Dr. Jinyuan (Stella) Sun Dept. of Electrical Engineering and Computer Science University of Tennessee Fall 2010

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1 Dr. Jinyuan (Stella) Sun Dept. of Electrical Engineering and Computer Science University of Tennessee Fall

2 Photuris and SKIP PHASE 1 IKE PHASE 2 IKE

3 How is SA established? How do parties negotiate a common set of cryptographic algorithms and keys to use? More than one SA can apply to a packet! E.g., end-to-end authentication (AH) and additional encryption (ESP) on the public part of the network

4 Purpose Mutual Authentication Shared Secret Establishment Crypto Algorithms Negotiation Security Association Establishment

5 Data transmitted needs to be secured IPsec SA, session keys, AH or ESP Messages for establishing IPsec SA need to be secured IKE SA, negotiated secret Negotiation for establishing IKE SA need to be authenticated Pre-shared secret key Public/private keys

6 Clogging attacks An opponent forges the source address of a legitimate user and send a public Diffie-Hellman key to the victim; The victim then performs a modular exponentiation to compute the secret key; Repeated messages of this type can clog the victim s system with useless work. First proposed in Photuris; Cookie A number chosen by responder; When receiving a request from S, send the cookie to S in clear; start the processing after the cookie comes back from the initiator. Stateless cookies The responder does not have to remember (store) the cookies he sent out; The cookie is a function of the IP address and a secret known to the responder

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9 Denial of service protection: Stateless cookie C B in message 2 Signed Diffie-Hellman exchange Signature on the previous message in message 5 and 6 Identity hiding Anonymous Diffie-Hellman Identities are encrypted in message 5 and 6 (for active man-in-the-middle, the initiator s identity is revealed, but not the responder)

10 SKIP: Simple Key Management for Internet Protocols SKIP uses long term Diffie-Hellman public keys Alice finds Bob s public key (g B mod p ) via a certificate from Bob or a directory. Bob finds Alice s public key (g A mod p ). Then they will have a common secret (g AB mod p). Data encryption Long term common secret should not be used to encrypt data. Instead each message has a SKIP header where the long term secret is used to encrypt a short-term data encryption key, which is used to encrypt the message.

11 Phase One Mutual authentication Session key establishment ISAKMP SA/IKE SA Phase Two Negotiating IPsec SAs (AH, ESP)

12 ISAKMP would be used by other protocols to set up SAs, not only to set up IPsec SAs. Phase 1 exchange is relatively expensive. ISAKMP/IKE SA has a longer timeout period. It can be used to negotiate multiple phase 2 IPsec SAs, which reduces the usage of pre-shared secret or private key.

13 Two modes Aggressive mode 3 messages Mutual authentication Session key establishment Main Mode 6 messages Mutual authentication Session key establishment Hiding endpoint identity Negotiating cryptographic algorithms

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16 Encryption algorithm (e.g., DES, 3DES, IDEA) Hash algorithm (e.g., MD5, SHA) Authentication method (e.g., pre-shared keys, RSA public key signature, DSS, RSA public key encryption) Diffie-Hellman group (e.g., g and p)

17 Alice sends Bob a list of proposals, each consisting of an encryption algorithm, a hash algorithm, authentication method, and a Diffie- Hellman group. Bob replies one as the accepted proposal. The parameters in the proposal are used in Phase 1 and Phase 2 (IKE SA), with hash algorithm used for various purposes.

18 Mutual authentication based on Pre-shared secret key Public encryption key Original protocol design Improved Protocol design Public signature key

19 IKE is stateful, starting from the first message. Alice s crypto proposal is in the identity proof ISAKMP requires randomly chosen cookies Identifier: <initiator cookie, responder cookie>

20 After Diffie-Hellman key exchange, each side knows g xy mod p Encryption key and Integrity key for the rest of IKE SA Keys for IPSec SAs

21 Pseudo Random function prf(key, data), e.g., CBC residue, HMAC SKEYID For signature public keys, prf(nonces, g xy ) For encryption public keys, prf(hash(nonces), cookies) For pre-shared secret keys, prf(pre-shared secret key, nonces) SKEYID_d: secret bits used to create other keys prf(skeyid, g xy cookies 0) SKEYID_a: the integrity protection key prf(skeyid, SKEYID_d g xy cookies 1) SKEYID_e: the encryption key prf(skeyid, SKEYID_d g xy cookies 2)

22 Proof of the key associated with the identity pre-shared secret key private encryption key private signature key Integrity-check on the previous messages, such as identity, Diffie-Hellman values, nonce, Alice s crypto proposal, and the cookies.

23 Alice s proof of identity prf ( SKEYID, g x g y cookies Alice s initial proposals Alice s identity ) Bob s proof of identity prf ( SKEYID, g x g y cookies Alice s initial proposal Bob s identity )

24 8 phase-1 protocols 2 modes 4 types of keys Common features Message 1 starts with Alice s cookie All other messages start with (initiator cookie, responder cookie), which serves as the IKE connection identifier

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33 Establish IPSec SAs (e.g., ESP and/or AH) Crypto parameters Diffie-Hellman numbers (optional) Traffic type (optional) All messages (except X,Y) encrypted and integrity protected

34 [Kaufman] Chapter 18

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