Cryptography

In slide 19, we describe a new puzzle mechanism that permits pre-computation of puzzles and puzzle sharing (i.e., the same puzzle can be used by different clients). Elaborate at least three design rationales (discussed in the class) that permit this feature.

Puzzle mechanism (Slide 19 is the image below):

Puzzle Construction S All clients (broadcast): Digitally sign: k, Ts. MS Client C S: C, MS, No X S: verify h(C, Ns, No X) has k leading zero's . the k first bits of the hash h (C. A's, Mc, X) 000 . . . 000, Y, the rest of the hash bits = a cryptographic hash function (e.g. MD5 or SHA) - the client indentitv the server's nonce Ns the client's nonce the solution of the puzzle the puzzle difficulty level the k first bits of the hash value; must be zero the rest of the hash value; may be anything = 000.. .000- - 19 Puzzle Construction S All clients (broadcast): Digitally sign: k, Ts. MS Client C S: C, MS, No X S: verify h(C, Ns, No X) has k leading zero's . the k first bits of the hash h (C. A's, Mc, X) 000 . . . 000, Y, the rest of the hash bits = a cryptographic hash function (e.g. MD5 or SHA) - the client indentitv the server's nonce Ns the client's nonce the solution of the puzzle the puzzle difficulty level the k first bits of the hash value; must be zero the rest of the hash value; may be anything = 000.. .000- - 19