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 Internet Local Network

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 Internet Local Network

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

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

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 Secure Communication IPsec Server

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

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 Protected Original Data Field Original IP Header Transport Security Header

IPsec Tunnel Mode –Adds a security header before 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 Protected Original Data Field Protected Original IP Header Tunnel Security Header

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

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

IPsec Modes and protection methods can be applied in any combination Tunnel Mode Transport Mode ESPSupported AHSupported

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 Host AHost B Negotiate Security Association

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

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 AHost B Security Association Negotiations Limited By Policies

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

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

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

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

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 AParty B

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 Party AParty B Nonce B Nonce A

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 Party AParty B Symmetric Key From nonces, independently compute same symmetric session key

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

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) Plaintext Block Previous Ciphertext Block To be Encrypted + Cipher Block Chaining

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

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

IPsec Adding Plaintext and Ciphertext together in DES-CBC –The bits are XORed –If the ciphertext is … –And the plaintext is … –The result is …

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

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

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

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 Original Plaintext HMAC

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 Original Plaintext Transmitted HMAC Key Computed HMAC Hashing Transmitted Original Plaintext

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