Cryptography - Cipher Block Chaining (CBC) Mode

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It is a A block cipher that encrypts a series of bits as a whole or as a block and applies a ciphertext or encrypted key to the full block of cryptography is known as a cipher block chaining (CBC) mode. Initialization vectors (IVs) of particular characters in length are used in cipher block chaining. One of its main features is that it makes use of chaining, a working technique that makes a ciphertext block's decryption dependent on every block that came before it. Therefore, the cryptography of the block that comes right before the ciphertext has all of the validity of all blocks that came before it.

A single-bit cryptography error in one block of ciphertext affects all subsequent block decryptions. The ciphertext blocks' order can be changed, which corrupts the decryption process. In cipher block chaining, each plaintext block is essentially encrypted using cryptography after being XORed (see XOR) with the block of ciphertext that comes right before it.

Only when the same plaintext or original text block is encrypted with the same encryption key and the initialization vector (IV) and the ciphertext block order is left unaltered will the identical ciphertext blocks provide the desired outcome. Because the XORing process masks plaintext patterns, it has an advantage over the Electronic Code Book mode.

When two communications are encrypted using the same encryption key, the initialization vector (IV) should differ. While some applications may find it useful and unique, the initialization vector does not necessarily need to be stored or kept secret.

Since ECB compromises some security or privacy criteria, cipher block chaining, or CBC, is an improved or more sophisticated version of ECB. After XORing with an initial plaintext block of the cryptography, the preceding cipher block in the CBC is sent as input to the subsequent encryption process. To put it simply, an XOR output of the previous cipher block and the current plaintext or original text block is encrypted to create a cipher block.

The process is shown below −

Operation

The above image shows how the CBC mode works. The following are the steps −

• The top register should be loaded with the n-bit Initialization Vector (IV).

• XOR the data value in the top register with the n-bit plaintext block.

• Use key K to encrypt the output of the XOR operation using the underlying block cipher.

• Continue the process until all plaintext blocks have been processed by feeding the ciphertext block into the top register.

• IV data is XORed with the first decrypted ciphertext block in order to decrypt it. In order to decrypt the next ciphertext block, the previous ciphertext block is added as well into the register that replaces IV.

Analysis of CBC Mode

In CBC mode, the previous ciphertext block and the current plaintext block are combined, and the outcome is then encrypted using the key. Decryption, then, is the opposite procedure, in which the previous ciphertext block is added to the result after the current ciphertext has been decrypted.

The benefit of CBC over ECB is that an identical message can use a different ciphertext when the IV is changed. On the negative side, the chaining effect causes the transmission error to propagate to a few further blocks during decryption.

It is important to note that CBC mode provides the base for a well-known data origin authentication system. Thus, it benefits applications which need both symmetric encryption and data origin authentication.

Bit-Width of CBC Mode

The following table show the bit-width of the interfaces that CBC mode offer −

A common block cipher mode of operation that makes use of the block cipher algorithm is the Cipher Block Chaining (CBC) mode. It has the ability to process both the Advanced Encryption Standard (AES) and the Data Encryption Standard (DES) in this version. The cipherkey length for AES should be 128/192/256 bits, and 64 bits for DES. Another drawback is that, but text in the real world comes in a range of lengths, our working mode only supports units of a fixed size (64 or 128 bits for a single block). Therefore, before encryption or decryption, the final block of text given to this primitive needs to be padded to 128 bits.

Formula for CBC mode

This method can be expressed as follows if it is put it into a formula −

Ci = EK(Bi ⊕ Ci-1)

where Ci-1 is the cipher that corresponds to Bi-1 and EK is the block encryption technique with key K.

Note: It is assuming that C0 is the initialization vector in the formula above.

In the same way, the CBC can be used for decryption by using −

Bi = DK(Ci)⊕(Ci-1)

Where DK stands for the block decryption technique with key K.

For decryption, the same initialization vector (C0) will be used.

Security Challenge

The primary attributes of this scheme are as follows −

• One additional particular block will be damaged if even a single bit of the conveyed message gets altered or lost. This harm wouldn't affect other blocks.

• In the event that a bit is lost or added to the ciphertext, the bits and block borders will move, producing an incorrect decoding of all ensuing blocks of the cryptography's ciphertext.

• The factor has the ability to append blocks to the end of the deciphered message, thereby enhancing it with either the original or plain text.

Advantages

Cipher Block Chaining (CBC) mode has several advantages −

• When input data is more than one block in size (usually 128 bits or 16 bytes for AES), CBC operates well. Messages of any length can be encrypted and decrypted by breaking them up into blocks and joining them.

• Although CBC itself cannot provide authentication, it can be used in combination with digital signatures or HMAC (Hash-based Message Authentication Code) in order to ensure the integrity and validity of the encrypted data. CBC can offer a complete security solution by combining authentication and encryption.

• When it comes to cryptanalysis, CBC is typically safer than ECB (Electronic Codebook) mode. An attacker finds it more difficult to identify patterns in the encrypted data when using CBC, because each block of ciphertext depends on the one before it. Due to this, CBC is more effective for securing private data from attacks or attempts to decipher it.

Disadvantages

The inability of Cipher Block Chaining (CBC) mode for sending out concurrent encryption is one of its drawbacks. Before encryption, each plaintext block in CBC mode is XORed with the previous ciphertext block. This creates a dependency on the previous ciphertext block for the current block's encryption. As each block's encryption depends on the previous block's encryption being finished, the following ones cannot be encrypted concurrently or in parallel. In comparison to modes like Electronic Codebook (ECB) or Counter (CTR) mode, which can carry out concurrent encryption of numerous blocks simultaneously, the sequential nature of encryption in CBC mode can result in slower encryption performance. But because of its security benefits and compatibility for a wide range of encryption scenarios, CBC mode is still widely used despite of this limitation.