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Cryptography Jason Gruber COSC 356-1

Outline Cryptography Terminology Types of encryption Symmetric Key Asymmetric Key Examples of different encryption methods Questions

Cryptography Terms Cryptography the study of ways to convert information from normal, comprehensible form into an incomprehensible format, rendering it illegible without ‘secret knowledge’ Cipher Algorithm for performing encryption and decryption Key Small amount of information used to encrypt / decrypt information (aka ‘secret knowledge’) Plain/Clear Text Information in its normal comprehensible form Ciphered Text Information that has been encrypted

2 Types of Encryption… The First Type Symmetric key ciphers use the same key for encryption and decryption "private-key", "one-key" and "single-key" cryptography block ciphers and stream ciphers Drawback: Key must be shared in a non-traditional communication medium else the key and encrypted data can be collected & decrypted by anyone

Stream Cipher Encrypts bit-per-bit RC4, FISH, and Helix The algorithm uses some function to combine a pseudorandom number generator (PRNG) with a plaintext stream (resulting in a key stream) Encryption consists of XORing the plaintext bits with the corresponding bits of the keystream; decryption consists of XORing the ciphertext bits with the corresponding keystream bits

Block Cipher Encrypts block-per-block Generally 64 or 128 bits DES, AES, IDEA

2 Types of Encryption… The Second Type Asymmetric key ciphers use a public and a private key, for encryption and decryption respectively Public key algorithms are usually based on hard mathematical problems. RSA, for example, relies on the (conjectured) difficulty of factorisation Although generally slower than symmetric, Asymmetric keys can be used to verify identity of sender.

Asymmetric Key Ciphers Plain text is encrypted using public key Corresponding private key is required to decrypt back to plain text Attempts to decrypt using public-key or any other key than specific private key will result in garbage. Key ranges from 160 – 510 bits

Caesar Cipher Key is a numerical value (1-26) Key value represents the number of “shifts” Original 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 G H I J K L M N O P Q R S T U V W X Y Z 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 HEY DOUG HOW ARE YOU? = NKE JUAM NUC GXK EUA

RC4 is the most widely-used software stream cipher It has become part of some commonly used encryption protocols and standards, including WEP and WPA for wireless cards and SSL A number of attacks on RC4 have been published, and have demonstrated that some ways of implementing RC4 within a cryptosystem are completely insecure. However, it is believed that RC4 can be secure when used appropriately RC4 was designed by Ron Rivest of RSA Security in 1987; while it is officially termed "Rivest Cipher 4", the RC acronym is alternatively understood to stand for "Ron's Code" http://en.wikipedia.org/wiki/RC4

RSA asymmetric algorithm widely used in electronic commerce protocols The algorithm was described in 1977 by Ron Rivest, Adi Shamir and Len Adleman who were all at MIT at the time the RSA system relies on the difficulty of factoring very large numbers As of 2004, there is no known method of attack which is feasible against the basic algorithm, and sufficiently long RSA keys make brute force attacks infeasible -- that is, effectively impossible http://en.wikipedia.org/wiki/RSA

Data Encryption Standard (DES) selected as an official Federal Information Processing Standard (FIPS) for the United States in 1976 initially controversial, with classified design elements, a relatively short key length, and suspicions about an NSA backdoor DES is now considered to be insecure for many applications. This is chiefly due to the 56-bit key size being too small; DES keys have been broken in less than 24 hours believed to be practically secure in the form of Triple DES the cipher has been superseded by the Advanced Encryption Standard (AES) http://en.wikipedia.org/wiki/DES

Triple DES (3DES) a block cipher formed from the Data Encryption Standard (DES) cipher developed by Walter Tuchman (the leader of the DES development team at IBM) is specified in FIPS Pub 46-3 Triple-DES is defined as performing a DES encryption, then a DES decryption, and then a DES encryption again 3DES=(DES(DES(DES(text)))) http://en.wikipedia.org/wiki/3DES

Advanced Encryption Standard (AES) also known as Rijndael (a combo of its inventor’s last names (Joan Daemen and Vincent Rijmen) ) block cipher adopted as an encryption standard by the US government, and is expected to be used worldwide and analyzed extensively adopted by National Institute of Standards and Technology (NIST) as US FIPS PUB 197 in November 2001 after a 5-year standardization process http://en.wikipedia.org/wiki/Advanced_Encryption_St andard

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