Compare EBC and CBC modes of operations? Why one should not use EBC? How the plaintext messages whose length is not a multiple of the block length is encrypted by using CBC?
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EBC (Electronic Codebook) and CBC (Cipher Block Chaining) are modes of operation used in block ciphers for encryption. Let's compare them:
1. EBC Mode:
- In EBC mode, each individual plaintext block is encrypted separately using the same key.
- It does not involve the XOR operation, meaning that the same plaintext block always produces the same ciphertext block.
- EBC mode is vulnerable to certain attacks, such as frequency analysis, because identical plaintext blocks result in identical ciphertext blocks.
2. CBC Mode:
- In CBC mode, each plaintext block is XORed with the previous ciphertext block before being encrypted with the key.
- To start the encryption process, an Initial Vector (IV) is required, which is XORed with the first plaintext block.
- The IV is usually a random value and changes for each encryption, adding randomness to the encryption process.
- CBC mode provides better security as identical plaintext blocks produce different ciphertext blocks due to the XOR operation and the use of the IV.
- It is resistant to certain attacks, including frequency analysis.
Why one should not use EBC?
- EBC mode is not recommended for many encryption purposes due to its vulnerability to attacks.
- Identical plaintext blocks yield identical ciphertext blocks, which can leak information and patterns.
- EBC mode lacks the necessary randomness and security that CBC mode provides.
How are plaintext messages with lengths not a multiple of the block length encrypted using CBC?
- If the plaintext message's length is not a multiple of the block length, padding is added to make it fit.
- Common padding schemes include PKCS7 or PKCS5, where the padding byte values represent the number of padding bytes added.
- Once padded, the plaintext message is divided into blocks equal to the cipher's block size.
- The initial IV is XORed with the first plaintext block, and the result is encrypted using the chosen encryption algorithm and key.
- The ciphertext block is then XORed with the next plaintext block and encrypted.
- This process continues until the last block, creating a chain of encrypted blocks.
- The final ciphertext is the concatenation of all the encrypted blocks, including the IV.
In summary, EBC and CBC modes have significant differences in terms of security. EBC is not recommended due to vulnerabilities, while CBC provides better security by using XOR and IV to create unique ciphertext blocks for identical plaintext blocks. When encrypting plaintext messages with lengths not divisible by the block size, padding is added, and CBC encrypts the blocks one by one, XORing them with the previous ciphertext block.
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