1. (a) Describe the scheme in steps.
2. (b) How do A and B know that the key is freshly generated?
3. (c) How could A and B know that the key is not available to other users in
the system?
4. (d) Does this scheme ensure the authenticity of both A and B? Justify your
answer.
5. Consider the following hash function based on RSA. The key < n, e > is
known to the public. A message M is represented by blocks of predefined
fixed size M1, M2, M3, …, Mm such that Mi < n. The hash is constructed by
XOR the results of encrypting all blocks. For example, the hash value of a
message consisting of three blocks is calculated by
H(M) = H(M1,M2,M3) = (M1
e mod n) ⊕ (M2
e mod n) ⊕ (M3
e mod n)
Does this hash function satisfy each of the following requirements? Justify
your an-swers (with examples if necessary).
(a) Variable input size (b) Fixed output size
(c) Efficiency (easy to calculate) (d) Preimage resistant
(e) Second preimage resistant (f) Collision resistant

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