HC-256 is a

stream cipher stream cipher is a symmetric key cipher where plaintext digits are combined with a pseudorandom cipher digit stream ( keystream). In a stream cipher, each plaintext digit is encrypted one at a time with the corresponding digit of the keystrea ...

designed to provide bulk encryption in

software Software consists of computer programs that instruct the Execution (computing), execution of a computer. Software also includes design documents and specifications. The history of software is closely tied to the development of digital comput ...

at high speeds while permitting strong confidence in its security. A 128-bit variant was submitted as an

eSTREAM eSTREAM is a project to "identify new stream ciphers suitable for widespread adoption", organised by the EU ECRYPT network. It was set up as a result of the failure of all six stream ciphers submitted to the NESSIE project. The call for primiti ...

cipher candidate and has been selected as one of the four final contestants in the software profile. The algorithm is designed by Hongjun Wu, and was first published in 2004. It is not patented.

Function

HC-256 has a 256 bit key and an

initialization vector In cryptography, an initialization vector (IV) or starting variable is an input to a cryptographic primitive being used to provide the initial state. The IV is typically required to be random or pseudorandom, but sometimes an IV only needs to be un ...

(nonce) of 256 bits. Internally, it consists of two secret tables (P and Q). Each table contains 1024 32-bit words. For each state update one 32-bit word in each table is updated using a non-linear update function. After 2048 steps all elements of the tables have been updated. It generates one 32-bit

word A word is a basic element of language that carries semantics, meaning, can be used on its own, and is uninterruptible. Despite the fact that language speakers often have an intuitive grasp of what a word is, there is no consensus among linguist ...

for each update step using a 32-bit to 32-bit mapping function similar to the output function of the

Blowfish Tetraodontidae is a family of marine and freshwater fish in the order Tetraodontiformes. The family includes many familiar species variously called pufferfish, puffers, balloonfish, blowfish, blowers, blowies, bubblefish, globefish, swellfish, ...

cipher. Finally a linear bit-masking function is applied to generate an output word. It uses the two message schedule functions in the

hash function A hash function is any Function (mathematics), function that can be used to map data (computing), data of arbitrary size to fixed-size values, though there are some hash functions that support variable-length output. The values returned by a ...

SHA-256 SHA-2 (Secure Hash Algorithm 2) is a set of cryptographic hash functions designed by the United States National Security Agency (NSA) and first published in 2001. They are built using the Merkle–Damgård construction, from a one-way compressi ...

internally, but with the tables P and Q as

S-box In cryptography, an S-box (substitution-box) is a basic component of symmetric key algorithms which performs substitution. In block ciphers, they are typically used to obscure the relationship between the key and the ciphertext, thus ensuring Clau ...

es. HC-128 is similar in function, and reduces each of key length, nonce, number of words in the tables P and Q, and number of table updating steps by half.

Performance

The performance of HC-256 is estimated by its author to be about 4

cycles per byte Encryption software is software that uses cryptography to prevent unauthorized access to digital information. Cryptography is used to protect digital information on computers as well as the digital information that is sent to other computers over t ...

on a

Pentium 4 Pentium 4 is a series of single-core central processing unit, CPUs for Desktop computer, desktops, laptops and entry-level Server (computing), servers manufactured by Intel. The processors were shipped from November 20, 2000 until August 8, 20 ...

processor. However the initialization phase of the cipher includes expanding the 256-bit key into the tables P, Q and then running the cipher for 4096 steps. The author of HC-256 estimates this process to take around 74,000 cycles. For HC-128 an encryption speed of about 3 cycles per byte on a

Pentium M The Pentium M is a family of mobile 32-bit single-core x86 microprocessors (with the modified Intel P6 (microarchitecture), P6 microarchitecture) introduced in March 2003 and forming a part of the Intel Centrino#Carmel platform (2003), Carmel no ...

processor are cited. The implementation of HC-128 on various computing structures is studied in detail, with significant performance gains compared to naive SW implementation.

References

External links

eSTREAM page on HC-256
{{Cryptography navbox , stream Stream ciphers