Key feedback mode

In cryptography key feedback mode (KFB) is a mode of operation for cryptographic block ciphers. It uses the block cipher output block to form the block cipher key of the next round, thereby creating a keystream generator that can be used as a synchronous stream cipher.

Key feedback mode

Operation

KFB mode outputs m bits (typically m = 8) at a time and uses a constant bitstring p as block cipher input, a key k of n bit length, and a n × m matrix with non-zero rows as IV. It is possible to reduce the size of the matrix and it does not need to be secret but must be random. If the output block size of the block cipher is not equal to the key size, a function is needed to form a valid key from the output block. The speed of KFB is a bit lower than for other modes of operation like OFB, mainly because the key schedule has to be done for each key.

Security

Hastad and Näslund proved KFB to be secure in terms of the theorems of complexity theory introduced by Blum, Micali, Levin and Goldreich, giving a quantitative relation between the effort of distinguishing the keystream from true randomness to the effort of retrieving the secret key.

Error properties

As the keystream is independent of plaintext and ciphertext, KFB mode turns a block cipher into a synchronous stream cipher. Just as with other synchronous stream ciphers, inverting a bit in the ciphertext produces an inverted bit in the plaintext at the same location, but does not affect further parts of the plaintext. This property allows many error correcting codes to function normally even when applied before encryption.

History

The concept of KFB has already been mentioned 1982 by Hellman and Reyneri,^[1] but only as a theoretical construct to analyse the cycle length of random functions with the purpose of comparing DES to a truly random function. The detailed specification was published and submitted to NIST in 2000 by J. Håstad and M. Naslund.

References

Seminar 'Modes of Operation for Block Ciphers and Hash Functions', Chair for communication security at the Ruhr-University Bochum, Germany

↑ Martin E. Hellman and Justin M. Reyneri. Drainage and the DES. In Advances in Cryptology: Proceedings of Crypto 82, pages 129–131, 1982

External links

KFB specification (NIST)

Block ciphers (security summary)
Common algorithms	AES Blowfish DES (Internal Mechanics, Triple DES) Serpent Twofish
Less common algorithms	Camellia CAST-128 IDEA RC2 RC5 RC6 SEED ARIA Skipjack TEA XTEA
Other algorithms	3-Way Akelarre Anubis BaseKing BassOmatic BATON BEAR and LION CAST-256 Chiasmus CIKS-1 CIPHERUNICORN-A CIPHERUNICORN-E CLEFIA CMEA Cobra COCONUT98 Crab Cryptomeria/C2 CRYPTON CS-Cipher DEAL DES-X DFC E2 FEAL FEA-M FROG G-DES GOST Grand Cru Hasty Pudding cipher Hierocrypt ICE IDEA NXT Intel Cascade Cipher Iraqi Kalyna KASUMI KeeLoq KHAZAD Khufu and Khafre KN-Cipher Kuznyechik Ladder-DES Libelle LOKI (97, 89/91) Lucifer M6 M8 MacGuffin Madryga MAGENTA MARS Mercy MESH MISTY1 MMB MULTI2 MultiSwap New Data Seal NewDES Nimbus NOEKEON NUSH PRESENT Prince Q RC6 REDOC Red Pike S-1 SAFER SAVILLE SC2000 SHACAL SHARK Simon SM4 Speck Spectr-H64 Square SXAL/MBAL Threefish Treyfer UES Xenon xmx XXTEA Zodiac
Design	Feistel network Key schedule Lai–Massey scheme Product cipher S-box P-box SPN Avalanche effect Block size Key size Key whitening (Whitening transformation)
Attack (cryptanalysis)	Brute-force (EFF DES cracker) MITM Biclique attack 3-subset MITM attack Linear (Piling-up lemma) Differential Impossible Truncated Higher-order Differential-linear Distinguishing (Known-key) Integral/Square Boomerang Mod n Related-key Slide Rotational Side-channel Timing Power-monitoring Electromagnetic Acoustic Differential-fault XSL Interpolation Partitioning Rubber-hose Black-bag Davies' Rebound Weak key Tau Chi-square Time/memory/data tradeoff
Standardization	AES process CRYPTREC NESSIE
Utilization	Initialization vector Mode of operation Padding

Cryptographic hash functions & message authentication codes
List Comparison Known attacks
Common functions	MD5 SHA-1 SHA-2 SHA-3 BLAKE2
SHA-3 finalists	BLAKE Grøstl JH Skein Keccak (winner)
Other functions	ECOH FSB GOST HAS-160 HAVAL Kupyna LM hash MD2 MD4 MD6 MDC-2 N-Hash RIPEMD RadioGatún SWIFFT Snefru Streebog Tiger VSH WHIRLPOOL
Key derivation functions	bcrypt crypt PBKDF2 scrypt Argon2 Lyra2
MAC functions	DAA CBC-MAC HMAC OMAC/CMAC PMAC VMAC UMAC Poly1305
Authenticated encryption modes	CCM CWC EAX GCM IAPM OCB
Attacks	Collision attack Preimage attack Birthday attack Brute-force attack Rainbow table Side-channel attack Length extension attack
Design	Avalanche effect Hash collision Merkle–Damgård construction Sponge function HAIFA construction Unique Block Iteration
Standardization	CRYPTREC NESSIE NIST hash function competition
Utilization	Hash-based cryptography Key stretching Merkle tree Message authentication Proof of work Salt

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