Security Principles and Cryptographic Primitives

Any complete security protocol will rely on a number of cryptographic primitives in order to guarantee all of the necessary security principles we discussed in class.

To recap, those principles are:

• confidentiality: only the sender and the intended receiver should “understand” the message contents
• authentication: the sender and receiver want to confirm each other’s identity
• message integrity: the sender and receiver want to ensure that the message is not altered (in transit, or afterwards) without detection
• access and availability: services must be accessible and available to users

Relevant cryptographic primitives are:

• a security protocol will start with a handshake to provide authentication and share a secret between the two parties
• the secret is then used as the input to a key derivation function to produce a shared session key or keys
• these keys are then used in a symmetric encryption algorithm which guarantees the confidentiality and integrity of the data that is being transmitted between the two parties
• the protocol needs a way to securely terminate the session so that an intruder can’t hijack a connection by tricking one participant into thinking that the connection has ended

Exercise: Last Friday, an intentional backdoor was discovered in liblzma leading to compromising ssh servers. The attack is explained in detail here. An attacker provides a payload - a ciphertext encrypted with a symmetric key. This ciphertext, when decrypted, returns a signature and a command to execute. This signature is derived by signing the command and the server's host key with the attacker's key.

Based on the description above, identify the cryptographic mechanisms and the security principles these mechanisms provide.

Diffie-Hellman Key Exchange:

Consider Diffie-Hellman Key Exchange. Both parties agree on some public parameters: a prime number p and a base g. Both parties choose some private keys. Let assume Alice picks a, and Bob picks b. The end goal is to establish a shared key without leaking it.

The next step is to send each other public keys. Exercise: What will be the public key and How do both parties get the shared key? Why is this hard for Trudy to decrypt the shared keys?

Exercise 1: What will be the public key and How do both parties get the shared key? Why is this hard for Trudy to decrypt the shared keys?

Exercise 2: Let's use p = 17, g = 10, a = 4, b = 3 to see Diffie-Hellman in action!