What is cipher block chaining?
Cipher block chaining (CBC) is a mode of operation for a block cipher — one in which a sequence of bits are encrypted as a single unit, or block, with a cipher key applied to the entire block. Cipher block chaining uses what is known as an initialization vector (IV) of a certain length. By using this along with a single encryption key, organizations and individuals can safely encrypt and decrypt large amounts of plaintext.
One of CBC’s key characteristics is that it uses a chaining process that causes the decryption of a block of ciphertext to depend on all the preceding ciphertext blocks. As a result, the entire validity of all preceding blocks is contained in the previous, adjacent ciphertext block. A single bit error in a ciphertext block affects the decryption of all subsequent blocks. Rearrangement of the order of the ciphertext blocks, for example, can cause the decryption process to become corrupted.
Essentially, in cipher block chaining, each plaintext block is XORed (numerically combined) with the previous ciphertext block and then encrypted. An XOR is a coding mechanism used to combine different inputs. Used in this case to simplify a combination of blank text blocks and encryption keys. The process repeats itself until all blank text blocks are successfully converted to ciphertext text blocks.
How does cipher block chaining work?
Cipher block chaining is a process used to encrypt and decrypt large plaintext inputs by creating a cryptographic chain wherein each ciphertext block is dependent on the last.
The first step to initiating a cipher block chain is to XOR the first of many plaintext blocks with an IV — a unique, fixed-length conversion function — to create a random, or pseudorandom, output. This XOR output is then encrypted using a cipher key to produce a ciphertext block, an encrypted text format that can be decrypted with the correct key.
For example, after the first plaintext block has been transformed into a ciphertext block, the subsequent plaintext block must be encrypted using a similar process. The only difference, however, is that the ciphertext block replaces the IV as one of the XOR inputs. This means that the encryption of the plaintext block after the first one is dependent on the encryption of the first plaintext block.For each encryption of an empty text block, a nearby encrypted text block should be used – as a chain. Therefore, the second ciphertext block is generated by XORing the first ciphertext block with a second plaintext block and using the same encryption key. This process can be repeated until there is no clear text left to be encrypted.
CBC Decryption Process
The CBC decryption process works in a similar but distinct way. Contrary to similar decryption methods, the process does not start with the final ciphertext block. In fact, it can all happen simultaneously because all inputs are present.
To invert the cipher block chaining procedure, one must essentially reverse the encryption process. To do that, one must first feed the first ciphertext block through the decryption process. This involves using the same encryption key as before but on the ciphertext block.
The product of this interaction is then XORed with the original IV to extract the original plaintext block. While similar, decrypting the second ciphertext block is different from decrypting the first one because an IV cannot be used.
After combining the second ciphertext block with the cipher key, the output is XORed with the first ciphertext block to produce the second plaintext block. In this case, the previous ciphertext block replaces the IV during the decryption process.
Remember, this is how the second ciphertext block was originally created; the second plaintext block and the first ciphertext block were XORed together. The process is complete once all ciphertexts have been successfully decrypted into plaintext.
Identical ciphertext blocks can only be produced if the same plaintext block is encrypted using the same key, IV and ciphertext block order. Patterns like this can make it that much easier for malicious hackers or cybercriminals to decrypt a series of responses because the decryption is more predictable. Though the IV doesn’t need to be a secret, some applications, such as security consultancy, may find this desirable.
The advantages of cipher block chaining
Cipher block chaining is one of the most used methods for encrypting large messages. As the more secure successor of electronic codebook (ECB) — the easiest block cipher mode of functioning — CBC can reliably encrypt large plaintext inputs but at a slower pace than some parallel encryption algorithms.
Here are some advantages to cipher block chaining.
Identical blocks do not share the same cipher
CBC has the advantage over the ECB mode in that the XORing process hides plaintext patterns. Even if the first plaintext block and third plaintext block were the exact same segment of plaintext, it is highly unlikely that the first ciphertext block and third ciphertext block would be the same. Essentially, this means that two identical pieces of plaintext, when encrypted, should not produce identical, or even similar, results. The only reason why it is not impossible is that there is a minuscule chance that XORing the second ciphertext block and the third plaintext block produces the same product as XORing the first plaintext block with the IV.
Because cipher block chaining relies on using previous ciphertext blocks to encrypt subsequent plaintext blocks, hackers and decryptors must have all ciphertext blocks available in order to successfully decrypt entire CBC outputs. This multistep encryption mechanism makes it difficult to deconstruct, thereby increasing the security of the messages it is trying to encrypt.
In a way, CBC is an example of effective challenge-response authentication. A user or group that requires access to a certain set of documents must be able to present the necessary ciphertext blocks to successfully decrypt the entire message or text.
The disadvantages of cipher block chaining
As effective as cipher block chaining is in securing large amounts of plaintext. It does have its challenges, especially when it comes to speed and convenience.
Not tolerant of block losses
Although different cipher text blocks are very useful in terms of encryption power. They can be dangerous depending on the reliability of encryption. If one or more cipher text blocks are lost, corrupted or corrupted. Then user will not be able to complete encryption. While this may be a minor and rare occurrence. It forces agencies to use secure backup systems to store all cipher text data.
Parallel CBC encryption is not possible
Due to the recurring nature of the CBC encryption process. It is not possible to simultaneously encrypt all empty text entries using CBC. Without collecting all previous ciphertext blocks, the user cannot get the same encryption The order is so important that, if exchanged during the CBC encryption process, it would result in a completely different set of ciphertext blocks.
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