Key Management

Symmetric cipher

Advantages.

• Fast and relatively secure.

  • Provides integrity and privacy.

• The larger key length provides larger security.

Disadvantages.

• Requires the share of a secret key.

  • How?

• Complex administration and non-scalable.

  • It is needed to distribute the keys.

  • A key for each receiver.

Triple symmetric mechanisms (e.g. “3- DES”)

Asymmetric cipher

Also known as PKE - public key encryption.

Advantages.

• It is not needed to share secret keys à priori.

• It is scalable and versatile.

Disadvantages.

• Generally computationally intensive.

• It may require a certificate of authority.

• Private keys have to be confidential.

Diffie-Hellman

Public-private pair for confidentiality

Public-private pair for authentication

Public-private pair for confidentiality and authentication

Alice and Bob agree on a prime number p and a base g.

Alice chooses the secret number a, and sends it to Bob (g^a modp).

ob chooses the secret number b, and sends it to Alice (g^b modp).

Alice calculates ((g^b modp)^a mod p).

Bob calculates ((g^a modp)^b mod p).

Alice and Bob use this value as their session key. p and g do not have to be protected.

Example Diffie-Hellman

Alice and Bob choose p = 23 e g = 5.

Alice chooses a = 6 and sends 56 mod 23 = 8.

Bob chooses b = 15 and sends 515 mod 23 = 19.

Alice calculates 196 mod 23 = 2.

Bob calculates 815 mod 23 = 2.

2 is the shared key.