IPsec

1. IPsec • IPsec (IP security) • Security for transmission over IP networks • The Internet • Internal corporate IP networks • IP packets sent over public switched data networks (PSDN) Local Network Local Network Internet

2. IPsec • Why do we need IPsec? • IP has no security • Add security to create a virtual private network (VPN) (Chapter 9) to give secure communication over the Internet or another IP network Local Network Local Network Internet

3. IPsec • Genesis • Being created by the Internet Engineering Task Force • For both IP version 4 and IP version 6

4. IPsec • Two Modes of operation • Tunnel Mode • IPsec server at each site • Secures messages going through the Internet Local Network Internet Local Network IPsec Server Secure Communication

5. IPsec • Tunnel Mode • Hosts operate in their usual way • Tunnel mode IPsec is transparent to the hosts • No security within the site networks Local Network Internet Local Network IPsec Server Secure Communication

6. IPsec • Two Modes of operation • Transport Mode • End-to-end security between the hosts • Security within site networks as well • Requires hosts to implement IPsec Local Network Internet Local Network Secure Communication

7. IPsec • Transport Mode • Adds a security header to IP packet • After the main IP header • Source and destination addresses of hosts can be learned by interceptor • Only the original data field is protected Original IP Header Transport Security Header Protected Original Data Field

8. IPsec • Tunnel Mode • Adds a security headerbefore the original IP header • Has IP addresses of the source and destination IPsec servers only, not those of the source and destination hosts • Protects the main IP header Tunnel Security Header Protected Original IP Header Protected Original Data Field

9. IPsec • Can combine the two modes • Transport mode for end-to-end security • Plus tunnel mode to hide the IP addresses of the source and destination hosts during passage through the Internet Local Network Internet Local Network Tunnel Mode Transport Mode

10. IPsec • Two forms of protection • Encapsulating Security Protocol (ESP) security provides confidentiality as well as authentication • Authentication Header (AH) security provides authentication but not confidentiality • Useful where encryption is forbidden by law • Provides slightly better authentication by providing authentication over a slightly larger part of the message, but this is rarely decisive

11. IPsec • Modes and protection methods can be applied in any combination

12. IPsec • Security Associations (SAs) are agreements between two hosts or two IPsec servers, depending on the mode • “Contracts” for how security will be performed • Negotiated • Governs subsequent transmissions Negotiate Security Association Host A Host B

13. IPsec • Security Associations (SAs) can be asymmetrical • Different strengths in the two directions • For instance, clients and servers may have different security needs SA for messages From A to B Host A Host B SA for messages From B to A

14. Security Association Negotiations Limited By Policies IPsec Policies may limit what SAs can be negotiated • To ensure that adequately strong SAs for the organization’s threats • Gives uniformity to negotiation decisions Host A Host B

15. IPsec • First, two parties negotiate IKE (Internet Key Exchange) Security Associations • IKE is not IPsec-specific • Can be used in other security protocols Communication Governed by IKE SA Host A Host B

16. IPsec • Under the protection of communication governed by this IKE SA, negotiate IPsec-specific security associations Communication Governed by IKE SA Host A Host B IPsec SA Negotiation

17. IPsec • Process of Creating IKE SAs (and other SAs) • Negotiate security parameters within policy limitations • Authenticate the parties using SA-agreed methods • Exchange a symmetric session key using SA-agreed method • Communicate securely with confidentiality, message-by-message authentication, and message integrity using SA-agreed method

18. IPsec • IPsec has mandatory security algorithms • Uses them as defaults if no other algorithm is negotiated • Other algorithms may be negotiated • But these mandatory algorithms MUST be supported

19. IPsec • Diffie-Hellman Key Agreement • To agree upon a symmetric session key to be used for confidentiality during this session • Also does authentication (not discussed) Party A Party B

20. IPsec • Diffie-Hellman Key Agreement • Each party sends the other a nonce (random number) • The nonces will almost certainly be different • Nonces are not sent confidentially Nonce B Party A Party B Nonce A

21. IPsec • Diffie-Hellman Key Agreement • From the different nonces, each party will be able to compute the same symmetric session key for subsequent use • No exchange of the key; instead, agreement on the key Symmetric Key Symmetric Key From nonces, independently compute same symmetric session key Party A Party B

22. IPsec • Mandatory algorithm for confidentiality is DES-CBC • DES with Cipher Block Chaining • An extension of DES (Data Encryption Standard) • Straight DES always gives the same ciphertext for the same plaintext and key • This allows certain types of attacks to guess passwords

23. IPsec • DES-CBC (DES Cipher Block Chaining) • DES works in blocks of 64 bits • DES-CBC begins with 64-bit plaintext to be encrypted • Combines with the ciphertext output from the previous block (cipher block chaining) Cipher Block Chaining Plaintext Block Previous Ciphertext Block Block To be Encrypted +

24. IPsec • DES-CBC • Encrypts the plaintext block plus previous ciphertext block to give ciphertext for the current block • This gives different ciphertexts for the same plaintext and key on different occasions Block To be Encrypted Ciphertext For Block DES Encryption

25. IPsec • Adding Plaintext and Ciphertext together in DES-CBC • The bits are XORed • The result is 1 if one bit (but not both) is 1 • 1 XOR 0 = 1 • 0 XOR 1 = 1 • The result is 0 if both bits are 1 or 0 • 1 XOR 1 = 0 • 0 XOR 0 = 0

26. IPsec • Adding Plaintext and Ciphertext together in DES-CBC • The bits are XORed • If the ciphertext is 111000 … • And the plaintext is 101010 … • The result is 010010 …

27. IPsec • HMAC • key-Hashed Message Authentication Code • Mandatory IKE message-by-message authentication and message integrity algorithm • Not a digital signature • HMAC does not use public key encryption • So it is faster than digital signature authentication, which uses public key encryption

28. IPsec • HMAC • Begins with original plaintext • Adds a secret HMAC key that only the communicating partners know • It is a shared secret • Usually different from the symmetric key used to send the entire message confidentiality Original Plaintext HMAC Key

29. IPsec • HMAC • Hashes the combination with MD5 or SHA1 • This gives the HMAC • Get different HMACs with different HMAC keys Original Plaintext HMAC Key HMAC Hashing

30. IPsec • HMAC • The HMAC is added to the original plaintext • Gives authentication and message integrity • HMAC is similar to digital signature • However, hashes instead of using public key encryption, so processing is faster HMAC Original Plaintext

31. IPsec • HMAC • Receiver again hashes plaintext message plus shared secret HMAC key • If the same as transmitted HMAC, sender is authenticated because the sender knows the shared secret HMAC key Transmitted HMAC Transmitted Original Plaintext Transmitted Original Plaintext HMAC Key Computed HMAC Hashing

32. IPsec • IPsec only uses symmetric key encryption and hashing, which are very fast • Avoids public key encryption, which is very slow • Diffie-Hellman key exchange instead of sending session key encrypted with receiver’s public key • HMAC instead of digital signatures • This allows IPsec to be fairly fast, reducing host or IPsec security server costs