crypto - Cryptography Cryptography The science and study of...

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1 1 Cryptography CS177 2011 Cryptography The science and study of secret writings Cipher – Is a secret method of writing that transforms plaintext into ciphertext The transformation is determined by a key Cryptographic systems One key Two key Public key Digital signatures 2 Cryptography CS177 2011 Cryptography • Comes in two flavors: Symmetric and Asymmetric • Best for protection of “online” communications • Good for archival data • So-so for electronic mail • Not good for active databases 3 Cryptography CS177 2011 Communication Security Secure communication should provide: • Privacy • Authentication • Integrity • Nonrepudiation Anna Bruno Carlo 4 Cryptography CS177 2011 Terminology • To lock (encipher): transforms into unintelligible form based on independent data element called a key • To unlock (decipher): transforms back into intelligible form, again using a key • Locked data is called ciphertext or black • Unlocked data is called plaintext , cleartext , or red • Keys are themselves data and can be locked and unlocked 5 Cryptography CS177 2011 Cryptography Enciphering Deciphering Clear Text Cipher Text 6 Cryptography CS177 2011 General Observations • Cryptography never solves a problem; it transforms a security problem into a key management problem • It takes a secret to keep a secret
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2 7 Cryptography CS177 2011 Cryptographic System (Cryptosystem) • A plaintext message space M • A ciphertext message space C • A key space K • A family of enciphering transformations Ek: M C • A family of deciphering transformations Dk: C M 8 Cryptography CS177 2011 Crypto Systems Should Guarantee Both – Secrecy – Authenticity Secrecy requirements 1. Should be computationally infeasible to systematically determine Dk from c, even if corresponding m is known 2. Should be computationally infeasible to determine m from intercepted c 9 Cryptography CS177 2011 Crypto Systems Should Guarantee Both – Secrecy – Authenticity Authenticity requirements 1. Should be computationally infeasible to systematically determine Ek from c, even if corresponding m is known 2. Should be computationally infeasible to find c' such that Dk(c') is valid plaintext in the set M 10 Cryptography CS177 2011 Desirable Properties of Crypto Systems • Enciphering and deciphering must be efficient for all keys • System must be easy to use • The security of the system should depend on the secrecy of the keys and not on the secrecy of the algorithms E or D 11 Cryptography CS177 2011 Cryptanalysis • Cryptanalysis attempts to discover the key or the plaintext of an encrypted message – Assume analyst knows the algorithm but not the key • Types of attack: – Ciphertext only • Given: C1 = Ek(M1), C2 = Ek(M2), . .., Ci = Ek(Mi) • Obtain: either M1,M2, . .., Mi or k – Known plaintext • Given: M1, C1 = Ek(M1), M2, C2 = Ek(M2), . .., Mi, Ci = Ek(Mi) • Obtain: either k or an algorithm to obtain Mi+1, from Ci+1 = Ek(Mi+1)
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crypto - Cryptography Cryptography The science and study of...

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