Threefish

Threefish
General

Designers	Bruce Schneier, Niels Ferguson, Stefan Lucks, Doug Whiting, Mihir Bellare, Tadayoshi Kohno, Jon Callas, Jesse Walker
First published	2008
Related to	Blowfish, Twofish
Cipher detail
Key sizes	256, 512 or 1024 bits (key size is equal to block size)
Block sizes	256, 512 or 1024 bits
Rounds	72 (80 for 1024-bit block size)
Speed	6.1 cpb on Core 2.^[1]

Threefish is a symmetric-key tweakable block cipher designed as part of the Skein hash function, an entry in the NIST hash function competition. Threefish uses no S-boxes or other table lookups in order to avoid cache timing attacks;^[1] its nonlinearity comes from alternating additions with exclusive ORs. In that respect, it is similar to Salsa20, TEA, and the SHA-3 candidates CubeHash and BLAKE.

Threefish and the Skein hash function were designed by Bruce Schneier, Niels Ferguson, Stefan Lucks, Doug Whiting, Mihir Bellare, Tadayoshi Kohno, Jon Callas, and Jesse Walker.

Description of the cipher^[1]

Threefish works on words of 64 bits (unsigned Little endian integers). $N_{w}\in \{4,8,16\}$ is the number of plaintext words and also of key words. The tweak consists of two words. All additions and subtractions are defined modulo $2^{{64}}$ .

Key schedule

Threefish uses ${\frac {N_{r}}{4}}+1$ different round keys ( $N_r$ : Number of rounds). To calculate these keys an additional key word $k_{{N_{w}}}$ is appended to the original key words $k_{0},k_{1},\dots ,k_{{N_{w}-1}}$ . An additional tweak word $t_{2}$ is also appended to the tweak words $t_{0},t_{1}$ .

k_{N_{w}}=C_{240}\oplus k_{0}\oplus k_{1}\oplus \dots \oplus k_{N_{w}-1};\quad C_{240}={\text{0x1BD11BDAA9FC1A22}}

t_{2}=t_{0}\oplus t_{1}

The purpose of the seemingly arbitrary constant $C_{240}$ is to frustrate some attacks that take advantage of the relationship between $k_{{N_{w}}}$ and the other keywords.

The round key words $k_{{s,i}}$ are now defined like this:

$k_{{s,i}}={\begin{cases}k_{{(s+i){\bmod (}N_{w}+1)}}&i=0,\dots ,N_{w}-4\\k_{{(s+i){\bmod (}N_{w}+1)}}+t_{{s{\bmod 3}}}&i=N_{w}-3\\k_{{(s+i){\bmod (}N_{w}+1)}}+t_{{(s+1){\bmod 3}}}&i=N_{w}-2\\k_{{(s+i){\bmod (}N_{w}+1)}}+s&i=N_{w}-1\end{cases}}$

Here $s=0,1,\dots ,N_{r}/4$ , and $4s$ is the number of the round in which the round key is used.

Mix function

The mix function takes a tuple of words $(x_{0},x_{1})$ and returns another tuple of words $(y_{0},y_{1})$ . The function is defined like this:

$y_{0}=x_{0}+x_{1}\mod 2^{{64}}$

$y_1 = (x_1 \lll R_{(d \bmod 8), j}) \oplus y_0$

$R_{{d,j}}$ is a fixed set of rotation constants chosen to achieve quick diffusion.

Permute

The permutation step swaps the positions of the words according to a constant pattern. Bit-level permutation is not achieved in this step, but this is not necessary since the MIX functions provides bit-level permutations in the form of bitwise rotations. The Permute step and rotation constants in the MIX functions are chosen in such a way that the overall effect is complete diffusion of all the bits in a data block.

Because this permutation is fixed and independent of the key, the time needed to compute it does not provide information about the key or plaintext. This is important because on most modern microprocessors performance optimisations can make the time taken to compute an array operation dependent on where the data is stored in memory. In ciphers where array lookup depends on either the key or plaintext (as is the case for the substitution step in AES), it can make the cipher vulnerable to timing attacks by examining the time required for encryption. The permutation is therefore deliberately designed to ensure that it should execute in the same fashion independent of the key being used or the data encrypted.

A full Threefish round

if $d\;{\bmod {\;}}4=0$ the round key $k_{d/4,i}$ is added to word $i$
the mix function is applied to consecutive words, the rotation widths depend on $d$ and the word number
the words are permutated using a permutation independent from the round number

Threefish256 and Threefish512 apply this round 72 times ( $d=0,1,\dots ,71$ ). Threefish1024 applies it 80 times( $d=0,1,\dots ,79$ ).

Final operations

After all rounds are applied, the last round key is added to the words and the words are converted back to a string of bytes.

Security

In October 2010, an attack that combines rotational cryptanalysis with the rebound attack was published. The attack mounts a known-key distinguisher against 53 of 72 rounds in Threefish-256, and 57 of 72 rounds in Threefish-512. It also affects the Skein hash function.^[2] This is a follow-up to the earlier attack published in February, which breaks 39 and 42 rounds respectively.^[3] In response to this attack, the Skein team tweaked the rotation constants used in Threefish and thereby the key schedule constants for round 3 of the NIST hash function competition.^[1]

In 2009, a related key boomerang attack against a reduced round Threefish version was published. For the 32-round version, the time complexity is $2^{226}$ and the memory complexity is $2^{12}$ ; for the 33-round version, the time complexity is $2^{352.17}$ with a negligible memory usage. The attacks also work against the tweaked version of Threefish: for the 32-round version, the time complexity is $2^{222}$ and the memory complexity is $2^{12}$ ; for the 33-round version, the time complexity is $2^{355.5}$ with a negligible memory usage.^[4]

References

1 2 3 4 Ferguson; et al. (2010-10-01). "The Skein Hash Function Family" (PDF). The paper in which Threefish was introduced.
↑ Dmitry Khovratovich; Ivica Nikolic; Christian Rechberger (2010-10-20). "Rotational Rebound Attacks on Reduced Skein".
↑ Dmitry Khovratovich & Ivica Nikolić (2010). "Rotational Cryptanalysis of ARX" (PDF). University of Luxembourg.
↑ Jiazhe Chen; Keting Jia (2009-11-01). "Improved Related-key Boomerang Attacks on Round-Reduced Threeﬁsh-512".

External links

"The Skein Hash Function Family" Homepage of the Skein Hash Function Family.

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 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 Timing XSL Interpolation Partitioning Davies' Rebound Weak key Tau Chi-square Time/memory/data tradeoff

Standardization	AES process CRYPTREC NESSIE

Utilization	Initialization vector Mode of operation Padding

Cryptography

History of cryptography Cryptanalysis Outline of cryptography

Symmetric-key algorithm Block cipher Stream cipher Public-key cryptography Cryptographic hash function Message authentication code Random numbers Steganography

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