Man-in-the-middle attack when Alice and Bob employ Diffie-Hellman key exchange. Here, Carol is the intruder. Compute and complete the unknown entries in the exchanges shown below. Alice Carol (Intruder) Bob p=23 and α= 5 are known to all Choose kpri,A =a = 7 Choose kpri,B =b = 13 Alice’s public key: kpub,A = A= αa mod p = ?? Bob’s public key: kpub,B = B=αb mod p = ?? Alice sends A to Bob; intercepted by Carol Bob sends B to Alice; intercepted by Carol Carol chooses c=5; computes A’ = B’ =αc mod p = ?? Carol sends A’ to Bob as if it is A from Alice Carol sends B’ to Alice as if it is from Bob Alice derives the shared secret key as K1= (B’)a mod p = ?? Carol derives K1= Ac mod p = ??, and K2= Bc mod p = ?? Bob derives the shared secret key as K2= (A’)b mod p = ?? Session 1 established with key K1: verify that Alice and Carol have derived the same key K1 Session 2 established with key K2; verify that Carol and Bob have derived the same key K2
Man-in-the-middle attack when Alice and Bob employ Diffie-Hellman key exchange. Here, Carol is the intruder. Compute and complete the unknown entries in the exchanges shown below.
Alice |
Carol (Intruder) |
Bob |
p=23 and α= 5 are known to all |
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Choose kpri,A =a = 7 |
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Choose kpri,B =b = 13 |
Alice’s public key: kpub,A = A= αa mod p = ?? |
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Bob’s public key: kpub,B = B=αb mod p = ?? |
Alice sends A to Bob; intercepted by Carol |
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Bob sends B to Alice; intercepted by Carol |
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Carol chooses c=5; computes A’ = B’ =αc mod p = ?? |
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Carol sends A’ to Bob as if it is A from Alice |
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Carol sends B’ to Alice as if it is from Bob |
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Alice derives the shared secret key as K1= (B’)a mod p = ?? |
Carol derives K1= Ac mod p = ??, and K2= Bc mod p = ?? |
Bob derives the shared secret key as K2= (A’)b mod p = ?? |
Session 1 established with key K1: verify that Alice and Carol have derived the same key K1 |
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Session 2 established with key K2; verify that Carol and Bob have derived the same key K2 |

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