May 4, 2005

Secure Socket Layer

Introduction
HTTPS is HTTP running over Secure Sockets Layer (SSL).
SSL (now up to version 3.0) is a standard protocol proposed by Netscape for implementing cryptography and enabling secure transmission on the Web

The primary goal of the SSL protocol is to

  • provide privacy and reliability between two communicating parties.

The two security aims of SSL are

  • To authenticate the server and the client using public key signatures and digital certificates.
  • To provide an encrypted connection for the client and server to exchange messages securely

SSL runs at the application layer.
SSL uses

  • certificates,
  • private/public key exchange pairs and
  • Diffie-Hellman key agreements

In SSL,

  • Symmetric cryptography is used for data encryption
  • Asymmetric or public key cryptography is used to authenticate the identities of
    the communicating parties and encrypt the shared encryption key when an SSL session is established.

SSL is comprised of three protocols:

  • record protocol
  • handshake protocol
  • alert protocol

The record protocol defines the way that messages passed between the client and servers are encapsulated. At any point in time it has a set of parameters associated with it, known as a cipher suite, which defines the cryptographic methods being used.

The handshake protocol runs on top of the SSL Record protocol. It defines a series of messages in which the client and server negotiate the type of connection that they can support, perform authentication, and generate a bulk encryption key. During a typical SSL session, the server and client exchange several Handshake protocol messages during the transaction. Depending on the chosen encryption type, a server using the SSL protocol uses public-key encryption technologies to authenticate itself to the client.

The alert protocol also runs over the SSL Record protocol. The SSL Alert protocol signals problems with the SSL session ranging from simple warnings (e.g., unknown certificate, revoked certificate, expired certificate) to fatal error messages that immediately terminate the SSL connection. For example, you might receive the You are about to leave a secure Internet connection warning because an SSL client received a closure_notify alert from an SSL server.

Operation of SSL

The client initiates an HTTP request for an SSL tunnel calling HTTPS directly.
By default, SSL uses a number of ports including 443, 643, 1443 and 2443.
For encryption SSL uses

  • RC4-128,
  • Diffie-Hellman 1024,
  • MD5 and
  • Null.

The encryption is carried out at layer 4 i.e. the socket layer.

The major elements in an SSL connection are:

1) The cipher suites that are enabled
2) The compression methods that can be used (the compression algorithms are used to compress the SSL data and should be lossless)
3) Digital certificates and private keys, used for authentication and verification
4) Trusted signers (the repository of trusted signer certificates, used to verify the other entities’ certificates)
5) Trusted sites (the repository of trusted site certificates)

SSL Handshake

The steps involved in an SSL transaction before the communication of data
begins are described in the following list:
1) The client sends the server a Client Hello message. This contains a request for a connection along with the client capabilities, like the version of SSL, the cipher suites and the data compression methods it supports.
2) The server responds with a Server Hello message. This includes the cipher suite and the compression method it has chosen for the connection and the session ID for the connection. Normally, the server chooses the strongest common cipher suite. If the server is unable to find a cipher suite that both the client and server support, it sends a handshake failure message and closes the connection.
3) The server sends its certificate if it is to be authenticated, and the client verifies it. Optionally the client sends its certificate and the server verifies it.
4) The client sends the ClientKeyExchange message. This is random key material, and it is encrypted with the server’s public key. This material is used to create the symmetric key to be used for this session, and the fact that it is encrypted with the server’s public key is to allow a secure transmission across the network. The server must verify that the same key is not already in use with any other client. If this is the case, the server asks the client for another random key.
5) When client and server agree on a common symmetric key for encrypting the communication, the client sends a ChangeCipherSpec message indicating the confirmation that it is ready to communicate. This message is followed by a Finished message.
6) In response, the server sends its own ChangeCipherSpec message indicating the confirmation that it is ready to communicate. This message is followed by a Finished message.

7) Client and Server exchange the encrypted data.

The problems associated with SSL are:

  • It prevents caching.
  • Using SSL imposes greater overheads on the server and the client.
  • Some firewalls and/or web proxies may not allow SSL traffic.
  • There is a financial cost associated with gaining a Certificate for the server/subject device.

2 comments:

ckarthi said...

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