DKIM Third-Party Authorization Label

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Abstract

TPA-Label is a DNS-based prefix mechanism for DKIM policy related records as a means to authorize Third-Party domains, such as mailing-lists. This mechanism allows first-party domains to autonomously authorize a range of third-party domains using scalable, individual DNS transactions. This authorization extends the scope of DKIM policy assertions as a means to supplant more difficult to administer mechanisms. Alternatives for facilitating third-party authorizations currently necessitate the coordination between two or more domains to synchronously set up selector/key DNS records, DNS zone delegations, and/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-Label transactions offer an efficient means for Author Domains to authorize specific third-party signing domains.

Requirements Language

1. Introduction
2. Evaluating Signing Domains
3. Authorization Scope
4. TPA-Label Tag Definitions
5. Scope
    5.1. From (Author) Header Field
    5.2. Other Originating Header Fields
    5.3. MailFrom Parameter
    5.4. SMTP Host domains
    5.5. NO-TPA
6. Authorized Signing Domain
7. Use of TPA-Label Resource Records
8. IANA Considerations
9. Security Considerations
10. Acknowledgements
11. References
    11.1. Normative References
    11.2. Informative References
Appendix A. DNS Example of TPA-Label policy record placement
Appendix B. C code for label generation
§ Author's Address

1. Introduction

Trust is an essential requisite before the DKIM signature header field's 'i=' semantics provide valuable advisory information. This advisory information is in regard to the "on-behalf-of" identity as a means to enable safer message annotations, and to better ensure trusted identities are recognized. However, in the case of third-party signatures, the 'i=' value will not directly reflect an email-address found within the From header, but would be in the form of an alias.

TPA-Labels convey which third-party domains are authoritative. However, third-party domains are unable to utilize DKIM signature's 'i=' semantics to directly assert which identifiers on whose behalf a signature was added. As such, no third-party domain should be authorized unless it is trusted to ensure submitting entities have demonstrated receipt of messages sent to the From header address contained within the domain's signed messages.

2. 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.

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 be beyond the control of the signing domain, message acceptance might remain dependent upon the reputation of the SMTP client's IP address and associated hosts, and perhaps that of the identity represented within the 'i=' parameter found within the DKIM signature header. Abuse reporting facilitated by DKIM signatures should therefore first select signing domains that correspond with domains administering the SMTP Client publicly transmitting the message. As such, a correspondence between SMTP Client host and DKIM signing domain with that of the Author domain can be affirmed by TPA-Label listed domains. Correspondence with the SMTP Client host may offer protection when located at consistently used IP addresses.

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 new or previously held domain names. Receivers may seek to limit the DKIM verification process, since acquiring policy records or DKIM keys may inadvertently leak valuable information that could benefit bad actors. Processing all DKIM signatures may also inundate a receiver's limited resources. As a result, validating DKIM signatures and obtaining related resource records might be limited to known trustworthy domains. Signing domains having good reputations that are listed by a TPA-Label might provide a means to safely extend limited verification resources to otherwise unknown domains.

3. Authorization Scope

Without using TPA-Labels, an authorization effort will likely involve sharing a number of details between the domain owner, and one or more email and DNS providers. Since there are many ways in which such authorizations can be accomplished, it is unlikely there will be consistent or standardized formats developed to exchange necessary, and at times, sensitive information. In addition, when there is a security breach, a wrong party might be held accountable for content they may have never seen nor logged. The TPA-Label authorization scheme permits the DKIM signature header to clarify who signed the message and on whose behalf, while also permitting greater control by the authorizing domain.

TPA-Label resource 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. In addition, to better secure services that might depend upon DKIM keys, the TPA-Label resource record allows authorizing domains an ability to limit the scope of their authorizations, without being mistaken for having authenticated the entity submitting the message, or for running ancillary services that make use of DKIM public keys.

When a signing domain differs from that of a domain within the header email-address, TPA-Label resource records safely extend policy compliance where a DNS publication is only required by the email-address domain, even when signing domains and the email-address domains differ. While offering only valid signatures does not ensure all possible spoofing is prevented, messages signed in this manner should not receive annotations indicating authenticated identities either. Authorizing domains play the role of only providing acceptable signatures which might be suitable for non-critical messages, when the goal is to improve delivery acceptance, such as those from mailing-lists.

4. TPA-Label Tag Definitions

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

Scope Values	Field or Parameter
F	From (Author) Header
O	Other than From (Author) Headers
M	MailFrom
H	SMTP Host
NO-TPA	All

The receiver obtains domain authorizations with a DNS query for an IN class TXT TPA-Label resource record located below the ADSP record location specified in [RFC5617] (Allman, E., Fenton, J., Delany, M., and J. Levine, “DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP),” August 2009.) section 4.3. The TPA-Label is generated by processing the domain found within the DKIM signature's "d=" parameter (does not include the trailing period). A TPA-Label is published below the normal ADSP policy record, for example below "._adsp.domainkey.<email-address domain>". The existence of a TPA-Label provides authorization for the listed domain.

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:

5. Scope

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

5.1. From (Author) Header Field

The "F" scope asserts that messages carrying the email-address domain within the From header field are authorized to be signed by the TPA-Label listed domain.

5.2. Other Originating Header Fields

The "O" scope asserts that messages with Sender or Resent-* header fields with email-address domains within the TPA-Label listed domain are also authorized.

5.3. MailFrom Parameter

5.4. SMTP Host domains

The "H" scope asserts that host names given in [RFC5321] (Klensin, J., “Simple Mail Transfer Protocol,” October 2008.) EHLO or HELO commands within TPA-Label listed domains are also authorized. This scope might be used to better ensure DKIM signatures within messages from these hosts are validated.

5.5. NO-TPA

6. Authorized Signing Domain

tpa= Authorized Signing Domain list (Optional). This tag, if present, MUST repeat 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.

7. Use of TPA-Label Resource Records

8. IANA Considerations

A registry has been established for DKIM policy record tags for RFC5617 which will need updated with the tags "tpa" and "scope".

9. 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.

Use of the TPA-Label rather simply listing the authorized domain ensures the maximal domain name size used with SMTP is reduced by less than 20%, rather than by an amount greater than 50% when attempting to include two domain names. The typical domain name size has been steadily increasing. This increase has been caused domain names that encode international character sets, and perhaps soon will be spurred by the expanse of TLDs having larger labels.

10. Acknowledgements

11. References

11.1. Normative References

11.2. Informative References

Appendix A. DNS Example of TPA-Label policy record placement

Appendix B. C code for label generation

/*
 * TPA-Label generation utility
 * Copyright (C) 2009 The IETF Trust & and the persons identified as
 * the document authors.  All rights reserved.
 * 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.
 */

#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>

#define TPA_LABEL_VERSION   101
#define MAX_DOMAIN_NAME     256
#define MAX_FILE_NAME       1024

static char base32[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567";
static char sign_on[] =
{"%s v%d.%02d Copyright (C) (2009)  The IETF Trust & Douglas Otis\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 file */
                 (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);
}

[FIPS.180-2.2002]	National Institute of Standards and Technology, “Secure Hash Standard,” FIPS PUB 180-2, August 2002.
[RFC2119]	Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (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).
[RFC5234]	Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” STD 68, RFC 5234, January 2008 (TXT).
[RFC5321]	Klensin, J., “Simple Mail Transfer Protocol,” RFC 5321, October 2008 (TXT).
[RFC5322]	Resnick, P., Ed., “Internet Message Format,” RFC 5322, October 2008 (TXT, HTML, XML).
[RFC5617]	Allman, E., Fenton, J., Delany, M., and J. Levine, “DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP),” RFC 5617, August 2009 (TXT).

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

DKIM Third-Party Authorization Label
draft-otis-dkim-tpa-label-01

Status of this Memo

Copyright Notice

Abstract

Requirements Language

Table of Contents

1. Introduction

2. Evaluating Signing Domains

3. Authorization Scope

4. TPA-Label Tag Definitions

5. Scope

5.1. From (Author) Header Field

5.2. Other Originating Header Fields

5.3. MailFrom Parameter

5.4. SMTP Host domains

5.5. NO-TPA

6. Authorized Signing Domain

7. Use of TPA-Label Resource Records

8. IANA Considerations

9. Security Considerations

10. Acknowledgements

11. References

11.1. Normative References

11.2. Informative References

Appendix A. DNS Example of TPA-Label policy record placement

Appendix B. C code for label generation

Author's Address

DKIM Third-Party Authorization Labeldraft-otis-dkim-tpa-label-01

Status of this Memo

Copyright Notice

Abstract

Requirements Language

Table of Contents

1. Introduction

2. Evaluating Signing Domains

3. Authorization Scope

4. TPA-Label Tag Definitions

5. Scope

5.1. From (Author) Header Field

5.2. Other Originating Header Fields

5.3. MailFrom Parameter

5.4. SMTP Host domains

5.5. NO-TPA

6. Authorized Signing Domain

7. Use of TPA-Label Resource Records

8. IANA Considerations

9. Security Considerations

10. Acknowledgements

11. References

11.1. Normative References

11.2. Informative References

Appendix A. DNS Example of TPA-Label policy record placement

Appendix B. C code for label generation

Author's Address

DKIM Third-Party Authorization Label
draft-otis-dkim-tpa-label-01