DKIM Third-Party Authorization for Sender Signer Practices

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Abstract

TPA-label is a DNS-based prefix mechanism for DKIM policy records as a means to authorize Third-Party domains. This mechanism allows first-party domains to autonomously authorize a range of third-party domains in a scalable, individual DNS transaction. This authorization extends the scope of DKIM policy assertions to supplant more difficult to administer mechanisms. Alternatives for facilitating third-party authorizations currently necessitate the coordination between two or more domains by setting up selector/key DNS records, DNS zone delegations, or the regular exchange of public/private keys.

Checking DKIM policies may occur when a From header email-address is not within the domain of a valid DKIM signature. When a Third-Party signature is found, TPA-labels offer an efficient means for email address domains to authorize specific third-party signing domains. The scope of the authorization may separately assert identity authentication for From and Sender and Resent-* headers for email-addresses within the authorizing domain. Identity authentication can be asserted by the scope of the authorization, even when signed by a Third-Party domain. In addition, the RFC2821.MailFrom domain can authorize domains for controlling DSNs.

Requirements Language

1. Introduction
2. Envelope Authorizations
3. Evaluating Signing Domains
4. Authorization Scope
5. TPA-labeled Policy Assertions
6. Scope
    6.1. From Originator Header Field
    6.2. MailFrom Parameter
    6.3. Other than From Originating Header Fields
    6.4. NO-TPA
7. Authorized Signing Domain
8. Use of TPA-labeled Policy
9. Restricted Local-part
10. IANA Considerations
11. Security Considerations
12. Acknowledgements
13. References
    13.1. Normative References
    13.2. Informative References
Appendix A. Change Log
Appendix B. DNS Example of TPA-label policy record placement
Appendix C. C code for label generation
§ Author's Address
§ Intellectual Property and Copyright Statements

1. Introduction

TPA-labels authorize signing domains to permit compliance with DKIM policies. TPA-label authorized domains are to be considered equivalent to the authorizing domain for the application of DKIM policies. This mechanism eliminates the complex coordination of selector/key DNS records, DNS delegation, or exchanges of public/private keys between two or more domains otherwise required to facilitate desired authorizations.

Trust is an essential requisite before the DKIM signature header field's 'i=' semantics or the TPA-label policy record's "-i" suffix scope assertions provide valuable advisory information. This advisory information regarding "on-behalf-of" identity authentication enables safer message annotations to better ensure trusted identities can be recognized.

TPA-labeled policy records also convey which third-party domains are authoritative. Although third-party domains are unable to utilize DKIM signature's 'i=' semantics to assert which identifiers on whose behalf a signature was added, the related identity can be constrained by the policy record's 'g=' parameter and by permitted header fields specified by the policy scope parameter. Policy scoping in this manner enables control over the recognition of trusted identities, as well as indicating when a message source may require added scrutiny.

2. Envelope Authorizations

DKIM is fully independent of the SMTP Client and the [RFC2821] (Klensin, J., “Simple Mail Transfer Protocol,” April 2001.).MailFrom email-address. Depending upon the normal policy assertions, allowing the [RFC2821] (Klensin, J., “Simple Mail Transfer Protocol,” April 2001.).MailFrom email-address to authorize signing domains and assert policies may prevent Delivery Status Notifications (DSNs) from being sent when the domain is not authorized. Conversely, authorization may ensure DSNs are not dropped when the signing domain is authorized. The application of TPA-labels at the MailFrom domain represents an entirely optional strategy which may or may not prove either effective or practical. The MailFrom scope is offered only for experimentation.

3. Evaluating Signing Domains

Regulatory agencies are unable to control Internet abuse by curtailing access. Unlike IPv4 addresses, there is virtually no limit on the number of domain-names available. Registrar pricing of domain-names need to remain uniform. Otherwise, fees based upon the intrinsic value of a name could cause name holders to become extortion targets. High initial costs for domain-names are also unlikely to represent a deterrent, largely due to high levels of payment fraud. Driven by market forces, registrars also offer the sampling of domain names (domain tasting) to generate additional revenue.

In addition, DKIM can not directly identify the domain transmitting the message, and can not prevent abusive message replay. Abusive message replay may prove indistinguishable from bulk mailings of various types. Since abuse may not be controlled by the signing domain, message acceptance will likely remain largely dependent upon the reputation of the SMTP client's IP address. Abuse reporting facilitated by DKIM signatures should therefore first select signing domains that correspond with domains administering the SMTP Client publicly transmitting the message. A correspondence between SMTP Client and the DKIM signing domain can be retained by using TPA-label authorizations.

The receiver's domain evaluation process will confront many domains with unknown reputations. New domains are constantly being introduced where registrars are unable to prevent bad actors from controlling either a new or previously held domain names. The receiver may seek to limit the DKIM verification process, since acquiring policy records or DKIM keys may inadvertently leak valuable information that benefit bad actors. Processing all DKIM signatures may also inundate a receiver's limited resources. As a result, validating DKIM signatures and obtaining policy resource records might become limited to known trustworthy domains. Signing domains authorized with TPA-labels by domains having good reputations provide a means to safely extend a limited verification resources.

4. Authorization Scope

An Authorization effort without using TPA-labels will likely involve sharing details between the domain owner, and one or more email and DNS providers. Since there are many ways in which authorizations can be accomplished, it is unlikely there will be consistent or standardized formats for exchanging the necessary information. In addition, in the case of security breaches, the wrong party might be held accountable for content never seen nor logged by them. The TPA-label authorization scheme clarifies who signed the message and on whose behalf, while permitting greater control by the authorizing domain.

TPA-labeled policy records replace domain delegations, selector/key record mirroring, or key exchanges. Significant amounts of detail is associated with selector/key records. These details include user limitations, suitable services, key resource record's Time-To-Live, revocation and update procedures, and how the DKIM Signature header field's 'i=' semantics are to be applied. The TPA-labeled policy records allow authorizing domains an improved ability to limit the scope of their authorizations and associated identities.

When a signing domain differs from that of a domain within the header email-address, TPA-labeled policy records can safely extend compliance where the action of only the email-address domain is required. In addition, just as a DKIM signature can assert an email-address has been authenticated via the "i" construct, a signing domain that only signs authenticated email-addresses from the authorizing domain can be denoted by the "-i" suffix on the scope assertion within the corresponding TPA-labeled policy record. A TPA-labeled policy record returning a scope with an "-i" suffix indicates the domain assures that email-addresses identities from the authorizing domain have been authenticated. This assertion remains valid even when signing domains and the email-address domains differ.

Without the "-i" suffix on the scope assertion within the TPA-labeled policy record, the authorized domain is designated as providing only acceptable signatures. The same would be true for a DKIM signature lacking the "i" parameter. While offering only valid signatures will not ensure all possible spoofing is prevented, messages signed in this manner should not receive annotations indicating authenticated identities either.

Choosing not to implement identity authentication may represent an economical means to administer domains employing DKIM signatures. Authorizing domains to play the role of only providing acceptable signatures may be suitable for non-critical messages, where the goal might be to improve delivery acceptance.

5. TPA-labeled Policy Assertions

The receiver obtains the TPA-labeled policy record for the email-address domain using a DNS query for an IN class TXT resource record. The tpa-label is created by processing the domain found within the signature's "d=" parameter (does not include the trailing period). The tpa-label would be placed below the normal policy records, for example "._adsp.domainkey.<email-address domain>". These labels would then be used to access the TPA-labeled policy TXT records. Character-strings contained within the TXT resource record are concatenated into forming a single string. A character-string is a single length octet followed by that number of characters treated as binary information. As an example, a TPA-labeled policy record may be located at these domains:

6. Scope

scope= Authorization Scope List (Optional). This tag defines a list of scoping assertions for various email-address locations within the message.

6.1. From Originator Header Field

Tag	Function
scope=	Authorization Scope List (as-list)
tpa=	Authorized Domains List (ad-list)
g=	Local-part restriction

Scope Values	Field or Parameter	Identity Authenticated
F	From (Author) Header	No
F-i	From (Author) Header	Yes
O	Other than From (Author) Headers	No
O-i	Other than From (Author) Headers	Yes
M	MailFrom	No
M-i	MailFrom	Yes
NO-TPA	All	No

The "F" or "F-i" scope asserts that messages carrying the email-address domain within the From header field are authorized to be signed by the tpa listed domain. When the "-i" suffix is included, it can be assumed identities for the scoped header have been authenticated.

6.2. MailFrom Parameter

This "M" or "M-i" scope asserts that messages carrying the email-address domain within the MailFrom parameter are authorized to be signed by the authorized domain. When the "-i" suffix is included, it can be assumed identities for the MailFrom have been authenticated.

6.3. Other than From Originating Header Fields

The "O" or "O-i" scope asserts that messages carrying the email-address domain within the Sender or Resent-* header fields are authorized to be signed by the authorized domain. When the "-i" suffix is included, it can be assumed identities for the scoped header have been authenticated.

6.4. NO-TPA

The "NO-TPA" scope asserts that domain does not publish TPA-labeled policy records.

7. Authorized Signing Domain

tpa= Authorized Signing Domain list (Optional). This tag repeats all or portions of the domain encoded within the tpa-label. This option ensures proper handling of possible hash collisions. When a domain is prefixed with the "*." ANY label, then all subdomains of this domain are to be considered included within the list.

8. Use of TPA-labeled Policy

9. Restricted Local-part

The "g=" tag provides a means for an authorizing domain to restrict authorization for local-parts appearing within the scoped headers.

g= Granularity of the TPA authorization (plain-text; OPTIONAL, default is "*"). This value MUST match the Local-part of the From header email-address. A single optional "*" character matches a sequence of zero or more arbitrary characters ("wildcarding"). An email with a From address local-part that does not match the value of this tag constitutes a failed TPA authorization. The intent of this tag is to constrain which email-addresses the domain can legitimately be authorized to sign. Wildcarding allows matching for addresses such as "user+*" or "*-offer". An empty "g=" value never matches any addresses.

10. IANA Considerations

Unless a registry is established for DKIM policy record tags, there are no IANA requirements in this draft.

Note to RFC Editor: this section may be removed on publication as an RFC and no request is desired or registration is not considered practical.

11. Security Considerations

The use of the SHA-1 hash algorithm does not represent a security concern. The hash simply ensures a deterministic domain-name size is achieved. Unexpected collisions can be detected and handled by using the extended TPA-label "tpa=" option.

12. Acknowledgements

13. References

13.1. Normative References

13.2. Informative References

Appendix A. Change Log

Appendix B. DNS Example of TPA-label policy record placement

Appendix C. C code for label generation

/*  tpa-label generation utility
 *  Copyright (C) The IETF Trust & Douglas Otis (2008).
 *  Mountain View, CA
 */

#include <stdio.h>
#include <sys/types.h>
#include <stddef.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <openssl/sha.h>

/*
 * Copyright (C) The IETF Trust & Douglas Otis (2008).
 * Redistributions of source code must retain the above copyright
 * notice and the following disclaimer.
 *
 * This document is subject to the rights, licenses and restrictions
 * contained in BCP 78, and except as set forth therein, the authors
 * retain all their rights.
 * This document and the information contained herein are provided on an
 * "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 * OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
 * THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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.
 */

#define TPA_LABEL_VERSION   100
#define MAX_DOMAIN_NAME     512
#define MAX_FILE_NAME       1024

static char base32[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567";
static char sign_on[] =
 {"%s v%d.%02d Copyright (C) The IETF Trust & Douglas Otis (2008)\n"};
char err_cmd[] =\
 "ERR: Command error with [%s]\n";
char use_txt[]=\
 "Usage: tpa-label [-i domain_input_file] [-o label_output_file][-v]\n";
char help_txt[]=\
"The options are as follows:\n"\
"-i  domain name input. Defaults to stdin. Removes trailing '.'\n"\
"-o  tpa-label output.  Defaults to stdout.\n"\
"-v  Specifies Verbose Mode.\n\n";

static void usage(void);
/*- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

static void
usage(void)
{
    (void) fprintf(stderr, "\n%s%s", use_txt, help_txt);
    exit(1);
}
/*- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

int
main (int argc, char * argv[])
{
    int  ret_val, in_mode, out_mode, verbose, done, i, j, k;
    char ch;
    unsigned long len;
    unsigned long long b_5;
    char in_fn[MAX_FILE_NAME], out_fn[MAX_FILE_NAME];
    unsigned char in_buf[MAX_DOMAIN_NAME + 2];
    unsigned char sha_res[20], tpa_label[33];
    FILE *in_file, *out_file;

    ret_val = in_mode = out_mode = verbose = done = 0;
    len = 0;

    while ((ch = getopt(argc, argv, "i:o:v")) != -1)
    {
        switch (ch)
        {
            case 'i':
                in_mode = 1;          /* input from file */
                (void) strncpy(in_fn, optarg, sizeof(in_fn));
                in_fn[sizeof(in_fn) - 1] = '\0';
                break;
            case 'o':
                 out_mode = 1;         /* out to time */
                 (void) strncpy(out_fn, optarg, sizeof(out_fn));
                 out_fn[sizeof(out_fn) - 1] = '\0';
                 break;
            case 'v':
                 verbose = 1;
                 break;
            case '?':
            default:
                (void) usage();
                break;
        }
    };

    if (in_mode)
    {
        if ((in_file = fopen(in_fn, "r")) == NULL)
        {
            (void) fprintf(stderr,
                           "ERR: Error opening [%s] input file.\n",
                           in_fn);
            exit(2);
        }
    }
    else
    {
        in_file = stdin;
    }

    if (out_mode)
    {
        if ((out_file = fopen(out_fn, "w")) == NULL)
        {
            (void) fprintf(stderr,
                           "ERR: Error opening [%s] output file.\n",
                           out_fn);
            exit(3);
        }
    }
    else
    {
        out_file = stdout;
    }

    if (out_mode && verbose)
    {
        (void) printf(sign_on, "tpa-label utility",
                      TPA_LABEL_VERSION / 100,
                      TPA_LABEL_VERSION % 100);
    }

    for (i = 0; i < MAX_DOMAIN_NAME && !done; i++)
    {
        if ((ch = fgetc(in_file)) == EOF)
        {
            ch = 0;
        }
        else  if (ch == '\n' || ch == '\r')
        {
            ch = 0;
        }

        in_buf[i] = tolower(ch);

        if (ch == 0)
        {
            len = i;         /* string length */
            done = 1;
        }
    }

    if (!done)
    {
        (void) fprintf(stderr, "ERR: Domain name too long.\n");
        exit (4);
    }

    if (len && in_buf[len - 1] == '.')    /* remove any trailing "." */
    {
        len--;
        in_buf[len] = 0;     /* replace trailing "." with 0 */
    }

    in_buf[len++] = '\n';    /* newline simulates commmand-line use */
    in_buf[len] = 0;         /* terminate string */

    if (len < 2)
    {
        (void)
        fprintf(stderr,
                "ERR: Domain name [%s] too short with %d length.\n",
                in_buf,
                len);
        exit (5);
    }

    SHA1(in_buf, len, sha_res);

    if (verbose)
    {
        printf("Normalized Domain = [%s] %d, SHA-1 = ", in_buf, len);

        for (i = 0; i < 20; i++)
        {
            printf("%02X", sha_res[i]);
        }
        printf("\nTPA-Label: 5 bit intervals left to right.\n");
    }

    /* process sha-1 results 4 times by 40 bits 0 to 160 */

    for (i = 0, j = 0; i < 4 ; i++)
    {
        b_5 =  (unsigned long long) sha_res[(i * 5)] << 32;
        b_5 |= (unsigned long long) sha_res[(i * 5) + 1] << 24;
        b_5 |= (unsigned long long) sha_res[(i * 5) + 2] << 16;
        b_5 |= (unsigned long long) sha_res[(i * 5) + 3] << 8;
        b_5 |= (unsigned long long) sha_res[(i * 5) + 4];

        if (verbose)
        {
            printf(" {%010llX}->", b_5);
        }

        for (k = 35; k >= 0; k-= 5, j++)    /* convert 40 bits (5x8) */
        {
            tpa_label[j] = base32[(b_5 >> k) & 0x1F];

            if (verbose)
            {
                 printf(" %02X:%c",
                        (unsigned int)(b_5 >> k) & 0x1F,
                        tpa_label[j]);
            }
        }
        if (verbose)
        {
            printf ("\n");
        }
    }
    if (verbose)
    {
        printf("\n");
    }

    tpa_label[j] = 0;   /* terminate label string */
    fprintf(out_file, "_%s", tpa_label);
    printf("\n");

    /* close */
    if (out_mode)
    {
        if (fclose (out_file) != 0)
        {
            (void) fprintf(stderr,
                           "ERR: Unable to close %s output file.\n",
                           out_fn);
            ret_val = 6;
        }
    }
    if (in_mode)
    {
        if (fclose (in_file) != 0)
        {
            (void) fprintf(stderr,
                           "ERR: Unable to close %s input file.\n",
                           in_fn);
             ret_val = 7;
        }
    }
    return (ret_val);
}

Author's Address

Full Copyright Statement

This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights.

This document and the information contained herein are provided on an “AS IS” basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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.

Intellectual Property

The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org.

[FIPS.180-2.2002]	National Institute of Standards and Technology, “Secure Hash Standard,” FIPS PUB 180-2, August 2002.
[I-D.ietf-dkim-ssp]	field, h., Domain, A., error, r., Allman, E., Fenton, J., Delany, M., and J. Levine, “DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP),” draft-ietf-dkim-ssp-10 (work in progress), May 2009 (TXT).
[RFC2119]	Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML).
[RFC2821]	Klensin, J., “Simple Mail Transfer Protocol,” RFC 2821, April 2001 (TXT).
[RFC2822]	Resnick, P., “Internet Message Format,” RFC 2822, April 2001 (TXT).
[RFC4234]	Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” RFC 4234, October 2005 (TXT, HTML, XML).
[RFC4648]	Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” RFC 4648, October 2006 (TXT).
[RFC4871]	Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, “DomainKeys Identified Mail (DKIM) Signatures,” RFC 4871, May 2007 (TXT).

	Douglas Otis
	Trend Micro, NSSG
	10101 N. De Anza Blvd
	Cupertino, CA 95014
	USA
Phone:	+1.408.257-1500
Email:	doug_otis@trendmicro.com

DKIM Third-Party Authorization for Sender Signer Practices
draft-otis-dkim-tpa-ssp-04

Status of this Memo

Abstract

Requirements Language

Table of Contents

1. Introduction

2. Envelope Authorizations

3. Evaluating Signing Domains

4. Authorization Scope

5. TPA-labeled Policy Assertions

6. Scope

6.1. From Originator Header Field

6.2. MailFrom Parameter

6.3. Other than From Originating Header Fields

6.4. NO-TPA

7. Authorized Signing Domain

8. Use of TPA-labeled Policy

9. Restricted Local-part

10. IANA Considerations

11. Security Considerations

12. Acknowledgements

13. References

13.1. Normative References

13.2. Informative References

Appendix A. Change Log

Appendix B. DNS Example of TPA-label policy record placement

Appendix C. C code for label generation

Author's Address

Full Copyright Statement

Intellectual Property

DKIM Third-Party Authorization for Sender Signer Practicesdraft-otis-dkim-tpa-ssp-04

Status of this Memo

Abstract

Requirements Language

Table of Contents

1. Introduction

2. Envelope Authorizations

3. Evaluating Signing Domains

4. Authorization Scope

5. TPA-labeled Policy Assertions

6. Scope

6.1. From Originator Header Field

6.2. MailFrom Parameter

6.3. Other than From Originating Header Fields

6.4. NO-TPA

7. Authorized Signing Domain

8. Use of TPA-labeled Policy

9. Restricted Local-part

10. IANA Considerations

11. Security Considerations

12. Acknowledgements

13. References

13.1. Normative References

13.2. Informative References

Appendix A. Change Log

Appendix B. DNS Example of TPA-label policy record placement

Appendix C. C code for label generation

Author's Address

Full Copyright Statement

Intellectual Property

DKIM Third-Party Authorization for Sender Signer Practices
draft-otis-dkim-tpa-ssp-04