HC-256 is a stream cipher

Stream cipher

In cryptography, a stream cipher is a symmetric key cipher where plaintext digits are combined with a pseudorandom cipher digit stream . In a stream cipher the plaintext digits are encrypted one at a time, and the transformation of successive digits varies during the encryption...

 designed to provide bulk encryption in software at high speeds while permitting strong confidence in its security. A 128-bit variant was submitted as an eSTREAM

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 primitives was first issued in November 2004. The project was...

 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

Initialization vector

In cryptography, an initialization vector is a fixed-size input to a cryptographic primitive that is typically required to be random or pseudorandom...

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

Blowfish (cipher)

Blowfish is a keyed, symmetric block cipher, designed in 1993 by Bruce Schneier and included in a large number of cipher suites and encryption products. Blowfish provides a good encryption rate in software and no effective cryptanalysis of it has been found to date...

 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

Hash function

A hash function is any algorithm or subroutine that maps large data sets to smaller data sets, called keys. For example, a single integer can serve as an index to an array...

 SHA-256 internally, but with the tables P and Q as S-boxes.

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

Cycles per byte

Cycles per byte is a unit of measurement which indicates the number of clock cycles a microprocessor will perform per byte of data processed in an algorithm. It is commonly used as a partial indicator of real-world performance in cryptographic functions....

 on a Pentium 4

Pentium 4

Pentium 4 was a line of single-core desktop and laptop central processing units , introduced by Intel on November 20, 2000 and shipped through August 8, 2008. They had a 7th-generation x86 microarchitecture, called NetBurst, which was the company's first all-new design since the introduction of the...

 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

Pentium M

The Pentium M brand refers to a family of mobile single-core x86 microprocessors introduced in March 2003 , and forming a part of the Intel Carmel notebook platform under the then new Centrino brand...

processor are cited.

External links

• eSTREAM page on HC-256