Online security (SSL and TLS)

Secure Socket Layer (SSL) and Transport Layer Security (TLS)

Secure Sockets Layer (SSL) and Transport Layer Security (TLS), its successor, are cryptographic protocols which provide secure communications on the Internet for such things as e-mail, Internet faxing, and other data transfers. There are slight differences between SSL 3.0 and TLS 1.0, but the protocol remains substantially the same. The term "SSL" as used here applies to both protocols unless clarified by context.

How it works

The SSL protocol exchanges records; each record can be optionally compressed, encrypted and packed with a message authentication code (MAC). Each record has a content_type field that specifies which upper level protocol is being used.

 

When the connection starts, the record level encapsulates another protocol, the handshake protocol, which has content_type 22.

 

The client sends and receives several handshake structures:

• It sends a ClientHello message specifying the list of cipher suites, compression methods and the highest protocol version it supports. It also sends random bytes which will be used later.

• Then it receives a ServerHello, in which the server chooses the connection parameters from the choices offered by the client earlier.

• When the connection parameters are known, client and server exchange certificates (depending on the selected public key cipher). These certificates are currently X.509, but there is also a draft specifying the use of OpenPGP based certificates.

• The server can request a certificate from the client, so that the connection can be mutually authenticated.

• Client and server negotiate a common secret called "master secret", possibly using the result of a Diffie-Hellman exchange, or simply encrypting a secret with a public key that is decrypted with the peer's private key. All other key data is derived from this "master secret" (and the client- and server-generated random values), which is passed through a carefully designed "Pseudo Random Function". TLS/SSL have a variety of security measures:

• Numbering all the records and using the sequence number in the MACs.
• Using a message digest enhanced with a key (so only with the key can you check the MAC). This is specified in RFC 2104).
• Protection against several known attacks (including man in the middle attacks), like those involving a downgrade of the protocol to previous (less secure) versions, or weaker cipher suites.
• The message that ends the handshake ("Finished") sends a hash of all the exchanged data seen by both parties.
• The pseudo random function splits the input data in 2 halves and processes them with different hashing algorithms (MD5 and SHA), then XORs them together. This way it protects itself in the event that one of these algorithms is found vulnerable.

  

Applications

SSL runs on layers beneath application protocols such as HTTP, FTP, SMTP and NNTP and above the TCP or UDP transport protocol, which form part of the TCP/IP protocol suite. While it can add security to any protocol that uses reliable connections (such as TCP), it is most commonly used with HTTP to form HTTPS. HTTPS is used to secure World Wide Web pages for applications such as electronic commerce. It uses public key certificates to verify the identity of endpoints.

 

While an increasing number of client and server products can support SSL natively, many still do not. In these cases, a user may wish to use standalone SSL products like Stunnel to provide encryption. However, the Internet Engineering Task Force recommended in 1997 that application protocols offer a way to upgrade to TLS from a plaintext connection, rather than use a separate port for encrypted communications - this prevents use of wrappers such as Stunnel.

 

SSL can also be used to tunnel an entire network stack to create a VPN, as is the case with OpenVPN.

TLS 1.1

As noted above, TLS 1.1 is the current approved version of the TLS protocol. TLS 1.1 clarifies some ambiguities and adds a number of recommendations. TLS 1.1 is very similar to TLS 1.0. The main reason for the new version number is a modified format for encrypted RSA pre-master secret, which is part of the client key-exchange message (if RSA is used), to use PKCS#1 v 2.1, as opposed to PKCS#1 v 1.5 in TLS 1.0. This is done to protect against an attack discovered by Daniel Bleichenbacher which can be launched against TLS 1.0 servers, using PKCS#1 v 1.5, which would fail in different ways depending if the decrypted format is correct or not. It also includes recommendations for avoiding remote timing attacks. A full list of differences between TLS 1.0 and TLS 1.1 is provided in RFC 4346 (Section 1.1).

 

TLS 1.1 is currently supported by Opera and GnuTLS.

 

11/20/2008 9:43:55 AM