Impersonating Attack

$E (A) \to B$ : “I’m Alice”

where E(A) means Elvis is impersonating Alice.

A simple protocol - Symmetric Key Encryption

Owning Shared Keys

A \to B : {"I’m Alice"}_{K_{ab}}

If Alice and Bob share a key $K_{ab}$ then Alice can encrypt the message.

A E (A) \to B : {"I’m Alice"}_{K_{ab}} \to B : {"I’m Alice"}_{K_{ab}}

Number that is only used once (often used in a challenge/response setting).

Protocol steps :

1. 2. 3. A \to B : A B \to A : {N_{a}}_{K_{ab}} A \to B : {N_{a} + 1}_{K_{ab}}, {Pay Elvis 5}_{K_{ab}}

Another example :

1. 2. 3. 4. 5. 6. A \to B : A B \to A : {N_{a}}_{K_{ab}} A \to B : {N_{a} + 1}_{K_{ab}}, {Pay Elvis 5}_{K_{ab}} A \to B : A B \to A : {N_{a 2}}_{K_{ab}} A \to B : {N_{a 2} + 1}_{K_{ab}}, {Pay Bob 5}_{K_{ab}}

1. 2. 3. A \to B : A B \to A : {N_{a}}_{K_{ab}} A \to B : {N_{a}, Pay Elvis 5}_{K_{ab}}

What Bob Can Be Sure Of :