Books / Practical Cryptography for Developers (Unfinished) / Chapter 6a
Cipher Block Modes
In cryptography block ciphers (like AES) are designed to encrypt a block of data of fixed size (e.g. 128 bits). The size of the input block is usually the same as the size of the encrypted output block, while the key length may be different.
Stream ciphers are more flexible: they are designed to encrypt data of arbitrary size (e.g. a PDF document), that may sometimes come as a stream (sequence of bytes or frames, e.g. video streaming).
Most of the popular symmetric key encryption algorithms are block ciphers, but cryptographers have proposed several schemes to transform a block cipher into a stream cipher and encrypt data of arbitrary size. These schemes are known as “block cipher modes of operation” and are applicable for most block ciphers like AES, RC6, Camellia, Serpent and many others.
When a symmetric cipher is combined with block mode of operation, the obtained cipher construction is denoted by the names of the cipher and the block mode and the key size. Examples:
- AES-256-GCM - the AES cipher with a 256-bit encryption key and GCM block mode
- AES-128-CTR - the AES cipher with a 128-bit encryption key and CTR block mode
- Serpent-128-CBC - the Serpent cipher with 128-bit encryption key and CBC block mode
Block Cipher Modes (CBC, CTR, GCM, …)
The main idea behind the block cipher modes (like CBC, CFB, OFB, CTR, EAX, CCM and GCM) is to repeatedly apply a cipher’s single-block encryption / decryption to securely encrypt / decrypt amounts of data larger than a block.
Some block modes (like CBC) require the input to be split into blocks and the final block to be padded to the block size using a padding algorithm (e.g. add a special padding character). Other block modes (like CTR, CFB, OFB, CCM, EAX and GCM) do not require padding at all, because they perform XOR between portions of the plaintext and the internal cipher’s state at each step.
Basically, encrypting a large input data works like this: the encryption algorithm state is initialized (using the encryption key + a random salt), then the first portion of data (e.g. a block or part of block) is encrypted, then the encryption state is transformed (using the encryption key and other parameters), then the next portion is encrypted, then the encryption state is transformed again and the next portion is then encrypted and so on, until all the input data is processed. The decryption works in a very similar way.
This is what developers should know about the “block cipher modes of operation” in order to use them correctly:
- Commonly used secure block modes are CBC (Cipher Block Chaining), CTR (Counter) and GCM (Galois/Counter Mode), which require a random (unpredictable) initialization vector (IV), known also as nonce or salt at the start.
- The “Counter (CTR)” block mode is a good choice in the most cases because of strong security, arbitrary input data length (without padding) and parallel processing capabilities. It does not provide authentication and integrity, just encryption.
- The GCM (Galois/Counter Mode) block mode takes all the advantages of the CTR mode and adds message authentication (produces a cryptographical message authentication tag). GCM is fast and efficient way to implement authenticated encryption in symmetric ciphers and it is highly recommended in the general case.
- The CBC mode works in block of fixed size. Thus a padding algorithm should be used to make the last block the same length after splitting the input data into blocks. Most applications use the PKCS7 padding scheme or ANSI X.923. In some scenarios the CBC block mode might be vulnerable to the “padding oracle” attack, so its is better to avoid the CBC mode.
- Well-known insecure block mode is ECB (Electronic Codebook), which encrypts equal input blocks as equal output blocks (does not provide cryptographic diffusion). Don’t use it! It may compromise the entire encryption.
- Most block like CBC, CTR and GCM modes supports “random access” decryption (e.g. seeking at arbitrary time offset in a video player, playing an encrypted video stream).
CTR (Counter) Block Mode
The diagram below illustrates how portions (blocks) of the plaintext are encrypted one after another in the CTR block mode of operation using a block cipher:
For each block in CTR mode a new unpredictable keystream block is generated based on the initial vector (IV, sometimes called “nonce”) + the current counter (01, 02, 03, …) + the secret encryption key and the input block is merged by XOR with the current keystream block to produce the output block. In the CTR mode the final portion of the input data can be shorter then the cipher block size, so padding is not needed. The input data (before encryption) and the output data (after encryption) have the same length.
GCM (Galois/Counter) Block Mode
The following diagram explains visually how the GCM block mode (Galois/Counter Mode) works:
The GCM mode uses a counter, which is increased for each block and calculated a message authentication tag (MAC code) after each processed block. The final authentication tag is calculated from the last block. Like all counter modes, GCM works as a stream cipher, and so it is essential that a different IV is used at the start for each stream that is encrypted.
Choosing a Cipher Block Mode
It is recommended to use either CTR (Counter) or GCM (Galois/Counter) block modes with symmetric ciphers like AES, RC6, Camellia, Serpent and many others. The others might be helpful in certain situations, but some of them are less secure, so use them only if you know well what are you doing.
The CTR and GCM encryption modes have many advantages: they are secure (no significant flaws are currently known), can encrypt data of arbitrary length without padding, can encrypt and decrypt the blocks in parallel (in multi-core CPUs) and provide random (unordered) access to the encrypted blocks, so they are suitable for encrypting crypto-wallets, documents and streaming video (where users can seek by time). GCM provides also message authentication and is the recommended choice for cipher block mode in the general case.
Note that the GCM, CTR and other block modes reveal the length of the original message. The length of the plaintext message is the same as the ciphertext length. If you want to avoid revealing the original plaintext length, you can add some random bytes to the plaintext before the encryption and remove them after decryption (this will be some kind of padding).
The Initialization Vector (IV)
Use a random and unpredictable IV (nonce) for each encrypted message. It is a common mistake to encrypt multiple messages with the same symmetric key and using the same IV. This opens a space for various crypto attacks for the most block modes. The size of the IV should be the same as the cipher block size, e.g. 128-bits for AES, Serpent and Camellia.
For the GCM mode the IV may not be secret and unpredictable, but should be different for each message.
In cryptography the concept of “authenticated encryption” (AE) refers to a scheme to encrypt data and simultaneously calculate an authentication code (authentication tag / MAC), used to provide message authenticity and integrity. If authenticated encryption scheme is used, at the moment of decryption it will be known if the decryption is successful (i.e. whether the decryption key / password was correct and whether the encrypted data was not tampered).
Authenticated encryption (AE) is related to the similar concept authenticated encryption with associated data (AEAD), which is a more secure variant of AE. AEAD binds associated data (AD) to the ciphertext and to the context where it’s supposed to appear, so that attempts to “cut-and-paste” a valid ciphertext into a different context can be detected and rejected. AEAD is used in scenarios where encrypted and unencrypted data is used together (e.g. in encrypted networking protocols) and ensures that the entire data stream is authenticated and integrity protected. In other words, AEAD adds
the ability to check the integrity and authenticity of some
Associated Data (AD), also called “Additional Authenticated Data”
(AAD), that is not encrypted.
Some encryption schemes (like ChaCha20-Poly1305 and AES-GCM) provide integrated authenticated encryption (AEAD), while others (like AES-CBC and AES-CTR) need authentication to be added additionally (if you need it).