Secure Sockets Layer

Reference:

• Calin Groza, "WWW Security," in WWW Beyond the Basics, Chapter 18.3.
• "SSL 3.0", http://home.netscape.com/eng/ssl3/index.html, 1996.

(accessed June 11, 1997)
• SSL, 1996, Secure Sockets Layer Discussion FAQ,  http://www.faqs.org/faqs/computer-security/ssl-talk-faq (accessed Nov. 13, 2000).
• E. Rescorla, A. Schiffman, The Secure HyperText Transfer Protocol, IETF Internet Draft, August 1999.
• L. Stein, How to Set up and Maintain a Web Server, 2nd. ed., 1997, pp. 746-747.
• Netscape, How SSL Works (accessed Nov. 13, 2000).

Introduction

Features

• SSL gives application programmer familiar socket interface, with cryptography added transparently.
• SSL lies between application and transport protocols to create secure and reliable communication.
• SSL is an open, nonproprietary protocol.
• SSL has been submitted to W3C working group on security for consideration as a standard

Goals

• Cryptographic security
• Interoperability:  SSL3.0 client from company A should work with SSL3.0 server company B
• Extensibility:
• "provide framework into which new public key and bulk encryption methods can be incorporated as necessary"
• avoids need to implement entirely new API when new cryptographic methods are developed.
• Permits efficiency:  Implementation of SSL can minimize CPU and network use:
• Try to use private key encryption for normal data transit, rather than public key.
• Optionally cache sessions to reduce connection establishment time

Basic Properties

1. No one can eavesdrop on connection. Encryption is used after an initial handshake to define cryptographic protocol. Secret-key cryptography is used for data encryption (e.g. DES, RC4, etc.).
2. No one can modify messages sent over connection. Message transport includes message integrity check using secure hash functions (e.g. SHA, MD5).
3. You can authenticate who you're connected to. Peer's identity can be authenticated using public-key cryptography (e.g., RSA, DSS).

Two Parts to SSL

1. SSL Handshake Protocol establishes secure channel.
2. SSL Application Data Protocol is used to exchange data over channel.

SSL Handshake Protocol

The protocol consists of a sequence of steps:

1. Client Hello

ClientHello(CipherSuite[], CompressionMethod[], ClientRandom)

One CipherSuite defines three encryption protocols:
1. key-exchange protocol: RSA, Diffie-Hellman
2. secret-key encryption algorithm: null, RC4, RC2, DES, DES40, fortezza
3. cryptographic hash algorithm: null, MD5, SHA

2. Server Hello

Server selects cipher suite and compression method, and informs client of decision by sending:

ServerHello(CipherSuite, CompressionMethod, ServerRandom)

Remaining protocol is used to agree on secret keys.

3. Key Exchange

• Server has certificate - so server sends to client certificate containing its public key:

ServerCertificate(Certificate)

The certificate contains the server's public key, which is used in the response message shown below.

However, the client won't use the server's public key for encryption of subsequent data.

• Instead, client generates 48 byte (time + random) master secret of this session.
• Future keys during session are generated from master secret. Random data is used to prevent an attacker from eavesdropping on the network, and then replaying a past transmission!
Client then sends master secret to server:

encrypt(MasterSecret) using ServerPublicKey

• Server does not have a certificate:

 

 

Server initiates key-exchange protocol, (e.g., Diffie-Hellman). After exchange of three messages, server and client have the master secret. Based on master secret both parties create keys used in communication.

4. Finished

Client to Server:
hash(AllMessagesSentByClient + MasterSecret)
Server to Client:
hash(AllMessagesSentByServer + MasterSecret)

SSL Application Data Protocol

1. Client sends a request to server as follows:

encrypt(ClientRequest + hash(ClientRequest+MasterSecret)) using ClientWriteKey

Example:

encrypt("Transfer $100 to my savings account" + hash("Transfer $100 to my savings account" + MasterSecret))
 

2. Server decrypts request, prepares the response, and sends it to client as follows:

Example:

encrypt("Transfer complete" + hash("Transfer complete"))

Alternative: S-HTTP

SHTTP was proposed in 1994 by Enterprise Integration Technologies (EIT) and later adopted by CommerceNet, a coalition of businesses interested in commerce on the Internet.

SHTTP only works with HTTP, in contrast to SSL.

SHTTP is implemented in few (lesser known) browsers and servers.

Another Protocol:  S/MIME

• Provides secure electronic mail
• Developed by RSA
• E-mail clients (e.g., Netscape) must implement S/MIME protocol
• S/MIME offers users the following features:

 
• message privacy
• digital signatures
• tamper detection
• interoperability among vendors/platforms
• seamless integration: decryption is transparent to user reading mail; encrypting mail you send is achieved by clicking a button