Transport Layer Security

Transport Layer Security (TLS) and its predecessor, Secure Sockets Layer (SSL), are cryptographic protocols that provide communication security over the Internet.[1] TLS and SSL encrypt the segments of network connections above the Transport Layer, using asymmetric cryptography for key exchange, symmetric encryption for privacy, and message authentication codes for message integrity.

Several versions of the protocols are in widespread use in applications such as web browsing, electronic mail, Internet faxing, instant messaging and voice-over-IP (VoIP).

TLS is an IETF standards track protocol, last updated in RFC 5246, and is based on the earlier SSL specifications developed by Netscape Communications.[2]

Internet protocol suite
Application layer
Transport layer
Internet layer
Link layer

Contents

Description

The TLS protocol allows client/server applications to communicate across a network in a way designed to prevent eavesdropping and tampering.

Since most protocols can be used either with or without TLS (or SSL) it is necessary to indicate to the server whether the client is making a TLS connection or not. There are two main ways of achieving this, one option is to use a different port number for TLS connections (for example port 443 for HTTPS). The other is to use the regular port number and have the client request that the server switch the connection to TLS using a protocol specific mechanism (for example STARTTLS for mail and news protocols).

Once the client and server have decided to use TLS they negotiate a stateful connection by using a handshaking procedure.[3] During this handshake, the client and server agree on various parameters used to establish the connection's security.

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.

History and development

Secure Network Programming API

Early research efforts toward transport layer security included the Secure Network Programming (SNP) application programming interface (API), which in 1993 explored the approach of having a secure transport layer API closely resembling Berkeley sockets, to facilitate retrofitting preexisting network applications with security measures.[4]

SSL 1.0, 2.0 and 3.0

The SSL protocol was originally developed by Netscape.[5] Version 1.0 was never publicly released; version 2.0 was released in February 1995 but "contained a number of security flaws which ultimately led to the design of SSL version 3.0".[6] SSL version 3.0, released in 1996, was a complete redesign of the protocol produced by Paul Kocher working with Netscape engineers Phil Karlton and Alan Freier. Newer versions of SSL/TLS are based on SSL 3.0. The 1996 draft of SSL 3.0 was published by IETF as a historic document in RFC 6101.

TLS 1.0

TLS 1.0 was first defined in RFC 2246 in January 1999 as an upgrade to SSL Version 3.0. As stated in the RFC, "the differences between this protocol and SSL 3.0 are not dramatic, but they are significant enough that TLS 1.0 and SSL 3.0 do not interoperate." TLS 1.0 does include a means by which a TLS implementation can downgrade the connection to SSL 3.0.

On September 23, 2011 researchers Thai Duong and Juliano Rizzo demonstrated a "proof of concept" called BEAST (using a Java Applet to violate "same origin policy" constraints) for a long-known Cipher block chaining (CBC) vulnerability in TLS 1.0.[7][8] Practical exploits had not been previously demonstrated for this vulnerability, which was originally discovered by Phillip Rogaway[9] in 2002.

Mozilla updated the development versions of their NSS libraries to mitigate BEAST-like attacks. NSS is used by Mozilla Firefox and Google Chrome to implement SSL. Some webservers that have a broken implementation of the SSL specification may stop working as a result.[10]

The TLS 1.0-specific BEAST attack can be prevented by removing all CBC ciphers from your list of allowed ciphers—leaving only the RC4 cipher, which is still widely supported on most websites.[11][12] Microsoft is releasing tools to support TLS 1.1 (which fixes this CBC attack vulnerability) on Microsoft servers and browsers.[13]

TLS 1.1

TLS 1.1 was defined in RFC 4346 in April 2006.[14] It is an update from TLS version 1.0. Significant differences in this version include:

TLS 1.2

TLS 1.2 was defined in RFC 5246 in August 2008. It is based on the earlier TLS 1.1 specification. Major differences include:

TLS 1.2 was further refined in RFC 6176 in March 2011 redacting its backward compatibility with SSL such that TLS sessions will never negotiate the use of Secure Sockets Layer (SSL) version 2.0.

Applications

In applications design, TLS is usually implemented on top of any of the Transport Layer protocols, encapsulating the application-specific protocols such as HTTP, FTP, SMTP, NNTP and XMPP. Historically it has been used primarily with reliable transport protocols such as the Transmission Control Protocol (TCP). However, it has also been implemented with datagram-oriented transport protocols, such as the User Datagram Protocol (UDP) and the Datagram Congestion Control Protocol (DCCP), usage which has been standardized independently using the term Datagram Transport Layer Security (DTLS).

A prominent use of TLS is for securing World Wide Web traffic carried by HTTP to form HTTPS. Notable applications are electronic commerce and asset management. Increasingly, the Simple Mail Transfer Protocol (SMTP) is also protected by TLS. These applications use public key certificates to verify the identity of endpoints.

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 a standard method to protect Session Initiation Protocol (SIP) application signaling. TLS can be used to provide authentication and encryption of the SIP signaling associated with VoIP and other SIP-based applications.

Security

TLS has a variety of security measures:

From a security standpoint, SSL 3.0 should be considered less desirable than TLS 1.0. The SSL 3.0 cipher suites have a weaker key derivation process; half of the master key that is established is fully dependent on the MD5 hash function, which is not resistant to collisions and is, therefore, not considered secure. Under TLS 1.0, the master key that is established depends on both MD5 and SHA-1 so its derivation process is not currently considered weak. It is for this reason that SSL 3.0 implementations cannot be validated under FIPS 140-2.[15]

A vulnerability in TLS 1.0 and all versions of SSL that was considered to be theoretical is to be demonstrated at the Ekoparty security conference on September 21, 2011. The demonstration will include the decryption of cookies used for website authentication.[16]

A vulnerability of the renegotiation procedure was discovered in August 2009 that can lead to plaintext injection attacks against SSL 3.0 and all current versions of TLS. For example, it allows an attacker who can hijack an https connection to splice their own requests into the beginning of the conversation the client has with the web server. The attacker can't actually decrypt the client-server communication, so it is different from a typical man-in-the-middle attack. A short-term fix is for web servers to stop allowing renegotiation, which typically will not require other changes unless client certificate authentication is used. To fix the vulnerability, a renegotiation indication extension was proposed for TLS. It will require the client and server to include and verify information about previous handshakes in any renegotiation handshakes.[17] When a user doesn't pay attention to their browser's indication that the session is secure (typically a padlock icon), the vulnerability can be turned into a true man-in-the-middle attack.[18] This extension has become a proposed standard and has been assigned the number RFC 5746. The RFC has been implemented in recent OpenSSL[19] and other libraries [20] .[21]

There are some attacks against the implementation rather than the protocol itself:[22]

SSL 2.0 is flawed in a variety of ways:

SSL 2.0 is disabled by default in Internet Explorer 7,[25] Mozilla Firefox 2, Mozilla Firefox 3, Mozilla Firefox 4[26] Opera and Safari. After it sends a TLS ClientHello, if Mozilla Firefox finds that the server is unable to complete the handshake, it will attempt to fall back to using SSL 3.0 with an SSL 3.0 ClientHello in SSL 2.0 format to maximize the likelihood of successfully handshaking with older servers.[27] Support for SSL 2.0 (and weak 40-bit and 56-bit ciphers) has been removed completely from Opera as of version 9.5.[28]

Modifications to the original protocols, like False Start (adopted and enabled by Google Chrome[29] ) or Snap Start, have been reported to introduce limited TLS protocol version rollback attacks[30] or to allow modifications to the cipher suite list sent by the client to the server (an attacker may be able influence the cipher suite selection in an attempt to downgrade the cipher suite strength, to use either a weaker symmetric encryption algorithm or a weaker key exchange[31] ).

TLS handshake in detail

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 messaging protocol — which has content type 22.

Simple TLS handshake

A simple connection example follows, illustrating a handshake where the server (but not the client) is authenticated by its certificate:

  1. Negotiation phase:
    • A client sends a ClientHello message specifying the highest TLS protocol version it supports, a random number, a list of suggested CipherSuites and suggested compression methods. If the client is attempting to perform a resumed handshake, it may send a session ID.
    • The server responds with a ServerHello message, containing the chosen protocol version, a random number, CipherSuite and compression method from the choices offered by the client. To confirm or allow resumed handshakes the server may send a session ID. The chosen protocol version should be the highest that both the client and server support. For example, if the client supports TLS1.1 and the server supports TLS1.2, TLS1.1 should be selected; SSL 3.0 should not be selected.
    • The server sends its Certificate message (depending on the selected cipher suite, this may be omitted by the server).[32]
    • The server sends a ServerHelloDone message, indicating it is done with handshake negotiation.
    • The client responds with a ClientKeyExchange message, which may contain a PreMasterSecret, public key, or nothing. (Again, this depends on the selected cipher.) This PreMasterSecret is encrypted using the public key of the server certificate.
    • The client and server then use the random numbers and PreMasterSecret to compute a common secret, called the "master secret". All other key data for this connection is derived from this master secret (and the client- and server-generated random values), which is passed through a carefully designed pseudorandom function.
  2. The client now sends a ChangeCipherSpec record, essentially telling the server, "Everything I tell you from now on will be authenticated (and encrypted if encryption parameters were present in the server certificate)." The ChangeCipherSpec is itself a record-level protocol with content type of 20.
    • Finally, the client sends an authenticated and encrypted Finished message, containing a hash and MAC over the previous handshake messages.
    • The server will attempt to decrypt the client's Finished message and verify the hash and MAC. If the decryption or verification fails, the handshake is considered to have failed and the connection should be torn down.
  3. Finally, the server sends a ChangeCipherSpec, telling the client, "Everything I tell you from now on will be authenticated (and encrypted, if encryption was negotiated)."
    • The server sends its authenticated and encrypted Finished message.
    • The client performs the same decryption and verification.
  4. Application phase: at this point, the "handshake" is complete and the application protocol is enabled, with content type of 23. Application messages exchanged between client and server will also be authenticated and optionally encrypted exactly like in their Finished message. Otherwise, the content type will return 25 and the client will not authenticate.

Client-authenticated TLS handshake

The following full example shows a client being authenticated (in addition to the server like above) via TLS using certificates exchanged between both peers.

  1. Negotiation phase:
    • A client sends a ClientHello message specifying the highest TLS protocol version it supports, a random number, a list of suggested cipher suites and compression methods.
    • The server responds with a ServerHello message, containing the chosen protocol version, a random number, cipher suite and compression method from the choices offered by the client. The server may also send a session id as part of the message to perform a resumed handshake.
    • The server sends its Certificate message (depending on the selected cipher suite, this may be omitted by the server).[32]
    • The server requests a certificate from the client, so that the connection can be mutually authenticated, using a CertificateRequest message.
    • The server sends a ServerHelloDone message, indicating it is done with handshake negotiation.
    • The client responds with a Certificate message, which contains the client's certificate.
    • The client sends a ClientKeyExchange message, which may contain a PreMasterSecret, public key, or nothing. (Again, this depends on the selected cipher.) This PreMasterSecret is encrypted using the public key of the server certificate.
    • The client sends a CertificateVerify message, which is a signature over the previous handshake messages using the client's certificate's private key. This signature can be verified by using the client's certificate's public key. This lets the server know that the client has access to the private key of the certificate and thus owns the certificate.
    • The client and server then use the random numbers and PreMasterSecret to compute a common secret, called the "master secret". All other key data for this connection is derived from this master secret (and the client- and server-generated random values), which is passed through a carefully designed pseudorandom function.
  2. The client now sends a ChangeCipherSpec record, essentially telling the server, "Everything I tell you from now on will be authenticated (and encrypted if encryption was negotiated)." The ChangeCipherSpec is itself a record-level protocol and has type 20 and not 22.
    • Finally, the client sends an encrypted Finished message, containing a hash and MAC over the previous handshake messages.
    • The server will attempt to decrypt the client's Finished message and verify the hash and MAC. If the decryption or verification fails, the handshake is considered to have failed and the connection should be torn down.
  3. Finally, the server sends a ChangeCipherSpec, telling the client, "Everything I tell you from now on will be authenticated (and encrypted if encryption was negotiated)."
    • The server sends its own encrypted Finished message.
    • The client performs the same decryption and verification.
  4. Application phase: at this point, the "handshake" is complete and the application protocol is enabled, with content type of 23. Application messages exchanged between client and server will also be encrypted exactly like in their Finished message. The application will never again return TLS encryption information without a type 32 apology.

Resumed TLS handshake

Public key operations (e.g., RSA) are relatively expensive in terms of computational power. TLS provides a secure shortcut in the handshake mechanism to avoid these operations. In an ordinary full handshake, the server sends a session id as part of the ServerHello message. The client associates this session id with the server's IP address and TCP port, so that when the client connects again to that server, it can use the session id to shortcut the handshake. In the server, the session id maps to the cryptographic parameters previously negotiated, specifically the "master secret". Both sides must have the same "master secret" or the resumed handshake will fail (this prevents an eavesdropper from using a session id). The random data in the ClientHello and ServerHello messages virtually guarantee that the generated connection keys will be different than in the previous connection. In the RFCs, this type of handshake is called an abbreviated handshake. It is also described in the literature as a restart handshake.

  1. Negotiation phase:
    • A client sends a ClientHello message specifying the highest TLS protocol version it supports, a random number, a list of suggested cipher suites and compression methods. Included in the message is the session id from the previous TLS connection.
    • The server responds with a ServerHello message, containing the chosen protocol version, a random number, cipher suite and compression method from the choices offered by the client. If the server recognizes the session id sent by the client, it responds with the same session id. The client uses this to recognize that a resumed handshake is being performed. If the server does not recognize the session id sent by the client, it sends a different value for its session id. This tells the client that a resumed handshake will not be performed. At this point, both the client and server have the "master secret" and random data to generate the key data to be used for this connection.
  2. The client now sends a ChangeCipherSpec record, essentially telling the server, "Everything I tell you from now on will be encrypted." The ChangeCipherSpec is itself a record-level protocol and has type 20 and not 22.
    • Finally, the client sends an encrypted Finished message, containing a hash and MAC over the previous handshake messages.
    • The server will attempt to decrypt the client's Finished message and verify the hash and MAC. If the decryption or verification fails, the handshake is considered to have failed and the connection should be torn down.
  3. Finally, the server sends a ChangeCipherSpec, telling the client, "Everything I tell you from now on will be encrypted."
    • The server sends its own encrypted Finished message.
    • The client performs the same decryption and verification.
  4. Application phase: at this point, the "handshake" is complete and the application protocol is enabled, with content type of 23. Application messages exchanged between client and server will also be encrypted exactly like in their Finished message.

Apart from the performance benefit, resumed sessions can also be used for single sign-on as it is guaranteed that both the original session as well as any resumed session originate from the same client. This is of particular importance for the FTP over TLS/SSL protocol which would otherwise suffer from a man in the middle attack in which an attacker could intercept the contents of the secondary data connections.[33]

TLS record protocol

This is the general format of all TLS records.

+ Byte +0 Byte +1 Byte +2 Byte +3
Byte
0		 Content type  
Bytes
1..4		 Version Length
(Major) (Minor) (bits 15..8) (bits 7..0)
Bytes
5..(m-1)		 Protocol message(s)
Bytes
m..(p-1)		 MAC (optional)
Bytes
p..(q-1)		 Padding (block ciphers only)
Content type
This field identifies the Record Layer Protocol Type contained in this Record.
Content types
Hex Dec Type
0x14 20 ChangeCipherSpec
0x15 21 Alert
0x16 22 Handshake
0x17 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 SSL 3.0
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 (16 KiB).
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 and Padding
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.
No MAC or Padding can be present at end of TLS records before all cipher algorithms and parameters have been negotiated and handshaked and then confirmed by sending a CipherStateChange record (see below) for signalling that these parameters will take effect in all further records sent by the same peer.

Handshake protocol

Most messages exchanged during the setup of the TLS session are based on this record, unless an error or warning occurs and needs to be signalled by an Alert protocol record (see below), or the encryption mode of the session is modified by another record (see ChangeCipherSpec protocol below).

+ Byte +0 Byte +1 Byte +2 Byte +3
Byte
0		 22  
Bytes
1..4		 Version Length
(Major) (Minor) (bits 15..8) (bits 7..0)
Bytes
5..8		 Message type Handshake message data length
(bits 23..16) (bits 15..8) (bits 7..0)
Bytes
9..(n-1)		 Handshake message data
Bytes
n..(n+3)		 Message type Handshake message data length
(bits 23..16) (bits 15..8) (bits 7..0)
Bytes
(n+4)..		 Handshake message data
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
Handshake message data 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.

Alert protocol

This record should normally not be sent during normal handshaking or application exchanges. However, this message can be sent at any time during the handshake and up to the closure of the session. If this is used to signal a fatal error, the session will be closed immediately after sending this record, so this record is used to give a reason for this closure. If the alert level is flagged as a warning, the remote can decide to close the session if it decides that the session is not reliable enough for its needs (before doing so, the remote may also send its own signal).

+ Byte +0 Byte +1 Byte +2 Byte +3
Byte
0		 21  
Bytes
1..4		 Version Length
(Major) (Minor) 0 2
Bytes
5..6		 Level Description  
Bytes
7..(p-1)		 MAC (optional)
Bytes
p..(q-1)		 Padding (block ciphers only)
Level
This field identifies the level of alert. If the level is fatal, the sender should close the session immediately. Otherwise, the recipient may decide to terminate the session itself, by sending its own fatal alert and closing the session itself immediately after sending it. The use of Alert records is optional, however if it is missing before the session closure, the session may be resumed automatically (with its handshakes).
Normal closure of a session after termination of the transported application should preferably be alerted with at least the Close notify Alert type (with a simple warning level) to prevent such automatic resume of a new session. Signalling explicitly the normal closure of a secure session before effectively closing its transport layer is useful to prevent or detect attacks (like attempts to truncate the securely transported data, if it intrinsically does not have a predetermined length or duration that the recipient of the secured data may expect).
Alert level types
Code Level type Connection state
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.
Alert description types
Code Description Level types Note
0 Close notify warning/fatal
10 Unexpected message fatal
20 Bad record MAC fatal Possibly a bad SSL implementation, or payload has been tampered with e.g. FTP firewall rule on FTPS server.
21 Decryption failed fatal TLS only, reserved
22 Record overflow fatal TLS only
30 Decompression failure fatal
40 Handshake failure fatal
41 No certificate warning/fatal SSL 3.0 only, reserved
42 Bad certificate warning/fatal
43 Unsupported certificate warning/fatal E.g. certificate has only Server authentication usage enabled and is presented as a client certificate
44 Certificate revoked warning/fatal
45 Certificate expired warning/fatal Check server certificate expire also check no certificate in the chain presented has expired
46 Certificate unknown warning/fatal
47 Illegal parameter fatal
48 Unknown CA (Certificate authority) fatal TLS only
49 Access denied fatal TLS only - E.g. no client certificate has been presented (TLS: Blank certificate message or SSLv3: No Certificate alert), but server is configured to require one.
50 Decode error fatal TLS only
51 Decrypt error warning/fatal TLS only
60 Export restriction fatal TLS only, reserved
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
110 Unsupported extension warning TLS only
111 Certificate unobtainable warning TLS only
112 Unrecognized name warning TLS only; client's Server Name Indicator specified a hostname not supported by the server
113 Bad certificate status response fatal TLS only
114 Bad certificate hash value fatal TLS only
115 Unknown PSK identity (used in TLS-PSK and TLS-SRP) fatal TLS only

ChangeCipherSpec protocol

+ Byte +0 Byte +1 Byte +2 Byte +3
Byte
0		 20  
Bytes
1..4		 Version Length
(Major) (Minor) 0 1
Byte
5		 CCS protocol type  
CCS protocol type
Currently only 1.

Application protocol

+ Byte +0 Byte +1 Byte +2 Byte +3
Byte
0		 23  
Bytes
1..4		 Version Length
(Major) (Minor) (bits 15..8) (bits 7..0)
Bytes
5..(m-1)		 Application data
Bytes
m..(p-1)		 MAC (optional)
Bytes
p..(q-1)		 Padding (block ciphers only)
Length
Length of Application data (excluding the protocol header and including the MAC and padding trailers)
MAC
20 bytes for the SHA-1-based HMAC, 16 bytes for the MD5-based HMAC.
Padding
Variable length ; last byte contains the padding length.

Support for name-based virtual servers

From the application protocol point of view, TLS belongs to a lower layer, although the TCP/IP model is too coarse to show it. This means that the TLS handshake is usually (except in the STARTTLS case) performed before the application protocol can start. The name-based virtual server feature being provided by the application layer, all co-hosted virtual servers share the same certificate because the server has to select and send a certificate immediately after the ClientHello message. This is a big problem in hosting environments because it means either sharing the same certificate among all customers or using a different IP address for each of them.

There are two known workarounds provided by X.509:

In order to provide the server name, RFC 4366 Transport Layer Security (TLS) Extensions allow clients to include a Server Name Indication extension (SNI) in the extended ClientHello message. This extension hints the server immediately which name the client wishes to connect to, so the server can select the appropriate certificate to send to the client.

Implementations

SSL and TLS have been widely implemented in several free and open source software projects. Programmers may use the PolarSSL, CyaSSL, 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. Comparison of TLS Implementations provides a brief comparison of features of different implementations.

Browser implementations

All the most recent web browsers support TLS:

Standards

The current approved version of TLS is version 1.2, which is specified in:

The current standard replaces these former versions, which are now considered obsolete:

Other RFCs subsequently extended TLS.

Extensions to TLS 1.0 include:

Extensions to TLS 1.1 include:

Extensions to TLS 1.2 include:

See also

Software

References and footnotes

  1. ^ T. Dierks, E. Rescorla (August 2008). "The Transport Layer Security (TLS) Protocol, Version 1.2". http://tools.ietf.org/html/rfc5246. 
  2. ^ The SSL Protocol: Version 3.0 Netscape's final SSL 3.0 draft (November 18, 1996)
  3. ^ "SSL/TLS in Detail". Microsoft TechNet. Updated July 31, 2003.
  4. ^ Thomas Y. C. Woo, Raghuram Bindignavle, Shaowen Su and Simon S. Lam, SNP: An interface for secure network programming Proceedings USENIX Summer Technical Conference, June 1994
  5. ^ "THE SSL PROTOCOL". Netscape Corporation. 2007. Archived from the original on 14 June 1997. http://web.archive.org/web/19970614020952/http://home.netscape.com/newsref/std/SSL.html. 
  6. ^ Rescorla 2001
  7. ^ Dan Goodin (2011-09-19). "Hackers break SSL encryption used by millions of sites". http://www.theregister.co.uk/2011/09/19/beast_exploits_paypal_ssl/. 
  8. ^ "Y Combinator comments on the issue". 2011-09-20. http://news.ycombinator.com/item?id=3015498. 
  9. ^ "Security of CBC Ciphersuites in SSL/TLS". 2004-05-20. http://www.openssl.org/~bodo/tls-cbc.txt. 
  10. ^ Brian Smith (2011-09-30). "(CVE-2011-3389) Rizzo/Duong chosen plaintext attack (BEAST) on SSL/TLS 1.0 (facilitated by websockets -76)". https://bugzilla.mozilla.org/show_bug.cgi?id=665814. 
  11. ^ "Safest ciphers to use with the BEAST? (TLS 1.0 exploit)". 2011-09-24. http://serverfault.com/questions/315042/. 
  12. ^ "First solutions for SSL/TLS vulnerability". 2011-09-26. http://www.h-online.com/security/news/item/First-solutions-for-SSL-TLS-vulnerability-1349813.html. 
  13. ^ "Microsoft releases fix-it tools for SSL/TLS vulnerability". 2011-09-27. http://www.h-online.com/security/news/item/Microsoft-releases-fix-it-tools-for-SSL-TLS-vulnerability-1350457.html. 
  14. ^ Dierks, T. and E. Rescorla (April 2006). "The Transport Layer Security (TLS) Protocol Version 1.1, RFC 4346". http://tools.ietf.org/html/rfc5246#ref-TLS1.1. 
  15. ^ National Institute of Standards and Technology (December 2010). "Implementation Guidance for FIPS PUB 140-2 and the Cryptographic Module Validation Program". http://csrc.nist.gov/groups/STM/cmvp/documents/fips140-2/FIPS1402IG.pdf. 
  16. ^ Dan Goodin (2011-09-19). "Hackers break SSL encryption used by millions of sites". The Register. http://www.theregister.co.uk/2011/09/19/beast_exploits_paypal_ssl/. Retrieved 2011-09-19. 
  17. ^ Eric Rescorla (2009-11-05). "Understanding the TLS Renegotiation Attack". Educated Guesswork. http://www.educatedguesswork.org/2009/11/understanding_the_tls_renegoti.html. Retrieved 2009-11-27. 
  18. ^ McMillan, Robert (2009-11-20). "Security Pro Says New SSL Attack Can Hit Many Sites". PC World. http://www.pcworld.com/article/182720/security_pro_says_new_ssl_attack_can_hit_many_sites.html. Retrieved 2009-11-27. 
  19. ^ "SSL_CTX_set_options SECURE_RENEGOTIATION". OpenSSL Docs. 2010-02-25. http://www.openssl.org/docs/ssl/SSL_CTX_set_options.html#SECURE_RENEGOTIATION. Retrieved 2010-11-18. 
  20. ^ "GnuTLS 2.10.0 released". GnuTLS release notes. 2010-06-25. http://article.gmane.org/gmane.network.gnutls.general/2046. Retrieved 2011-07-24. 
  21. ^ "NSS 3.12.6 release notes". NSS release notes. 2010-03-03. https://developer.mozilla.org/NSS_3.12.6_release_notes. Retrieved 2011-07-24. 
  22. ^ Various (2002-08-10). "IE SSL Vulnerability". Educated Guesswork. http://www.mail-archive.com/bugtraq@securityfocus.com/msg08807.html. Retrieved 2010-11-17. 
  23. ^ "Defeating SSL". https://www.blackhat.com/presentations/bh-dc-09/Marlinspike/BlackHat-DC-09-Marlinspike-Defeating-SSL.pdf. 
  24. ^ Adrian, Dimcev. "SSL/TLS version rollbacks and browsers". Random SSL/TLS 101. http://www.carbonwind.net/blog/post/Random-SSLTLS-101–SSLTLS-version-rollbacks-and-browsers.aspx. Retrieved 9 March 2011. 
  25. ^ Lawrence, Eric (2005-10-22). "IEBlog : Upcoming HTTPS Improvements in Internet Explorer 7 Beta 2". MSDN Blogs. http://blogs.msdn.com/ie/archive/2005/10/22/483795.aspx. Retrieved 2007-11-25. 
  26. ^ "Bugzilla@Mozilla — Bug 236933 - Disable SSL2 and other weak ciphers". Mozilla Corporation. https://bugzilla.mozilla.org/show_bug.cgi?id=236933. Retrieved 2007-11-25. 
  27. ^ "Firefox still sends SSLv2 handshake even though the protocol is disabled". 2008-09-11. https://bugzilla.mozilla.org/show_bug.cgi?id=454759. 
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  31. ^ Adrian, Dimcev. "False Start". Random SSL/TLS 101. http://www.carbonwind.net/blog/post/Random-SSLTLS-101-False-Start.aspx. Retrieved 9 March 2011. 
  32. ^ a b These certificates are currently X.509, but there is also a draft specifying the use of OpenPGP based certificates.
  33. ^ vsftpd-2.1.0 released Using TLS session resume for FTPS data connection authentication. Retrieved on 2009-04-28.
  34. ^ Named-based SSL virtual hosts: how to tackle the problem, SWITCH.
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Further reading

External links

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

Software (closed source)
Software (open source)
Mechanisms
SSTP · IPsec · L2TP · L2TPv3 · PPTP · Split tunneling · SSL/TLS · (Opportunistic : tcpcrypt)
Vendor-driven