]> Trend Micro

10101 N. De Anza Blvd Cupertino CA 95014 USA +1.408.257-1500 doug_otis@trendmicro.com

Internet Area DKIM Working Group DKIM-TPA-Label Draft 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. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .

This document describes how any email-address domain that is publishing DKIM policy records, such as those defined in , can also autonomously authorize signing by specific third-party domains. TPA-Label listed domains offer secondary policy compliance options when no valid Author Domain Signature is present within the message. Recommended or suggested actions for DKIM receivers are not included, and are considered "out-of-scope" for this document. The receiver is assumed to better understand their environment's impact upon the performance of DKIM signatures and how the transactional results are best utilized. TPA-Labels authorize third-party signing domains as a means to extend DKIM policy compliance options defined by . TPA-Label listed domains are to be considered equivalent to the authorizing domain in the application of DKIM policies. The TXT records associated with TPA-Labels start with the 'dkim' tag as defined by , and may contain tags specifically defined for TPA-Labels. This mechanism can eliminate the complex coordination of selector/key DNS records, DNS delegation, or exchanges of public/private keys between two or more domains that would be otherwise required to facilitate transparent authorizations. 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.

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.

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.

Every TPA-Label TXT resource record MUST start with an outbound signing-practices tag, so the first four characters of the record are lowercase "dkim", followed by optional whitespace and "=". In addition to tags defined by , TPA-Label syntax descriptions use the form described in Augmented BNF for Syntax Specifications . The "base32" function is defined in and the "sha-1" hash function is defined in . The TPA-Label TXT resource records follow the tag-value syntax described in section 4.2.1 of and section 3.2 of . Unrecognized tags and tags with illegal values MUST be ignored. In the ABNF below, the WSP token is inherited from . The ALPHA and DIGIT tokens are imported from . The function "lcase" converts upper-case ALPHA characters to lower-case. The function "sha-1" returns a hash that is converted to a base32 character set. The terminating period is not included in domain-name conversions. In addition, a newline character (0x0A) is appended to the end of the string to match a command line generation of SHA1 values. The tags used in TPA-Label resource records are as follows:

asterisk = %x2A ; "*" dash = %x2D ; "-" dot = %x2E ; "." underscore = %x5F ; "_" ANY = asterisk dot ; "*." dns-char = ALPHA / DIGIT / dash id-prefix = ALPHA / DIGIT label = id-prefix [*61dns-char id-prefix] sldn = label dot label base-char = (dns-char / underscore) domain = *(label dot) sldn tpa-label = underscore base32( sha-1( lcase(signing-domain)))

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 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: <tpa-label>._adsp._domainkey.<email-address domain>.

scope= Authorization Scope List (Optional). This tag defines a list of scoping assertions for various email-address locations within the message. scope = "F" / "O" / "M" / "H" / "NO-TPA" as-list = "scope" [WSP] "=" [WSP] scope 0*([WSP] ":" [WSP] scope)

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.

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.

This "M" scope asserts that an email-address domain that is within a TPA-Label listed domain used in the MAIL command is also authorized.

The "H" scope asserts that host names given in 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.

The "NO-TPA" scope asserts that the authorizing domain does not publish TPA-Labeled policy records. This scope is intended to be used in normal ADSP records as a means to inhibit subsequent TPA-Label transactions.

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. ad = [ANY] domain ad-list = "tpa" [WSP] "=" [WSP] ad 0*([WSP] ":" [WSP] ad)

Use of TPA-Label resource record assertions need not be subsequent to the discovery of the policy record specified by . When an acceptable First-Party signature was not discovered, and the From domain's resource record contains the "scope" tag then: When one or more valid Third-Party Signatures are present in the message, and a scope tag exists within the normal policy record, and the scope tag does not contain "NO-TPA", then: When a TPA-Label TXT resource record within the From header domain has a scope tag of "F", and the email-address domain within the From headers is within the TPA-Label listed domain, then the message is considered signed with an Author's Domain Acceptable Third-Party Signature. When a TPA-Label TXT resource record within the From header domain has a scope tag of "O", and the email-address domain within the Sender, or Resent-* headers are within the TPA-Label listed domain, use of these headers by this domain is authorized by the Author's Domain. When a TPA-Label TXT resource record within the From header domain has a scope tag of "M", and the email-address domain within the MAIL command is within the TPA-Label listed domain, use of this command by this domain is authorized by the Author's Domain. When a TPA-Label TXT resource record within the From header domain has a scope tag of "H", and a host domain given by EHLO or HELO command is within the TPA-Label listed domain, the SMTP client is authorized by the Author's Domain. When no TPA-Label TXT resource record is found or published, and a valid Third-party signature is acceptable to the verifier, then the message is considered signed by a Verifier Acceptable Third-Party Signature.

A registry has been established for DKIM policy record tags for RFC5617 which will need updated with the tags "tpa" and "scope". Note to RFC Editor: this section may be removed on publication as an RFC.

This draft extends signing policies related to . Security considerations are mostly related to attempts on the part of malicious senders to represent themselves as other senders, often in an attempt to defraud either the recipient or the alleged originator. Additional security considerations regarding DKIM signing practices may be found in the DKIM threat analysis . 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.

Frank Ellermann and Wietse Venema.

&RFC2119; &RFC4648; &RFC4871; &RFC5234; &RFC5321; &RFC5322; &RFC5617; &FIPS.180-2.2002; &RFC4686;

#### # Policies for Example.com email domain using example.com, isp.com, # and example.com.isp.com as signing domains. #### #### 5322.From authorization for TP domains #### _adsp._domainkey.example.com. IN TXT "dkim=all; scope=F:O:M;" ## "isp.com" TPA-Label ## _HGSSD3SNMI6635J5743VDJHAJKMPMFIF._adsp._domainkey.example.com. IN TXT "dkim=all; tpa=isp.com; scope=F;" #### 5322.From/Originator/MailFrom authorization for TP domains #### ## "example.com.isp.com" TPA-Label ## _ZZHFFXWCFI7RPDDQDIGYHPBTAA7VWITU._adsp._domainkey.example.com. IN TXT "dkim=all; tpa=*.isp.com; scope=F:O:M;"

The following utility can be compiled as tpa-label.c using the following: gcc -lcrypto tpa-label.c -o tpa-label

/* * 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); }