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 The FROG Encryption Algorithm   Dianelos Georgoudis 02.07.1998

Алгоритм шифрования FROG использует нестандартный подход. Секретный ключ строится не как простая текстовая строка, а как программа с набором примитивных инструкций. Цель такого подхода - как можно сильнее усложнить жизнь дешифровальщику. Программа плюс описание алгоритма [html].
FROG is a new cipher with an unorthodox structure. Any symmetrical cipher's job is to conceal the plaintext's information through a computational process of confusion and diffusion. The basic idea behind the design of FROG is to conceal the definition of most of this process in a secret internal key. The actual encryption algorithm operates as an interpreter that regards the secret internal key as a program and executes it as if it were a series of primitive instructions.



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B.1. Introduction FROG is a new cipher with an unorthodox structure. Any symmetrical cipher's job is to conceal the plaintext's information through a computational process of confusion and diffusion. The basic idea behind the design of FROG is to conceal the definition of most of this process in a secret internal key. The actual encryption algorithm operates as an interpreter that regards the secret internal key as a program and executes it as if it were a series of primitive instructions. The goal is to deny the attacker as much knowledge as possible about the actual process being performed and therefore defeat any attack, whether publicly known or not. The encryption and decryption operations used in FROG are extremely simple. All the complexity lies within the internal key the details of which are unknown to an attacker. FROG is very easy to implement (the reference C version has only about 150 lines of code). Much of the code needed to implement FROG is used to generate the secret internal key, the internal cipher itself is a very short piece of code. The implementation will run well on 8 bit processors because it uses only byte level instructions - the only arithmetic operation used is exclusive-OR and, optionally, 1 byte modulus. Also no bit specific operations are used. The algorithm is fairly fast, a version implemented using 8086 assembler achieves processing speeds of over 2.2 Mbytes per second when run on a 200 MHz Pentium PC. A version implemented using Pentium specific assembler would run even faster. The FROG reference program included in this submission allows for user keys of any length between 5 bytes and 125 bytes (i.e. between 40 bits and 1000 bits in multiples of 8 bits). The block size is defined as a constant and is set equal to 16 bytes which is the minimum requirement for AES. Nevertheless, this constant can be given any value (even odd values) between 8 and 128 bytes in which case the re-compiled code will encrypt blocks whose sizes vary from a minimum of 64 bits up to a maximum of 1024 bits. ------------- Pascal reference implementation for the FROG symmetric block cipher submitted by TecApro to NIST as an AES candidate algorithm. This implementation of FROG is more flexible than the C and Java reference implementations submitted, because it permits the use of any block size between 8 bytes and 128 bytes. The full FROG documentation can be found in www.tecapro.com This code was written by Dianelos Georgoudis (email: dianelos@tecapro.com) in June 1998. This source code has three parts: A. Definition of the pure FROG algorithm. B. Implementation of the NIST specified API, including ECB, CBC, and CFB 1-bit modes. C. Examples of use in ECB, CBC and CFB mode as well as code for the creation of the Known Answer Tests and the Monte Carlo Tests, as specified by NIST. This algorithm is put in the public domain. Disclaimer: TecApro is not responsible for any damage, direct or indirect, resulting from the use of this source code, and makes no claims about its fitness for any particular purpose. By using this software you are solely responsible for all consequences. *)