How to Add and Remove PKCS#7 Padding in AES Encryption?

Considerations for Adding and Removing PKCS#7 Padding in AES Encryption

In AES-128 encryption using the ECB mode (Electronic Codebook), PKCS#7 (Public Key Cryptography Standard #7) padding is commonly employed to ensure that the message to be encrypted conforms to the AES block size, which is 128 bits. The padding process involves appending a specific number of bytes to the message to make its length divisible by 128.

To add PKCS#7 padding to a message, determine the number of padding bytes needed. This is calculated by taking the difference between the current message length and the block size, and then adding 1. The padding bytes added will consist of a repeated character whose ASCII value is the number of padding bytes. For instance, if four padding bytes are needed, the character 0x04 (ASCII code for 4) will be repeated four times and appended to the end of the message.

To remove PKCS#7 padding from a decrypted message, examine the last byte of the decrypted message. This byte indicates the number of padding bytes to be removed. Subtract the value of this byte from the length of the decrypted message to determine the actual message length. Then, remove the last few bytes from the message, ensuring that the number of removed bytes matches the value indicated by the last byte.

For more detailed implementation examples, refer to the provided user-contributed notes in the mcrypt documentation, which demonstrate specific implementations of PKCS#7 padding for various encryption algorithms, including AES. It is important to note that instead of merely stripping off the padding bytes, it is recommended to verify their correctness by confirming that they all have the same value as the last byte. This serves as a basic check against potential data corruption or manipulation.

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