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Cryptography General Interview Questions

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SAFER (Secure And Fast Encryption Routine) is a non-proprietary block cipher developed by Massey in 1993 for Cylink Corporation. It is a byte-oriented algorithm with a 64-bit block size and, in one version, a 64-bit key size. It has a variable number of rounds (maximum of 10), but a minimum of six rounds is recommended. Unlike most recent block ciphers, SAFER has slightly different encryption and decryption procedures. Only byte-based operations are employed to ensure its utility in smart card-based applications that have limited processing power. When first announced, SAFER was intended to be implemented with a key of length 64 bits and it was accordingly named SAFER K-64. Another version of SAFER was designed that could handle 128-bit keys and was named SAFER K-128. 

The Fast Data Encipherment Algorithm (FEAL) was presented by Shimizu and Miyaguchi as an alternative to DES. The original cipher (called FEAL-4) was a four-round cryptosystem with a 64-bit block size and a 64-bit key size and it was designed to give high performance in software. Soon a variety of attacks against FEAL-4 were announced including one attack that required only 20 chosen plaintexts. Several results in the cryptanalysis of FEAL-8 (eight-round version) led the designers to introduce a revised version, FEAL-N, where N denoted the number of rounds. Biham and Shamir developed differential cryptanalytic attacks against FEAL-N for up to 31 rounds. In 1994, Ohta and Aoki presented a linear cryptanalytic attack against FEAL-8 that required 225 known plaintexts, and other improvements followed. In the wake of these numerous attacks, FEAL and its derivatives should be considered insecure. 

Skipjack is the encryption algorithm contained in the Clipper chip, and it was designed by the NSA. It uses an 80-bit key to encrypt 64-bit blocks of data. Skipjack can be more secure than DES, since it uses 80-bit keys and scrambles the data for 32 steps, or "rounds"; by contrast, DES uses 56-bit keys and scrambles the data for only 16 rounds.

The details of Skipjack are classified. The decision not to make the details of the algorithm publicly available has been widely criticized. Many people are suspicious that Skipjack is not secure, either due to oversight by its designers, or by the deliberate introduction of a secret trapdoor. By contrast, there have been many attempts to find weaknesses in DES over the years, since its details are public. These numerous attempts (and the fact that they have failed) have made people confident in the security of DES. Since Skipjack is not public, the same scrutiny cannot be applied towards it, and thus a corresponding level of confidence may not arise. 

A stream cipher is a symmetric encryption algorithm. Stream ciphers can be designed to be exceptionally fast, much faster in fact than any block cipher. While block ciphers operate on large blocks of data, stream ciphers typically operate on smaller units of plaintext, usually bits. The encryption of any particular plaintext with a block cipher will result in the same ciphertext when the same key is used. With a stream cipher, the transformation of these smaller plaintext units will vary, depending on when they are encountered during the encryption process.

A stream cipher generates what is called a keystream and encryption is provided by combining the keystream with the plaintext, usually with the bitwise exclusive-or operation. The generation of the keystream can be independent of the plaintext and ciphertext (yielding what is termed a synchronous stream cipher) or it can depend on the data and its encryption (in which case the stream cipher is said to be self-synchronizing). Most stream cipher designs are for synchronous stream ciphers. 

The Software-optimized Encryption Algorithm (SEAL) was designed by Rogaway and Coppersmith in 1993 as a fast stream cipher for 32-bit machines. SEAL has a rather involved initialization phase during which a large set of tables is initialized using the Secure Hash Algorithm. However, the use of look-up tables during keystream generation helps to achieve a very fast performance with just five instructions required per byte of output generated.

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