Introduction to Cryptography - Introduction to Cryptography History is punctuated with codes They have decided the outcomes of battles and led to the

Introduction to Cryptography - Introduction to Cryptography...

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History is punctuated with codes. They have decided the outcomes of battles and led to the deaths of kings and queens. — Simon Singh, The Code Book Introduction to Cryptography
Greek: “krypto” = hide Cryptology: science of hiding cryptography + cryptanalysis + steganography Cryptography: secret writing Cryptanalysis: analyzing (breaking) secrets Decipher (decryption) is what we do Cryptanalysis is what they do W hat is cryptology? 2
“Covered” messages Technical steganography Invisible ink, shaved heads, microdots Linguistic steganography “Open code”: secret message appears innocent “East wind rain” = war with USA Broken dolls in weII Hide message in low-order bits in images Steganography 3
Cryptology is a branch of mathematics Lots of formal representation Proofs about encryption are possible Security is a system issue Easiest way to violate security is through people! Security uses cryptology and other tools Difficult to prove things about security Cryptology vs. security 4
Terminology 5 5 Alice Bob Eve Insecure Channel C = E( P ) P = D( C ) E must be invertible Encrypt Ciphertext Decrypt Plaintext Plaintext
Cryptography always involves two things Transformation Secret Security should depend only on the secrecy of the key Assume the enemy can get the algorithm Can capture machines (or people), disassemble programs, etc. Very expensive and difficult to invent a new algorithm if the old one might have been compromised Security through obscurity isn’t Look at history of examples Better to have scrutiny by open experts “The enemy knows the system being used.” (Claude Shannon) Kerckhoff’s Principle 6
Alice and Bob 7 Alice Bob KE KD KE = KD symmetric encryption KE KD asymmetric encryption C = E(KE, P) = E KE (P) P = D(KD, C) = D KD (C) Insecure Channel Encrypt Ciphertext Decrypt Plaintext Plaintext
Three basic types of algorithms Symmetric (shared) key encryption Asymmetric (public key) encryption Secure hash functions For each type of algorithm, many choices Symmetric key: DES, AES, Blowfish, RC5, RC6 Asymmetric key: RSA, El-Gamal, elliptic curve Secure hash function: MD5, SHA-1, SHA-256, RIPEMD Different implementations within a type of algorithm share many characteristics Goal, approach are similar Specific implementation details may differ Good books on algorithms include Applied Cryptography (somewhat dated) and Cryptography Engineering Overview of modern cryptography 8
Encryption key and decryption key are identical Strength of algorithm is usually proportional to 2 key_ length Assumes a truly random key! Algorithm is usually fast Under 20 cycles per byte for many algorithms 1000+ MB/s possible on today’s CPUs (often limited by memory speed!) Straightforward to build hardware to run the algorithm Newer CPUs often have instructions to help (e.g., AES on x86) Decryption may be the same algorithm as encryption, but isn’t always Symmetric key encryption 9 KS KS Alice Bob Insecure Channel Encrypt Ciphertext Decrypt Plaintext Plaintext

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