11-Basic Cryptography Dr. John P. Abraham Professor UTPA

Cryptography “Hidden writing” - Scramble the data Steganography – hides the existence of the data –Terrorist use this to hide what they are transmitting. Julius Ceasar shifted each letter of his message down to another character. Plaintext  encryption  ciphertext

In cryptography, the encryption/decryption algorithms are public; the keys are secret.

Cryptography and security Can protect the confidentiality of info – only the authorized party can view it. Protect integrity of info. Ensures that info is correct, no one altered it. Ensure availability of info – authorized persons can view Can verify authenticity of the sender Non-repudiation

Symmetric-key In symmetric-key cryptography, the same key is used by the sender (for encryption) and the receiver (for decryption). The key is shared.

Security+ Guide to Network Security Fundamentals, Third Edition Asymmetric Cryptographic Algorithms Asymmetric cryptographic algorithms –Also known as public key cryptography –Uses two keys instead of one The public key is known to everyone and can be freely distributed The private key is known only to the recipient of the message Asymmetric cryptography can also be used to create a digital signature 7

Asymmetric-key Public key

Security+ Guide to Network Security Fundamentals, Third Edition Asymmetric Cryptographic Algorithms (continued) A digital signature can: –Verify the sender –Prove the integrity of the message –Prevent the sender from disowning the message 9

Figure Signing the whole document

Security+ Guide to Network Security Fundamentals, Third Edition11

Security+ Guide to Network Security Fundamentals, Third Edition12 Asymmetric Cryptographic Algorithms (continued)

Security+ Guide to Network Security Fundamentals, Third Edition RSA The most common asymmetric cryptography algorithm RSA multiplies two large prime numbers p and q –To compute their product (n=pq) A number e is chosen that is less than n and a prime factor to (p-1)(q-1) Another number d is determined, so that (ed-1) is divisible by (p-1)(q-1) The public key is the pair (n,e) while the private key is (n,d) 13

Security+ Guide to Network Security Fundamentals, Third Edition Cryptographic Algorithms There are three categories of cryptographic algorithms: –Hashing algorithms –Symmetric encryption algorithms –Asymmetric encryption algorithms 14

Security+ Guide to Network Security Fundamentals, Third Edition Hashing Algorithms Hashing –Also called a one-way hash –A process for creating a unique “signature” for a set of data This signature, called a hash or digest, represents the contents Hashing is used only for integrity to ensure that: –Information is in its original form –No unauthorized person or malicious software has altered the data Hash created from a set of data cannot be reversed Hash values are often posted on Internet sites –In order to verify the file integrity of files that can be downloaded 15

Security+ Guide to Network Security Fundamentals, Third Edition Hashing Algorithms (continued) 16 The hashed value is stored on the card.

Security+ Guide to Network Security Fundamentals, Third Edition Hashing Algorithms (continued) A hashing algorithm is considered secure if it has these characteristics: –The ciphertext hash is a fixed size –Two different sets of data cannot produce the same hash, which is known as a collision –It should be impossible to produce a data set that has a desired or predefined hash –The resulting hash ciphertext cannot be reversed The hash serves as a check to verify the message contents 17

Security+ Guide to Network Security Fundamentals, Third Edition18 Man in the middle defeated

Message Digest algorithms Secure Hash Algorithm (SHA) –A more secure hash than MD –A family of hashes SHA-1 –Patterned after MD4, but creates a hash that is 160 bits in length instead of 128 bits SHA-2 –Comprised of four variations, known as SHA-224, SHA-256, SHA-384, and SHA-512 –Considered to be a secure hash Passwords are stored hashed MD5