2.3 Modes of Block Cipher
Encryptions
Ram Datta Bhatta
Modes of Block Cipher Encryptions
✓Different techniques or strategies used to apply a block cipher algorithm to
encrypt data of varying lengths.
✓Block ciphers operate on fixed-size blocks of data (e.g., 64 or 128 bits), but in
practice, data to be encrypted can come in various lengths. Modes of encryption
define how these variable-length data are transformed using the block cipher,
ensuring confidentiality, integrity, and security
✓Block cipher is an encryption algorithm that takes a fixed size of input say b
bits and produces a ciphertext of b bits again. If the input is larger than b bits it
can be divided further.
Modes of Block Cipher Encryptions
There are several modes of operations for a block cipher as follows:
1. Electronic Code Book (ECB)
2. Cipher Block Chaining (CBC)
3. Cipher Feedback Mode (CFB)
4. Output Feedback Mode (OFM)
5. Counter Mode
https://www.geeksforgeeks.org/block-cipher-modes-of-operation/
1. Electronic Code Book (ECB)
✓The plaintext is handled one block at a time and each block of plaintext is
encrypted using the same key
✓For a message longer than b bits, the procedure is simply to break the message
into b-bit blocks, padding the last block if necessary
Drawbacks
• An attacker could uncover the key and thus have the key to decrypt all the
blocks of data, or
• An attacker could gather the ciphertext and plaintext of each block and build
the code book that was used, without needing the key.
ECB
2. Cipher Block Chaining (CBC)
✓ In CBC mode, the ciphertext from the previous block of data is used in
encrypting the next block of data,
✓The input to the encryption algorithm is the XOR of the current plaintext
block and the preceding ciphertext block; the same key is used for each block.
✓ In effect, we have chained together the processing of the sequence of
plaintext blocks.
✓To produce the first block of ciphertext, an initialization vector (IV) is XORed
with the first block of plaintext.
✓Parallel encryption is not possible since every encryption requires a previous
cipher
CBC
3. Cipher Feedback Mode (CFB)
✓In this mode the cipher is given as feedback to the next block of
encryption with some new specifications: first, an initial vector IV is
used for first encryption and output bits are divided as a set of s and
b-s bits.
✓The left-hand side s bits are selected along with plaintext bits to
which an XOR operation is applied. The result is given as input to a
shift register having b-s bits to LHS, s bits to RHS and the process
continues.
4. Output Feedback Mode
• The output feedback mode follows nearly the same process as the
Cipher Feedback mode except that it sends the encrypted output as
feedback instead of the actual cipher which is XOR output.
5. Counter Mode
• A counter equal to the plaintext block size is used.
• Every time a counter-initiated value is encrypted and given as input to
XOR with plaintext which results in ciphertext block.
• The CTR mode is independent of feedback use and thus can be
implemented in parallel
• The counter is initialized to some value and then incremented by 1 for
each subsequent block
• https://www.youtube.com/watch?v=iaeKeZCHA1I&t=292s
• https://www.youtube.com/watch?v=XgeBUPaTqPY