Kerberos: Give description, short dialogue between user and server

Explain the Hash function by using the general formula. Give an example of hash algorithm.

can be applied to any sized message M

produces fixed-length output h

is easy to compute h=H(M) for any message M

given h is infeasible to find x s.t. H(x)=h

one-way property

given x is infeasible to find y s.t. H(y)=H(x)

weak collision resistance

is infeasible to find any x,y s.t. H(y)=H(x)

strong collision resistance

ex:Bithday algorithm

Describe the Direct Digital Signature.

involve only sender & receiver

assumed receiver has sender’s public-key

digital signature made by sender signing entire message or hash with private-key

can encrypt using receivers public-key

important that sign first then encrypt message & signature

security depends on sender’s private-key

Kerberos: Give description, short dialogue between user and server.

trusted key server system from MIT

provides centralised private-key third-party authentication in a distributed network

allows users access to services distributed through network

without needing to trust all workstations

rather all trust a central authentication server

two versions in use: 4 & 5

1. obtain ticket granting ticket from AS

• once per session

1. obtain service granting ticket from TGT

• for each distinct service required

1. client/server exchange to obtain service

• on every service request