F-Cryptography - A PPENDIX F C RYPTOGRAPHIC A L G O R I T H...

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A A PPENDIX PPENDIX F F C C RYPTOGRAPHIC RYPTOGRAPHIC A A LGORITHMS LGORITHMS William Stallings Copyright 2008 F.1 SYMMETRIC ENCRYPTION .......................................................................................... 2 The Data Encryption Standard (DES) ............................................................................ 4 Advanced Encryption Standard ..................................................................................... 4 F.2 PUBLIC-KEY CRYPTOGRAPHY ................................................................................... 4 Rivest-Shamir-Adleman (RSA) Algorithm ................................................................... 7 F.3 SECURE HASH FUNCTIONS .......................................................................................... 7 Supplement to Operating Systems: Internals and Design Principles, Sixth Edition William Stallings Prentice Hall 2008 ISBN-10: 0-13-600632-9 ISBN-13: 978-0-13-600632-9 http://williamstallings.com/OS/OS6e.html
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F-2 The essential technology underlying virtually all automated network and computer security applications is cryptography. Two fundamental approaches are in use: symmetric encryption, also known as conventional encryption, and public-key encryption, also known as asymmetric encryption. This appendix provides an overview of both types of encryption, together with a brief discussion of some important encryption algorithms. F.1 SYMMETRIC ENCRYPTION Symmetric encryption was the only type of encryption in use prior to the introduction of public- key encryption in the late 1970s. Symmetric encryption has been used for secret communication by countless individuals and groups, from Julius Caesar to the German U-boat force to present- day diplomatic, military, and commercial users. It remains by far the more widely used of the two types of encryption. A symmetric encryption scheme has five ingredients (Figure F.1): Plaintext: This is the original message or data that is fed into the algorithm as input. Encryption algorithm: The encryption algorithm performs various substitutions and transformations on the plaintext. Secret key: The secret key is also input to the encryption algorithm. The exact substitutions and transformations performed by the algorithm depend on the key. Ciphertext: This is the scrambled message produced as output. It depends on the plaintext and the secret key. For a given message, two different keys will produce two different ciphertexts. Decryption algorithm: This is essentially the encryption algorithm run in reverse. It takes the ciphertext and the secret key and produces the original plaintext. There are two requirements for secure use of symmetric encryption: 1. We need a strong encryption algorithm. At a minimum, we would like the algorithm to be such that an opponent who knows the algorithm and has access to one or more ciphertexts would be unable to decipher the ciphertext or figure out the key. This requirement is usually stated in a stronger form: The opponent should be unable to decrypt ciphertext or discover the key even if he or she is in possession of a number of ciphertexts together with the plaintext that produced each ciphertext.
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F-3 2. Sender and receiver must have obtained copies of the secret key in a secure fashion and must keep the key secure. If someone can discover the key and knows the algorithm, all communication using this key is readable.
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