Question

Why in Cipher Block Chaining (CBC) mode, an initialization vector is needed in addition to the Key?

Answer 1

Why in Cipher Block Chaining (CBC) mode, an initialization vector is needed in addition to the Key?

• In cryptography, an initialization vector or starting variable is a fixed-size input to a cryptographic primitive that is typically required to be random or pseudorandom. Randomization is crucial for encryption schemes to achieve semantic security, a property whereby repeated usage of the scheme under the same key does not allow an attacker to infer relationships between segments of the encrypted message. For block ciphers, the use of an IV is described by the modes of operation. Randomization is also required for other primitives, such as universal hash functions and message authentication codes.

• Cipher block chaining (CBC) mode is most common legacy encryption mode. It is simple to understand and trivial to implement around an existing ECB mode cipher implementation. It is often mistakenly attributed with providing authenticity for the reason that “a change in the ciphertext will make a nontrivial change in the plaintext.” It is true that changing a single bit of ciphertext will alter two blocks of plaintext in a nontrivial fashion. It is not true that this provides authenticity.

• A block cipher is one of the most basic primitives in cryptography, and frequently used for data encryption. However, by itself, it can only be used to encode a data block of a predefined size, called the block size. For example, a single invocation of the AES algorithm transforms a 128-bit plaintext block into a ciphertext block of 128 bits in size. The key, which is given as one input to the cipher, defines the mapping between plaintext and ciphertext. If data of arbitrary length is to be encrypted, a simple strategy is to split the data into blocks each matching the cipher's block size, and encrypt each block separately using the same key. This method is not secure as equal plaintext blocks get transformed into equal ciphertexts, and a third party observing the encrypted data may easily determine its content even when not knowing the encryption key.

• To hide patterns in encrypted data while avoiding the re-issuing of a new key after each block cipher invocation, a method is needed to randomize the input data.

• In 1980, the National Institute of Standards and Technology published a national standard document designated Federal Information Processing Standard(FIPS) PUB 81, which specified four so-called block cipher modes of operation, each describing a different solution for encrypting a set of input blocks. The first mode implements the simple strategy described above, and was specified as the electronic codebook (ECB) mode. In contrast, each of the other modes describe a process where ciphertext from one block encryption step gets intermixed with the data from the next encryption step.

• To initiate this process, an additional input value is required to be mixed with the first block, and which is referred to as an initialization vector.

• “‘For example, the cipher-block chaining (CBC) mode requires an unpredictable value of the cipher's block size as additional input, and adds it to the first plaintext block before subsequent encryption. In turn, the ciphertext produced in the first encryption step is added to the second plaintext block, and so on. The ultimate goal for encryption schemes is to provide semantic security: by this property, it is practically impossible for an attacker to draw any knowledge from observed ciphertext. It can be shown that each of the three additional modes specified by the National Institute of Standards and Technology are semantically secure under so-called chosen-plaintext attacks.”’

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