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Unformatted text preview: on that the user’s password is much more sensitive than the host’s password. This falls to a more complicated attack, also described in . SKID
SKID2 and SKID3 are symmetric cryptography identification protocols developed for RACE’s RIPE project  (See Section 25.7). They use a MAC (see Section 2.4) to provide security and both assume that both Alice and Bob share a secret key, K. SKID2 allows Bob to prove his identity to Alice. Here’s the protocol: (1) Alice chooses a random number, RA. (The RIPE document specifies a 64-bit number). She sends it to Bob. (2) Bob chooses a random number, RB. (The RIPE document specifies a 64-bit number). He sends Alice: RB,HK(RA,RB,B) HK is the MAC. (The RIPE document suggests the RIPE-MAC function—see Section 18.14.) B is Bob’s name. (3) Alice computes HK(RA,RB,B) and compares it with what she received from Bob. If the results are identical, then Alice knows that she is communicating with Bob. SKID3 provides mutual authentication between Alice and Bob. Steps (1) through (3) are identical to SKID2, and then the protocol proceeds with: (4) Alice sends Bob: HK(RB,A) A is Alice’s name. (5) Bob computes HK(RB,A), and compares it with what he received from Alice. If the results are identical, then Bob knows that he is communicating with Alice. This protocol is not secure against a man-in-the-middle attack. In general, a man-in-the-middle attack can defeat any protocol that doesn’t involve a secret of some kind. Message Authentication
When Bob receives a message from Alice, how does he know it is authentic? If Alice signed her message, this is easy. Alice’s digital signature is enough to convince anyone that the message is authentic. Symmetric cryptography provides some authentication. When Bob receives a message from Alice encrypted in their shared key, he knows it is from Alice. No one else knows their key. However, Bob has no way of convincing a third party of this fact. Bob can’t show the message to Trent and c...
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- Fall '10