๐Ÿ“Output Feedback (OFB)

OFB is a block cipher mode of operation which makes a block cipher into a synchronous stream cipher. OFB does not require padding.

Encryption algorithm:

  • Encrypt IV and use that as IV for the next block.
  • Xor encrypted IV with the block of plaintext.

Decryption algorithm:

  • Because of symmetry of Xor, the decryption algorithm is exactly the same as encryption.
  • Encrypt IV and use that as IV for the next block.
  • Xor encrypted IV with the block of ciphertext.
  • The stream of encrypted IVs (keystream) can be computed beforehand and applied in parallel.
  • The keystream can be obtained by using CBC with a constant stream of zeros as input. That is useful as it allows using hardware acceleration.
  • Using OFB mode with partial blocks as feedback reduces the average cycle length by factor of 2322^{32} or more. For this reason, support for truncated feedback was removed from the specification of OFB.

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