I'm studying the Lamport's Hash one-time password scheme.
This is the scheme:
Alice wants to authenticate herself to Bob from a workstation that knows nothing about her. Alice only knows her password. Bob knows
- Alice types her name and her password into her Workstation
- The Workstation sends "Alice" to Bob
- Bob sends n to the Workstation
- The workstation computes x = hash^(n-1) (pwd) and sends it to Bob
- Bob hashes it once and compares it with database. If it matches, Bob considers the response valid, replaces the stored quantity with the received quantity, and replace n by n-1.
This scheme has a security weakness called small n attack.
This is the explanation given in "Network Security: Private Communications in a Public World":
Suppose an intruder, Trudy, were to impersonate Bob's network address and wait for Alice to attempt to log in. When Alice attempts to log into Bob, Trudy sends back a small value for n, say 50. When Alice responds with hash^(50) (pwd), Trudy will have enough information to impersonate Alice for some time, assuming that the actual n at Bob is greater than 50.
I don't understand this. I would need a different explanation.