The Scream cipher is a word-based 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...

 developed by Shai Halevi, Don Coppersmith

Don Coppersmith

Don Coppersmith is a cryptographer and mathematician. He was involved in the design of the Data Encryption Standard block cipher at IBM, particularly the design of the S-boxes, strengthening them against differential cryptanalysis...

 and Charanjit Jutla from IBM

IBM

International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...

.

The cipher is designed as a software efficient stream cipher. The authors describe the goal of the cipher to be a more secure version of the SEAL

SEAL (cipher)

In cryptography, SEAL is a very fast stream cipher optimised for machines with a 32-bit word size and plenty of RAM. SEAL is actually a pseudorandom function family in that it can easily generate arbitrary portions of the keystream without having to start from the beginning...

 cipher.

The general design of Scream is close to the design of SEAL with block cipher

Block cipher

In cryptography, a block cipher is a symmetric key cipher operating on fixed-length groups of bits, called blocks, with an unvarying transformation. A block cipher encryption algorithm might take a 128-bit block of plaintext as input, and output a corresponding 128-bit block of ciphertext...

-like round functions. There are two versions of Scream. One of them, Scream-F, reuses the S-boxes from the Advanced Encryption Standard

Advanced Encryption Standard

Advanced Encryption Standard is a specification for the encryption of electronic data. It has been adopted by the U.S. government and is now used worldwide. It supersedes DES...

(AES) block cipher, while the other, Scream, internally generates new, key-dependent S-boxes as part of the initialization phase. The round function is also based on the AES-round function, but is narrower, 64 bits instead of 128 bits.

The cipher uses a 128-bit key and a 128-bit nonce. It is efficient in software, running at 4-5 cycles per byte on modern processors.

The cipher was presented at the Fast Software Encryption (FSE) conference in 2002.