References

Garfinkel and Spafford, Web Security and Commerce, chap. 12, O'Reilly 1997

Tanenbaum 3rd edition section 7.1.6, 7.4.5

Web pages at Netscape Communications Corporation - Technical Documentation

Web pages about PGP.

Applications of hybrid encryption for security for network applications

The combination of private key (symmetric) and public key (asymmetric) encryption appears in two widely used security enhancements for electronic mail - PGP - and Web browsing - SSL.

PGP - Pretty Good Privacy

PGP is a hybrid system for sending enciphered, digitally signed messages usually by email.

Features:

• combination of algorithms in a set of utility software for
• encryption of messages, digests, and keys
• message digest used for digital signature
• key generation for private session key
• key generation for users' public/private key pairs
• key management and certification

References

G&S p. 213 (brief)

Tanenbaum pp 664-667

<URL: http://world.std.com/~franl/pgp/> - collection of resources on PGP

PGP for Absolute Beginners

User's Guide

Beginner's Guide

Sending a message

1. Attach message signature

128 bit message digest plus timestamp
is enciphered with sender's private key

2. Compress message + digest

Removes redundancy - makes more secure (harder to attack)
and makes message smaller

3. Create session key

4. Symmetric encryption for message contents and signature

Method is not DES - uses IDEA (see Tanenbaum p 596).

Uses session key.

5. Encrypt session key

The session key is encrypted using RSA on receiver's public key.

6. Transmit (4) & (5)

Session key - digest and encryption

• a session key is generated for each message sent
• changes every time
• made different by timestamp
• made different by timing how user types input on the spot
• key is sent to recipient using receiver's public key + RSA (asymmetric)
• message is sent using session key +IDEA (symmetric encryption)
• message includes digital signature using MD5 and RSA
• authentication
• non-repudiation
• integrity

PGP key management summary

• a public key/private key pair
  is generated by PGP utility software for user on request
• user's private key is stored securely on user's disk,
  encrypted using user's pass phrase
  (from user's wetware memory)
• user's public key is made available in a record for giving away to others
  (user id, public key, timestamp)

Certification is a key concept

Client/user will trust a public key if can get a Certificate for that key

Certificates can be checked by decryption with this trusted person's public key.

This person may be a CA (Certificate Authority).

Any accepted certificates are kept as trusted public keys in a public keyring file
and can be used automatically to check any later incoming certificates.

Leads to a network of trust building up.

PGP Standardisation and legal issues

Legal Issues

Not allowed to use within some countries by law (e.g. France)

Early version violates some USA patents (in USA only)

Zimmerman may be on trial for "exporting munitions"
but see New Scientist this week,

Standards

PGP is not a standard, but is freely available.

Algorithms are open.

Techno-politics

A political agenda is evident in the documentation.

Privacy - independence from government

SSL - Secure Socket Layer

Netscape Communications Corporation proprietary protocol.

see <URL:http://home.netscape.com/newsref/std/ssl_2.0_certificate.html>
and <URL:http://home.netscape.com/newsref/ref/128bit.html>

Protocol is built into Netscape browsers (and servers).

Provides

• privacy by encryption
• authentication by certificates and public/private keys
• data integrity by digital signature

SSL is a replacement for the socket layer - i.e. transport layer -
not specific to HTTP alone.

Available servers listen on different ports.

Transparent to user in operation

Appears within HTML as replacement of URL

http://somewhere.org/thing.html

with

https://somewhere.org/thing.html

or use in HTML form for ACTION

<form method=POST
action="https://abc.com/cgi_bin/get_credit-card_no" ...>

SSL operation

Initial handshake between client and server when making connection

• server sends certificate containing ID and public key to client
  - certificate is RSA public key encrypted with a CA's private key
• client checks certificate against own certificate database
  or else checks signature of CA on the certificate - Certificate Authority

User can accept certificates and build up own list in client database.

Netscape builds in initial list of trusted Certificate Authorities
see Netscape browser->options->Security preferences->Site certificates

• length/type of secret session keys and algorithm to use are negotiated between client and server
• client creates 4 x private keys for session with a server
• client sends 4 keys to server encrypted with server's public key (known from certificate)
• client sends requests
  - encrypted - RC2, RC4 or DES (using key K1)
  - digitally signed - MD5 or SHA-1, RSA (using key K2)
• server sends responses similarly encrypted (K3), digitally signed (K4)

For efficiency a session can contain several HTTP requests with a server.

SSL - standardisation and legal issues

Legal issues

SSL is exported as crippled outside USA
(< 512 bit RSA keys; <40 bit secret keys)
so is not barred by USA export restrictions.

Standard

SSL has been submitted to Internet Engineering Task Force (IETF) as a draft standard - like an RFC.

Techno-politics

Netscape interests in commerce on the Web - want to build trust for transmission of Credit Card details etc.

- commercial reasons for them to make the protocol as (apparently) openly trustworthy as possible