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AES-128-CFB ENCRYPTION TOOL

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The AES-128-CFB (Advanced Encryption Standard with 128-bit key in Cipher Feedback mode) is a symmetric key encryption method designed to provide confidentiality of data through a combination of block cipher operations and feedback mechanisms. AES operates on fixed-size blocks of 128 bits, applying a series of transformations to transform plaintext into ciphertext using a secret key of 128 bits.

Key Components

  • Key Schedule: AES-128 generates a round key for each encryption round from the original 128-bit key using a key expansion algorithm. The schedule ensures that each round uses a unique subkey derived from the master key.
  • SubBytes: A non-linear substitution step where each byte of the state matrix is replaced with a corresponding value from the S-box, providing confusion.
  • ShiftRows: A transposition step where rows of the state are cyclically shifted to the left by different offsets, increasing diffusion across the block.
  • MixColumns: A linear transformation that combines bytes within each column of the state using Galois field multiplication, further enhancing diffusion. In CFB mode, this step is not applied to plaintext but affects the keystream generation.
  • AddRoundKey: Each byte of the state is combined with the round key using bitwise XOR, integrating key material into the block.

Cipher Feedback Mode (CFB)

CFB transforms a block cipher into a self-synchronizing stream cipher. It encrypts an initialization vector (IV) with AES and then XORs the output with the plaintext to produce ciphertext. For subsequent blocks, the previous ciphertext segment is used as input to the AES encryption function. This allows encryption of data in units smaller than the block size and provides error propagation only over a limited number of blocks.

Encryption Process

  1. The 128-bit key and an initialization vector (IV) are provided as inputs.
  2. The IV is encrypted using the AES-128 block cipher.
  3. The output of AES encryption is XORed with the first plaintext segment to produce ciphertext.
  4. The ciphertext segment is then fed back as the input for encrypting the next plaintext segment.
  5. This process continues iteratively for the entire message, ensuring that the ciphertext depends on both the key and all previous plaintext segments.

Security Features

  • Maintains confidentiality using a 128-bit key resistant to brute-force attacks.
  • Provides self-synchronizing error handling, allowing recovery after partial data corruption.
  • Supports variable-length plaintext encryption while preserving the block cipher structure.

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