How to Add and Remove PKCS7 Padding from AES-Encrypted Strings?

How to Add/Remove PKCS7 Padding from AES Encrypted String?

In cryptography, PKCS7 padding is used to ensure the confidentiality of sensitive data by adding additional bytes to the end of a message. This padding allows for the encrypted data to be a multiple of the block size of the encryption algorithm being used, typically AES (Advanced Encryption Standard) in 128-bit mode.

Understanding PKCS7 Padding

PKCS7 padding follows a specific algorithm:

1. Calculate the number of bytes needed to pad the message to a multiple of the block size.
2. Append that number of bytes to the end of the message.
3. Set each padding byte to the value of the number of padding bytes.

Adding PKCS7 Padding

To add PKCS7 padding, you can follow these steps:

import base64
from Crypto.Cipher import AES
from Crypto.Util.Padding import pad

# Sample data to encrypt
data = "Hello, World!"

# Define the AES key (128 bits = 16 bytes)
key = 'YOUR_AES_KEY_16_BYTES_LONG'

# Initialization vector (IV) for ECB mode is not used and ignored
iv = '0' * 16

# Create an AES cipher in ECB mode
cipher = AES.new(key, AES.MODE_ECB)

# Pad the data before encryption
padded_data = pad(data, 16)  # 16 is the block size for AES-128

# Encrypt the padded data
ciphertext = cipher.encrypt(padded_data)

# Encode the ciphertext in base64 for transmission
ciphertext_base64 = base64.b64encode(ciphertext).decode('utf-8')

# Print the encrypted and base64-encoded ciphertext
print(ciphertext_base64)

Removing PKCS7 Padding

To remove PKCS7 padding, you can follow these steps:

import base64
from Crypto.Cipher import AES
from Crypto.Util.Padding import unpad

# Sample base64-encoded ciphertext to decrypt
ciphertext_base64 = 'ENCRYPTED_AND_BASE64_ENCODED_DATA'

# Decode the base64 ciphertext
ciphertext = base64.b64decode(ciphertext_base64)

# Define the AES key (128 bits = 16 bytes)
key = 'YOUR_AES_KEY_16_BYTES_LONG'

# Initialization vector (IV) for ECB mode is not used and ignored
iv = '0' * 16

# Create an AES cipher in ECB mode
cipher = AES.new(key, AES.MODE_ECB)

# Decrypt the ciphertext
decrypted = cipher.decrypt(ciphertext)

# Remove the padding from the decrypted data
data = unpad(decrypted, 16)

# Print the decrypted and unpadded data
print(data.decode('utf-8'))

The above is the detailed content of How to Add and Remove PKCS7 Padding from AES-Encrypted Strings?. For more information, please follow other related articles on the PHP Chinese website!