Transport Layer Security (TLS) and its predecessor, Secure Sockets Layer (SSL), are cryptographic protocols that provide secure communications on the Internet for such things as web browsing, e-mail, Internet faxing, instant messaging and other data transfers. There are slight differences between SSL and TLS, but the protocol remains substantially the same.
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:
During the first phase, the client and server negotiate cipher suites, which determinethe ciphers to be used, the key exchange and authentication algorithms, as well as the message authentication codes (MACs). The key exchange and authentication algorithms are typically public key algorithms, or as in TLS-PSK preshared keys could be used. The message authentication codes are made up from cryptographic hash functions using the HMAC construction.
Typical algorithms could be:
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.
The TLS protocol exchanges records, which 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.
| + | Bits 0–7 | 8-15 | 16-23 | 24–31 |
|---|---|---|---|---|
| 0 | Content Type | Version (MSB) | Version (LSB) | Length (MSB) |
| 32 | Length (LSB) | Protocol Message(s) | ||
| ... | Protocol Message (cont.) | |||
| ... | MAC (optional) | |||
| ... | Padding (optional) | |||
; Content Type: This field identifies the Record Layer Protocol Type contained in this Record.
| Content Types | |
|---|---|
| 20 | ChangeCipherSpec |
| 21 | Alert |
| 22 | Handshake |
| 23 | Application |
; 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.
| Versions | ||
|---|---|---|
| Major Version | Minor Version | Version Type |
| 3 | 0 | SSLv3 |
| 3 | 1 | TLS 1.0 |
| 3 | 2 | TLS 1.1 |
| 3 | 3 | TLS 1.2 |
; 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.
| + | Bits 0–7 | 8-15 | 16-23 | 24–31 | ||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 20 | Version (MSB) | Version (LSB) | 0 | ||||||||||||||||||||||||||||
| 32 | 1 | 1 (CCS protocol type) | ||||||||||||||||||||||||||||||
| + | Bits 0–7 | 8-15 | 16-23 | 24–31 |
|---|---|---|---|---|
| 0 | 21 | Version (MSB) | Version (LSB) | 0 |
| 32 | 2 | Level | Description |
; Level: This field identifies the level of alert.
| Level Types | |
|---|---|
| Code | Description |
| 1 | Warning - connection or security may be unstable |
| 2 | Fatal - connection or security may be compromised, or an unrecoverable error has occurred |
; Description: This field identifies which type of alert is being sent.
| Description Types | |
|---|---|
| Code | Description |
| 0 | Close notify (warning or fatal) |
| 10 | Unexpected message (fatal) |
| 20 | Bad record MAC (fatal) |
| 21 | Decryption failed (fatal, TLS only) |
| 22 | Record overflow (fatal, TLS only) |
| 30 | Decompression failure (fatal) |
| 40 | Handshake failure (fatal) |
| 41 | No certificate (SSL v3 only) (warning or fatal) |
| 42 | Bad certificate (warning or fatal) |
| 43 | Unsupported certificate (warning or fatal) |
| 44 | Certificate revoked (warning or fatal) |
| 45 | Certificate expired (warning or fatal) |
| 46 | Certificate unknown (warning or fatal) |
| 47 | Illegal parameter (fatal) |
| 48 | Unknown CA (fatal, TLS only) |
| 49 | Access denied (fatal, TLS only) |
| 50 | Decode error (fatal, TLS only) |
| 51 | Decrypt error (TLS only) (warning or fatal) |
| 60 | Export restriction (fatal, TLS only) |
| 70 | Protocol version (fatal, TLS only) |
| 71 | Insufficient security (fatal, TLS only) |
| 80 | Internal error (fatal, TLS only) |
| 90 | User cancelled (fatal, TLS only) |
| 100 | No renegotiation (warning, TLS only) |
| + | Bits 0–7 | 8-15 | 16-23 | 24–31 | ||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 22 | Version (MSB) | Version (LSB) | Length (MSB) | ||||||||||||||||||||||||||||
| 32 | Length (LSB) | Message type | Message length | |||||||||||||||||||||||||||||
| 64 | Message length (cont.) | Handshake message | ||||||||||||||||||||||||||||||
| ... | Handshake message | Message type | Message length | |||||||||||||||||||||||||||||
| ... | Message length (cont.) | Handshake message | ||||||||||||||||||||||||||||||
; Message type: This field identifies the Handshake message type.
| Message Types | |
|---|---|
| Code | Description |
| 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.
| + | Bits 0–7 | 8-15 | 16-23 | 24–31 | ||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 23 | Version (MSB) | Version (LSB) | Length (MSB) | ||||||||||||||||||||||||||||
| 32 | Length (LSB) | Application data | ||||||||||||||||||||||||||||||
| 64 | Application data (cont.) | |||||||||||||||||||||||||||||||
| ... | MAC (20B for SHA-1, 16B for MD5) | |||||||||||||||||||||||||||||||
| ... | Variable length padding (block ciphers only) | Padding length, (block ciphers only)(1B) | ||||||||||||||||||||||||||||||
TLS does not provide a mechanism for a client to tell a server the
name of the server it is contacting. It is often desirable for clientsto provide this information to facilitate secure connections to
servers that host multiple Virtual Servers sharing a single IP address.
In order to provide the server name, RFC 4366 Transport Layer Security (TLS) Extensions allow clients to include a "server_name" extension in the extended client hello. The
TLS server, in response, should provide the appropriate certificate for the requested Virtual Server.
TLS/SSL have a variety of security measures:
SSL v2 is flawed in a variety of ways:
SSL v2 is disabled by default in Internet Explorer 7,[1] Mozilla Firefox 2,[2] Opera 9[3] and Safari. Support for SSL v2 (and weak 40-bit and 56-bit ciphers) will be removed completely from the upcoming Opera 9.5 (code-named Kestrel).[4]
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 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.
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 and other SIP-based applications.
Early research efforts toward transport layer security included the Secure Network Programming (SNP) API, which in 1993 explored the approach of having a secure transport layer API closely resembling sockets, to facilitate retrofitting preexisting network applications with security measures.
[5] The SNP project received the 2004 ACM Software System Award.[6]
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 Internet Engineering Task Force (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.
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, 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.
The first definition of TLS appeared in:
The current approved version is 1.1, which is specified in
The next version is proposed:
Other RFCs subsequently extended TLS, including:
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.
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).