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Transport Layer Security ('''TLS''') and its predecessor, '''Secure Sockets Layer''' ('''SSL'''), are Cryptographic Protocol s that provide Secure communications on the Internet for such things as Web Browsing , E-mail , Internet Fax ing, Instant Messaging and other data transfers. There are slight differences between SSL and TLS, but the protocol remains substantially the same. The term "TLS" as used here applies to both protocols unless clarified by context. DESCRIPTION The TLS protocol allows applications to communicate across a network 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 (whether an individual or an application, such as a Web browser) can be sure with whom they are communicating. The next level of security—in which both ends of the "conversation" are sure with whom they are communicating—is known as Mutual Authentication . Mutual authentication requires Public Key Infrastructure (PKI) deployment to clients unless TLS-PSK or TLS-SRP are used, which provide strong mutual authentication without needing to deploy a PKI. TLS involves three basic phases: # Peer negotiation for''' algorithm support # Public Key exchange and certificate-based authentication # Symmetric Cipher encryption During the first phase, the client and server negotiate ''cipher suites'', which combine one cipher from each of the following:
HOW IT WORKS A TLS client and server negotiate a stateful connection by using a handshaking procedure. During this handshake, the client and server agree on various parameters used to establish the connection's security.
The client may contact the server that issued the certificate (the trusted CA as above) and confirm that the certificate is authentic before proceeding.
This concludes the handshake and begins the secured connection, which is encrypted and decrypted with the key material until the connection closes. ''If any one of the above steps fails, the TLS handshake fails, and the connection is not created.'' TLS Handshake in Detail The TLS protocol exchanges ''records'' that encapsulate the data to be exchanged. Each record can be compressed, padded, appended with a Message Authentication Code (MAC), or encrypted, all depending on the state of the connection. Each record has a ''content type'' field that specifies the record, a length field, and a TLS version field. When the connection starts, the record encapsulates another protocol, the handshake protocol, which has ''content type'' 22. A simple connection example follows:
:: ''These certificates are currently X.509 , but there is also a draft specifying the use of OpenPGP based certificates.''
TLS Record Protocol ; Content Type: This field identifies the Record Layer Protocol Type contained in this Record. ; Version: This field identifies the major and minor version of TLS for the contained message. For a ClientHello message, this need not be the ''highest'' version supported by the client. ; Length: The length of Protocol message(s), not to exceed 214 bytes. ; Protocol message(s): One or more messages identified by the Protocol field. Note that this field may be encrypted depending on the state of the connection. ; MAC: A message authentication code computed over the Protocol message, with additional key material included. Note that this field may be encrypted, or not included entirely, depending on the state of the connection. ChangeCipherSpec Protocol Alert Protocol ; Level: This field identifies the level of alert. ; Description: This field identifies which type of alert is being sent. Handshake Protocol ; Message type: This field identifies the Handshake message type. Handshake Types are: 0 HelloRequest 1 ClientHello 2 ServerHello 11 Certificate 12 ServerKeyExchange 13 CertificateRequest 14 ServerHelloDone 15 CertificateVerify 16 ClientKeyExchange 20 Finished ; Message length: This is a 3-byte field indicating the length of the handshake data, not including the header. Note that multiple Handshake messages may be combined within one record. Application Protocol SECURITY TLS/SSL have a variety of security measures:
APPLICATIONS TLS runs on layers beneath application protocols such as HTTP , FTP , SMTP , NNTP , and XMPP and above a reliable transport protocol, TCP for example. 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 and Asset Management . SMTP is also an area in which TLS has been growing and is specified in RFC 3207. These applications use Public Key Certificate s 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. 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. TLS is also increasingly being used as the standard method for protecting SIP application signaling. TLS can be used to provide authentication and encryption of the SIP signalling associated with VOIP (Voice over IP) and other SIP-based applications. HISTORY AND DEVELOPMENT The SSL protocol was originally developed by Netscape . Version 1.0 was never publicly released; version 2.0 was released in 1994 but "contained a number of security flaws which ultimately led to the design of SSL version 3.0", which was released in 1996 (Rescorla 2001). This later served as the basis for TLS version 1.0, an IETF standard Protocol first defined in RFC 2246 in January 1999. 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 . Early short keys Some early implementations of SSL used 40-bit Symmetric Key s because of US government restrictions on the Export Of Cryptographic Technology . The US government explicitly imposed a 40-bit keyspace, which was 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 cryptographic products with longer key sizes produced outside the US, the authorities relaxed some aspects of the export restrictions. The 40-bit Key Size limitation has mostly gone away, and modern implementations use 128-bit (or longer) keys for symmetric key ciphers. STANDARDS The first definition of TLS appeared in:
The current approved version is 1.1, which is specified in
The next version is proposed:
Other RFC s subsequently extended TLS, including:
IMPLEMENTATION Programmers may use the OpenSSL , NSS , or GnuTLS libraries for SSL/TLS functionality. Microsoft Windows includes an implementation of SSL and TLS as part of its Secure Channel package. Delphi programmers may use a library called Indy. 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, but remains very similar to TLS 1.0. A full list of differences is provided in RFC 4346 (Section 1.1). CERTIFICATE PROVIDERS A 2005 Netcraft survey determined that VeriSign and its acquisitions such as Thawte have a 53% share of the certificate authority market, followed by GeoTrust (25%), Comodo (12%), GoDaddy (4%) and Entrust (2%). The Netcraft Secure Server Survey (GeoTrust has since been acquired by VeriSign.) A more recent market share report from Security Space as of April 2007 determined that (1.3%) and Network Solutions (1.1%). CAcert.org is a community-driven certificate authority that issues free public key certificates. SEE ALSO
Software
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