PKCS7 padding from a ciphertext string:

• Decrypt the ciphertext to obtain the padded plaintext.
• Calculate the padding length as the value of the last byte of the plaintext.
• Verify that the last padding_length bytes of the plaintext are all equal to the padding length.
• Remove the last padding_length bytes from the plaintext.

Example in PHP (Mcrypt)

<?php
$ciphertext = ...; // Encrypted ciphertext with PKCS7 padding
$key = ...; // Encryption key
$decrypted_plaintext = mcrypt_decrypt('rijndael_128', $key, $ciphertext, 'ecb');
$padding_length = ord($decrypted_plaintext[strlen($decrypted_plaintext) - 1]);
if (str_repeat(chr($padding_length), $padding_length) === substr($decrypted_plaintext, -1 * $padding_length)) {
    $plaintext = substr($decrypted_plaintext, 0, -1 * $padding_length); // Remove padding
} else {
    // Invalid padding
}
?>

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