Cryptography

Historical uses of cryptography

-Caesar cipher -shifted character 3 spaces to the left
-Scytale -spartans used this cipher to communicate messages to generals in the field, wrapped tape around a rod, diameter of the rod is the pre-agreed upon secret
-Vignere -a polyalphabetic cipher, key word is agreed upon, first letter of the key is matched against first letter of the message
-Vernam cipher -the only mathematically unbreakable form of cryptography, one time key pad

Security services provided by cryptography

-privacy, authenticity, integrity, non-repudiation

plain text + initialization vector + algorithm + key = cipher text

Desirable qualities of an algorithm

-confusion, diffusion, avalanche, permutations, open-Kerchkoff's principle

Desirable qualities of a key
-long, random, secret

Symmetric cryptography

-the same key is used by both parties to both encrypt and decrypt
-two types of symmetric ciphers or algorithms --> stream and block
-stream is very efficient, however generally considered to be not as secure
-RC-4 is a stream cipher

-block ciphers are slower but more secure
-two main block ciphers used today are AES and 3DES

-drawbacks of symmetric cryptography -out of band key exchange, not scalable, no authenticity, integrity, or non-repudiation

Asymmetric cryptography

-every user has a key pair -public key is made available to anyone who requests it, private key is only available to that user and must not be disclosed or shared
-the keys are mathematically related so that anything encrypted with one key can only be decrypted by the other