📝Output Feedback (OFB)

tag: § Cryptography

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 $2^{32}$ or more. For this reason, support for truncated feedback was removed from the specification of OFB.

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