2 Symmetric Key Protocol (B)

Exercises Applied Cryptography Protocols (31.05.2012)
The exercizes serve two goals. Firstly, they are preparations for the KU examination. Secondly,
they are meant as a help to understand the VO lectures. The Method is at least as
important as the final result.
1 Symmetric Key Protocol (A)
In the following protocol the participants share no secrets. It is used for transmitting data
over an insecure channel. It assumes that encryption is commutative. Show how an attacker
can retrieve M.
A → B : E KA (M) (1)
B → A : E KB (E KA (M)) (2)
A → B : E KB (M) (3)
2 Symmetric Key Protocol (B)
This protocol is intended to distribute a new session key between two principals A and B.
N A , N B , N B ′ are nonces. In the final message (4) the nonce N B ′ is a handshake number to be
used in future messages. What is the problem of this protocol?
A → B : A, E KAB (N A ) (1)
B → A : E KAB (N A + 1, N B ) (2)
A → B : E KAB (N B + 1) (3)
B → A : E KAB (K AB, ′ N B) ′ (4)
3 Symmetric Key Protocols with Trusted Third Parties (A)
Consider this protocol, explain the purpose of the protocol and show why it is not secure.
A → S : A, B, N A (1)
S → A : E KAS (N A , B, K AB , E KBS (K AB , A)) (2)
A → B : E KBS (K AB , A) (3)
B → A : E KAB (N B ) (4)
A → B : E KAB (N B − 1) (5)
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Exercises Applied Cryptography Protocols (31.05.2012)
4 Symmetric Key Protocols with Trusted Third Parties (B)
Consider the following protocol, explain the purpose of the protocol and show why it is not
secure. Hint: Consider that parts of a known message can be reused for a different purpose.
A → B : N, A, B, E KAS (N A , N, A, B) (1)
B → S : N, A, B, E KAS (N A , N, A, B), E KBS (N B , N, A, B) (2)
S → B : N, E KAS (N A , K AB ), E KBS (N B , K AB ) (3)
B → A : N, E KAS (N A , K AB ) (4)
5 Symmetric Key Protocols with Trusted Third Parties (C)
Consider the following authentication protocol. Explain who is getting authenticated, and
why this protocol is not secure. How can this protocol be fixed?
A → B : A (1)
B → A : N BA (2)
A → B : E KAS (N BA ) (3)
B → S : B, E KBS (A, E KAS (N BA )) (4)
S → B : E KBS (N BA ) (5)
6 Asymmetric Key Protocol
Consider the following key exchange protocol based on an asymmetric cryptosystem (K
A + is
the public key of A, KA − is the private key of A). Explain why this protocol is not secure.
How can the protocol be fixed?
A → S : A, B (1)
S → A : CA, CB (2)
A → B : CA, CB, E K
+ (E
B K
−(K AB , T A )) (3)
A
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