CSCI-235 Micro-Computer Applications Privacy & Security II

The Encryption Debate Encryption is the coding and scrambling process by which a message is made unreadable except by the intended recipient Encryption is needed for electronic commerce

Simplified Communications Model - Diagram

Simplified Data Communications Model

Encryption Basics A readable message is called plaintext An encryption algorithm is a formula used to make plaintext unreadable The coded message is called ciphertext I LOVE YOU V YBIR LBH

Encryption Basics Symmetric key encryption are encryption techniques that use the same key to encrypt and decrypt a message Strong encryption refers to encryption methods that are used by banks and military agencies and are nearly impossible to break

Symmetric Encryption or conventional / private-key / single-key sender and recipient share a common key all classical encryption algorithms are private-key was only type prior to invention of public-key in 1970’s

Basic Terminology plaintext - the original message ciphertext - the coded message cipher - algorithm for transforming plaintext to ciphertext key - info used in cipher known only to sender/receiver encipher (encrypt) - converting plaintext to ciphertext decipher (decrypt) - recovering ciphertext from plaintext cryptography - study of encryption principles/methods cryptanalysis (codebreaking) - the study of principles/ methods of deciphering ciphertext without knowing key cryptology - the field of both cryptography and cryptanalysis

Symmetric Cipher Model

Requirements two requirements for secure use of symmetric encryption: a strong encryption algorithm a secret key known only to sender / receiver Y = EK(X) X = DK(Y) assume encryption algorithm is known implies a secure channel to distribute key

Classical Substitution Ciphers where letters of plaintext are replaced by other letters or by numbers or symbols

Caesar Cipher earliest known substitution cipher by Julius Caesar first attested use in military affairs replaces each letter by k-th letter on Example ( what is k ? ): meet me after the toga party PHHW PH DIWHU WKH WRJD SDUWB

Caesar Cipher can define transformation (with k = 3) as: a b c d e f g h i j k l m n o p q r s t u v w x y z D E F G H I J K L M N O P Q R S T U V W X Y Z A B C mathematically give each letter a number a b c d e f g h i j k l m 0 1 2 3 4 5 6 7 8 9 10 11 12 n o p q r s t u v w x y Z 13 14 15 16 17 18 19 20 21 22 23 24 25 then have Caesar cipher as: Y = EK(X) = (X + k) mod 26 X = DK(Y) = (Y – k) mod 26 EXAMPLE: Encrypt “howdy” using key k = 5

Cryptanalysis of Caesar Cipher only have 26 possible ciphers A maps to A,B,..Z could simply try each in turn a brute force search given ciphertext, just try all shifts of letters

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Private-Key Cryptography traditional private/secret/single key cryptography uses one key shared by both sender and receiver if this key is disclosed communications are compromised also is symmetric, parties are equal

Public-Key Cryptography probably most significant advance in the 3000 year history of cryptography uses two keys – a public & a private key asymmetric since parties are not equal uses clever application of number theoretic concepts to function complements rather than replaces private key cryptography

Public-Key Cryptography public-key/two-key/asymmetric cryptography involves the use of two keys: a public-key, which may be known by anybody, and can be used to encrypt messages, and verify signatures a private-key, known only to the recipient, used to decrypt messages, and sign (create) signatures is asymmetric because those who encrypt messages or verify signatures cannot decrypt messages or create signatures

Public-Key Cryptography

Public-Key Characteristics Public-Key algorithms rely on two keys with the characteristics that it is: computationally infeasible to find decryption key knowing only algorithm & encryption key computationally easy to en/decrypt messages when the relevant (en/decrypt) key is known

Digital Signatures and Certificates Digital signatures are a technique used to guarantee that a message has not been tampered with Digital certificates are a technique used to validate one’s identity