tailieunhanh - Chapter 3: Traditional Symmetric-Key Ciphers

Objectives of Chapter 3: To define the terms and the concepts of symmetric key ciphers; to emphasize the two categories of traditional ciphers: substitution and transposition ciphers; to describe the categories of cryptanalysis used to break the symmetric ciphers. | Cryptography and Network Security Chapter 3 Objectives Chapter 3 Traditional Symmetric-Key Ciphers To define the terms and the concepts of symmetric key ciphers To emphasize the two categories of traditional ciphers substitution and transposition ciphers To describe the categories of cryptanalysis used to break the symmetric ciphers To introduce the concepts of the stream ciphers and block ciphers To discuss some very dominant ciphers used in the past such as the Enigma machine 3-1 INTRODUCTION Continued Figure shows the general idea behind a symmetric-key cipher. The original message from Alice to Bob is called plaintext the message that is sent through the channel is called the ciphertext. To create the ciphertext from the plaintext Alice uses an encryption algorithm and a shared secret key. To create the plaintext from ciphertext Bob uses a decryption algorithm and the same secret key. Topics discussed in this section Kerckhoff s Principle Cryptanalysis Categories of Traditional Ciphers Figure General idea of symmetric-key cipher Continued If P is the plaintext C is the ciphertext and K is the key Encryption c E . P Decryption p DA. C In which D E x EẢ. DÁ. x X We assume that Bob creates P1 we prove that P1 P Alice c Ek P Bob Pj Dk C Dk E P p 1 Kerckhoff s Principle Cryptanalysis Based on Kerckhoff s principle one should always assume that the adversary Eve knows the encryption decryption algorithm. The resistance of the cipher to attack must be based only on the secrecy of the key. As cryptography is the science and art of creating secret codes cryptanalysis is the science and art of breaking those codes. Figure Cryptanalysis attacks Continued Ciphertext-Only Attack Continued Known-Plaintext Attack Figure Ciphertext-only attack Figure Known-plaintext attack Continued Chosen-Plaintext Attack Continued Chosen-Ciphertext Attack Figure Chosen-plaintext attack Figure .