This document discusses man-in-the-middle attacks against cryptographic protocols. It describes two classes of man-in-the-middle attacks - passive attacks where the attacker can only monitor communications, and active attacks where the attacker can modify messages. It then gives an example of how a man-in-the-middle attack works against key exchange protocols by replacing one party's public key. Finally, it provides an example of how the Diffie-Hellman key exchange protocol allows two parties to securely generate a shared secret key to prevent such man-in-the-middle attacks.

2. MAN-IN-THE-MIDDLE
• Cryptographic Protocol attacks are often analyzed in terms of
a man-in-the-middle
• This is an agent who is able to listen to and potentially add,
delete, or change messages being sent over an open channel.

3. CLASSES OF MITM ATTACK
•We can divide attacks roughly into two classes:
•A passive attack is one in which the attacker is only able to monitor the communications
channel.
• Threatens confidentiality
•An active attack is one in which the attacker attempts to add, delete, or modify messages.
• Threatens both confidentiality and data integrity.

4. ATTACKING KEY EXCHANGE
•Key exchange is one of the most common places for a man-in-the-middle attack.
•A sends b its public key.
•Man-in-the-middle replaces a’s public key with a false one.
•Man-in-the-middle is now able to intercept and decrypt secret messages from b to a.

5. PUBLIC-KEY INFRASTRUCTURE
•For real-world applications, a complex web of software systems is required to ensure security.
•This is referred to as a public key infrastructure (PKI).
•Focus shifts from provable protocol properties to system design.

6. CERTIFICATE AUTHORITY
•A certificate authority has a number of functions within a PKI
•Authentication
•Key generation
•Key revocation
•Many commercial entities serve as CAs

7. CERTIFICATE AUTHORITIES
•A certificate authority will wrap a users public key in a certificate.
• X.509 is most common standard.
• Contains the user’s identity and public key.
• Signed with the CAs private key.
• Risk is shifted:
• Previously: could unknown user A be compromised?
• Now: could the CA be compromised?

8. EXAMPLE
•USE OF DIFFIE–HELLMAN KEY EXCHANGE

9. INTRODUCTION
•Discovered by whitfield diffie and martin hellman
• “New directions in cryptography”
•Diffie-hellman key agreement protocol
• Exponential key agreement
• Allows two users to exchange a secret key
• Requires no prior secrets
• Real-time over an untrusted network

10. DIFFIE–HELLMAN KEY EXCHANGE
• Is a specific method of exchanging cryptographic keys.
• One of the earliest practical examples of key exchange implemented within the field of cryptography.
• Allows two parties that have no prior knowledge of each other to jointly establish a shared secret key over
an insecure communications channel.
• This key can then be used to encrypt subsequent communications using a symmetric key cipher.

11. IMPLEMENTATION
•P and G are both publicly available numbers
• P is at least 512 bits
•Users pick private values a and b
•Compute public values
• X = ga mod p
• Y = gb mod p
•Public values x and y are exchanged

12. IMPLEMENTATION
•Compute shared, private key
• Ka = ya mod p
• Kb = xb mod p
•Algebraically it can be shown that ka = kb
• Users now have a symmetric secret key to encrypt

13. IMPLEMENTATION

14. EXAMPLE
•Two internet users, alice and bob wish to have a secure conversation.
•They decide to use the diffie-hellman protocol

15. EXAMPLE
• Alice and bob get public numbers
• P = 23, g = 9
• Alice and bob compute public values
• X = 94 mod 23 = 6561 mod 23 = 6
• Y = 93 mod 23 = 729 mod 23 = 16
• Alice and bob exchange public numbers.

16. EXAMPLE
• Alice and bob compute symmetric keys
• Ka = ya mod p = 164 mod 23 = 9
• Kb = xb mod p = 63 mod 23 = 9
• Alice and bob now can talk securely!

17. APPLICATION
•Diffie-hellman is currently used in many protocols, namely:
• Secure sockets layer (SSL)/transport layer security (TLS)
• Secure shell (SSH)
• Internet protocol security (ipsec)
• Public key infrastructure (PKI)

18. CONCLUSION
•Authenticated diffie-hellman key agreement
• Defeats middleperson attack
•Diffie-hellman continues to play large role in secure protocol creation.