Transport Layer Security

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Transport Layer Security (TLS) and its predecessor, Secure Sockets Layer (SSL), are cryptographic protocols which provide secure communications on the Internet for such things as web browsing, 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 "TLS" as used here applies to both protocols unless clarified by context.

Contents

[edit] Description

The TLS protocol(s) allow client/server applications to communicate in a way designed to prevent eavesdropping, tampering, and message forgery. TLS provides endpoint authentication and communications privacy over the Internet using cryptography. Typically, only the server is authenticated (i.e., its identity is ensured) while the client remains unauthenticated; this means that the end user (be that a person, or an application such as a web browser), can be sure of who they are "talking" to. The next level of security - both ends of the "conversation" being sure of who they are "talking" to - is known as mutual authentication. Mutual authentication requires public key infrastructure (PKI) deployment to clients.

TLS involves three basic phases:

  1. Peer negotiation for algorithm support
  2. Public key encryption -based key exchange and certificate-based authentication
  3. Symmetric cipher -based traffic encryption

During the first phase, the client and server negotiation uses cryptographic algorithms. Current implementations support the following choices:

[edit] How it works

The TLS 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, the 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.
  • The client and server negotiate a common secret called the "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 "pseudorandom 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 a previous (less secure) version or a weaker cipher suite.
  • The message that ends the handshake ("Finished") sends a hash of all the exchanged data seen by both parties.
  • The pseudorandom function splits the input data in half and processes each one with a different hashing algorithm (MD5 and SHA), then XORs them together. This provides protection if one of these algorithms is found to be vulnerable.

[edit] Applications

TLS 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. SMTP is also an area in which TLS has been growing and is specified in RFC 3207. These applications use public key certificates to verify the identity of endpoints.

An increasing number of client and server products support TLS natively, but many still lack support. As an alternative, users may wish to use standalone TLS products like Stunnel. Wrappers such as Stunnel rely on being able to obtain a TLS connection immediately, by simply connecting to a separate port reserved for the purpose. For example, by default the TCP port for HTTPS is 443, to distinguish it from HTTP on port 80. However, in 1997 the Internet Engineering Task Force recommended that application protocols always start unsecured and instead offer a way to upgrade to TLS - which a pure wrapper like Stunnel cannot cope with.

TLS can also be used to tunnel an entire network stack to create a VPN, as is the case with OpenVPN. Many vendors now marry TLS's encryption and authentication capabilities with authorization. There has also been substantial development since the late 1990s in creating client technology outside of the browser to enable support for client/server applications. When compared against traditional IPSec VPN technologies, TLS has some inherent advantages in firewall and NAT traversal that make it easier to administer for large remote access populations. Vendors like Aventail, F5, Juniper, and others have been developing in this space for some time.

[edit] History and development

Developed by Netscape, SSL version 3.0 was released in 1996, which later served as the basis for TLS version 1.0, an IETF standard protocol first defined in RFC 2246. Visa, MasterCard, American Express and many leading financial institutions have endorsed SSL for commerce over the Internet.

SSL operates in modular fashion. It is extensible by design, with support for forward and backward compatibility and negotiation between peers.

[edit] Early short keys

Some early implementations of SSL used 40-bit symmetric keys because of US government restrictions on the export of cryptographic technology. The US government explicitly imposed a 40-bit keyspace small enough to be broken by brute-force search by law enforcement agencies wishing to read the encrypted traffic, while still presenting obstacles to less-well-funded attackers. A similar limitation applied to Lotus Notes in export versions. After several years of public controversy, a series of lawsuits, and eventual US government recognition of changes in the market availability of 'better' cryptographic products produced outside the US, the authorities relaxed some aspects of the export restrictions. The 40-bit key size limitation has mostly gone away. Modern implementations use 128-bit (or longer) keys for symmetric key ciphers.

[edit] Incorrect uses

Some websites have been criticized for incorrectly using TLS and therefore negating its security benefits [1]. Such incorrect uses include:

  • Only securing the form submission page, while failing to secure the login page [2]
  • Displaying a secure page mixed with non-secure media [3]

Both practices have been found present in many commercial websites such as those of Bank of America, Washington Mutual, JPMorgan Chase & Co. [4], and Paypal [5].

[edit] Standards

The first definition of TLS appeared in RFC 2246: "The TLS Protocol Version 1.0". The current approved version is 1.1, which is specified in RFC 4346: "The Transport Layer Security (TLS) Protocol Version 1.1".

Other RFCs subsequently extended TLS, including:

  • RFC 2712: "Addition of Kerberos Cipher Suites to Transport Layer Security (TLS)". The 40-bit ciphersuites defined in this memo appear only for the purpose of documenting the fact that those ciphersuite codes have already been assigned.
  • RFC 2817: "Upgrading to TLS Within HTTP/1.1", explains how to use the Upgrade mechanism in HTTP/1.1 to initiate Transport Layer Security (TLS) over an existing TCP connection. This allows unsecured and secured HTTP traffic to share the same well known port (in this case, http: at 80 rather than https: at 443).
  • RFC 2818: "HTTP Over TLS", distinguishes secured traffic from insecure traffic by the use of a different 'server port'.
  • RFC 3207: "SMTP Service Extension for Secure SMTP over Transport Layer Security". Specifies an extension to the SMTP service that allows an SMTP server and client to use transport-layer security to provide private, authenticated communication over the Internet.
  • RFC 3268: "AES Ciphersuites for TLS". Adds Advanced Encryption Standard (AES) ciphersuites to the previously existing symmetric ciphers.
  • RFC 3546: "Transport Layer Security (TLS) Extensions", adds a mechanism for negotiating protocol extensions during session initialisation and defines some extensions.
  • RFC 4132: "Addition of Camellia Cipher Suites to Transport Layer Security (TLS)".
  • RFC 4162: "Addition of SEED Cipher Suites to Transport Layer Security (TLS)".
  • RFC 4279: "Pre-Shared Key Ciphersuites for Transport Layer Security (TLS)", adds three sets of new ciphersuites for the TLS protocol to support authentication based on pre-shared keys.
  • RFC 4347: "Datagram Transport Layer Security" specifies a TLS variant that works over datagram protocols (such as UDP).
  • RFC 4366: "Transport Layer Security (TLS) Extensions" describes both a set of specific extensions, and a generic extension mechanism.
  • RFC 4492: "Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS)".

[edit] Implementation

Programmers may use the OpenSSL, NSS, or GnuTLS libraries for SSL/TLS functionality. Delphi programmers may use the library called Indy, which has ways of connecting components to an TLS intercept using the OpenSSL libraries. This enables the development of secure Web browsers and Web servers using Delphi/Indy/OpenSSL. The protocols supported are SSLv2, SSLv3, and TLS v1.

[edit] 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. 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.

[edit] See also

Portal:Cryptography
 Cryptography Portal

[edit] Software

[edit] References

  • David Wagner and Bruce Schneier, Analysis of the SSL 3.0 Protocol, The Second USENIX Workshop on Electronic Commerce Proceedings, USENIX Press, November 1996, pp29–40. http://www.schneier.com/paper-ssl.pdf

[edit] Certificate Providers

[edit] External links

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.

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