How Does PKCS7 Padding Work with AES Encryption, and How Can I Add and Remove It in PHP?

PKCS7 Padding for AES Encryption

When using AES encryption, it's common to employ PKCS7 padding to ensure the encrypted data length aligns with the block size of the underlying encryption algorithm. However, adding and removing this padding may not be handled by the Mcrypt extension, leaving it as a manual task.

PKCS7 Padding Explained

As defined in RFC 5652, PKCS #7 padding works by appending bytes with a value equal to the padding length. For instance, if we need to pad three bytes, the appended bytes would all have the value 0x03.

Adding PKCS7 Padding

To add the padding manually, calculate the difference between the current data length and the desired block size. This value represents the number of padding bytes required. Then, append that many bytes with the padding length as value to the end of the data.

Sample PHP Function for Adding Padding

function addPKCS7Padding($data)
{
    $block_size = 16;
    $pad_length = $block_size - (strlen($data) % $block_size);
    $padding = str_repeat(chr($pad_length), $pad_length);
    return $data . $padding;
}

Removing PKCS7 Padding

To remove the padding, read the last byte of the decrypted data. This value represents the length of the padding. Then, simply remove that number of bytes from the end of the data.

Sample PHP Function for Removing Padding

function removePKCS7Padding($data)
{
    $pad_length = ord($data[strlen($data) - 1]);
    return substr($data, 0, -1 * $pad_length);
}

Implementation Notes

• The example PHP functions use a block size of 16 bytes for AES-128.
• It's crucial to verify that the padding is correct before using the decrypted data by ensuring all padding bytes have the same value as the padding length.
• ECB mode is insecure. Encourage your client to consider using a more secure mode like CBC or GCM for this encryption task.

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