cmsc 414 computer and network security lecture 26
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CMSC 414 Computer and Network Security Lecture 26

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CMSC 414 Computer and Network Security Lecture 26. Jonathan Katz. Administrivia. Final exam reminder + study guide DSS students contact me. Network security protocols in practice. Network layers. Application Transport Network Data link Physical. Roughly….

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Presentation Transcript
  • Final exam reminder + study guide
    • DSS students contact me
network layers
Network layers
  • Application
  • Transport
  • Network
  • Data link
  • Physical
  • Application layer: the communicating processes themselves and the actual ‘content’ transmitted
  • Transport layer (TCP/UDP): “end-to-end” communication; reliability; flow control
  • Network layer (IP): “host-to-host” communication; routing
  • Data link layer (Ethernet/WiFi): transmission of frames over a single hop
example security protocols
Example security protocols
  • Application layer: PGP, SSH
  • Transport layer: SSL/TLS
  • Network layer: IPsec
  • Data link layer: IEEE 802.11 (WEP, WPA)
security in what layer
Security in what layer?
  • Depends on the purpose…
    • How are keys provisioned/shared?
    • Should the (human) user be involved?
    • Semantics: authenticate user-to-user, or host-to-host?
security in what layer1
Security in what layer?
  • Depends on what’s available
    • E.g., consider a user connecting to a website from a café (over a wireless network)
    • End-to-end encryption might be unavailable (e.g., if website does not support encryption)
    • Eavesdropping on Internet backbone less likely than eavesdropping on wireless link in café
    • Encrypt link from user to wireless router
    • Link-layer encryption more appropriate
      • Link-layer authentication also possible
security in what layer2
Security in what layer?
  • Depends on the threat model/what threats are being addressed
    • What information needs to be protected? (Ports, IP addresses?)
    • E.g., network-layer authentication will not prevent DoS attacks at link level (e.g., ARP spoofing, replay disconnect messages, overloading access point)
    • E.g., an application-layer protocol cannot protect IP header information
    • End-to-end, or hop-by-hop?
security in what layer3
Security in what layer?
  • Security interactions with various layers
    • E.g., if TCP accepts a packet which is rejected by the application above it, then TCP will reject the “correct” packet (detecting a replay) when it arrives!
    • E.g., if higher-layer header data is used by a firewall to make decisions, this is incompatible with network-layer encryption (if it encrypts headers)
  • When security is placed at lower levels, it can provide automatic, “blanket” coverage…
    • …but it can take a long time before it is widely adopted
    • Can be inefficient to encrypt everything
  • When security is placed at higher levels, individual users can choose when to use it…
    • …but users who are not security-conscious may not take advantage of it
    • Can encrypt only what is necessary
example pgp vs ssl vs ipsec
Example: PGP vs. SSL vs. IPsec
  • PGP is an application-level protocol for “secure email”
    • Can provide security over insecure networks
    • 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 secures “the connection” from Alice’s computer; would need additional mechanisms to authenticate the user
    • Communication with off-line party (i.e., email)
example pgp vs ssl vs ipsec1
Example: PGP vs. SSL vs. IPsec
  • SSL sits at the transport layer, “above” TCP
    • Packet stream authenticated/encrypted
    • End-to-end security, best for connection-oriented sessions (e.g., http traffic)
    • User does not need to be involved
    • The OS does not have to change, but applications do if they want to communicate securely
example pgp vs ssl vs ipsec2
Example: PGP vs. SSL vs. IPsec
  • IPsec sits at the network layer
    • Individual packets authenticated/encrypted
    • End-to-end or hop-by-hop security
    • Need to modify OS
    • All applications “protected” by default, without requiring any change to applications or actions on behalf of users
    • Only authenticates hosts, not users
    • User can be completely unaware that IPsec is running
brief history
Brief history…
  • SSLv2 deployed in Netscape 1.1 (1995)
  • Modified version of SSLv3 standardized as TLS
  • This overview will not focus on the differences; I just say “SSL” for convenience
  • SSL is a major success story!
    • Used extensively and (almost) exclusively to secure web traffic
broad overview
Broad overview
  • SSL runs on top of TCP
    • Advantage: does not require changes to TCP
  • From the programmer’s point of view, it sits at the transport layer
    • Same API as for TCP
    • Runs only with TCP, not UDP
  • Primarily used for HTTP traffic
ssl overview
SSL overview
  • Three phases
    • Handshake
    • Key derivation
    • Data transfer
handshake key derivation
Handshake + key derivation
  • Client sends list of supported crypto algorithms and nonce RC
  • Server sends a certificate, selects a crypto algorithm, and sends nonce RS
    • Nonce protects against client impersonation
  • Client encrypts random K with server’s public key
  • Client/server derive session keys from RC, RS, K
    • Prevents replay attacks
  • Client sends a MAC of the handshake; server responds with the same
sessions and connections
Sessions and connections
  • One session may have multiple simultaneous connections
  • Can derive per-connections keys by exchanging fresh RC, RS, and using session (master) key K
data transfer
Data transfer
  • Client and server use K to establish four keys: encryption and authentication, for each direction
  • SSL breaks data stream into records; appends a MAC to each record; and then encrypts the result
    • Mac-then-encrypt…
    • What would have been a better choice?
  • The MAC is computed over the record plus a sequence number
    • Prevents replay, re-ordering, or dropping packets
ssl security
SSL security
  • Provides confidentiality, integrity, and unidirectional authentication
    • Client wants to make sure they are talking to the right merchant; merchant does not care who client is
  • Mutual authentication supported
    • If server requests it, and client has a certificate
    • Note that mutual authentication may be done at application level (e.g., password-based login)