项目文献原文-SMTP Service Extension for Authentication

外文文献原文

SMTP Service Extension for Authentication

This document specifies an Internet standards track protocol for the Internet community,and requests discussion and suggestions for improvements.Please refer to the current edition ofthe"Internet O fficial Protocol Standards"(STD 1)for the standardization state and status o fthisprotoco l.Distribution o f this memo is unlimited.

Copyright Notice

Copyright(C)The Internet Society(1999).All Rights Reserved.

1. Intro d uc t io n

This document defines an SMTP service extension[ESMTP]whereby an SMTP clientmay indicate an authentication mechanism to the server,perform an authentication protocolexchange, and optionally negotiatea security layer for subsequent protocol interactions.Thisextension is a profile of the Simple Authentication and Security Layer[SASL].

2.Conventions Used in this Document

In examples, "C:"and"S:" indicate lines sentby the client and serverrespectively.The keywords "MUS T", "MUST NOT", "SHOULD", "SHOULD NOT",and"MAY" in this documentare to be interpreted as defined in"Key words for use in RFCs to Indicate Requirement Levels"[KEYWORDS].

3.The Authentication service extension

(1) the name of the SMTP service extension is "Authentication"

(2) the EHLO keyword value associated with this extension is "AUTH"

(3)The AUTH EHLO keyword contains as a parameter a space separated list of the namesof supported SASL mechanisms.

(4)a new SMTP verb"AUTH" is defined

(5) an optional parameter using the keyword "AUTH" is added to the MAIL FROMcommand, and extends the maximum line length of the MAIL FROM command by 500characte rs.

(6) this extension is appropriate for the submission protocol [SUBMIT].

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4.The AUTH command AUTH mechanism[initial-response]

Ar gume nts:a string identifying a SASL authentication mechanism. an optional base64-encodedre sp o nse

Restrictions:

After an AUTH command has successfully completed,no more AUTH commands may beissued in the same session.After a successful AUTH command completes, a server MUSTreject any further AUTH commands with a 503 reply.The AUTH command is not permittedduring a mail transaction.

Discussion:

The AUTH command indicates an authentication mechanism to the server. If the serversupports the requested authentication mechanism, it performs an authentication protocolexchange to authenticate and identify the user.Optionally, it also negotiates a security layer forsubsequent protocol interactions. If the requested authentication mechanism is not supported,the server rejects the AUTH command with a 504 reply.

The authentication protocol exchange consists of a series of server challenges and clientanswers that are specific to the authentication mechanism.A server challenge,otherwise knownas a ready response, is a 334 reply with the text part containing a BASE64 encoded string.Theclient answer consists of a line containing a BASE64 encoded string. If the client wishes tocancel an authentication exchange, it issues a line with a single "*". If the server receives suchan answer, it MUST reject the AUTH command by sending a 501 reply.

The optional initial-response argument to the AUTH command is used to save a round tripwhen using authentication mechanisms that are defined to send no data in the initial challenge.

When the initial-response argument is used with such a mechanism, the initial emptychallenge is not sent to the client and the server uses the data in the initial-response argument asif it were sent in response to the empty challenge.Unlike a zero-length client answer to a 334reply,a zero- length initial response is sent as a single equals sign("="). If the client uses aninitial-response argument to the AUTH command with a mechanism that sends data in theinitial challenge, the server rejects the AUTH command with a 535 reply.

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If the server cannot BASE64 decode the argument, it rejects the AUTH command with a501 reply. If the server rejects the authentication data, it SHOULD reject the AUTH commandwith a 535 reply unless a more specific error code, such as one listed in section 6, is appropriate.Should the client successfully complete the authentication exchange, the SMTP server issues a235 rep ly.

The service name specified by this protocol's profile of SASL is "smtp".

If a security layer is negotiated through the SASL authentication exchange, it takes effectimmediately following the CRLF that concludes the authentication exchange for the client,andthe CRLF of the success reply for the server.Upon a security layer's taking effect, the SMTPprotocol is reset to the initial state(the state in SMTP after a server issues a 220 service readygreeting).The server MUST discard any knowledge obtained from the client, such as theargument to the EHLO command,which was not obtained from the SASL negotiation itself.The client MUST discard any knowledge obtained from the server, such as the list of SMTPservice extensions,which was not obtained from the SASL negotiation itself (with theexception that a client MAY compare the list ofadvertised SASL mechanisms before and afterauthentication in order to detect an active down-negotiation attack).The client SHOULD sendan EHLO command as the first command after a successful SASL negotiation which results inthe enabling ofa security layer.

The server is not required to support any particular authentication mechanism,nor areauthentication mechanisms required to support any security layers. If an AUTH command fails,the client may try another authentication mechanismby issuing another AUTH command.

If an AUTH command fails, the server MUST behave the same as if the client had notissued the AUTH command.

The BASE64 string may in general be arbitrarily long.Clients and servers MUST be ableto support challenges and responses that are as long as are generated by the authenticationmechanisms they support, independent of any line length limitations the client or server mayhave in other parts of its protocol implementation.

Examples:

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S:220 smtp.example.com ESMTP server ready

C:EHLO j gm.example.com

S:250-smtp.example.com

S:250 AUTH CRAM-MD5 DIGEST-MD5

C:AUTH FOOBAR

S:504 Unrecognized authentication type.

C:AUTH CRAM-MD5

S:334

PENCeUxFREJoU0NnbmhN WitOMjNGNndAZWx3b29kLmlubm9 zb2 Z0 LmN vb T4=

C:ZnJlZC A5ZTk 1 YWVlMD ljNDBhZjJiODRhM GMyYjN iYmF lN zg2 ZQ==

S:235 Authentication successful.

5.The AUTH parameter to the MAILFROM command

AUTH=addr-spec

Ar gume nts:

An addr-spec containing the identity which submitted the message to the delivery system,or the two character sequence "<>" indicating such an identity is unknown or insufficientlyauthenticated.To comply with the restrictions imposed on ESMTP parameters, the addr-spec isencoded inside an xtext.The syntax of an xtext is described in section 5 of[ESMTP-DSN].

Discussion:

The optional AUTH parameter to the MAIL FROM command allows cooperating agentsin a trusted environment to communicate the authentication o f individual message s.

Ifthe server trusts the authenticated identity of the client toassert that the message was originally submitted by the supplied addr-spec, then the serverSHOULD supply the same addr-spec in an AUTH parameter when relaying the message to any参考设计材料包含项目源代码屏幕录像指导、项目运行截图、项目设计说明书、任务书、报告书以及文献参考翻译等完整的

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server which supports the AUTH extension.

A MAIL FROM parameter of AUTH=<> indicates that the original submitter of themessage is not known.The server MUST NOT treat the message as having been originallysubmitted by the c lient.

If the AUTH parameter to the MAIL FROM is not supplied, the client has authenticated,and the server believes the message is an original submission by the client, the server MAYsupply the client's identity in the addr-spec in an AUTH parameter when relaying the messageto any server which supports the AUTH extension.

If the server does not sufficiently trust the authenticated identity of the client, or if theclient is not authenticated, then the server MUST behave as if the AUTH=<>parameter wassupplied.The server MAY,however,write the value of the AUTH parameter to a log file.

If anAUTH=<>parameter was supplied,either explicitly or due to the requirement in theprevious paragraph, then the server MUST supply the AUTH=<>parameter when relaying themessage to any server which it has authenticated to using the AUTH extension.

A server MAY treat expansion of a mailing list as a new submission, setting the AUTHparameter to the mailing list address or mailing list administration address when relaying themessage to list subscribers.

It is conforming for an implementation to be hard-coded to treat all clients as beinginsufficiently trusted. In that case, the implementation does nothing more than parse anddiscard syntactically valid AUTH parameters to the MAIL FROM command and supplyAUTH=<>parameters to any servers to which it authenticates using the AUTH extension.

Examples:

C:MAILFROM:<e=mc2@example.com>AUTH=e+3Dmc2@example.com

S:250 OK

6.Error Codes

The following error codes maybe used to indicate various conditions as described.

432 A password transition is needed

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This response to the AUTH command indicates that the user needs to transition to theselected authentication mechanism. This typically done by authenticating once using thePLAIN authentication mechanism.

534 Authentication mechanism is too weak

This response to the AUTH command indicates that the selected authenticationmechanism is weaker than server policy permits for that user.

538 Encryption required for requested authentication mechanism

This response to the AUTH command indicates that the selected authenticationmechanism may only be used when the underlying SMTP connection is encrypted.

454 Temporary authentication failure

This response to the AUTH command indicates that the authentication failed due to atemporary server failure.

530 Authentication required

This response maybe returned by any command other than AUTH,EHLO,HELO,NOOP,RSET,or QUIT. It indicates that server policy requires authentication in order to perform therequested action.

7.Formal S yntax

The following syntax specification uses the augmented Backus-Naur Form(BNF)notation as specified in[ABNF].

Except as noted otherwise,all alphabetic characters are case- insensitive.The use of upperor lower case characters to define token strings is for editorial clarity only. ImplementationsMUST accept these strings in a case-insensitive fashion.

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UPALPHA=%x41-5A; ;Uppercase:A-Z

LO ALPHA=%x61-7A;;Low ercase:a-z

ALPHA=UPALPHA/LOALPHA;;case insensitive

DIGIT=%x30-39;;Digits 0-9

HEXDIGIT=%x41-46/DIGIT; ;hexidec imal digit(upp ercase)hexchar="+"HEXDIGIT HEXDIGITxchar=%x21-2A/%x2C-3C/%x3E-7E; ;US-ASCII except for "+", "=",SPACE andCTLxtext=*(xchar/hexchar)

AUTH_CHAR=ALPHA/DIGIT/ "-" / "_"

auth_type=1*20AUTH_CHARauth_command="AUTH"SPAC E auth_type [SPAC E(base64/ "=")]*(CRLF[base64])CRLFauth_param= "AUTH=" xtext;; The decoded form of the xtext MUST be either;; anaddr-spec or the two characters "<>"base64=base64_terminal/( 1*(4b ase64_CHAR) [base64_terminal] )base64_c har=UPALP HA/LOALPHA/DI GIT/ "+"/ "/"; ;C ase-se ns itivebase64_terminal=(2bas e64_char "==")/(3b ase64_char "=")continue_req="334"SPACE[base64]C RLF

CR=%x0C;;ASCII CR,carriage return

CRLF=CR LF

CTL=%x00-1F/%x7F; ;any ASCII control character and DEL

LF=%x0A; ;ASCII LF, line feed

SPACE=%x20; ;ASCII SP, space

8.References

[ABNF]Crocker,D.and P.Overell, "Augmented BNF for Syntax Specifications:ABNF",RFC2234,November 1997.

[CRAM-MD5]Klensin, J., Catoe, R. and P.Krumviede, "IMAP/POP AUTHorize参考设计材料包含项目源代码屏幕录像指导、项目运行截图、项目设计说明书、任务书、报告书以及文献参考翻译等完整的

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Extension for Simple Challenge/Response",RFC 2195,September 1997.

[ESMTP]Klensin,J.,Freed,N.,Rose,M., Stefferud,E. and D.Crocker, "SMTP ServiceExtensions",RFC1869,November 1995.

[ESMTP-DSN]Moore,K, "SMTP Service Extension for Delivery Status Notifications",RFC 1891,January 1996.

[KEYWORDS]Bradner,S., "Key words for use in RFCs to Indicate Requirement Levels",BCP 14,RFC2119,March 1997

[SASL]Myers, J., "Simple Authentication and Security Layer (SASL)",RFC2222,October 1997.

[SUBMIT]Gellens,R. and J.Klensin, "Message Submission",RFC 2476,December

1998.

[RFC821]Postel,J., "Simple Mail Transfer Protocol",STD 10,RFC 821,August 1982.

[RFC822]Crocker,D., "Standard for the Format of ARPA Internet Text Messages",STD11,RFC822,August 1982.

9.Security Considerations

Security issues are discussed throughout this memo.

If a client uses this extension to get an encrypted tunnel through an insecure network to acooperating server, it needs to be configured to never send mail to that server when theconnection is not mutually authenticated and encrypted.Otherwise,an attacker could steal theclient's mail by hijacking the SMTP connection and either pretending the server does notsupport the Authentication extension or causing all AUTH commands to fail.

Before the SASL negotiation has begun, any protocol interactions are performed in theclear and may be modified by an active attacker.For this reason, clients and servers MUSTdiscard anyknowledge obtained prior to the start ofthe SASL negotiation upon completion of aSASL negotiation which results in a security layer.

This mechanism does not protect the TCP port, so an active attacker may redirect a relayconnection attempt to the submis sion port [SUBMIT].The AUTH=<>parameter prevents suchan attack from causing an relayed message without an envelope authentication to pick up theauthentication ofthe relay client.

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A message submission client may require the user to authenticate whenever a suitableSASL mechanism is advertised.Therefore, it may not be desirable for a submission server[SUBMIT] to advertise a SASL mechanism when use of that mechanism grants the client nobenefits over anonymous submis sion.

This extension is not intended to replace or be used instead of end- to-end messagesignature and encryption systems such as S/MIME or PGP.This extension addresses a differentproblem than end-to-end systems; it has the following key differences:

(1) it is generally useful only within a trusted enclave

(2) it protects the entire envelope of a message,not just the message's body.

(3) it authenticates the message submission,not authorship of the message content

(4) it can give the sender some assurance the message was delivered to the next hop in thecase where the sender mutually authenticates with the next hop and negotiates an appropriatesecurity layer.

Additional security considerations are mentioned in the SASL specification[SASL].参考设计材料包含项目源代码屏幕录像指导、项目运行截图、项目设计说明书、任务书、报告书以及文献参考翻译等完整的

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