Question

1. [Public key revocation procedure] Suppose that a certain authority is running a (publicly accessible) database of users’ public keys. Let us define the following procedure for revoking a public key from the database. When a user Bob claims that the secret key sk_B corresponding to his public key pk_B was stolen, he sends to the authority the statement “Revoke Bob’s public key pk_B”
   which is signed using sk_B . Upon receiving such the statement, the authority revokes pk_B from the database.

Explain why it is not necessary for the authority to check Bob’s identity in this case.
In particular, explain why it is not a problem that an adversary who has stolen Bob’s secret key sk_B can forge signatures for this key.

Hint: Consider the following two cases:

1) Bob’s secret key sk_B was not stolen;

2) Bob’s secret key sk_B was stolen.

Answer 1

In the database there has a secret key and public key.Here the pulic ke has a secret par and a public part.

for example bob has published his public key on his homepage for anyone to download.another user obtain his public ke encrpts a message using this ke and then send it to bob and bob able t decrypt the message using the secret part of his own key.

one one part must be kept secret.

there is no need to change your public/private key pair unless someone finds our public key.

For N people to communicate there need only be N public/private ke pairs.

There is no need for initial ke exchange and i can serve as a digital signature.

disadvantages they have he slow do to the enoromous amount of computation involved.

keys must be long   (1024 bits )

It has not been around long enough to be tested as much.

here is no proof for hat any public key scheme is secure x=secret key

y=public key

users i derives y in :

y_i=a^(x_i) mod g

users j derives in:

y_j=a^(x_i) mod g

z_ij=y_j^(x_i)=(a^(x_j))^(x_i) mod g = (a^(x_i))^(x_j) mod g =z_ji