CS 4803 Computer and Network SecurityAlexandra (Sasha) BoldyrevaIPsec1Network layers•Application•Transport•Network•Lower level2Roughly…•Application layer: the communicating processes themselves and the actual messages transmitted•Transport layer: handles transmissions on an “end-to-end” basis•Network layer: handles transmissions on a “hop-by-hop” basis3Examples•Application layer: PGP, SSH•Transport layer: SSL/TLS•Network layer: IPsec•Security at the lower layer? 4Security in what layer?•Depends on the purpose…•What information needs to be protected?•What is the attack model?•Who shares keys in advance?•Should the user be involved?•E.g., a network-layer protocol cannot authenticate two end-users to each other•An application-layer protocol cannot protect IP header information•Also affects efficiency, ease of deployment, etc.5Example: PGP vs. SSL vs. IPsec•PGP is an application-level protocol for “secure email”•Can provide security on “insecure” systems•Users choose when to use PGP; user must be involved•Alice’s signature on an email proves that Alice actually generated the message, and it was received unaltered; also non-repudiation•In contrast, SSL would secure “the connection” from Alice’s computer6Example: PGP vs. SSL vs. IPsec•SSL sits “on top of” the transport layer•End-to-end security, best for connection-oriented sessions•User does not need to be involved•The OS does not have to be changed•Easy to modify applications to use SSL•If SSL rejects packet accepted by TCP, then TCP rejects “correct” packet when it arrives!• SSL must then close the connection…7Example: PGP vs. SSL vs. IPsec•IPsec sits “on top of” the network layer•End-to-end or hop-by-hop security• Best for connectionless channels•Need to modify OS•All applications are “protected” by default, without requiring any change to applications or actions on behalf of users•Can only authenticate hosts, not users•User completely unaware that IPsec is running8Take home message…•Best solution may involve changes at both the OS and applications layers•The “best” solution is not to run SSL and IPsec!•Would have been better to design system with security in mind from the beginning…•(Keep in mind for future systems…)9OverviewIPSec = AH + ESP + IKEProtection for IP trafficAH provides integrity and origin authenticationESP also confidentialitySets up keys and algorithmsfor AH and ESP10Security associations (SAs)•An SA is a crypto-protected connection•One SA in each direction…•At each end, the SA contains a key, the identity of the other party, the sequence number, and crypto parameters•IPsec header indicates which SA to use•Parties will maintain a database of SAs for currently-open connections•Used both to send and receive packets11AH vs. ESP•Authentication header (AH)•Provides integrity only•Encapsulating security payload (ESP)•Provides encryption and/or integrity•Both provide cryptographic protection of everything beyond the IP headers •AH additionally provides integrity protection of some fields of the IP header12Transport vs. tunnel mode•Transport mode: add IPsec information between IP header and rest of packet•Most logical when IPsec used end-to-endIP header(real dest)IPSec header TCP/UDP header + data13Transport vs. tunnel mode•Tunnel mode: keep original IP packet intact; add new header information• •Can be used when IPSec is applied at intermediate point along path (e.g., for firewall-to-firewall traffic)• E.g., change source/destination info…•Results in slightly longer packetIP header(gateway)IPSec header TCP/UDP header + dataIP header(real dest)14Tunnel mode illustrationImplementsIPSecImplementsIPSec15More on AH•AH provides integrity protection on header•But some fields change en route!•Only immutable fields are included in the integrity check•Mutable but predictable fields are also included in the integrity check•E.g., payload length•The final value of the field is used16More on AH vs. ESP•Recall that ESP provides encryption and/or authentication•So why do we need AH?•AH also protects the IP header•Export restrictions•Firewalls need some high-level data to be unencrypted•None of these are compelling…17The future of IPsec?•In the long run, it seems that AH will become obsolete•Better to encrypt everything anyway•No real need for AH•Certain performance disadvantages•AH is complex…•Etc.•IPsec is still
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