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Symmetric Message Authentication Codes Prof. Ravi Sandhu
2 © Ravi Sandhu MESSAGE AUTHENTICATION CODES MAC Algorithm M Verification Algorithm V Plain- text Yes/No Plaintext + MAC INSECURE CHANNEL K A A B B K MAC = MD of plaintext + K
3 © Ravi Sandhu CURRENT GENERATION MAC ALGORITHMS HMAC-MD5, HMAC-SHA IETF standard general technique for constructing a MAC from a message digest algorithm Older MACs are based on secret key encryption algorithms (notably DES) and are still in use DES based MACs are 64 bit and not considered strong anymore
4 © Ravi Sandhu HMAC HMAC computation HMAC K (M) = h(K opad || h(K ipad || M) ) h is any message digest function M message K secret key opad, ipad: fixed outer and inner padding HMAC-MD5, HMAC-SHA
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MAC Raushan. DES simple fiestel network 3131 PlainText Blocks 2*4=8bits 31 f f =0011 xor 0011=0000 = 0 f(r,k)=(2*r+k^2)%8 f(1,5)=(2*1+5^2)%8=3 xor 3 3.
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