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AES-CBC with Pointy Castle

This article describes how to perform AES-CBC encryption and decryption using the Pointy Castle package, which is an implementation of cryptographic algorithms for use with the Dart programming language.

Overview

The Advanced Encryption Standard (AES) is a symmetric encryption algorithm. As a symmetric algorithm, the same secret key is used to encrypt and decrypt. It is also a block cipher algorithm, which means the algorithm processes the data in fixed-size blocks.

Cipher Block Chaining (CBC) is a mode of operation where each block is combined with the previous block before it is encrypted. Since the first block doesn't have a previous block, it is combined with an Initialization Vector (IV).

There are three algorithms in the AES family: AES-128, AES-192 and AES-256, corresponding to the length of the keys in bits. For all the algorithms, the block size is always 128-bits (16 bytes).

To encrypt using AES-CBC:

  1. Instantiate the CBC block cipher class with the AES implementation class.
  2. Initialize it with the key and Initialization Vector (IV) for encryption.
  3. Process each block of the padded plaintext being encrypted.

To decrypt using AES-CBC:

  1. Instantiate the CBC block cipher class with the AES implementation class.
  2. Initialize it with the key and Initialization Vector (IV) for decryption.
  3. Process each block of the ciphertext being decrypted.

These functions encrypts and decrypts using AES-CBC:

import 'dart:convert';
import 'dart:math';
import 'dart:typed_data';

import "package:pointycastle/export.dart";

Uint8List aesCbcEncrypt(
    Uint8List key, Uint8List iv, Uint8List paddedPlaintext) {
  assert([128, 192, 256].contains(key.length * 8));
  assert(128 == iv.length * 8);
  assert(128 == paddedPlaintext.length * 8);

  // Create a CBC block cipher with AES, and initialize with key and IV

  final cbc = CBCBlockCipher(AESEngine())
    ..init(true, ParametersWithIV(KeyParameter(key), iv)); // true=encrypt

  // Encrypt the plaintext block-by-block

  final cipherText = Uint8List(paddedPlaintext.length); // allocate space

  var offset = 0;
  while (offset < paddedPlaintext.length) {
    offset += cbc.processBlock(paddedPlaintext, offset, cipherText, offset);
  }
  assert(offset == paddedPlaintext.length);

  return cipherText;
}

Uint8List aesCbcDecrypt(Uint8List key, Uint8List iv, Uint8List cipherText) {
  assert([128, 192, 256].contains(key.length * 8));
  assert(128 == iv.length * 8);
  assert(128 == cipherText.length * 8);

  // Create a CBC block cipher with AES, and initialize with key and IV

  final cbc = CBCBlockCipher(AESEngine())
    ..init(false, ParametersWithIV(KeyParameter(key), iv)); // false=decrypt

  // Decrypt the cipherText block-by-block

  final paddedPlainText = Uint8List(cipherText.length); // allocate space

  var offset = 0;
  while (offset < cipherText.length) {
    offset += cbc.processBlock(cipherText, offset, paddedPlainText, offset);
  }
  assert(offset == cipherText.length);

  return paddedPlainText;
}

The key must be exactly 128-bits, 192-bits or 256-bits (i.e. 16, 24 or 32 bytes). This is what determines whether AES-128, AES-192 or AES-256 is being performed.

The iv must be exactly 128-bits (16 bytes) long, which is the AES block size.

The paddedPlainText must be a multiple of the block size (128-bits). If the data being encrypted is not the correct length, it must be padded before it can be processed by AES. The implementations of padding algorithms in Pointy Castle can be used for this.

Details

Implementation

Using the registry

If using the registry, invoke the BlockCipher factory with the name of the encryption algorithm and block cipher mode: "AES/CBC".

final aesCbc = BlockCipher('AES/CBC');

Without the registry

If the registry is not used, invoke the block cipher's constructor, passing in the AES implementation as a parameter.

final aesCbc = CBCBlockCipher(AESEngine());

Initialize with key and IV

The first parameter determines if the object is used for encryption or decryption.

Initialize for encryption:

aesCbc.init(true, ParametersWithIV(KeyParameter(key), iv)); // true=encrypt

Initialize for decryption:

aesCbc.init(false, ParametersWithIV(KeyParameter(key), iv)); // false=decrypt

Processing each block

Invoke the processBlock method for each block, in order from the first block to the last.

The method takes four parameters:

  • Source Uint8List where the block comes from;
  • Offset into that source where the block starts;
  • Destination Uint8List to write the calculated block to;
  • Offset into that destination where the output block will start.

Since each block is processed into another block, the destination is the exact same size as the source.

For example,

final destination = Uint8List(source.length); // allocate space

var offset = 0;
while (offset < paddedPlaintext.length) {
  offset += cbc.processBlock(source, offset, destination, offset);
}
assert(offset == source.length);

The process is the same for encryption and decryption. With encryption, the source is the padded plaintext. With decryption, the source is the ciphertext (which doesn't need padding, since it is guaranteed to be a multiple of the block size).

External dependencies

There is no single standard for how the IV, ciphertext, and the other necessary information (e.g. the key length) is stored or transmitted. Different programs and standards do it differently. To be able to interoperate, the encrypting and decrypting programs must agree to use the same method.

There is also no single standard for how the keys are obtained, the IV is generated, or how the data is padded. Again, for interoperability, the encrypting and decrypting programs must agree to use the same method.

  • The key is normally derived from a text passphrase, using a secure key derivation algorithm. While the key must be kept secret, the algorithm (with any parameters) used by the encrypting software must provided to or known by the decrypting software. One of those parameters is the size of the key, since that determines which AES algorithm is used.

  • The Initialization Vector (IV) is normally randomly generated. It must be stored or transmitted to the decrypting software, since the same IV must be used in decryption.

  • The padding must be identified by the decrypting software, so it can remove it from the decrypted blocks.

It is important to securely derive the key and generate the IV. While AES itself is secure, a system is only as secure as its weakest link.