Symmetric Cryptosystems and Authentication

1. Symmetric Cryptosystems and Authentication

2. Secret Key Cryptography • Secret key cryptography involves two functions: • encryption: E(key, message) -> enciphered text • decryption: D(key, ciphertext) -> message.

3. Secret Key Cryptography • These functions are inverses of each other, • message = D(key, E(key, message)), or • k{m} to say that message m was encrypted under key k.

4. Secret Key Cryptography • Uses for secret key cryptography include: • transmitting secret messages in the presence of passive eavesdroppers, • storing information in encrypted form on insecure media, and • authentication (determining who is involved in a given dialogue.) • Note: if you are using encryption to store data on insecure media, most editors work on a decrypted form of data. Thus, they may store temporary unencrypted versions of your data.

5. Secret Key Cryptography • Authentication is often done with a password. • involves revealing the 'secret' (password) in order to prove one's identity. • In strong authentication • provide proof of identity without revealing the secret. • involves proving knowledge of a secret (key/password) without revealing the secret itself.

6. Secret Key Cryptography • Suppose we would like user A to prove to user B that A is A without revealing the secret that is the essence of being A, and vice versa. • Thus, we desire a protocol that allows A and B to authenticate themselves to each other. • Assume that there is a shared, secret key: k. • Knowing k is proof of being either A or B.

7. The desired mutual authentication protocol works as outlined below. The arrows indicate sending a message.

8. Secret Key Cryptography • There are many subtle errors that can arise when designing cryptographic protocols. • First, note in the above protocol that B sends A two messages in a row. • An obvious optimization might be to combine those messages into one, as well as • having A announce itself and • send a random bit string for B to encrypt at the same time.

9. Secret Key Cryptography

10. Secret Key Cryptography • This protocol is susceptible to what is known as a reflection attack. • It is possible for an eavesdropper, T, to convince B that T is A. • T exploits the fact that B seems willing to encrypt challenges. • Therefore, T can pretend to be A as follows:

11. Secret Key Cryptography

12. Secret Key Cryptography • After B sends R1 to T, T needs to get it encrypted under k in order to convince B that T is A. • T therefore starts another session, sending R1 which B then returns encrypted. • T now learns the encrypted version of R1 and can convince B that B is communicating with A.

13. Secret Key Cryptography:There are several possible approaches to repairing the protocol • Use two different keys, one from A to B and another from B to A. This introduces additional keys to keep track of. • Insist that the challenge from the initiator look different from the challenge from the responder. If this holds, then T can never get B to encrypt the 'right thing.' For instance, suppose A generates random even numbers and B generates random odd numbers. Then B would never encrypt a random number that it had generated (as in the above example.)

14. Secret Key Cryptography:There are several possible approaches to repairing the protocol • Initiator must prove its identity first. This is based on the assumption that the initiator in a protocol is likely to be the attacker. In the above examples, B would respond with a challenge of its own before responding to A's (T's) challenge. The reflection attack in this type of protocol would not work.