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Network Working Group                                          C. NewmanRequest for Comments: 2595                                      InnosoftCategory: Standards Track                                      June 1999Using TLS with IMAP, POP3 and ACAPStatus of this Memo   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 of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (1999).  All Rights Reserved.1. Motivation   The TLS protocol (formerly known as SSL) provides a way to secure an   application protocol from tampering and eavesdropping.  The option of   using such security is desirable for IMAP, POP and ACAP due to common   connection eavesdropping and hijacking attacks [AUTH].  Although   advanced SASL authentication mechanisms can provide a lightweight   version of this service, TLS is complimentary to simple   authentication-only SASL mechanisms or deployed clear-text password   login commands.   Many sites have a high investment in authentication infrastructure   (e.g., a large database of a one-way-function applied to user   passwords), so a privacy layer which is not tightly bound to user   authentication can protect against network eavesdropping attacks   without requiring a new authentication infrastructure and/or forcing   all users to change their password.  Recognizing that such sites will   desire simple password authentication in combination with TLS   encryption, this specification defines the PLAIN SASL mechanism for   use with protocols which lack a simple password authentication   command such as ACAP and SMTP.  (Note there is a separate RFC for the   STARTTLS command in SMTP [SMTPTLS].)   There is a strong desire in the IETF to eliminate the transmission of   clear-text passwords over unencrypted channels.  While SASL can be   used for this purpose, TLS provides an additional tool with different   deployability characteristics.  A server supporting both TLS withNewman                      Standards Track                     [Page 1]

RFC 2595           Using TLS with IMAP, POP3 and ACAP          June 1999   simple passwords and a challenge/response SASL mechanism is likely to   interoperate with a wide variety of clients without resorting to   unencrypted clear-text passwords.   The STARTTLS command rectifies a number of the problems with using a   separate port for a "secure" protocol variant.  Some of these are   mentioned insection 7.1.1. Conventions Used in this Document   The key words "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD NOT",   "MAY", and "OPTIONAL" in this document are to be interpreted as   described in "Key words for use in RFCs to Indicate Requirement   Levels" [KEYWORDS].   Terms related to authentication are defined in "On Internet   Authentication" [AUTH].   Formal syntax is defined using ABNF [ABNF].   In examples, "C:" and "S:" indicate lines sent by the client and   server respectively.2. Basic Interoperability and Security Requirements   The following requirements apply to all implementations of the   STARTTLS extension for IMAP, POP3 and ACAP.2.1. Cipher Suite Requirements   Implementation of the TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA [TLS] cipher   suite is REQUIRED.  This is important as it assures that any two   compliant implementations can be configured to interoperate.   All other cipher suites are OPTIONAL.2.2. Privacy Operational Mode Security Requirements   Both clients and servers SHOULD have a privacy operational mode which   refuses authentication unless successful activation of an encryption   layer (such as that provided by TLS) occurs prior to or at the time   of authentication and which will terminate the connection if that   encryption layer is deactivated.  Implementations are encouraged to   have flexability with respect to the minimal encryption strength or   cipher suites permitted.  A minimalist approach to this   recommendation would be an operational mode where the   TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA cipher suite is mandatory prior to   permitting authentication.Newman                      Standards Track                     [Page 2]

RFC 2595           Using TLS with IMAP, POP3 and ACAP          June 1999   Clients MAY have an operational mode which uses encryption only when   it is advertised by the server, but authentication continues   regardless.  For backwards compatibility, servers SHOULD have an   operational mode where only the authentication mechanisms required by   the relevant base protocol specification are needed to successfully   authenticate.2.3. Clear-Text Password Requirements   Clients and servers which implement STARTTLS MUST be configurable to   refuse all clear-text login commands or mechanisms (including both   standards-track and nonstandard mechanisms) unless an encryption   layer of adequate strength is active.  Servers which allow   unencrypted clear-text logins SHOULD be configurable to refuse   clear-text logins both for the entire server, and on a per-user   basis.2.4. Server Identity Check   During the TLS negotiation, the client MUST check its understanding   of the server hostname against the server's identity as presented in   the server Certificate message, in order to prevent man-in-the-middle   attacks.  Matching is performed according to these rules:   - The client MUST use the server hostname it used to open the     connection as the value to compare against the server name as     expressed in the server certificate.  The client MUST NOT use any     form of the server hostname derived from an insecure remote source     (e.g., insecure DNS lookup).  CNAME canonicalization is not done.   - If a subjectAltName extension of type dNSName is present in the     certificate, it SHOULD be used as the source of the server's     identity.   - Matching is case-insensitive.   - A "*" wildcard character MAY be used as the left-most name     component in the certificate.  For example, *.example.com would     match a.example.com, foo.example.com, etc. but would not match     example.com.   - If the certificate contains multiple names (e.g. more than one     dNSName field), then a match with any one of the fields is     considered acceptable.   If the match fails, the client SHOULD either ask for explicit user   confirmation, or terminate the connection and indicate the server's   identity is suspect.Newman                      Standards Track                     [Page 3]

RFC 2595           Using TLS with IMAP, POP3 and ACAP          June 19992.5. TLS Security Policy Check   Both the client and server MUST check the result of the STARTTLS   command and subsequent TLS negotiation to see whether acceptable   authentication or privacy was achieved.  Ignoring this step   completely invalidates using TLS for security.  The decision about   whether acceptable authentication or privacy was achieved is made   locally, is implementation-dependent, and is beyond the scope of this   document.3. IMAP STARTTLS extension   When the TLS extension is present in IMAP, "STARTTLS" is listed as a   capability in response to the CAPABILITY command.  This extension   adds a single command, "STARTTLS" to the IMAP protocol which is used   to begin a TLS negotiation.3.1. STARTTLS Command   Arguments:  none   Responses:  no specific responses for this command   Result:     OK - begin TLS negotiation               BAD - command unknown or arguments invalid      A TLS negotiation begins immediately after the CRLF at the end of      the tagged OK response from the server.  Once a client issues a      STARTTLS command, it MUST NOT issue further commands until a      server response is seen and the TLS negotiation is complete.      The STARTTLS command is only valid in non-authenticated state.      The server remains in non-authenticated state, even if client      credentials are supplied during the TLS negotiation.  The SASL      [SASL] EXTERNAL mechanism MAY be used to authenticate once TLS      client credentials are successfully exchanged, but servers      supporting the STARTTLS command are not required to support the      EXTERNAL mechanism.      Once TLS has been started, the client MUST discard cached      information about server capabilities and SHOULD re-issue the      CAPABILITY command.  This is necessary to protect against      man-in-the-middle attacks which alter the capabilities list prior      to STARTTLS.  The server MAY advertise different capabilities      after STARTTLS.      The formal syntax for IMAP is amended as follows:Newman                      Standards Track                     [Page 4]

RFC 2595           Using TLS with IMAP, POP3 and ACAP          June 1999        command_any   =/  "STARTTLS"   Example:    C: a001 CAPABILITY               S: * CAPABILITY IMAP4rev1 STARTTLS LOGINDISABLED               S: a001 OK CAPABILITY completed               C: a002 STARTTLS               S: a002 OK Begin TLS negotiation now               <TLS negotiation, further commands are under TLS layer>               C: a003 CAPABILITY               S: * CAPABILITY IMAP4rev1 AUTH=EXTERNAL               S: a003 OK CAPABILITY completed               C: a004 LOGIN joe password               S: a004 OK LOGIN completed3.2. IMAP LOGINDISABLED capability   The current IMAP protocol specification (RFC 2060) requires the   implementation of the LOGIN command which uses clear-text passwords.   Many sites may choose to disable this command unless encryption is   active for security reasons.  An IMAP server MAY advertise that the   LOGIN command is disabled by including the LOGINDISABLED capability   in the capability response.  Such a server will respond with a tagged   "NO" response to any attempt to use the LOGIN command.   An IMAP server which implements STARTTLS MUST implement support for   the LOGINDISABLED capability on unencrypted connections.   An IMAP client which complies with this specification MUST NOT issue   the LOGIN command if this capability is present.   This capability is useful to prevent clients compliant with this   specification from sending an unencrypted password in an environment   subject to passive attacks.  It has no impact on an environment   subject to active attacks as a man-in-the-middle attacker can remove   this capability.  Therefore this does not relieve clients of the need   to follow the privacy mode recommendation insection 2.2.   Servers advertising this capability will fail to interoperate with   many existing compliant IMAP clients and will be unable to prevent   those clients from disclosing the user's password.4. POP3 STARTTLS extension   The POP3 STARTTLS extension adds the STLS command to POP3 servers.   If this is implemented, the POP3 extension mechanism [POP3EXT] MUST   also be implemented to avoid the need for client probing of multiple   commands.  The capability name "STLS" indicates this command is   present and permitted in the current state.Newman                      Standards Track                     [Page 5]

RFC 2595           Using TLS with IMAP, POP3 and ACAP          June 1999      STLS         Arguments: none         Restrictions:             Only permitted in AUTHORIZATION state.         Discussion:             A TLS negotiation begins immediately after the CRLF at the             end of the +OK response from the server.  A -ERR response             MAY result if a security layer is already active.  Once a             client issues a STLS command, it MUST NOT issue further             commands until a server response is seen and the TLS             negotiation is complete.             The STLS command is only permitted in AUTHORIZATION state             and the server remains in AUTHORIZATION state, even if             client credentials are supplied during the TLS negotiation.             The AUTH command [POP-AUTH] with the EXTERNAL mechanism             [SASL] MAY be used to authenticate once TLS client             credentials are successfully exchanged, but servers             supporting the STLS command are not required to support the             EXTERNAL mechanism.             Once TLS has been started, the client MUST discard cached             information about server capabilities and SHOULD re-issue             the CAPA command.  This is necessary to protect against             man-in-the-middle attacks which alter the capabilities list             prior to STLS.  The server MAY advertise different             capabilities after STLS.         Possible Responses:             +OK -ERR         Examples:             C: STLS             S: +OK Begin TLS negotiation             <TLS negotiation, further commands are under TLS layer>               ...             C: STLS             S: -ERR Command not permitted when TLS activeNewman                      Standards Track                     [Page 6]

RFC 2595           Using TLS with IMAP, POP3 and ACAP          June 19995. ACAP STARTTLS extension   When the TLS extension is present in ACAP, "STARTTLS" is listed as a   capability in the ACAP greeting.  No arguments to this capability are   defined at this time.  This extension adds a single command,   "STARTTLS" to the ACAP protocol which is used to begin a TLS   negotiation.5.1. STARTTLS Command   Arguments:  none   Responses:  no specific responses for this command   Result:     OK - begin TLS negotiation               BAD - command unknown or arguments invalid      A TLS negotiation begins immediately after the CRLF at the end of      the tagged OK response from the server.  Once a client issues a      STARTTLS command, it MUST NOT issue further commands until a      server response is seen and the TLS negotiation is complete.      The STARTTLS command is only valid in non-authenticated state.      The server remains in non-authenticated state, even if client      credentials are supplied during the TLS negotiation.  The SASL      [SASL] EXTERNAL mechanism MAY be used to authenticate once TLS      client credentials are successfully exchanged, but servers      supporting the STARTTLS command are not required to support the      EXTERNAL mechanism.      After the TLS layer is established, the server MUST re-issue an      untagged ACAP greeting.  This is necessary to protect against      man-in-the-middle attacks which alter the capabilities list prior      to STARTTLS.  The client MUST discard cached capability      information and replace it with the information from the new ACAP      greeting.  The server MAY advertise different capabilities after      STARTTLS.      The formal syntax for ACAP is amended as follows:        command_any   =/  "STARTTLS"   Example:    S: * ACAP (SASL "CRAM-MD5") (STARTTLS)               C: a002 STARTTLS               S: a002 OK "Begin TLS negotiation now"               <TLS negotiation, further commands are under TLS layer>               S: * ACAP (SASL "CRAM-MD5" "PLAIN" "EXTERNAL")Newman                      Standards Track                     [Page 7]

RFC 2595           Using TLS with IMAP, POP3 and ACAP          June 19996. PLAIN SASL mechanism   Clear-text passwords are simple, interoperate with almost all   existing operating system authentication databases, and are useful   for a smooth transition to a more secure password-based   authentication mechanism.  The drawback is that they are unacceptable   for use over an unencrypted network connection.   This defines the "PLAIN" SASL mechanism for use with ACAP and other   protocols with no clear-text login command.  The PLAIN SASL mechanism   MUST NOT be advertised or used unless a strong encryption layer (such   as the provided by TLS) is active or backwards compatibility dictates   otherwise.   The mechanism consists of a single message from the client to the   server.  The client sends the authorization identity (identity to   login as), followed by a US-ASCII NUL character, followed by the   authentication identity (identity whose password will be used),   followed by a US-ASCII NUL character, followed by the clear-text   password.  The client may leave the authorization identity empty to   indicate that it is the same as the authentication identity.   The server will verify the authentication identity and password with   the system authentication database and verify that the authentication   credentials permit the client to login as the authorization identity.   If both steps succeed, the user is logged in.   The server MAY also use the password to initialize any new   authentication database, such as one suitable for CRAM-MD5   [CRAM-MD5].   Non-US-ASCII characters are permitted as long as they are represented   in UTF-8 [UTF-8].  Use of non-visible characters or characters which   a user may be unable to enter on some keyboards is discouraged.   The formal grammar for the client message using Augmented BNF [ABNF]   follows.   message         = [authorize-id] NUL authenticate-id NUL password   authenticate-id = 1*UTF8-SAFE      ; MUST accept up to 255 octets   authorize-id    = 1*UTF8-SAFE      ; MUST accept up to 255 octets   password        = 1*UTF8-SAFE      ; MUST accept up to 255 octets   NUL             = %x00   UTF8-SAFE       = %x01-09 / %x0B-0C / %x0E-7F / UTF8-2 /                     UTF8-3 / UTF8-4 / UTF8-5 / UTF8-6   UTF8-1          = %x80-BF   UTF8-2          = %xC0-DF UTF8-1   UTF8-3          = %xE0-EF 2UTF8-1Newman                      Standards Track                     [Page 8]

RFC 2595           Using TLS with IMAP, POP3 and ACAP          June 1999   UTF8-4          = %xF0-F7 3UTF8-1   UTF8-5          = %xF8-FB 4UTF8-1   UTF8-6          = %xFC-FD 5UTF8-1   Here is an example of how this might be used to initialize a CRAM-MD5   authentication database for ACAP:   Example:    S: * ACAP (SASL "CRAM-MD5") (STARTTLS)               C: a001 AUTHENTICATE "CRAM-MD5"               S: + "<1896.697170952@postoffice.reston.mci.net>"               C: "tim b913a602c7eda7a495b4e6e7334d3890"               S: a001 NO (TRANSITION-NEEDED)                  "Please change your password, or use TLS to login"               C: a002 STARTTLS               S: a002 OK "Begin TLS negotiation now"               <TLS negotiation, further commands are under TLS layer>               S: * ACAP (SASL "CRAM-MD5" "PLAIN" "EXTERNAL")               C: a003 AUTHENTICATE "PLAIN" {21+}               C: <NUL>tim<NUL>tanstaaftanstaaf               S: a003 OK CRAM-MD5 password initialized   Note: In this example, <NUL> represents a single ASCII NUL octet.7. imaps and pop3s ports   Separate "imaps" and "pop3s" ports were registered for use with SSL.   Use of these ports is discouraged in favor of the STARTTLS or STLS   commands.   A number of problems have been observed with separate ports for   "secure" variants of protocols.  This is an attempt to enumerate some   of those problems.   - Separate ports lead to a separate URL scheme which intrudes into     the user interface in inappropriate ways.  For example, many web     pages use language like "click here if your browser supports SSL."     This is a decision the browser is often more capable of making than     the user.   - Separate ports imply a model of either "secure" or "not secure."     This can be misleading in a number of ways.  First, the "secure"     port may not in fact be acceptably secure as an export-crippled     cipher suite might be in use.  This can mislead users into a false     sense of security.  Second, the normal port might in fact be     secured by using a SASL mechanism which includes a security layer.     Thus the separate port distinction makes the complex topic of     security policy even more confusing.  One common result of this     confusion is that firewall administrators are often misled intoNewman                      Standards Track                     [Page 9]

RFC 2595           Using TLS with IMAP, POP3 and ACAP          June 1999     permitting the "secure" port and blocking the standard port.  This     could be a poor choice given the common use of SSL with a 40-bit     key encryption layer and plain-text password authentication is less     secure than strong SASL mechanisms such as GSSAPI with Kerberos 5.   - Use of separate ports for SSL has caused clients to implement only     two security policies: use SSL or don't use SSL.  The desirable     security policy "use TLS when available" would be cumbersome with     the separate port model, but is simple with STARTTLS.   - Port numbers are a limited resource.  While they are not yet in     short supply, it is unwise to set a precedent that could double (or     worse) the speed of their consumption.8. IANA Considerations   This constitutes registration of the "STARTTLS" and "LOGINDISABLED"   IMAP capabilities as required bysection 7.2.1 of RFC 2060 [IMAP].   The registration for the POP3 "STLS" capability follows:   CAPA tag:                   STLS   Arguments:                  none   Added commands:             STLS   Standard commands affected: May enable USER/PASS as a side-effect.     CAPA command SHOULD be re-issued after successful completion.   Announced states/Valid states: AUTHORIZATION state only.   Specification reference:    this memo   The registration for the ACAP "STARTTLS" capability follows:   Capability name:            STARTTLS   Capability keyword:         STARTTLS   Capability arguments:       none   Published Specification(s): this memo   Person and email address for further information:       see author's address section below   The registration for the PLAIN SASL mechanism follows:   SASL mechanism name:        PLAIN   Security Considerations:    Seesection 9 of this memo   Published specification:    this memo   Person & email address to contact for further information:       see author's address section below   Intended usage:             COMMON   Author/Change controller:   see author's address section belowNewman                      Standards Track                    [Page 10]

RFC 2595           Using TLS with IMAP, POP3 and ACAP          June 19999. Security Considerations   TLS only provides protection for data sent over a network connection.   Messages transferred over IMAP or POP3 are still available to server   administrators and usually subject to eavesdropping, tampering and   forgery when transmitted through SMTP or NNTP.  TLS is no substitute   for an end-to-end message security mechanism using MIME security   multiparts [MIME-SEC].   A man-in-the-middle attacker can remove STARTTLS from the capability   list or generate a failure response to the STARTTLS command.  In   order to detect such an attack, clients SHOULD warn the user when   session privacy is not active and/or be configurable to refuse to   proceed without an acceptable level of security.   A man-in-the-middle attacker can always cause a down-negotiation to   the weakest authentication mechanism or cipher suite available.  For   this reason, implementations SHOULD be configurable to refuse weak   mechanisms or cipher suites.   Any protocol interactions prior to the TLS handshake are performed in   the clear and can be modified by a man-in-the-middle attacker.  For   this reason, clients MUST discard cached information about server   capabilities advertised prior to the start of the TLS handshake.   Clients are encouraged to clearly indicate when the level of   encryption active is known to be vulnerable to attack using modern   hardware (such as encryption keys with 56 bits of entropy or less).   The LOGINDISABLED IMAP capability (discussed insection 3.2) only   reduces the potential for passive attacks, it provides no protection   against active attacks.  The responsibility remains with the client   to avoid sending a password over a vulnerable channel.   The PLAIN mechanism relies on the TLS encryption layer for security.   When used without TLS, it is vulnerable to a common network   eavesdropping attack.  Therefore PLAIN MUST NOT be advertised or used   unless a suitable TLS encryption layer is active or backwards   compatibility dictates otherwise.   When the PLAIN mechanism is used, the server gains the ability to   impersonate the user to all services with the same password   regardless of any encryption provided by TLS or other network privacy   mechanisms.  While many other authentication mechanisms have similar   weaknesses, stronger SASL mechanisms such as Kerberos address this   issue.  Clients are encouraged to have an operational mode where all   mechanisms which are likely to reveal the user's password to the   server are disabled.Newman                      Standards Track                    [Page 11]

RFC 2595           Using TLS with IMAP, POP3 and ACAP          June 1999   The security considerations for TLS apply to STARTTLS and the   security considerations for SASL apply to the PLAIN mechanism.   Additional security requirements are discussed insection 2.10. References   [ABNF]     Crocker, D. and P. Overell, "Augmented BNF for Syntax              Specifications: ABNF",RFC 2234, November 1997.   [ACAP]     Newman, C. and J. Myers, "ACAP -- Application              Configuration Access Protocol",RFC 2244, November 1997.   [AUTH]     Haller, N. and R. Atkinson, "On Internet Authentication",RFC 1704, October 1994.   [CRAM-MD5] Klensin, J., Catoe, R. and P. Krumviede, "IMAP/POP              AUTHorize Extension for Simple Challenge/Response",RFC2195, September 1997.   [IMAP]     Crispin, M., "Internet Message Access Protocol - Version              4rev1",RFC 2060, December 1996.   [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [MIME-SEC] Galvin, J., Murphy, S., Crocker, S. and N. Freed,              "Security Multiparts for MIME: Multipart/Signed and              Multipart/Encrypted",RFC 1847, October 1995.   [POP3]     Myers, J. and M. Rose, "Post Office Protocol - Version 3",              STD 53,RFC 1939, May 1996.   [POP3EXT]  Gellens, R., Newman, C. and L. Lundblade, "POP3 Extension              Mechanism",RFC 2449, November 1998.   [POP-AUTH] Myers, J., "POP3 AUTHentication command",RFC 1734,              December 1994.   [SASL]     Myers, J., "Simple Authentication and Security Layer              (SASL)",RFC 2222, October 1997.   [SMTPTLS]  Hoffman, P., "SMTP Service Extension for Secure SMTP over              TLS",RFC 2487, January 1999.   [TLS]      Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",RFC 2246, January 1999.Newman                      Standards Track                    [Page 12]

RFC 2595           Using TLS with IMAP, POP3 and ACAP          June 1999   [UTF-8]    Yergeau, F., "UTF-8, a transformation format of ISO              10646",RFC 2279, January 1998.11. Author's Address   Chris Newman   Innosoft International, Inc.   1050 Lakes Drive   West Covina, CA 91790 USA   EMail: chris.newman@innosoft.comA. Appendix -- Compliance Checklist   An implementation is not compliant if it fails to satisfy one or more   of the MUST requirements for the protocols it implements.  An   implementation that satisfies all the MUST and all the SHOULD   requirements for its protocols is said to be "unconditionally   compliant"; one that satisfies all the MUST requirements but not all   the SHOULD requirements for its protocols is said to be   "conditionally compliant".   Rules                                                 Section   -----                                                 -------   Mandatory-to-implement Cipher Suite                      2.1   SHOULD have mode where encryption required               2.2   server SHOULD have mode where TLS not required           2.2   MUST be configurable to refuse all clear-text login     commands or mechanisms                                 2.3   server SHOULD be configurable to refuse clear-text     login commands on entire server and on per-user basis  2.3   client MUST check server identity                        2.4   client MUST use hostname used to open connection         2.4   client MUST NOT use hostname from insecure remote lookup 2.4   client SHOULD support subjectAltName of dNSName type     2.4   client SHOULD ask for confirmation or terminate on fail  2.4   MUST check result of STARTTLS for acceptable privacy     2.5   client MUST NOT issue commands after STARTTLS      until server response and negotiation done        3.1,4,5.1   client MUST discard cached information             3.1,4,5.1,9   client SHOULD re-issue CAPABILITY/CAPA command       3.1,4   IMAP server with STARTTLS MUST implement LOGINDISABLED   3.2   IMAP client MUST NOT issue LOGIN if LOGINDISABLED        3.2   POP server MUST implement POP3 extensions                4   ACAP server MUST re-issue ACAP greeting                  5.1Newman                      Standards Track                    [Page 13]

RFC 2595           Using TLS with IMAP, POP3 and ACAP          June 1999   client SHOULD warn when session privacy not active and/or     refuse to proceed without acceptable security level    9   SHOULD be configurable to refuse weak mechanisms or     cipher suites                                          9   The PLAIN mechanism is an optional part of this specification.   However if it is implemented the following rules apply:   Rules                                                 Section   -----                                                 -------   MUST NOT use PLAIN unless strong encryption active     or backwards compatibility dictates otherwise         6,9   MUST use UTF-8 encoding for characters in PLAIN          6Newman                      Standards Track                    [Page 14]

RFC 2595           Using TLS with IMAP, POP3 and ACAP          June 1999Full Copyright Statement   Copyright (C) The Internet Society (1999).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Newman                      Standards Track                    [Page 15]