Part 1  Cryptography 1 Integrity

Part 1  Cryptography 2 Data Integrity  Integrity  detect unauthorized writing (i.e., modification of data)  Example: Inter-bank fund transfers o Confidentiality may be nice, integrity is critical  Encryption provides confidentiality (prevents unauthorized disclosure)  Encryption alone does not provide integrity o One-time pad, ECB cut-and-paste, etc.

Message Authentication Code (MAC)  Message Authentication Code ( MAC ) o Used for data integrity o Integrity not the same as confidentiality Part 1  Cryptography 3 plaintext MAC compute

Part 1  Cryptography 4 Computation of MAC  There are two ways of computing MAC.  One way is to compute MAC as CBC residue o That is, compute CBC encryption, saving only final ciphertext block, the MAC  The other way is to use the hash function.(we will cover it ch 5)

Part 1  Cryptography 5 MAC Computation  MAC computation (assuming N blocks) C 0 = E(IV  P 0, K), C 1 = E(C 0  P 1, K), C 2 = E(C 1  P 2, K),… C N  1 = E(C N  2  P N  1, K) = MAC  MAC sent with IV and plaintext  Receiver does same computation and verifies that result agrees with MAC  Note: receiver must know the key K

Part 1  Cryptography 6 Does a MAC work?  Suppose Alice has 4 plaintext blocks  Alice computes C 0 = E(IV  P 0,K), C 1 = E(C 0  P 1,K), C 2 = E(C 1  P 2,K), C 3 = E(C 2  P 3,K) = MAC  Alice sends IV,P 0,P 1,P 2,P 3 and MAC to Bob  Suppose Trudy changes P 1 to X  Bob computes C 0 = E(IV  P 0,K), C 1 = E(C 0  X,K), C 2 = E(C 1  P 2,K), C 3 = E(C 2  P 3,K) = MAC  MAC  That is, error propagates into MAC  Trudy can’t make MAC == MAC without K

Part 1  Cryptography 7 Confidentiality and Integrity  Encrypt with one key, MAC with another key  Why not use the same key? o Send last encrypted block ( MAC ) twice? o This cannot add any security!  Using different keys to encrypt and compute MAC works, even if keys are related o But, twice as much work as encryption alone o Can do a little better  about 1.5 “encryptions”  Confidentiality and integrity with same work as one encryption is a research topic

CBC and CTR(1)  Can we achieve both confidentiality and integrity through the same encryption? o CBC is possible, but CTR is not.  But, the drawback of CBC is o Not possible to do pipelined and parallelized implementation. o So, common method is the composition of CBC with HMAC-SHA1. Part 1  Cryptography 8

CBC and CTR(2)  CTR is possible to do pipelined and parallelized implementation.  But there is no suitable message authentication algorithm.  So, the question is, o Can we encrypt at high speed and provide message authentication that can keep up with our cipher? Part 1  Cryptography 9

Part 1  Cryptography 10 Uses for Symmetric Crypto  Confidentiality o Transmitting data over insecure channel o Secure storage on insecure media  Integrity ( MAC )  Authentication protocols (later…)  Anything you can do with a hash function (upcoming chapter…)