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- RockLv 61 decade agoFavorite Answer
A4 is cipher developed by R. Rivest for RSA Data Security, Inc. It is proprietary, and its detail have not been officially published, and RC4 avariable-key-size stream cipher (the key size may vary between 1 up to 2048 bits). RC4 is used a lot today; you can find it, for example, in secure socket layer/SSL, in WiFi security protocols, etc.
A5/1 is the symmetric cipher used for encrypting over-the air transmission in the GSM standard. A5/1 is used in most European countries, whereas a weaker cipher, called A5/2, is used in other countries. The description of A5/1 was first kept secret but its design was revered engineered in 1999 by Briceno, Golgerg, and Wagner. A5/1 is a synchronous stream cipher based on linear feedback shift registers (LFSRs)Source(s): Encyclopedia of Cryptography and Security, Myself.