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SSL and HTTPS for Web Communication. CSCI 5857: Encoding and Encryption. Web Security Problems. Major concerns: Encryption of sensitive data sent between client and server Authentication of server How does client know who they are dealing with? Information integrity

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Ssl and https for web communication

SSL and HTTPS for Web Communication

CSCI 5857: Encoding and Encryption

Web security problems
Web Security Problems

Major concerns:

  • Encryption of sensitive data sent between client and server

  • Authentication of server

    • How does client know who they are dealing with?

  • Information integrity

    • How do we know third party has not altered data en route?

Bob’s web site

Alice thinks she is at Bob’s site, but Darth is spoofing it

Bob’s web site

Address information

Change so item shipped to Darth


  • Web sites that deal in ecommerce must have certificates for authentication

    • Installed at server

    • Transmitted to client for authentication

    • Validated using CA’s public key

Server machine


Request for secure session

Client machine

Web Container



Certificatesigned by CA


  • Public keys stored in browser

    • Can request from other CAs via public key infrastructure as needed

Secure socket layer protocol
Secure Socket Layer Protocol

  • Secure Socket Layer protocol for web communication

    • Latest upgrade: Transport Layer Security (TLS)

    • Same structure as SSL, somewhat more secure

Ssl protocol phase 1
SSL Protocol: Phase 1

Phase 1: Information exchange

  • Problem: Large number of encryption algorithms in use

    • How do client and server agree on which to use?

    • How does client tell server which ones it supports?

Ssl protocol phase 11
SSL Protocol: Phase 1

  • Client passes preferred algorithms to server via https request

    • Public key encryption algorithms

    • Private key encryption algorithms

    • Hash algorithms

    • Compression algorithms

    • Also random number for key generation

  • Server replies with algorithms that will be used

    • Also passes own random number

  • Ssl protocol phase 2
    SSL Protocol: Phase 2

    Phase 2: Server Identification and Key Exchange

    • Server passes their certificates to client

      • Client uses issuer public key to verify identity

      • Client retrieves server public key from certificate

      • Server may pass many certificates for authentication

    Ssl protocol phase 21
    SSL Protocol: Phase 2

    • If no certificate containing a public key, separate public key must be passed

    Certificate contains RSA public key, so no separate key passed

    No certificate, so Diffie-Hellman key exchange parameters passed

    Ssl protocol phase 22
    SSL Protocol: Phase 2

    • Server can also request appropriate client certificates to authenticate client

      • Online banking

      • Remote access to company database

    Ssl protocol phase 3
    SSL Protocol: Phase 3

    Phase 3: Client Identification and Key Exchange

    • Client sends certificate or public key if requested by server

    Ssl key generation
    SSL Key Generation

    • Client generates “pre-master key”

    • Sends to server encrypted with server public key

    • Client and server use to generate master key used to create cipher keys

      • Also use client, server random numbers exchanged in phase 1

    Ssl key generation3
    SSL Key Generation

    • Key material used to generate:

      • Keys for encryption and authentication (MAC)

      • IV’s for cipher block chaining

    Phase 4 final handshake
    Phase 4: Final Handshake

    Client and server verify protocols and keys

    • Sender signs/encrypts “finished” message

    • Receiver decrypts/verifies message to confirm keys

    Ssl data transmission
    SSL Data Transmission

    • Message broken into blocks

    • Block compressed

    • Compressed block hashed with authentication key to get MAC (message integrity)

    • Compressed block + MAC encrypted with cipher key

    • Encrypted block + record protocol header with version/length information sent

    Ssl data transmission1
    SSL Data Transmission

    • MAC algorithm is modifiedHMAC

      • Two stage hash with secret MAC key inserted at each stage

      • Values similar to IPAD and OPAD also inserted

    Sessions and ssl
    Sessions and SSL

    • Connection: single transmission between client and server

    • Session: set of connections for some purpose

      • Example: Ecommerce payment session: Credit card, Address, etc.

      • Often involves same https sessions

    • Can reuse same keys for all connections in session

      • Much more efficient than restarting SSL protocol each connection

      • Can still require for maximum security

    Https protocol
    Https Protocol

    • When started, requests secure session from server

      • Uses separate port in some servers

    • Invokes SSL protocol

    Https protocol1
    Https Protocol

    • Https protocol is expensive

      • Should not do unless necessary

    • Once done with secure transactions, should go back to using non-secure channel

      • Return to non-secure port