 VEST

VEST
High Level Structure of VESTGeneral Designers Sean O'Neil First published June 13, 2005 Cipher detail Key sizes any Security claims 80–256 bits State size 256 bits (VEST4) to 768 (VEST32) Structure NLFSR, SPN, Tfunction VEST (Very Efficient Substitution Transposition) ciphers are a set of families of generalpurpose hardwarededicated ciphers that support single pass authenticated encryption and can operate as collisionresistant hash functions designed by Sean O'Neil, Benjamin Gittins and Howard Landman.^{[1]} VEST cannot be implemented efficiently in software.
VEST is based on a balanced Tfunction that can also be described as a bijective nonlinear feedback shift register with parallel feedback (NLPFSR) or as a substitutionpermutation network, which is assisted by a nonlinear RNS based counter. The four VEST family trees described in the cipher specification are VEST4, VEST8, VEST16, and VEST32. VEST ciphers support keys and IVs of variable sizes and instant rekeying. All VEST ciphers release output on every clock cycle.
All the VEST variants are covered by European Patent Number EP 1820295(B1), owned by Synaptic Laboratories.
VEST was a Phase 2 Candidate in the eSTREAM competition in the hardware portfolio, but was not a Phase 3 or Focus candidate and so is not part of the final portfolio.
Contents
Overview
Cipher: VEST4 VEST8 VEST16 VEST32 AES128 Output, bits per call: 4 8 16 32 128 Claimed security, bits: 80 128 160 256 128 Recommended key length, bits: 160 256 320 512 128 Recommended hash length, bits: 160 256 320 512 Counter size, bits: 163 163 171 171 Core size, bits: 83 211 331 587 State size, bits: 256 384 512 768 128 Design
Overall Structure
VEST ciphers consist of four components: a nonlinear counter, a linear counter diffusor, a bijective nonlinear accumulator with a large state and a linear output combiner (as illustrated by the image on the topright corner of this page). The RNS counter consists of sixteen NLFSRs with prime periods, the counter diffusor is a set of 5to1 linear combiners with feedback compressing outputs of the 16 counters into 10 bits while at the same time expanding the 8 data inputs into 9 bits, the core accumulator is an NLPFSR accepting 10 bits of the counter diffusor as its input, and the output combiner is a set of 6to1 linear combiners.
Accumulator
The core accumulator in VEST ciphers can be seen as a SPN constructed using nonlinear 6to1 feedback functions, one for each bit, all of which are updated simultaneously. The VEST4 core accumulator is illustrated below:
It accepts 10 bits (d_{0}  d_{9}) as its input. The least significant five bits (p_{0}  p_{4}) in the accumulator state are updated by a 5x5 substitution box and linearly combined with the first five input bits on each round. The next five accumulator bits are linearly combined with the next five input bits and with a nonlinear function of four of the less significant accumulator bits. In authenticated encryption mode, the ciphertext feedback bits are also linearly fed back into the accumulator (e_{0}  e_{3}) with a nonlinear function of four of the less significant accumulator bits. All the other bits in the VEST accumulator state are linearly combined with nonlinear functions of five less significant bits of the accumulator state on each round. The use of only the less significant bits as inputs into the feedback functions for each bit is typical of Tfunctions and is responsible for the feedback bijectivity. This substitution operation is followed by a pseudorandom transposition of all the bits in the state (see picture below).
Data authentication
VEST ciphers can be executed in their native authenticated encryption mode similar to that of Phelix but authenticating ciphertext rather than plaintext at the same speed and occupying the same area as keystream generation. However, unkeyed authentication (hashing) is performed only 8 bits at a time by loading the plaintext into the counters rather than directly into the core accumulator.
Family keying
The four root VEST cipher families are referred to as VEST4, VEST8, VEST16, and VEST32. Each of the four family trees of VEST ciphers supports family keying to generate other independent cipher families of the same size. The familykeying process is a standard method to generate cipher families with unique substitutions and unique counters with different periods. Family keying enables the enduser to generate a unique secure cipher for every chip.
Periods
VEST ciphers are assisted by a nonlinear RNS counter with a very long period. According to the authors, determining average periods of VEST ciphers or probabilities of the shortest periods of VEST16 and VEST32 falling below their advertised security ratings for some keys remains an open problem and is computationally infeasible. They believe that these probabilities are below 2^{−160} for VEST16 and below 2^{−256} for VEST32. The shortest theoretically possible periods of VEST4 and VEST8 are above their security ratings as can be seen from the following table.
Period: VEST4 VEST8 VEST16 VEST32 Guaranteed Minimum 2^{134} 2^{134} 2^{143} 2^{143} Longest Possible 2^{251} 2^{383} 2^{519} 2^{791} Performance
Computational Efficiency in Software
The core accumulator in VEST ciphers has a complex, highly irregular structure that resists its efficient implementation in software.
The highly irregular input structure coupled with a unique set of inputs for each feedback function hinders efficient software execution. As a result, all the feedback functions need to be calculated sequentially in software, thus resulting in the hardwaresoftware speed difference being approximately equal to the number of gates occupied by the feedback logic in hardware (see the column "Difference" in the table below).
Implementation: Clock VEST4 VEST8 VEST16 VEST32 Hardware 250 MHz ~1 Gbit/s ~2 Gbit/s ~4 Gbit/s ~8 Gbit/s Software 250 MHz < 1.0 Mbit/s < 0.8 Mbit/s < 1.1 Mbit/s < 1.3 Mbit/s Difference > 1000 x > 2300 x > 3500 x > 6000 x The large differential between VEST's optimised hardware execution and equivalently clocked software optimised execution offers a natural resistance against low cost generalpurpose software processor clones masquerading as genuine hardware authentication tokens.
In bulk challengeresponse scenarios such as RFID authentication applications, bitsliced implementations of VEST ciphers on 32bit processors which process many independent messages simultaneously are 24 times slower per message byte than AES.
Hardware Performance
VEST is submitted to the eStream competition under the Profile II as designed for "hardware applications with restricted resources such as limited storage, gate count, or power consumption", and shows high speeds in FPGA and ASIC hardware according to the evaluation by ETH Zurich.
The authors claim that according to their own implementations using "conservative standard RapidChip design frontend signoff process", "VEST32 can effortlessly satisfy a demand for 256bit secure 10 Gbit/s authenticated encryption @ 167 MHz on 180ηm LSI Logic RapidChip platform ASIC technologies in less than 45K Gates and zero SRAM". On the 110ηm Rapidchip technologies, VEST32 offers 20 Gbit/s authenticated encryption @ 320 MHz in less than 45 K gates". They also state that unrolling the round function of VEST can halve the clockspeed and reduce power consumption while doubling the output per clockcycle, at the cost of increased area.
Key Agility
VEST ciphers offer 3 keying strategies:
 Instantly loading the entire cipher state with a cryptographically strong key (100% entropy) supplied by a strong key generation or key exchange process;
 Instant reloading of the entire cipher state with a previously securely initialised cipher state;
 Incremental key loading (of an imperfect key) beginning with the least significant bit of the key loaded into the counter 15, sliding the 16bit window down by one bit on each round until the single bit 1 that follows the most significant bit of the key is loaded into the counter 0. The process ends with 32 additional sealing rounds. The entire cipher state can now be stored for instant reloading.
Key Bits Rounds to load a key 80 128 160 208 256 304 320 368 512 560 VEST ciphers offer only 1 resynchronisation strategy:
 Hashing the (IV) by loading it incrementally 8bits at a time into the first 8 RNS counters, followed by additional 32 sealing rounds.
IV Bits Rounds to load an IV 64 40 128 48 256 64 History
VEST was designed by Sean O'Neil and submitted to the eStream competition in June 2005. This was the first publication of the cipher.^{[citation needed]}
Security
The authors say that VEST security margins are inline with the guidelines proposed by Lars Knudsen in the paper "Some thoughts on the AES process" and the more conservative guidelines recently proposed by Nicolas Courtois in the paper “Cryptanalysis of Sfinks”. Although the authors are not publishing their own cryptanalysis, VEST ciphers have survived more than a year of public scrutiny as a part of the eStream competition organised by the ECRYPT. They were advanced to the second phase, albeit not as part of the focus group.
Attacks
At SASC 2007, Joux and Reinhard published an attack that recovered 53 bits of the counter state. By comparing the complexity of the attack to a parallelized bruteforce attack, Bernstein evaluated the resultant strength of the cipher as 100 bits [1], somewhat below the design strength of most of the VEST family members. The designers of VEST claimed the attack is due to a typographical error in the original cipher specification and published a correction on the Cryptology ePrint archive on the 21st of January 2007, a few days prior to publication of the attack.
References
 "Some thoughts on the AES process" paper by Lars R. Knudsen
 "Cryptanalysis of Sfinks" paper by Nicolas Courtois
 "Rediscovery of Time Memory Tradeoffs" paper by J. Hong and P. Sarkar
 "Understanding Brute Force" paper by Daniel J. Bernstein
 "Comments on the Rediscovery of Time Memory Data Tradeoffs" paper by C. De Cannière, J. Lano and B. Preneel
 IdealtoRealized Security Assurance In Cryptographic Keys by Justin Troutman
Notes
 ^ Sean O’Neil, Benjamin Gittins, Howard Landman (20051025). "VEST, HardwareDedicated Stream Ciphers" (PDF). eSTREAM Round 1 Submission. http://www.ecrypt.eu.org/stream/vest.html. Retrieved 20070515.
External links
 The official Synaptic Laboratories VEST website
 eSTREAM page on VEST
 VEST eStream Phase II specification
 VEST C reference source code and test vectors
 ETH Zurich Hardware Performance Review
Cryptographic hash functions and message authentication codes (MACs) Common functions Functions SHA3 finalists BLAKE · Grøstl · JH · Keccak · SkeinMAC algorithms Authenticated
encryption modesAttacks Misc. Standardization Stream ciphers Theory: Shift register · LFSR · NLFSR · Shrinking generator · Tfunction · IV
Attacks: Correlation attack · Correlation immunity
Cryptography Categories: Stream ciphers
 Message authentication codes
 Cryptographic hash functions
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