Security in Networks CPSC 363 Computer Networks Ellen Walker Hiram College Includes figures from Computer Networking by Kurose Ross Addison Wesley 2002 2006 Goals for Secure Communication Only the designated recipient can read the message Confidentiality The sender and recipient are correctly identified to each other Authentication The message is received as it was sent Message Integrity and Nonrepudiation The network infrastructure is available for communication by legitimate users Availability and Access Control Aspects of Security Encryption Authentication Ensuring message integrity Network protection Firewalls Attacks Countermeasures Security crosses many layers of the network stack Alice Bob and Trudy Alice wants to communicate with Bob Trudy wants to intercept or disrupt communication Alice channel data secure sender data control messages secure receiver Trudy Bob data Who are Bob and Alice Humans Customer store credit card transaction Customer bank Store supplier Bank central data warehouse DNS servers exchanging addresses What might Trudy do Eavesdrop intercept messages Insert messages into conversation Impersonate someone Alice Bob or someone else Hijack the conversation replace Alice or Bob Denial of service prevent this conversation and any others Cryptography Disguise data so that only the designated recipients can read it with a key Long history significant advances in past 30 years The Association for Computing Machinery ACM awarded the 2002 A M Turing Award considered the Nobel Prize of Computing to Leonard M Adleman Ronald L Rivest and Adi Shamir for their role in the creation of the world s most widely used public key cryptography system which has become known by their initials RSA Cryptography Definitions plaintext Alice s K encryptio A n key encryption ciphertext algorithm Bob s K decryptio Bn key decryption plaintext algorithm Decrypt Encrypt text Akey Bkey text Two kinds of keys Symmetric key systems Encryption sender s key decryption recipient s key Key must be exchanged privately first Public key systems Encryption and decryption keys are different Each individual has a public and private key Alice uses Bob s public key to send to Bob Bob uses his private key to read the message Substitution Ciphers Symmetric Key Caesar Cipher Substitute each letter by one k away in the alphabet with wrap If k 1 HAL becomes IBM 25 different options assuming 0 is not reasonable Monoalphabetic Cipher Substitute each letter by another arbitrarily Example A X B A C F Y O Z T 26 1 different options all orders of 26 letters Polyalphabetic Cipher Use multiple monoalphabetic ciphers in a pattern e g c1 c2 c2 c1 for each 4 letter sequence Attacking Substitution Ciphers Ciphertext only Use letter frequency statistics e g e is most frequent in English text Use common patterns e g ing ion ed Known plaintext Get pairings from known words e g bob give b and o use those to get more words Chosen plaintext If Trudy can get Alice to send a known message containing all 26 letters Steal the key DES A Modern Symmetric key System 64 bit key really 56 8 parity bits Operates directly on binary data 64 bit chunks The goal is to completely scramble the data and key there should be no correlation between the ciphertext and either the original data or the key NIST 1999 DES Encryption 56 bit 2 permutation steps first and last 16 rounds each using different 48 bits of key Took 4 months to break by brute force in 1997 Broken in 22 hours in 1999 Strengthening DES Cipher block chaining Each 64 bit block is XOR ed with the encrypted previous block before encoding Triple DES 3DES Run the algorithm 3 times with 3 different keys Advanced Encryption System AES 2001 128 bit blocks Keys are 128 192 and 256 bits long If brute force takes 1 sec on DES it takes 149 TRILLION YEARS for AES Problem with Symmetric Key How does Alice communicate to Bob what key to use Cannot send it in plaintext it is too valuable Use another symmetric key same problem Use prior knowledge Use the name of my dog when I was little Exchange key physically by secure courier One time pad two identical sequences of secret keys each is used for one message and thrown away Public Key Encryption Every individual has two keys public private My private key unlocks message encrypted with my public key and vice versa Private key cannot be computed from public key KB Bob s public key K plaintext encryption ciphertext message m algorithm K m B Bob s private B key decryption plaintext algorithm message m KB K m B Advantages No shared secrets No couriers needed Anyone can send an encrypted message to anyone also a disadvantage If public key unlocks private key as well we have a way of authenticating signatures Bob signs with his private key Alice verifies it s Bob by decrypting with Bob s public key RSA Keys Choose 2 large prime numbers p and q Compute n p q and z p 1 q 1 Pick another number e less than n and relatively prime to z Find a number d so that e d 1 z 0 Public key is n e and Private key is n d RSA Encryption Decryption Let m be the message text as a binary c pow m e n encryption m pow c d n decryption Because of the choices of d and e pow m d e n m see p 619 Also d e e d so we can use public private key in either order Authentication Goal Bob wants Alice to prove her identity to him Protocol ap1 0 Alice says I am Alice I am Alice Failure scenario Authentication Goal Bob wants Alice to prove her identity to him Protocol ap1 0 Alice says I am Alice I am Alice in a network Bob can not see Alice so Trudy simply declares herself to be Alice Authentication another try Protocol ap2 0 Alice says I am Alice in an IP packet containing her source IP address Alice s I am Alice IP address Failure scenario Authentication another try Protocol ap2 0 Alice says I am Alice in an IP packet containing her source IP address Trudy can create a packet spoofing Alice s I am Alice Alice s address IP address Authentication another try Protocol ap3 0 Alice says I am Alice and sends her secret password to prove it Alice s Alice s I m Alice IP addr password Alice s IP addr OK Failure scenario Authentication another try Protocol ap3 0 Alice says I am Alice and sends her secret password to prove it Alice s Alice s I m Alice IP addr password Alice s IP addr OK playback attack Trudy records Alice s packet and later plays it back to Bob Alice s Alice s I m Alice IP addr password Authentication yet another try Protocol ap3 1 Alice says I am Alice and sends her encrypted