1. Secure Messenger Protocol using AES (Rijndael) Sang won, Lee 2001807 swlee@icu.ac.kr

2. 2 Contents Introduction Security Consideration AES (Rijndael) SIM Protocol System model Conclusion

3. 3 Introduction Instant Messenger How does it work? Through server routing Direct communication Combination Security of Instant messenger Messenger with security module is rare. Security of instant messenger is important. Because using messenger is spread to business market.

4. 4 Security Consideration Privacy User’s information should be kept safely. Confidentiality All message are encrypted before sending. Authentication Only authentication user can use SIM service Integrity Message sent by user should not be modified or forged by other. Efficiency SIM use hybrid cryptosystem. Asymmetric cryptosystem is used in key agreement and digital signature. Symmetric cryptosystem is used in message encryption and decryption.

5. 5 AES (Rijndael) SIM use Rijndael for encryption and decryption of message. 4x4 byte array Input whiteningByteSubShiftRowMixColumnAddRoundKey ByteSubShiftRowAddRoundKey Nr-1 rounds Final round

6. 6 SIM Protocol User Registration User know {n, g} User generates RSA public key and private key. User sends public key with user information. Server receives and saves this information. Server responses to client if registration is ok or not. If server’s response is ok, User saves one’s key pair. If fail, retry. {n, g} ClientServer Generate PK, SK {PK, Info} Check Info Save {PK, Info} R If Ok, Save If Fail, Retry

7. 7 SIM Protocol User Login User loads one’s key pair. User know {n, g} 1. User computes RSA Dec{ID} Sk. 2. User sends ID and decryption result. 3. Server verify ClientServer Load Key pair {PK,SK} H = H(ID) Y = Dec{H} SK ID, Y R Verify H(ID) = Enc{Y} PK {n, g}

8. 8 SIM Protocol Key Agreement 1. User generates random integer x. 2. User computes g x. 3. User request friends list to server. 4. Server confirms whether user’s friends is online or not and sends friends list with online conformation information. 5. User receives one’s friends list. 6. If friend is online. User sends g x to friend. User receives g y. User computes session key g xy. User saves session key with ID. Request ClientServer Compute g x Friends List gygy S = g xy Generate Random x Confirm Friends List gxgx Client Diffie-Hellman Key agreement S = g xy

9. 9 SIM Protocol Message Send & Receive Message can be encrypted or decrypted with shared session key by Rijndael

10. 10 System model Client Messenger Encrypted Message Diffie-Hellman Key Agreement Key Client Friend List Messenger Server Storage Login Session Key Session Key Encryption Decryption

11. 11 Conclusion Multiparty key agreement should be supported. Two party key agreement does not support something like chatting. Database should be used for key management. Key management using Database is important for security. Key size should be longer than 128bit. The size of RSA for signature is 128bit in SIM. That is short. Exchange with off-line user should be considered SIM only can exchange message with on-line user.