Transport Layer Security

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 they are essentially the same.

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 it is 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 the Secure Remote Password (SRP) protocol are used, which provide strong mutual authentication without needing to deploy a PKI.

TLS involves three basic phases:
# Peer negotiation for"' algorithm support
# Key exchange and authentication
# Symmetric cipher encryption and message authentication

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 for TLS, and a non-standard pseudorandom function for SSL.

Typical algorithms could be:
* For key exchange: RSA, Diffie-Hellman, ECDH, SRP, PSK
* For authentication: RSA, DSA, ECDSA
* Symmetric ciphers: RC4, Triple DES, AES, IDEA, DES, or Camellia. In older versions of SSL, RC2 was also used.
* For cryptographic hash function: HMAC-MD5 or HMAC-SHA are used for TLS, MD5 and SHA for SSL, while older versions of SSL also used MD2 and MD4.

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 handshake begins when a client connects to a TLS-enabled server requesting a secure connection, and presents a list of supported ciphers and hash functions.
* From this list, the server picks the strongest cipher and hash function that it also supports and notifies the client of the decision.
* The server sends back its identification in the form of a digital certificate. The certificate usually contains the server name, the trusted certificate authority (CA), and the server's public encryption key.The client may contact the server that issued the certificate (the trusted CA as above) and confirm that the certificate is authentic before proceeding.
* In order to generate the session keys used for the secure connection, the client encrypts a random number with the server's public key, and sends the result to the server. Only the server can decrypt it (with its private key): this is the one fact that makes the keys hidden from third parties, since only the server and the client have access to this data.
* From the random number, both parties generate key material for encryption and decryption.

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", 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, illustrating a "full" handshake, follows:

* 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).These certificates are currently X.509, but there is also a draft specifying the use of OpenPGP based certificates.]
* 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.)
* 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".
* The Client now sends a ChangeCipherSpec record, essentially telling the Server, "Everything I tell you from now on will be encrypted." Note that 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.
* Finally, the Server sends a ChangeCipherSpec and its encrypted Finished message, and the Client performs the same decryption and verification.
* 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 be encrypted.

Client-Authenticated TLS handshake

The following example shows a client being authenticated via TLS using certificates.

* 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).
* 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".
* The Client now sends a ChangeCipherSpec record, essentially telling the Server, "Everything I tell you from now on will be encrypted." Note that 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.
* Finally, the Server sends a ChangeCipherSpec and its encrypted Finished message, and the Client performs the same decryption and verification.
* 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 be encrypted.
* The encryption protocol 324.22-9 is impemented only in instances where the handshake goes awry.

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.

* 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.
* The Server now sends a ChangeCipherSpec record, essentially telling the Client, "Everything I tell you from now on will be encrypted." Note that the ChangeCipherSpec is itself a record-level protocol, and has type 20, and not 22.
* The Server sends an encrypted Finished message, containing a hash and MAC over the previous handshake messages.
* The Client will attempt to decrypt the Servers'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.
* Finally, the Client sends a ChangeCipherSpec and its encrypted Finished message, and the Server performs the same decryption and verification. The Finished messages are used to validate that the Client and Server are using the same keys. If the validation of the Finished messages fail, then the resumed handshake fails and both the Client and Server purge the "session id" information and do not use it again.
* 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 be encrypted.
* If encryption fails, the handshake will not occur.

TLS record protocol

; 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: 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

; Description: This field identifies which type of alert is being sent.

:

; 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

upport for name-based virtual servers

From the application protocol point of view, TLS belongs to a lower layer, although theTCP/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 theapplication layer, all co-hosted virtual servers share the same certificate becausethe 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 thesame certificate among all customers or using a different IP address for each of them.

There are two known workarounds provided by X.509:
* If all virtual servers belongs to the same domain, you can use a [http://wiki.cacert.org/wiki/WildcardCertificates wildcard certificate] . Besides the loose host name selection that might be a problem or not, there is no common agreement about how to match wildcard certificates. Different rules are applied depending on the application protocol or software used. [https://www.switch.ch/pki/meetings/2007-01/namebased_ssl_virtualhosts.pdf Named-based SSL virtual hosts: how to tackle the problem] , SWITCH.]
* Add every virtual host name in the subjectAltName extension. The major problem being that you need to reissue a certificate whenever you declare a new virtual server.

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 servercan select the appropriate certificate to send to the client.

ecurity

TLS/SSL have a variety of security measures:

* The client may use the certificate authority's (CA's) "public" key to validate the CA's digital signature on the server certificate. If the digital signature can be verified, the client accepts the server certificate as a valid certificate issued by a trusted CA.
* The client verifies that the issuing CA is on its list of trusted CAs.
* The client checks the server's certificate validity period. The authentication process stops if the current date and time fall outside of the validity period.
* Protection against a downgrade of the protocol to a previous (less secure) version or a weaker cipher suite.
* Numbering all the Application records with a sequence number, and using this sequence number in the message authentication codes (MACs).
* Using a message digest enhanced with a key (so only a key-holder can check the MAC). This is specified in RFC 2104. "TLS only."
* The message that ends the handshake ("Finished") sends a hash of all the exchanged handshake messages 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-1), then XORs them together. This provides protection even if one of these algorithms is found to be vulnerable. "TLS only."
* SSL v3 improved upon SSL v2 by adding SHA-1 based ciphers, and support for certificate authentication. Additional improvements in SSL v3 include better handshake protocol flow and increased resistance to man-in-the-middle attacks.

SSL v2 is flawed in a [http://www.eucybervote.org/Reports/MSI-WP2-D7V1-V1.0-02.htm variety] of ways:
* Identical cryptographic keys are used for message authentication and encryption.
* MACs are unnecessarily weakened in the "export mode" required by U.S. export restrictions (symmetric key length was limited to 40 bits in Netscape and Internet Explorer).
* SSL v2 has a weak MAC construction and relies solely on the MD5 hash function.
* SSL v2 does not have any protection for the handshake, meaning a Man-in-the-middle downgrade attack can go undetected.
* SSL v2 uses the TCP connection close to indicate the end of data. This means that truncation attacks are possible: the attacker simply forges a TCP FIN, leaving the recipient unaware of an illegitimate end of data message (SSL v3 fixes this problem by having an explicit closure alert).

SSL v2 is disabled by default in Internet Explorer 7, [cite web
url = http://blogs.msdn.com/ie/archive/2005/10/22/483795.aspx
title = IEBlog : Upcoming HTTPS Improvements in Internet Explorer 7 Beta 2
accessdate = 2007-11-25
last = Lawrence
first = Eric
date = 2005-10-22
publisher = MSDN Blogs] Mozilla Firefox 2 and Mozilla Firefox 3, [cite web
url = https://bugzilla.mozilla.org/show_bug.cgi?id=236933
title = Bugzilla@Mozilla - Bug 236933 - Disable SSL2 and other weak ciphers
accessdate = 2007-11-25
publisher = Mozilla Corporation] 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 v2 format to maximize the likelihood of successfully handshaking with older servers. [cite web
url = https://bugzilla.mozilla.org/show_bug.cgi?id=454759
title = Firefox still sends SSLv2 handshake even though the protocol is disabled
date = 2008-09-11] . Support for SSL v2 (and weak 40-bit and 56-bit ciphers) has been removed completely from Opera as of version 9.5. [cite web
url = http://my.opera.com/yngve/blog/2007/04/30/10-years-of-ssl-in-opera
title = 10 years of SSL in Opera - Implementer's notes
accessdate = 2007-11-25
last = Pettersen
first = Yngve
date = 2007-04-30
publisher = Opera Software]

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 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.

History and development

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.Woo, Thomas Y. C. and Bindignavle, Raghuram and Su, Shaowen and Lam, Simon S. 1994. "SNP: An interface for secure network programming" In Usenix Summer Technical Conference] The SNP project received the 2004 ACM Software System Award.Association for Computing Machinery, [http://campus.acm.org/public/pressroom/press_releases/3_2005/ss_award_3_15_2005.cfm "ACM: Press Release, March 15, 2005"] , "campus.acm.org", accessed December 26, 2007. (English version).]

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.Fact|date=December 2007

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 keys because of US government restrictions on the export of cryptographic technology. 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.

tandards

The current approved version is 1.2, which is specified in:
* RFC 5246: “The Transport Layer Security (TLS) Protocol Version 1.2”.

The current standard obsoletes these former versions:
* RFC 2246: “The TLS Protocol Version 1.0”.
* RFC 4346: “The Transport Layer Security (TLS) Protocol Version 1.1”.

Other RFCs subsequently extended TLS, including:
* RFC 2595: “Using TLS with IMAP, POP3 and ACAP”. Specifies an extension to the IMAP, POP3 and ACAP services that allow the server and client to use transport-layer security to provide private, authenticated communication over the Internet.
* 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. Made obsolete by RFC 4366.
* RFC 3749: “Transport Layer Security Protocol Compression Methods”, specifies the framework for compression methods and the DEFLATE compression method.
* RFC 3943: “Transport Layer Security (TLS) Protocol Compression Using Lempel-Ziv-Stac (LZS)”.
* 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 4217: “Securing FTP with 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)”.
* RFC 4507: “Transport Layer Security (TLS) Session Resumption without Server-Side State”.
* RFC 4680: “TLS Handshake Message for Supplemental Data”.
* RFC 4681: “TLS User Mapping Extension”.
* RFC 4785: “Pre-Shared Key (PSK) Ciphersuites with NULL Encryption for Transport Layer Security (TLS)”.

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.

ee also

* Certificate authority
* Digital Certificate
* Extended Validation Certificate
* SSL acceleration
* Datagram Transport Layer Security
* Multiplexed Transport Layer Security
* X.509
* Virtual private network
* SEED
* Server gated cryptography

oftware

* OpenSSL: a free (and very popular) implementation (BSD license with some extensions)
* GnuTLS: a free LGPL licensed implementation
* JSSE: a Java implementation included in the Java Runtime Environment
* Network Security Services (NSS): FIPS 140 validated open source library
* [http://sourceforge.net/projects/perl-openssl/ Crypt::OpenSSL] : Perl wrapper modules for OpenSSL
* [http://msdn2.microsoft.com/en-us/library/system.net.security.aspx System.Net.Security] : a Microsoft implementation for the .NET Common Language Runtime
* [http://www.go-mono.com/docs/monodoc.ashx?tlink=0@N%3aMono.Security.Protocol.Tls Mono.Security] a free software implementation for the Common Language Runtime
* [http://www.yassl.com yaSSL] : a free implementation (GPL and commercial licenses available)

References and footnotes

Further reading

* cite conference
first = David
last = Wagner
coauthors = Schneier, Bruce
title = Analysis of the SSL 3.0 Protocol
booktitle = The Second USENIX Workshop on Electronic Commerce Proceedings
publisher = USENIX Press
month = November
year = 1996
page s= 29–40
url = http://www.schneier.com/paper-ssl.pdf
* cite book
author = Eric Rescorla,
title = SSL and TLS: Designing and Building Secure Systems
publisher = Addison-Wesley Pub Co
location = United States
year =
pages =
isbn = 0-201-61598-3
oclc =
doi =
* cite book
author = Stephen A. Thomas
title = SSL and TLS essentials securing the Web
publisher = Wiley
location = New York
year = 2000
pages =
isbn = 0-471-38354-6
oclc =
doi =
* cite journal|title=A Challenging But Feasible Blockwise-Adaptive Chosen-Plaintext Attack On Ssl|journal=International Association for Cryptologic Research|date=2006|first= Gregory|last=Bard|coauthors=|volume=|issue=136|pages=|id= |url=http://citeseer.ist.psu.edu/bard04vulnerability.html|format=|accessdate=2007-04-20
* cite web|url=http://lasecwww.epfl.ch/memo/memo_ssl.shtml |title=Password Interception in a SSL/TLS Channel |accessdate=2007-04-20 |last=Canvel |first=Brice

External links

* [http://www.mozilla.org/projects/security/pki/nss/ssl/draft02.html SSL 2 specification] (published 1994)
* [http://www.freesoft.org/CIE/Topics/ssl-draft/3-SPEC.HTM SSL 3.0 specification] (published 1996)
* [http://www.mozilla.org/projects/security/pki/nss/ssl/draft302.txt Netscape's final SSL 3.0 draft (1996)]
* [http://www.ietf.org/html.charters/tls-charter.html The IETF TLS Workgroup]
* [http://www2.rad.com/networks/2001/ssl/index.htm SSL tutorial]
* [http://apacheworld.org/ty24/site.chapter17.html Introduction to SSL/TLS and related Apache configuration]
* [http://ardoino.com/40-openssl-thread-safe-secure-connections/ OpenSSL thread safe connections tutorial with example source code]
* [http://www.semnanweb.com/ecmast-ecmascript-secure-transform/ ECMAScript Secure Transform (Web 2 Secure Transform Method)]