Message Authentication Codes CSCI 5857: Encoding and Encryption

Outline Message Authentication Code Adding a key to an existing hash function –Prefix/postfix MAC –Nested MAC –HMAC algorithm Creating a MAC using a block cipher (CMAC) Combining confidentiality and information integrity

3 Digests and Networks Same hash applied to message by sender and recipient –Sender creates digest and sends along with message –Recipient creates digest from received message, and compares to received digest –If no match, message has been tampered with en route M

4 Digests and Networks Problem: Adversary can easily intercept digest and change it to match new message –Must assume adversary knows hash function we use! M h(M )

5 Message Authentication Codes Using secret key to create digest –Creates MAC as h(M, k) –Without k, Darth can’t substitute M and then duplicate the h(M, k) that recipient will use to check message integrity –k must be large enough to prevent exhaustive search

6 Prefix/Postfix MAC Key = “extra bits” at beginning or end of message h(M, k) = h(M | k) or h(k | M) Hash algorithm used must have strong “avalanche effect” –Changing few bits at beginning/end changes most bits of MAC even if rest of message is the same –Better if key “spread out” over message rather than at known fixed location Message

7 Nested MAC Hashing applied multiple times –Concatenate key with message: k | M –Run through hash: h(k | M) –Concatenate key again: k | h(k | M) –Run through hash again: MAC = h(k | h(k | M)) Changes in key have greater avalanche effect on final MAC

8 Hashed MAC (HMAC) 2-stage nested MAC Different “round keys” generated for each hash –Stage 1: k1 = k  ipad –Stage 2: k2 = k  opad

9 Hashed MAC (HMAC) Stage 1: k 1 = k  ipad –Key k padded out to b bits with extra 0’s –ipad = … repeated to b bits Stage : k 2 = k  opad –opad = … repeated to b bits Key idea: ipad and opad differ in half of possible bits  k 1 and k 2 will differ very greatly

10 Chained MAC (CMAC) “Hashless” MAC –Uses an encryption algorithm (DES, AES, etc.) to generate MAC

11 Chained MAC (CMAC) Based on same idea as cipher block chaining –Message broken into N blocks –Each block fed into an encryption algorithm with key –Result XOR’d with next block before encryption to make final MAC depend on all blocks Compresses result to size of single block (unlike encryption)

12 Chained MAC (CMAC) Final stage uses “additional key” –Derived from cipher key but hides relationship to key: Encrypting all 0’s Multiplying by x or x 2 over GF(2 n )

13 Chained MAC (CMAC) Additional key XOR’d with final block Crucial to use different key for last XOR –Avoids differential cryptanalysis of 2 messages with same beginning MAC = leftmost n bits of result

14 Chained MAC (CMAC) Advantages: –Can use existing encryption functions –Encryption functions have properties that resist preimage and collision attacks Ciphertext designed to appear like “random noise” – good approximation of random oracle model Most exhibit strong avalanche effect – minor change in message gives great change in resulting MAC Disadvantage: –Encryption algorithms (particularly when chained) can be much slower than hash algorithms

15 Message Integrity and Confidentiality Can encrypt and hash message with different keys –Hash plaintext before encryption –Hash ciphertext after encryption Allows authentication to take place without decryption (usually much faster) h h h h h h h