Introduction to Information Security

Authentication and Key-Exchange Protocols

• A weakness of the Denning and Sacco protocol
• Harry can trick Diane into thinking that she is communicating with Carla when she is really communicating with Harry
• Harry establishes a session key, KCH, with Carla

C: => H(Encrypt(Encrypt((KCH,TC),CPrivate),HPublic));

• Harry decrypts Carla’s message and learns KCH
• Harry encrypts Carla’s signed message with Diane’s public key, and sends the result to Diane claiming to be Carla

H: => D(Encrypt(Encrypt((KCH,TC),CPrivate),DPublic));

• Diane will decrypt the message, check the signature and timestamp, and believe that she is talking to Carla with KCH as the session key

Authentication and Key-Exchange Protocols

• Fixing the Denning and Sacco protocol:
• Add the other party’s name to the key exchange message:
• C: => D(Encrypt(Encrypt((D,KCD,TC),CPrivate),DPublic));

Computer Systems Security

Introduction to cryptanalysis

Crypto

• Cryptology  The art and science of making and breaking “secret codes”
• Cryptography  making “secret codes”
• Cryptanalysis  breaking “secret codes”
• Crypto  all of the above (and more)

How to Speak Crypto

• A cipher or cryptosystem is used to encrypt the plaintext
• The result of encryption is ciphertext
• We decrypt ciphertext to recover plaintext
• A key is used to configure a cryptosystem
• A symmetric key cryptosystem uses the same key to encrypt as to decrypt
• A public key cryptosystem uses a public key to encrypt and a private key to decrypt
• Private key can be used to sign and public key used to verify signature (more on this later…)