General

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Web Security (I)
CS 6262 Spring 04
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How the Web Works - HTTP

• Hypertext transfer protocol (http).
• Clients request documents (or scripts) through
  URL.
• Server response with documents.
• Stateless protocol, requests are independent.

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How the Web Works Other Elements

• Hyper-text markup language (html).
• Other application specific document.
• E.G., MIME, graphics, video/audio, postscript,
  Java applets, etc.
• Browsers.
• Display html documents and embedded graphics.
• Run Java program.
• Start helper applications.
• ...

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Web Vulnerabilities

• http//www.w3.org/Security/Faq
• Revealing private information on server
• Intercept of client information
• Execute unauthorized programs
• Denial of service
• ...

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Web Vulnerabilities cgi-bin

• cgi-bin server-side includes
• If random arguments are allowed
• Web server program should run under a particular
  UID (e.g., www, nobody)

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Web Security

• Authentication
• Basic (username, password)
• Can be used along with cookie
• Digest
• Access control via addresses
• Multi-layered
• S-http (secure http), just for http
• Proposed by CommerceNet, pretty much dead
• SSL (TLS), generic for TCP
• https http over SSL
• IPSec

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HTTP Authentication - Basic

• Client doesnt know which method
• Client attempts access (GET, PUT, ) normally
• Server returns
• 401 unauthorized
• Realm protection space
• Client tries again with (userpassword)
• Passwords in the clear
• Repeated for each access

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From Basic Authentication to Forms and Cookies

• Not all sites use basic authentication
• Many instead ask the user to type
  username/password into a HTML form
• Server looks up the user and sends back a cookie
• The browser (client) resends the cookie on
  subsequent requests

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HTTP Access Control - Digest

• Server sends www-authenticate parameters
• Realm
• Domain
• Nonce, new for each 401 response
• E.G. H(client-IPtimestampserver-secret)
• Algorithm
• E.G., MD5

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HTTP Access Control - Digest

• Client sends authorization response
• Same nonce
• H(A1), where A1userrealmpassword, and other
  information
• Steal H(A1)
• Only good for realm

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Web Server Access Configuration

• http//hoohoo.ncsa.uiuc.edu/docs/tutorials/user.ht
  ml
• .htaccess per directory
• Global configuration file access.conf

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SSL Overview

• For any TCP protocol HTTP (https// port 443),
  NNTP, telnet, etc.
• Secure byte stream.
• Optional (but common) public key server
  authentication.
• Optional client authentication.
• Hash combined MD5 and SHA.
• Encryption optional.
• Now TLS (IETF WG).

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SSL Architecture

• Two layers
• SSL record protocol provides basic security
  services
• 3 higher-layer protocols
• Handshake, change cipher spec, alert
• Connection
• A transport with some service, associated with a
  session
• Session
• Created by handshake, defines cryptographic
  security parameters for multiple connections

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Session and Connection

• Session parameters
• ID, peer certificate, compression method, cipher
  spec, master secret, is resumable.
• Connection parameters
• Server and client random, server write MAC
  secret, client write MAC secret, server write
  key, client write key, IV, sequence number.

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SSL Record Protocol

• 2 services
• Confidentiality, message integrity
• Layered protocol
• Fragment application data into blocks
• Compress data
• Apply message authentication code (MAC) h(sm)
  for message m and secret s
• Encrypt with client (cw) or server (sw) write key
• Transmit over TCP
• Specify content type for higher protocols

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Handshake Protocol

• Establish security capabilities
• Protocol version, session ID, cipher suite,
  compression method, IV
• Server authentication and key exchange
• Send certificate, key exchange, request client
  certificate
• Client authentication and key exchange
• Send certificate, key exchange, certificate
  verification
• Finish

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Cryptographic Computations

• Master secret creation.
• A pre-master-secret is exchanged first.
• RSA, or Diffie-Hellman.
• Both sides compute master secret based on
  pre-master-secret.
• Generation of cryptographic parameters.
• Client/server write MAC secrets, client/server
  write keys, client/server write IV are generated
  from master secret.

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Cryptographic Computations Details (1)

• Client generates a 48-byte pre-master-secret sp
• Master secret
• smMD5(spSHA(A sprcrs)) MD5(spSHA(BB
  sprcrs)) MD5(spSHA(CCC sprcrs))
• Where rc,s client, server random

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Cryptographic Computations Details (2)

• Session key same as above, but use the master
  secret in place of sp to generate byte stream to
  cut out
• Client, server MAC secret
• Client, server write key
• Client, server IV