TAILIEUCHUNG - Bảo mật thông tin: Các phương pháp mã hóa - phần 1

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. | Bảo mật hệ thống thông tin CÁC PHƯƠNG PHÁP MÃ HÓA PHẦN 1 1/2011 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 ❏ 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 Objectives Chapter 3 3. 3-1 INTRODUCTION 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. Kerckhoff’s Principle Cryptanalysis Categories of Traditional Ciphers Topics discussed in this section: 3. Figure General idea of symmetric-key cipher Continued 3. Continued If P is the plaintext, C is the ciphertext, and K is the key, We assume that Bob creates P1; we prove that P1 = P: 3. Figure Locking and unlocking with the same key Continued 3. Kerckhoff’s Principle 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. 3. Cryptanalysis As cryptography is the science and art of creating secret codes, cryptanalysis is the science and art of breaking those codes. Figure Cryptanalysis attacks 3. Continued Figure Ciphertext-only attack Ciphertext-Only Attack 3. Continued Figure Known-plaintext attack Known-Plaintext Attack 3. Continued Figure .

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