Computer Networking Lecture Notes

15 April 2013 $ Transport Security

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Outline

• Transport security, motivations and origins.
• SSL service.
• Connection establishment.
  • Cryptographic choices.
  • SSL transport protocol.
• Transport-layer security.

Transport Security

• Security between the end-points.
  • Not security within.

Motiviations

• Pioneered (in some sense) by the Web.
• End-points engaging in sensitive transactions.

  

Origins

• Netscape introduced the Secure Sockets Layer (SSL) in 1995.
• Taken up by other browsers in due course.
• Eventually turned over to the IETF to become Transport-Layer Security (TLS).

Rules of the Game

• SSL security consists of
  • Responder authenticates to the initiator.
    • Usually “responder” means “server” and “initiator” means “client”.
    • Optionally, initiator authenticates to the responder.
  • Data are encrypted for privacy.
  • Communication integrity.
• Parameter negotiation between end-points.

SSL Service

• HTTP through SSL is called HTTPS (Secure HTTP).
  • Via port 443, not 80.
• SSL version 3 in what follows.

SSL Parameters

• During set-up, end-points can negotiate
  • Data compression or not.
  • Cryptographic algorithms used.
  • Cryptographic strength (to satisfy U.S.-imposed export restrictions).
    • Big-key (> 40 bits) encryption can’t cross U.S. boarders.

SSL Protocols

• SSL involves two sub-protocols:
  1. Establish a secure connection.
  2. Use the secure connection.

Connection Establishment

Connecting, Part 1

I: SSL version, preferences, N_I
R: SSL version, choices, N_R
R: X.509 certificate chain
R: Responder done

• N_I and N_R are nonces, random values.
• The initiator has a list of certificate authorities.
• Optionally, the responder can request the initiator’s X.509 certificate.

Connecting, Part 2

I: E_R premaster key
I: Change cypher
I: Finished

• The premaster key is a 384-bit random number.
  • The premaster key’s encrypted with the responder’s public key.
• The session key is a combination of the premaster key and the nonces.

Connecting, Part 3

R: Change cypher
R: Finished

• Each side has a new session key; secure connection established.
• The responder may use other mechanisms to further authenticate the initiator.

Cryptographic Choices

• Most SSL implementations use
  • Triple DES with three encryption keys, and
  • SHA-1 for data integrity.
• These are expensive and secure (?) algorithms. SSL implementations may also offer
  • RC4 with 128-bit keys, 88 bits revealed, for encryption, and
  • MD5 for data integrity.

SSL Transport Protocol

Comments

• The protocol operates the same way on both directions.
• MAC = hash(fragment ++ session key).
  • Hash function chosen during set-up.
• The last header identifies fragments.
• The result gets fed to the TCP connection.

Transport-Layer Security

• In 1996 Netscape gave SSL to the IETF to be standardized.
• The result was Transport-Layer Security (TLS, rfc 5246).
  • TLS is incompatible with SSL.
  • Most browsers provide both TLS and SSL.

Summary

• Transport security provides end-to-end security.
• Secure sockets layer (SSL) is an early and widely used secure transport service.
  • And not just for browsers.
• Most SSL protocol features are parameters.
• The IETF took over SSL development from Netscape, resulting in Transport-layer security (TLS).

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This page last modified on 2012 October 4.