Network Working Group D. Crocker
Internet-Draft Brandenburg InternetWorking
Intended status: Informational M. Kucherawy
Expires: October 27, 2011 Cloudmark
April 25, 2011
DomainKeys Security Tagging (DOSETA)
draft-crocker-doseta-base-02
Abstract
DomainKeys Security Tagging (DOSETA) is a component mechanism
enabling development of security-related services, such as for
authentication or encryption; it uses self-certifying keys based on
domain names. The domain name owner can be any actor involved in the
handling of the data, such as the author's organization, a server
operator or one of their agents. The DOSETA Library provides a
collection of common capabilities, including canonicalization,
parameter tagging and key retrieval. The DOSETA Signing Template
creates common framework for a signature of data that are in a
"header/content" form. Defining the meaning of a signature is the
responsibility of the service that incorporates DOSETA. Data
security is enforced through the use of cryptographic algorithms.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 27, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Comments and Issues . . . . . . . . . . . . . . . . . . . 5
2. Framework . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. DOSETA Architecture . . . . . . . . . . . . . . . . . . . 5
2.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. DOSETA Library . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1. Normalization for Transport Robustness . . . . . . . . . . 10
3.2. Canonicalization . . . . . . . . . . . . . . . . . . . . . 11
3.3. Tag=Value Parameters . . . . . . . . . . . . . . . . . . . 15
3.4. Key Management . . . . . . . . . . . . . . . . . . . . . . 17
3.5. Selectors for Keys . . . . . . . . . . . . . . . . . . . . 17
3.6. DNS Binding for Key Retrieval . . . . . . . . . . . . . . 19
3.7. Stored Key Data . . . . . . . . . . . . . . . . . . . . . 20
4. DOSETA H/C Signing Template . . . . . . . . . . . . . . . . . 23
4.1. Cryptographic Algorithms . . . . . . . . . . . . . . . . . 23
4.2. Signature Data Structure . . . . . . . . . . . . . . . . . 25
4.3. Signature Calculations . . . . . . . . . . . . . . . . . . 31
4.4. Signer Actions . . . . . . . . . . . . . . . . . . . . . . 35
4.5. Verifier Actions . . . . . . . . . . . . . . . . . . . . . 38
4.6. Requirements for Tailoring the Signing Service . . . . . . 45
5. Semantics of Multiple Signatures . . . . . . . . . . . . . . . 46
5.1. Example Scenarios . . . . . . . . . . . . . . . . . . . . 46
5.2. Interpretation . . . . . . . . . . . . . . . . . . . . . . 47
6. DOSETA Claims Registry Definition . . . . . . . . . . . . . . 48
7. Considerations . . . . . . . . . . . . . . . . . . . . . . . . 48
7.1. IANA Considerations . . . . . . . . . . . . . . . . . . . 49
7.2. Security Considerations . . . . . . . . . . . . . . . . . 49
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 50
8.1. Normative References . . . . . . . . . . . . . . . . . . . 50
8.2. Informative References . . . . . . . . . . . . . . . . . . 51
Appendix A. Creating a Public Key . . . . . . . . . . . . . . . . 52
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 53
Appendix C. Example -- DKIM Using DOSETA . . . . . . . . . . . . 53
C.1. Signing and Verification Protocol . . . . . . . . . . . . 53
C.2. Extensions to DOSETA Template . . . . . . . . . . . . . . 54
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 61
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1. Introduction
DomainKeys Security Tagging (DOSETA) is a component mechanism
enabling development of security-related services, such as for
authentication or encryption; it uses self-certifying keys based on
domain names [RFC1034]. The domain name owner can be any actor
involved in the handling of the data, such as the author's
organization, a server operator or one of their agents. The DOSETA
Library provides a collection of common capabilities, including
canonicalization, parameter tagging and key retrieval. The DOSETA
Signing Template creates common framework for a signature of data
that are in a "header/content" form. Defining the meaning of a
signature is the responsibility of the service that incorporates
DOSETA. Data security is enforced through the use of cryptographic
algorithms.
The approach taken by DOSETA differs from previous approaches to data
signing -- such as, Secure/Multipurpose Internet Mail Extensions
(S/MIME) [RFC1847], OpenPGP [RFC4880] -- in that:
o The message signature can be packaged independently of the data it
is signing, so that neither human viewers of the data nor existing
data handling software is confused by security-related content
appearing in the Content.
o There is no dependency on having public and private key pairs
being issued by well-known, trusted certificate authorities.
o There is no dependency on the deployment of any new Internet
protocols or services for public key distribution or revocation.
o Specific security services can be limited to those needed by the
service using them.
DOSETA:
o enables compatibility with the existing data handling
infrastructure and is transparent to the fullest extent possible
o requires minimal new infrastructure
o can support a variety of implementation configurations, in order
to reduce deployment time
o can be deployed incrementally
o allows delegation of signing to third parties
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DOSETA derives from Domain Keys Identified Mail (DKIM) [RFC5672] and
has extracted the core portions of the its signing specification
[DKIMSign], so that they can be applied to other security-related
services. For example, the core could support a DKIM-like signing
service for web pages, and it could support a data encryption
mechanism using the same DNS-based, self-certified key service as
DKIM.
DOSETA features include:
Identity: DOSETA distinguishes the identity of the DOSETA
producer from that associated with any other identifier, such
as the data's purported author. In particular, the DOSETA
header field includes the DOSETA Domain Identifier (DDI), per
Section 2.2.1. DOSETA consumers can use the DDI decide how
they want to process the data. The DDI can be directly
included in the attributes of the data or can be recorded
elsewhere.
NOTE: DOSETA does not, itself, specify that the identity it
uses is required to match any other associated identifier.
Those other identifiers already carry their own semantics
which might conflict with the use of the identifier needed
by DOSETA. However a particular DOSETA-based security
service that might choose to add constraints on the choice
of identifier, such as having it match another identifier
that is associated with the data.
Scalability: DOSETA is designed to easily support the extreme
scaling requirements that characterize Internet data
identification.
Key Management: DOSETA differs from traditional hierarchical
public-key systems in that no Certificate Authority
infrastructure is required; the verifier requests the public
key from a repository under the domain name associated with the
use of DOSETA directly, rather than requiring consultation of a
certificate authority. That is, DOSETA provides self-
certifying keys.
The DNS is the initial mechanism for DOSETA public keys. Thus,
DOSETA currently depends on DNS administration and the security
of the DNS system. DOSETA is designed to be extensible to
other key fetching services as they become available.
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Data Integrity: When DOSETA is used to sign data -- independent
of the semantics of that signature -- there is a computed hash
of some or all of the data that ensures detection of changes to
that data, between the times of signing and verifying.
1.1. Comments and Issues
[RFC EDITOR] Remove this sub-section prior to publication.
Possible applications:
o JSON structure
o XMPP message
o XML object
o vCard
o vCal
o Web page signing?
o Web ad authentication
o Handle System
Discussion Venue: Discussion of this draft should take place on the
doseta-discuss mailing list. It is located at:
http://www.trusteddomain.org/mailman/listinfo/doseta-discuss
2. Framework
This section provides the technical background for the remainder of
the document.
2.1. DOSETA Architecture
As component technology, DOSETA is meant to be incorporated into a
service. This specification provides an underlying set of common
features and a template for using them to provide a signing service,
such as for authenticating an identifier. Hence, the pieces can be
depicted as follows, with DKIM being shown as a specific service that
incorporates DOSETA:
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+--------+ +----------+ +-----------------+
| DKIM | | MIMEAUTH | | Message Privacy |
+---+----+ +-----+----+ +--------+--------+
| | |
++=====V==================V========++ |
|| || |
|| Header/Content Signing Template || |
|| || |
++================+================++ |
| |
++=================V=================================V===============++
|| ||
|| D O S E T A L I B R A R Y ||
|| +------------------+ +------------+ +-------------+ +-----------+ ||
|| | | | Key | | Common | | Data Tags | ||
|| | Canonicalization | | Management | | Parameters | | Header | ||
|| | | | (DNS) | | (tab=value) | | | ||
|| +------------------+ +------------+ +-------------+ +-----------+ ||
|| ||
++===================================================================++
2.2. Terminology
Within the specification, the label "[TEMPLATE]" is used to indicate
actions that are required for tailoring the use of DOSETA into a
specific service.
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 [RFC2119].
Additional terms for this document are divided among Identity and
Actors.
2.2.1. Identity
Identity: A person, role, or organization. In the context of
DOSETA, examples include author, author's organization, an ISP
along the handling path, an independent trust assessment service,
and a data processing intermediary operator.
Identifier: A label that refers to an identity. The primary
example is a domain name.
DOSETA Domain Identifier (DDI): A single domain name that serves as
an identifier, referring to the DOSETA key owner's identity. The
DDI is specified in Section 4.2. Within this specification, the
name has only basic domain name semantics; any possible owner-
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specific semantics MUST be provided in the specification that
incorporates DOSETA.
Identity Assessor: A module that consumes DOSETA's payload output.
The module is dedicated to the assessment of the delivered
identifier. Optionally, other DOSETA (and non-DOSETA) values can
also be delivered to this module as well as to a more general
message evaluation filtering engine. However, this additional
activity is outside the scope of the DOSETA specification.
2.2.2. Actors
Producer: An element in the data handling system that produces a
cryptographic encoding, on behalf of a domain, is referred to as a
Producer. For example, a signer is a type of producer.
Consumer: An element in the data handling system that processes an
existing cryptographic encoding, on behalf of a domain, is
referred to as a consumer. For example, a verifier is a type of
consumer.
Signer: An element in the data handling system that creates a
digital signature, on behalf of a domain, is referred to as a
signer. This element specifies an actor that is a type of DOSETA
producer. The actor might operate through a client, server or
other agent such as a reputation service. The core requirement is
that the data MUST be signed before it leaves the control of the
signer's administrative domain.
Verifier: An element in the data handling system that verifies
signatures is referred to as a verifier. This element is a
consumer of a signing service. It might be a client, server, or
other agent, such as a reputation service. In most cases it is
expected that a verifier will be close to an end user of the data
or some consuming agent such as a data processing intermediary.
2.3. Syntax
This section specifies foundational syntactic constructs used in the
remainder of the document.
Syntax descriptions use Augmented BNF (ABNF) [RFC5234].
2.3.1. Whitespace
There are three forms of whitespace:
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WSP: represents simple whitespace, that is, a space or a tab
character (formally defined in [RFC5234]).
LWSP: is linear whitespace, defined as WSP plus CRLF (formally
defined in [RFC5234]).
FWS: is folding whitespace. It allows multiple lines to be
joined with each a separated by a sequence having CRLF followed
by at least one whitespace.
The formal syntax for these are (WSP and LWSP are given for
information only):
ABNF:
WSP = SP / HTAB
LWSP = *(WSP / CRLF WSP)
FWS = [*WSP CRLF] 1*WSP
The definition of FWS is identical to that in [RFC5322] except for
the exclusion of obs-FWS.
2.3.2. Common ABNF Tokens
The following tokens are used in this document:
ABNF:
hyphenated-word = ALPHA
[ *(ALPHA / DIGIT / "-")
(ALPHA / DIGIT) ]
ALPHADIGITPS = (ALPHA / DIGIT / "+" / "/")
base64string = ALPHADIGITPS *([FWS] ALPHADIGITPS)
[ [FWS] "=" [ [FWS] "=" ] ]
hdr-name = field-name
qp-hdr-value = D-quoted-printable
; with "|" encoded
2.3.3. Imported ABNF Tokens
The following tokens are imported from other RFCs as noted. Those
RFCs SHOULD be considered definitive.
From [RFC5321]:
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<local-part> Implementation Warning: This permits quoted
strings)
<sub-domain>
From [RFC5322]:
<field-name> (name of a header field)
From [RFC2045]:
<qp-section> a single line of quoted-printable-encoded text
<hex-octet> a quoted-printable encoded octet)
NOTE: Be aware that the ABNF in [RFC2045] does not obey the rules
of [RFC5234] and MUST be interpreted accordingly, particularly as
regards case folding.
Other tokens not defined herein are imported from [RFC5234]. These
are intuitive primitives such as SP, HTAB, WSP, ALPHA, DIGIT, CRLF,
etc.
2.3.4. D-Quoted-Printable
The D-Quoted-Printable encoding syntax resembles that described in
Quoted-Printable [RFC2045], Section 6.7:
o Any character MAY be encoded as an "=" followed by two hexadecimal
digits from the alphabet "0123456789ABCDEF" (no lowercase
characters permitted) representing the hexadecimal-encoded integer
value of that character.
o All control characters (those with values < %x20), 8-bit
characters (values > %x7F), and the characters DEL (%x7F), SPACE
(%x20), and semicolon (";", %x3B) MUST be encoded.
o All whitespace, including SPACE, CR, and LF characters, MUST be
encoded.
o After encoding, FWS MAY be added at arbitrary locations in order
to avoid excessively long lines; such whitespace is NOT part of
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the value, and MUST be removed before decoding.
The formal syntax for D-Quoted-Printable is:
ABNF:
D-quoted-printable = *(FWS / hex-octet / D-safe-char)
; hex-octet is from RFC2045
D-safe-char = %x21-3A / %x3C / %x3E-7E
; '!' - ':', '<', '>' - '~'
; Characters not listed as "mail-safe"
; in [RFC2049] are also not
; recommended.
D-Quoted-Printable differs from Quoted-Printable as defined in
[RFC2045] in several important ways:
1. Whitespace in the input text, including CR and LF, MUST be
encoded. [RFC2045] does not require such encoding, and does not
permit encoding of CR or LF characters that are part of a CRLF
line break.
2. Whitespace in the encoded text is ignored. This is to allow tags
encoded using D-Quoted-Printable to be wrapped as needed. In
particular, [RFC2045] requires that line breaks in the input be
represented as physical line breaks; that is not the case here.
3. The "soft line break" syntax ("=" as the last non-whitespace
character on the line) does not apply.
4. D-Quoted-Printable does not require that encoded lines be no more
than 76 characters long (although there might be other
requirements depending on the context in which the encoded text
is being used).
3. DOSETA Library
DOSETA's library of functional components is distinguished by a DNS-
based, self-certifying public key mechanism, common data
normalization and canonicalization algorithms, and a common parameter
encoding mechanism.
3.1. Normalization for Transport Robustness
Some messages, particularly those using 8-bit characters, are subject
to modification during transit, notably from conversion to 7-bit
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form. Such conversions will break DOSETA signatures. Similarly,
data that is not compliant with its associated standard, might be
subject to corrective efforts intermediaries. See Section 8 of
[RFC4409] for examples of changes that are commonly made to email.
Such "corrections" might break DOSETA signatures or have other
undesirable effects.
In order to minimize the chances of such breakage, signers convert
the data to a suitable encoding, such as quoted-printable or base64,
as described in [RFC2045] before signing. Specification and use of
such conversions is outside the scope of DOSETA.
If the data is submitted to a DOSETA process with any local encoding
that will be modified before transmission, that modification to a
canonical form MUST be done before DOSETA processing. For Text data
in particular, bare CR or LF characters (used by some systems as a
local line separator convention) MUST be converted to the CRLF
sequences before the data is signed. Any conversion of this sort
SHOULD be applied to the data actually sent to the recipient(s), not
just to the version presented to the signing algorithm.
More generally, a DOSETA producer MUST use the data as it is expected
to be received by the DOSETA consumer rather than in some local or
internal form.
3.2. Canonicalization
Some data handling systems modify the original data during transit,
potentially invalidating a cryptographic function. In some cases,
mild modification of data can be immaterial to the validity of a
DOSETA-based service. In these cases, a canonicalization algorithm
that survives modest handling modification is preferred.
In other cases, preservation of the exact, original bits is required;
even minor modifications need to result in a failure. Hence a
canonicalization algorithm is needed that does not tolerate any in-
transit modification of the data.
To satisfy basic requirements, two canonicalization algorithms are
defined: a "simple" algorithm that tolerates almost no modification
and a "relaxed" algorithm that tolerates common modifications such as
whitespace replacement and data line rewrapping.
Data presented for canonicalization MUST already be in "network
normal" format -- text is ASCII encoded, lines are separated with
CRLF characters, etc.) See Section 3.1 for information about
normalizing data.
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Data handling systems sometimes treat different portions of text
differentially and might be subject to more or less likelihood of
breaking a signature. DOSETA currently covers two types of data:
Header: Attribute:value sets, in the style of Internet Mail
header fields or MIME header fields
Content: Lines of ASCII text
Some DOSETA producers might be willing to accept modifications to
some portions of the data, but not other portions. For DOSETA, a
producer MAY specify one algorithm for the header and another for the
content.
If no canonicalization algorithm is specified, the "simple" algorithm
defaults for each part. DOSETA producers MUST implement both of the
base canonicalization algorithms. Because additional
canonicalization algorithms might be defined in the future, producers
MUST ignore any unrecognized canonicalization algorithms.
Canonicalization simply prepares the data for presentation to the
DOSETA processing algorithm.
NOTE: Canonicalization operates on a copy of the data; it MUST NOT
change the transmitted data in any way. Canonicalization of
distinct data portions is described below.
3.2.1. Header Canonicalization Algorithms
This section describes basic entries for the Header Canonicalization
IANA registry defined in [DKIMSign], , which also applies to DOSETA
header canonicalization.
simple: The "simple" header canonicalization algorithm is for a set
of "attribute:value" textual data structures, such as email header
fields [RFC5322]. It does not change the original Header fields
in any way. Header fields MUST be presented to the processing
algorithm exactly as they are in the data being processed. In
particular, header field names MUST NOT be case folded and
whitespace MUST NOT be changed.
relaxed: The "relaxed" header canonicalization algorithm is for a
set of "attribute:value" textual data structures, such as email
header fields [RFC5322]. It does not change the original Header
fields in any way. The following steps MUST be applied in order:
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* Convert all header field names (not the header field values) to
lowercase. For example, convert "SUBJect: AbC" to "subject:
AbC".
* Unfold all header field continuation lines as described in
[RFC5322]; in particular, lines with terminators embedded in
continued header field values (that is, CRLF sequences followed
by WSP) MUST be interpreted without the CRLF. Implementations
MUST NOT remove the CRLF at the end of the header field value.
* Convert all sequences of one or more WSP characters to a single
SP character. WSP characters here include those before and
after a line folding boundary.
* Delete all WSP characters at the end of each unfolded header
field value.
* Delete any WSP characters remaining before and after the colon
separating the header field name from the header field value.
The colon separator MUST be retained.
3.2.2. Content Canonicalization Algorithms
This section describes basic entries for the Message Canonicalization
IANA registry defined in [DKIMSign], which also applies to DOSETA
Content.
simple: The "simple" Content canonicalization algorithm is for
lines of ASCII text, such as occur in the body of email [RFC5322].
It ignores all empty lines at the end of the Content. An empty
line is a line of zero length after removal of the line
terminator. If there is no Content or no trailing CRLF on the
Content, a CRLF is added. It makes no other changes to the
Content. In more formal terms, the "simple" Content
canonicalization algorithm converts "0*CRLF" at the end of the
Content to a single "CRLF".
Note that a completely empty or missing Content is canonicalized
as a single "CRLF"; that is, the canonicalized length will be 2
octets.
The sha1 value (in base64) for an empty Content (canonicalized to
a "CRLF") is:
uoq1oCgLlTqpdDX/iUbLy7J1Wic=
The sha256 value is:
frcCV1k9oG9oKj3dpUqdJg1PxRT2RSN/XKdLCPjaYaY=
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relaxed: The "relaxed" Content canonicalization algorithm is for
lines of ASCII text, such as occur in the body of email [RFC5322].
It MUST apply the following steps (a) and (b) in order:
A. Reduce whitespace:
+ Ignore all whitespace at the end of lines. Implementations
MUST NOT remove the CRLF at the end of the line.
+ Reduce all sequences of WSP within a line to a single SP
character.
B. Ignore all empty lines at the end of the Content. "Empty
line" is defined in Section 3.2.2. If the Content is non-
empty, but does not end with a CRLF, a CRLF is added. (For
email, this is only possible when using extensions to SMTP or
non-SMTP transport mechanisms.)
The sha1 value (in base64) for an empty Content (canonicalized to
a null input) is:
2jmj7l5rSw0yVb/vlWAYkK/YBwk=
The sha256 value is:
47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=
NOTE: The relaxed Content canonicalization algorithm can enable
certain types of extremely crude "ASCII Art" attacks in which a
message can be conveyed, by adjusting the spacing between
words. If this is a concern, the "simple" Content
canonicalization algorithm is more appropriate for use.
3.2.3. Canonicalization Examples
In the following examples, actual whitespace is used only for
clarity. The actual input and output text is designated using
bracketed descriptors: "<SP>" for a space character, "<HTAB>" for a
tab character, and "<CRLF>" for a carriage-return/line-feed sequence.
For example, "X <SP> Y" and "X<SP>Y" represent the same three
characters.
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Example 1: An email message reading:
A: <SP> X <CRLF>
B <SP> : <SP> Y <HTAB><CRLF>
<HTAB> Z <SP><SP><CRLF>
<CRLF>
<SP> C <SP><CRLF>
D <SP><HTAB><SP> E <CRLF>
<CRLF>
<CRLF>
when canonicalized using relaxed canonicalization for both Header and
Content results in a Header reading:
a:X <CRLF>
b:Y <SP> Z <CRLF>
and a Content reading:
<SP> C <CRLF>
D <SP> E <CRLF>
Example 2: The same message canonicalized using simple
canonicalization for both Header and Content results in a header
reading:
A: <SP> X <CRLF>
B <SP> : <SP> Y <HTAB><CRLF>
<HTAB> Z <SP><SP><CRLF>
and a Content reading:
<SP> C <SP><CRLF>
D <SP><HTAB><SP> E <CRLF>
Example 3: When processed using relaxed Header canonicalization and
simple Content canonicalization, the canonicalized version has a
header of:
a:X <CRLF>
b:Y <SP> Z <CRLF>
and a Content reading:
<SP> C <SP><CRLF>
D <SP><HTAB><SP> E <CRLF>
3.3. Tag=Value Parameters
DOSETA uses a simple "tag=value" parameter syntax in several
contexts, such as when representing associated cryptographic data and
domain key records.
Values are a series of strings containing either plain text, "base64"
text (as defined in [RFC2045], Section 6.8), "qp-section" (ibid,
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Section 6.7), or "D-quoted-printable" (as defined in Section 2.6).
The definition of a tag will determine the specific encoding for its
associated value. Unencoded semicolon (";") characters MUST NOT
occur in the tag value, since that separates tag-specs.
NOTE: The "plain text" defined below, as "tag-value", only supports
use of 7-bit characters. However, it is likely that support of
UTF-8 Unicode [UTF8] data will eventually be deemed important.
Formally the syntax rules are as follows:
ABNF:
tag-list = tag-spec 0*( ";" tag-spec ) [ ";" ]
tag-spec = [FWS] tag-name [FWS] "=" [FWS] tag-value [FWS]
tag-name = ALPHA 0*ALNUMPUNC
tag-value = [ tval 0*( 1*(WSP / FWS) tval ) ]
; WSP and FWS prohibited at beginning and end
tval = 1*VALCHAR
VALCHAR = %x21-3A / %x3C-7E
; EXCLAMATION to TILDE except SEMICOLON
ALNUMPUNC = ALPHA / DIGIT / "_"
NOTE: WSP is allowed anywhere around tags. In particular, any WSP
after the "=" and any WSP before the terminating ";" is not part
of the value. However, WSP inside the value is significant.
Tags MUST interpret a VALCHAR as case-sensitive, unless the specific
tag description of semantics specifies case insensitivity.
Tags MUST be unique; duplicate names MUST NOT occur within a single
tag-list. If a tag name does occur more than once, the entire tag-
list is invalid.
Whitespace within a value MUST be retained unless explicitly excluded
by the specific tag description.
Tag=value pairs that represent the default value MAY be included to
aid legibility.
Unrecognized tags MUST be ignored.
Tags that have an empty value are not the same as omitted tags. An
omitted tag is treated as having the default value; a tag with an
empty value explicitly designates the empty string as the value.
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3.4. Key Management
Applications require some level of assurance that a producer is
authorized to use a cited public. Many applications achieve this by
using public key certificates issued by a trusted authority. For
applications with modest certification requirements, DOSETA achieves
a sufficient level of security, with excellent scaling properties, by
simply having the consumer query the purported producer's DNS entry
(or a supported equivalent) in order to retrieve the public key. The
safety of this model is increased by the use of DNSSEC [RFC4033] for
the key records in the DNS.
DOSETA keys might be stored in multiple types of key servers and in
multiple formats. As long as the key-related information is the same
and as long as the security properties of key storage and retrieval
are the same, DOSETA's operation is unaffected by the actual source
of a key.
The abstract key lookup algorithm is:
public-key = D-find-key(q-val, d-val, s-val)
where:
q-val: The type of the lookup, as specified in the "q" parameter
(Section 4.2)
d-val: The domain of the signature, as specified in the "d"
parameter (Section 4.2)
s-val: The selector of the lookup as specified in the "s"
parameter (Section 4.2)
D-find-key: A function that uses q-val to determine the specific
details for accessing the desired stored Key record.
This document defines a single binding between the abstract lookup
algorithm and a physical instance, using DNS TXT records, per
Section 3.6. Other bindings can be defined.
3.5. Selectors for Keys
It can be extremely helpful to support multiple DOSETA keys for the
same domain name. For example:
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* Rolling over from one key to another is a common security
administration requirement; for an operational service this is
made far easier when the old and new keys are supported
simultaneously.
* Domains that want to delegate signing capability for a specific
address for a given duration to a partner, such as an
advertising provider or other outsourced function.
* Domains that want to allow frequent travelers to generate
signed data locally without the need to connect to a particular
server.
* "Affinity" domains (such as, college alumni associations) that
provide data forwarding, but that do not operate a data
origination agent for outgoing data.
To these ends, DOSETA includes a mechanism that supports multiple
concurrent public keys per signing domain. The key namespace is
subdivided using "selectors". For example, selectors might indicate
the names of office locations (for example, "sanfrancisco",
"coolumbeach", and "reykjavik"), the signing date (for example,
"january2005", "february2005", etc.), or even an individual user.
For further administrative convenience, sub-division of selectors is
allowed, distinguished as dotted sub-components of the selector name.
When keys are retrieved from the DNS, periods in selectors define DNS
label boundaries in a manner similar to the conventional use in
domain names. Selector components might be used to combine dates
with locations, for example, "march2005.reykjavik". In a DNS
implementation, this can be used to allow delegation of a portion of
the selector namespace.
ABNF:
selector = sub-domain *( "." sub-domain )
The number of public keys and the corresponding selectors for each
domain are determined by the domain owner. Many domain owners will
be satisfied with just one selector, whereas administratively
distributed organizations might choose to manage disparate selectors
and key pairs in different regions or on different servers.
As noted, selectors make it possible to seamlessly replace public
keys on a routine basis. If a domain wishes to change from using a
public key associated with selector "january2005" to a public key
associated with selector "february2005", it merely makes sure that
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both public keys are advertised in the public-key repository
concurrently for the transition period during which data might be in
transit prior to verification. At the start of the transition
period, the outbound servers are configured to sign with the
"february2005" private key. At the end of the transition period, the
"january2005" public key is removed from the public-key repository.
NOTE: A key can also be revoked as described below. The
distinction between revoking and removing a key selector record
is subtle. When phasing out keys as described above, a signing
domain would probably simply remove the key record after the
transition period. However, a signing domain could elect to
revoke the key (but maintain the key record) for a further
period. There is no defined semantic difference between a
revoked key and a removed key.
While some domains might wish to make selector values well known,
others will want to take care not to allocate selector names in a way
that allows harvesting of data by outside parties. For example, if
per-user keys are issued, the domain owner will need to make the
decision as to whether to associate this selector directly with the
name of a registered end-user, or make it some unassociated random
value, such as a fingerprint of the public key.
NOTE: The ability to reuse a selector with a new key (for
example, changing the key associated with a user's name) makes
it impossible to tell the difference between data that didn't
verify because the key is no longer valid versus a data that is
actually forged. For this reason, reuse of selectors with
different keys is ill-advised. A better strategy is to assign
new keys to new selectors.
3.6. DNS Binding for Key Retrieval
This section defines a binding using DNS TXT records as a key
service. All implementations MUST support this binding.
3.6.1. Namespace
A DOSETA key is stored in a subdomain named:
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ABNF:
dns-record = s "._domainkey." d
where:
s: is the selector of the lookup as specified in the "s"
parameter (Section 4.2)
d: is the domain of the signature, as specified in the "d"
parameter (Section 4.2)
NOTE: The string constant "_domainkey" is used to mark a sub-tree
that contains unified DOSETA key information. This string is a
constant, rather than being a different string for different key-
based services, with the view that keys are agnostic about the
service they are used for. That is, there is no semantic or
security benefit in having a different constant string for
different key services. That said, a new service is certainly
free to define a new constant and maintain and entirely
independent set of keys.
Given a DOSETA-Signature field with a "d" parameter of "example.com"
and an "s" parameter of "foo.bar", the DNS query will be for:
foo.bar._domainkey.example.com
Wildcard DNS records (for example, *.bar._domainkey.example.com) do
not make sense in the context of DOSETA and their presence can be
problematic. Hence DNS wildcards with DOSETA SHOULD NOT be used.
Note also that wildcards within domains (for example,
s._domainkey.*.example.com) are not supported by the DNS.
3.6.2. Resource Record Types for Key Storage
The DNS Resource Record type used is specified by an option to the
query-type ("q") parameter. The only option defined in this base
specification is "txt", indicating the use of a DNS TXT Resource
Record (RR), as defined in Section 3.7. A later extension of this
standard might define another RR type.
Strings in a TXT RR MUST be concatenated together before use, with no
intervening whitespace. TXT RRs MUST be unique for a particular
selector name; that is, if there are multiple records in an RRset,
the results are undefined.
3.7. Stored Key Data
This section defines a syntax for encoding stored key data within an
unstructured environment such as the simple text environment of a DNS
TXT record.
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[TEMPLATE] (Key Retrieval) A service that incorporates DOSETA
MAY define the specific mechanism by which consumers can obtain
associated public keys. This might be as easy as referencing
an existing key management system or it might require a new set
of conventions.
Absent an explicit specification for key retrieval, the default
mechanism is specified in Section 3.6. Use of this means
sharing the set of public keys with DKIM and other DOSETA-based
services.
The overall syntax is a tag-list as described in Section 3.3. The
base set of valid tags is described below. Other tags MAY be present
and MUST be ignored by any implementation that does not understand
them.
v= Version of the DOSETA key record (plain-text; RECOMMENDED,
default is "DKIM1"). If specified, this tag MUST be set to
"DKIM1" (without the quotes). This tag MUST be the first tag
in the record. Records beginning with a "v" parameter with any
other value MUST be discarded. Note that verifiers MUST do a
string comparison on this value; for example, "DKIM1" is not
the same as "DKIM1.0".
ABNF:
key-v-tag = %x76 [FWS] "=" [FWS] %x44 %x4B %x49 %x4D %x31
NOTE: The version string "DKIM1" is retained from the
original DKIM specification, in order to preserve the
installed base of records. In addition, there is no
functional benefit in defining a new string, since the key
record is not application-specific. Another use of the
string is to distinguish the Domainkeys use of a TXT RR from
other, unrelated uses.
k= Key type (plain-text; OPTIONAL, default is "rsa"). Signers
and verifiers MUST support the "rsa" key type. The "rsa" key
type indicates that an ASN.1 DER-encoded [ITU-X660-1997]
RSAPublicKey [RFC3447] (see Sections Section 3.5 and A.1.1) is
being used in the "p" parameter. (Note: the "p" parameter
further encodes the value using the base64 algorithm.)
Unrecognized key types MUST be ignored.
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ABNF:
key-k-tag = %x76 [FWS] "=" [FWS] key-k-tag-type
key-k-tag-type = "rsa" / x-key-k-tag-type
x-key-k-tag-type = hyphenated-word ; for future extension
n= Notes that might be of interest to a human (qp-section;
OPTIONAL, default is empty). No interpretation is made by any
program. This tag should be used sparingly in any key server
mechanism that has space limitations (notably DNS). This is
intended for use by administrators, not end users.
ABNF:
key-n-tag = %x6e [FWS] "=" [FWS] qp-section
p= Public-key data (base64; REQUIRED). An empty value means
that this public key has been revoked. The syntax and
semantics of this tag value before being encoded in base64 are
defined by the "k" parameter.
ABNF:
key-p-tag = %x70 [FWS] "=" [ [FWS] base64string]
NOTE: If a private key has been compromised or otherwise
disabled (for example, an outsourcing contract has been
terminated), a signer might want to explicitly state that it
knows about the selector, but also have all queries using
that selector result in a failed verification. Verifiers
SHOULD ignore any DOSETA-Signature header fields with a
selector referencing a revoked key.
NOTE: A base64string is permitted to include white space
(FWS) at arbitrary places; however, any CRLFs MUST be
followed by at least one WSP character. Implementors and
administrators are cautioned to ensure that selector TXT
records conform to this specification.
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4. DOSETA H/C Signing Template
This section specifies the basic components of a signing mechanism;
it is similar to the one defined for DKIM. This template for a
signing service can be mapped to a two-part -- header/content -- data
model. As for DKIM this separates specification of the signer's
identity from any other identifiers that might be associated with
that data.
NOTE: The use of hashing and signing algorithms by DOSETA
inherently provides a degree of data integrity protection,
between the signing and verifying steps. However it does not
necessarily "authenticate" the data that is signed. For
example, it does not inherently validate the accuracy of the
data or declare that the signer is the author or owner of the
data. To the extent that authentication is meant by the
presence of a signature, that needs to be specified as part of
the semantics for the service based upon this template.
[TEMPLATE] (Header/Content Mapping) The service incorporating
this mechanism MUST define the precise mappings onto the
template provided in this section. (Data lacking a header
component might still be possible to cast in a header/content
form, where the header comprises on the DOSETA Signature
information.)
The service also MUST define the precise meaning of a
signature.
4.1. Cryptographic Algorithms
DOSETA supports multiple digital signature algorithms:
rsa-sha1: The rsa-sha1 Signing Algorithm computes a message hash
as described in Section 4.3 below using SHA-1 [FIPS-180-2-2002]
as a hashing algorithm. That hash is then signed by the signer
using the RSA algorithm (defined in PKCS#1 version 1.5
[RFC3447]) as the crypt-alg and the signer's private key. The
hash MUST NOT be truncated or converted into any form other
than the native binary form before being signed. The signing
algorithm SHOULD use a public exponent of 65537.
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rsa-sha256: The rsa-sha256 Signing Algorithm computes a message
hash as described in [RFC5451] below using SHA-256
[FIPS-180-2-2002] as the hash-alg. That hash is then signed by
the signer using the RSA algorithm (defined in PKCS#1 version
1.5 [RFC3447]) as the crypt-alg and the signer's private key.
The hash MUST NOT be truncated or converted into any form other
than the native binary form before being signed.
Other: Other algorithms MAY be defined in the future. Verifiers
MUST ignore any signatures using algorithms that they do not
implement.
Signers MUST implement and SHOULD sign using rsa-sha256. Verifiers
MUST implement rsa-sha256.
NOTE: Although sha256 is strongly encouraged, some senders of low-
security messages (such as routine newsletters) might prefer to
use sha1 because of reduced CPU requirements to compute a sha1
hash. In general, sha256 is always preferred, whenever possible.
Selecting appropriate key sizes is a trade-off between cost,
performance, and risk. Since short RSA keys more easily succumb to
off-line attacks, signers MUST use RSA keys of at least 1024 bits for
long-lived keys. Verifiers MUST be able to validate signatures with
keys ranging from 512 bits to 2048 bits, and they MAY be able to
validate signatures with larger keys. Verifier policies might use
the length of the signing key as one metric for determining whether a
signature is acceptable.
Factors that ought to influence the key size choice include the
following:
* The practical constraint that large (for example, 4096 bit)
keys might not fit within a 512-byte DNS UDP response packet
* The security constraint that keys smaller than 1024 bits are
subject to off-line attacks
* Larger keys impose higher CPU costs to verify and sign data
* Keys can be replaced on a regular basis, thus their lifetime
can be relatively short
* The security goals of this specification are modest compared to
typical goals of other systems that employ digital signatures
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See [RFC3766] for further discussion on selecting key sizes.
4.2. Signature Data Structure
A signature of data is stored into an data structure associated with
the signed data. This structure contains all of the signature- and
key-fetching data. This DOSETA-Signature structure is a tag-list as
defined in Section 3.3.
[TEMPLATE] (Signature Association) A service that incorporates
DOSETA MUST define the exact means by which the Signature
structure is associated with the data.
When the DOSETA-Signature structure is part of a sequence of
structures -- such as being added to an email header -- it SHOULD NOT
be reordered and SHOULD be pre-pended to the message. (This is the
same handling as is given to email trace Header fields, defined in
Section 3.6 of [RFC5322].)
The tags are specified below. Tags described as <qp-section> are
encoded as described in Section 6.7 of MIME Part One [RFC2045], with
the additional conversion of semicolon characters to "=3B";
intuitively, this is one line of quoted-printable encoded text. The
D-quoted-printable syntax is defined in Section 2.3.4.
Tags on the DOSETA-Signature structure along with their type and
requirement status are shown below. Unrecognized tags MUST be
ignored.
v= Version (MUST be included). This tag defines the version of
this specification that applies to the signature record. It
MUST have the value "1". Note that verifiers MUST do an exact
string comparison on this value; for example, "1" is not the
same as "1.0".
ABNF:
sig-v-tag = %x76 [FWS] "=" [FWS] "1"
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NOTE: DOSETA-Signature version numbers are expected to
increase arithmetically as new versions of this
specification are released.
a= The algorithm used to generate the signature (plain-text;
REQUIRED). Verifiers MUST support "rsa-sha1" and "rsa-sha256";
signers SHOULD sign using "rsa-sha256". See Section 4.1 for a
description of algorithms.
ABNF:
sig-a-tag = %x61 [FWS] "=" [FWS] sig-a-tag-alg
sig-a-tag-alg = sig-a-tag-k "-" sig-a-tag-h
sig-a-tag-k = "rsa" / x-sig-a-tag-k
sig-a-tag-h = "sha1" / "sha256" / x-sig-a-tag-h
x-sig-a-tag-k = ALPHA *(ALPHA / DIGIT)
; for later extension
x-sig-a-tag-h = ALPHA *(ALPHA / DIGIT)
; for later extension
b= The signature data (base64; REQUIRED). Whitespace is
ignored in this value and MUST be ignored when reassembling the
original signature. In particular, the signing process can
safely insert FWS in this value in arbitrary places to conform
to line-length limits. See Signer Actions (Section 4.4) for
how the signature is computed.
ABNF:
sig-b-tag = %x62 [FWS] "=" [FWS] sig-b-tag-data
sig-b-tag-data = base64string
bh= The hash of the canonicalized Content (body), as limited by
the "l" parameter (base64; REQUIRED). Whitespace is ignored in
this value and MUST be ignored when reassembling the original
signature. In particular, the signing process can safely
insert FWS in this value in arbitrary places to conform to
line-length limits. See Section 4.3 for how the Content hash
is computed.
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ABNF:
sig-bh-tag = %x62 %x68 [FWS] "=" [FWS] sig-bh-tag-data
sig-bh-tag-data = base64string
c= Data canonicalization (plain-text; OPTIONAL, default is
"simple/simple"). This tag informs the verifier of the type of
canonicalization used to prepare the message for signing. It
consists of two names separated by a "slash" (%d47) character,
corresponding to the header and Content canonicalization
algorithms respectively:
ABNF:
sig-bh-tag = %x63 [FWS] "=" [FWS] sig-c-header "/" sig-c-content
where:
sig-c-header: A value from Header Canonicalization IANA
registry defined in [DKIMSign]
sig-c-content: A value from Message Canonicalization IANA
registry defined in [DKIMSign]
These algorithms are described in Section 3.2. If only one
algorithm is named, that algorithm is used for the header and
"simple" is used for the Content. For example, "c=relaxed" is
treated the same as "c=relaxed/simple".
ABNF:
sig-c-tag = %x63 [FWS] "=" [FWS] sig-c-tag-alg
["/" sig-c-tag-alg]
sig-c-tag-alg = "simple" / "relaxed" / x-sig-c-tag-alg
x-sig-c-tag-alg = hyphenated-word ; for later extension
cl= A list of semantic claims, asserting the set of "meanings"
for the signature, such as for author validity or content
validity. The list of supported claims comprises values from
the DOSETA Claims IANA registry, defined in Section 6
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ABNF:
sig-cl-tag = %x63 %x6C [FWS] "=" [FWS]
sig-cl-tag-claim
["/" sig-c-tag-claim]
sig-c-tag-claim = hyphenated-word
; per DOSETA Claims Registry
d= The DDI doing the signing (plain-text; REQUIRED). Hence,
the DDI value is used to form the query for the public key.
The DDI MUST correspond to a valid DNS name under which the
DOSETA key record is published. The conventions and semantics
used by a signer to create and use a specific DDI are outside
the scope of the DOSETA Signing specification, as is any use of
those conventions and semantics. When presented with a
signature that does not meet these requirements, verifiers MUST
consider the signature invalid.
Internationalized domain names MUST be encoded as described in
[RFC5890].
[TEMPLATE] (Semantics) The service incorporating DOSETA
MUST define the semantics of a signature.
ABNF:
sig-d-tag = %x64 [FWS] "=" [FWS] domain-name
domain-name = sub-domain 1*("." sub-domain)
; from RFC 5321 Domain,
; but excluding address-literal
h= Signed Header fields (plain-text, but see description;
REQUIRED). A colon-separated list of header field names that
identify the Header fields presented to the signing algorithm.
The field MUST contain the complete list of Header fields in
the order presented to the signing algorithm. The field MAY
contain names of Header fields that do not exist when signed;
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nonexistent Header fields do not contribute to the signature
computation (that is, they are treated as the null input,
including the header field name, the separating colon, the
header field value, and any CRLF terminator). The field MUST
NOT include the DOSETA Signature header field that is being
created or verified, but might include others. Folding
whitespace (FWS) MAY be included on either side of the colon
separator. Header field names MUST be compared against actual
header field names in a case-insensitive manner. This list
MUST NOT be empty. See Section 4.4.3 for a discussion of
choosing Header fields to sign.
ABNF:
sig-h-tag = %x68 [FWS] "=" [FWS] hdr-name
0*( [FWS] ":" [FWS] hdr-name )
By "signing" Header fields that do not actually exist, a signer
can prevent insertion of those Header fields before
verification. However, since a signer cannot possibly know all
possible Header fields that might be created in the future, the
security of this solution is not total.
The exclusion of the header field name and colon as well as the
header field value for non-existent Header fields prevents an
attacker from inserting an actual header field with a null
value.
q= A colon-separated list of query methods used to retrieve the
public key (plain-text; OPTIONAL, default is "dns/txt"). Each
query method is of the form "type[/options]", where the syntax
and semantics of the options depend on the type and specified
options. If there are multiple query mechanisms listed, the
choice of query mechanism MUST NOT change the interpretation of
the signature. Implementations MUST use the recognized query
mechanisms in the order presented. Unrecognized query
mechanisms MUST be ignored.
Currently, the only valid value is "dns/txt", which defines the
DNS TXT record lookup algorithm described elsewhere in this
document. The only option defined for the "dns" query type is
"txt", which MUST be included. Verifiers and signers MUST
support "dns/txt".
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ABNF:
sig-q-tag = %x71 [FWS] "=" [FWS] sig-q-tag-method
*([FWS] ":" [FWS] sig-q-tag-method)
sig-q-tag-method = "dns/txt" / x-sig-q-tag-type
["/" x-sig-q-tag-args]
x-sig-q-tag-type = hyphenated-word ; for future extension
x-sig-q-tag-args = qp-hdr-value
s= The selector subdividing the namespace for the "d=" (domain)
tag (plain-text; REQUIRED).
ABNF:
sig-s-tag = %x73 [FWS] "=" [FWS] selector
t= Signature Timestamp (plain-text unsigned decimal integer;
RECOMMENDED, default is an unknown creation time). The time
that this signature was created. The format is the number of
seconds since 00:00:00 on January 1, 1970 in the UTC time zone.
The value is expressed as an unsigned integer in decimal ASCII.
This value is not constrained to fit into a 31- or 32-bit
integer. Implementations SHOULD be prepared to handle values
up to at least 10^12 (until approximately AD 200,000; this fits
into 40 bits). To avoid denial-of-service attacks,
implementations MAY consider any value longer than 12 digits to
be infinite. Leap seconds are not counted. Implementations
MAY ignore signatures that have a timestamp in the future.
ABNF:
sig-t-tag = %x74 [FWS] "=" [FWS] 1*12DIGIT
x= Signature Expiration (plain-text unsigned decimal integer;
RECOMMENDED, default is no expiration). The format is the same
as in the "t" parameter, represented as an absolute date, not
as a time delta from the signing timestamp. The value is
expressed as an unsigned integer in decimal ASCII, with the
same constraints on the value in the "t=" tag. Signatures MAY
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be considered invalid if the verification time at the verifier
is past the expiration date. Ideally verification time is when
a message is first received at the administrative domain of the
verifier; otherwise the current time SHOULD be used. The value
of the "x" parameter MUST be greater than the value of the "t"
parameter if both are present.
NOTE: The "x" parameter is not intended as an anti-replay
defense.
NOTE: Due to clock drift, the receiver's notion of when to
consider the signature expired might not match exactly when
the sender is expecting. Receivers MAY add a 'fudge factor'
to allow for such possible drift.
ABNF:
sig-x-tag = %x78 [FWS] "=" [FWS]
1*12DIGIT
4.3. Signature Calculations
Hashing and cryptographic signature algorithms are combined into a
procedure for computing a digital signature. Producers will choose
appropriate parameters for the signing process. Consumers will use
the tags that are then passed as an associated DOSETA-Signature
header field. Section 4.2. In the following discussion, the names
of the tags are parameters in that field.
The basic operations for producing a signature are canonicalization,
hashing and signing. Canonicalization removes irrelevant variations.
Hashing produces a very short representation for the data and signing
produces a unique, protected string to be exchanged.
NOTE: Canonicalization (see Section 3.2) prepares a separate
representation of the data for additional processing; it does
not affect the original, transmitted data in any way.
Producers MUST compute hashes in the order defined. Consumers MAY
compute them in any order convenient to the producer, provided that
the result is semantically identical to the semantics that would
occur, had they been computed in this order.
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The combined hashing and signing algorithms are:
Content Hash:
1. Truncate the content to the length specified in the "l"
parameter.
2. Canonicalize the truncated content, using the algorithm
specified in the "c" parameter.
3. Hash the canonicalized content, using the hashing algorithm
specified in the "a" parameter.
4. Convert the resulting hash to base64 form.
5. Signers then insert the base64 result into the "bh"
parameter of the DOSETA-Signature field; verifiers compare
their hash with the value in the "bh" parameter.
Header Hash:
1. Select the header fields specified by the "h" parameter.
2. Ensure that each field is terminated by a single CRLF.
3. Canonicalize each of these fields, using the header
canonicalization algorithm specified in the "c" parameter.
4. Select the DOSETA-Signature field that exists (verifying),
or will be inserted (signing), in the header.
5. From that field, delete the value portion of the "b"
parameter, including all surrounding whitespace; that is,
treat the "b" parameter as containing an empty string.
6. Canonicalize the resulting field, using the Header
canonicalization algorithm specified in the "c" parameter,
but remove the trailing CRLF, if present.
7. Using the hashing algorithm specified in the "a" parameter,
hash the sequence:
1. the canonicalized header fields, in the order specified
by the "h" parameter,
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2. the output from the content hash, and
3. the canonicalized DOSETA-Signature field.
Signature:
1. Obtain the relevant key associated with the relevant domain
and selector; for signing this is a private key; for
verifying this is a public key.
2. Obtain the header hash produced by the previous
calculation.
3. Using the signing algorithm specified in the "a" parameter,
and the relevant key, process the hash.
All tags cited in the "h" parameter MUST be included even if they are
not understood by the verifier. Note that the DOSETA-Signature field
is presented to the hash algorithm after the content hash is
processed, rather than with the rest of the header fields that are
processed before the content hash. The DOSETA-Signature header
structure MUST NOT be cited in its own h= tag. If present, other
DOSETA-Signature header fields MAY be cited and included in the
signature process (see Section 5).
When calculating the hash on data that will be transmitted using
additional encoding, such as base64 or quoted-printable, signers MUST
compute the hash after the encoding. Likewise, the verifier MUST
incorporate the values into the hash before decoding the base64 or
quoted-printable text. However, the hash MUST be computed before
transport level encodings such as SMTP "dot-stuffing" (the
modification of lines beginning with a "." to avoid confusion with
the SMTP end-of-message marker, as specified in [RFC5321]).
With the exception of the canonicalization procedure described in
Section 3.2, the DOSETA signing process treats the content as a
simple string of octets. DOSETA content MAY be either simple lines
of plain-text or as a MIME object; no special treatment is afforded
to MIME content.
Formally, the algorithm for the signature is as follows:
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ABNF:
content-hash = hash-alg (canon-content, l-param)
data-hash = hash-alg (h-headers, D-SIG, content-hash)
signature = sig-alg (d-domain, selector, data-hash)
where:
content-hash: is the output from hashing the content, using
hash-alg.
hash-alg: is the hashing algorithm specified in the "a"
parameter.
canon-content: is a canonicalized representation of the content.
l-param: is the length-of-content value of the "l" parameter.
data-hash: is the output from using the hash-alg algorithm, to
hash the header including the DKIM-Signature header, and the
content hash.
h-headers: is the list of headers to be signed, as specified in
the "h" parameter.
D-SIG: is the canonicalized DOSETA-Signature field without the
signature value portion of the parameter, itself; that is, an
empty parameter value.
canon-content: is the canonicalized data content (respectively)
as defined in Section 3.2 (excluding the DOSETA-Signature
field).
signature: is the signature value produced by the signing
algorithm.
sig-alg: is the signature algorithm specified by the "a"
parameter.
d-domain: is the domain name specified in the "d" parameter.
selector: is the selector value specified in the "s" parameter.
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NOTE: Many digital signature APIs provide both hashing and
application of the RSA private key using a single "sign()"
primitive. When using such an API, the last two steps in the
algorithm would probably be combined into a single call that would
simultaneously perform both "a-hash-alg" and the "sig-alg".
4.4. Signer Actions
The following steps are performed in order by signers.
4.4.1. Determine Whether the Data Should Be Signed and by Whom
A signer can obviously only sign data using domains for which it has
a private key and the necessary knowledge of the corresponding public
key and selector information. However, there are a number of other
reasons beyond the lack of a private key why a signer could choose
not to sign the data.
NOTE: Signing modules can be incorporated into any portion of a
service, as deemed appropriate, including end-systems, servers and
intermediaries. Wherever implemented, signers need to beware of
the semantics of signing data. An example of how this can be
problematic is that within a trusted enclave the signing address
might be derived from the data according to local policy; the
derivation is based on local trust rather than explicit
validation.
If the data cannot be signed for some reason, the disposition of that
data is a local policy decision.
4.4.2. Select a Private Key and Corresponding Selector Information
This specification does not define the basis by which a signer ought
to choose which private key and selector information to use.
Currently, all selectors are equal, with respect to this
specification. So the choices ought to largely be a matter of
administrative convenience. Distribution and management of private
keys is also outside the scope of this document.
NOTE: A signer SHOULD use a private key with an associated selector
record that is expected to still be valid by time the verifier is
likely to have an opportunity to validate the signature. The
signer SHOULD anticipate that verifiers can choose to defer
validation, perhaps until the message is actually read by the
final recipient. In particular, when rotating to a new key pair,
signing SHOULD immediately commence with the new private key, but
the old public key SHOULD be retained for a reasonable validation
interval before being removed from the key server.
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4.4.3. Determine the Header Fields to Sign
Signers SHOULD NOT sign an existing header field that is likely to be
legitimately modified or removed in transit. Signers MAY include any
other Header fields present at the time of signing at the discretion
of the signer.
NOTE: The choice of which Header fields to sign is non-obvious.
One strategy is to sign all existing, non-repeatable Header
fields. An alternative strategy is to sign only Header fields
that are likely to be displayed to or otherwise be likely to
affect the processing of the Content at the receiver. A third
strategy is to sign only "well known" headers. Note that
verifiers might treat unsigned Header fields with extreme
skepticism, including refusing to display them to the end user or
even ignoring the signature if it does not cover certain Header
fields.
The DOSETA-Signature header field is always implicitly signed and
MUST NOT be included in the "h" parameter except to indicate that
other preexisting signatures are also signed.
Signers MAY claim to have signed Header fields that do not exist
(that is, signers MAY include the header field name in the "h"
parameter even if that header field does not exist in the message).
When computing the signature, the non-existing header field MUST be
treated as the null string (including the header field name, header
field value, all punctuation, and the trailing CRLF).
NOTE: This allows signers to explicitly assert the absence of a
header field; if that header field is added later the signature
will fail.
NOTE: A header field name need only be listed once more than the
actual number of that header field in a message at the time of
signing in order to prevent any further additions. For example,
if there is a single Comments header field at the time of signing,
listing Comments twice in the "h" parameter is sufficient to
prevent any number of Comments Header fields from being appended;
it is not necessary (but is legal) to list Comments three or more
times in the "h" parameter.
Signers choosing to sign an existing header field that occurs more
than once in the message (such as Received) MUST sign the physically
last instance of that header field in the header block. Signers
wishing to sign multiple instances of such a header field MUST
include the header field name multiple times in the h= tag of the
DOSETA-Signature header field, and MUST sign such Header fields in
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order from the bottom of the header field block to the top. The
signer MAY include more instances of a header field name in h= than
there are actual corresponding Header fields to indicate that
additional Header fields of that name SHOULD NOT be added.
EXAMPLE:
If the signer wishes to sign two existing Received Header fields,
and the existing header contains:
Received: <A>
Received: <B>
Received: <c>
then the resulting DOSETA-Signature header field ought to read:
DKIM-Signature: ... h=Received : Received :...
and Received Header fields <C> and <B> will be signed in that
order.
Signers need to be careful of signing Header fields that might have
additional instances added later in the delivery process, since such
Header fields might be inserted after the signed instance or
otherwise reordered. Trace Header fields (such as Received) and
Resent-* blocks are the only fields prohibited by [RFC5322] from
being reordered. In particular, since DOSETA-Signature Header fields
might be reordered by some intermediate MTAs, signing existing
DOSETA-Signature Header fields is error-prone.
NOTE: Despite the fact that [RFC5322] permits Header fields to be
reordered (with the exception of Received Header fields),
reordering of signed Header fields with multiple instances by
intermediate MTAs will cause DOSETA signatures to be broken; such
anti-social behavior ought to be avoided.
NOTE: Although not required by this specification, all end-user
visible Header fields SHOULD be signed to avoid possible "indirect
spamming". For example, if the Subject header field is not
signed, a spammer can resend a previously signed mail, replacing
the legitimate subject with a one-line spam.
4.4.4. Compute the Message Signature
The signer MUST compute the message hash as described in Section 4.3
and then sign it using the selected public-key algorithm. This will
result in a DOSETA-Signature header field that will include the
Content hash and a signature of the header hash, where that header
includes the DOSETA-Signature header field itself.
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Entities such as mailing list managers that implement DOSETA and that
modify the message or a header field (for example, inserting
unsubscribe information) before retransmitting the message SHOULD
check any existing signature on input and MUST make such
modifications before re-signing the message.
The signer MAY elect to limit the number of bytes of the Content that
will be included in the hash and hence signed. The length actually
hashed SHOULD be inserted in the "l=" tag of the DOSETA-Signature
header field.
4.4.5. Insert the DOSETA-Signature Header Field
Finally, the signer MUST insert the DOSETA-Signature header field
created in the previous step prior to transmitting the data. The
DOSETA-Signature header field MUST be the same as used to compute the
hash as described above, except that the value of the "b" parameter
MUST be the appropriately signed hash computed in the previous step,
signed using the algorithm specified in the "a" parameter of the
DOSETA-Signature header field and using the private key corresponding
to the selector given in the "s=" tag of the DOSETA-Signature header
field, as chosen above in Section 4.4.2
The DOSETA-Signature header field MUST be inserted before any other
DOSETA-Signature fields in the header block.
NOTE: The easiest way to achieve this is to insert the
DOSETA-Signature header field at the beginning of the header
block. In particular, it might be placed before any existing
Received Header fields. This is consistent with treating
DOSETA-Signature as a trace header field.
4.5. Verifier Actions
Since a signer MAY remove or revoke a public key at any time, it is
recommended that verification occur in a timely manner. In many
configurations, the most timely place is during acceptance by the
border MTA or shortly thereafter. In particular, deferring
verification until the message is accessed by the end user is
discouraged.
A border or intermediate server MAY verify the data signature(s). An
server that has performed verification MAY communicate the result of
that verification by adding a verification header field to incoming
data.
A verifying server MAY implement a policy with respect to
unverifiable data, regardless of whether or not it applies the
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verification header field to signed messages.
Verifiers MUST produce a result that is semantically equivalent to
applying the following steps in the order listed. In practice,
several of these steps can be performed in parallel in order to
improve performance.
4.5.1. Extract Signatures from the Message
The order in which verifiers try DOSETA-Signature Header fields is
not defined; verifiers MAY try signatures in any order they like.
For example, one implementation might try the signatures in textual
order, whereas another might try signatures by identities that match
the contents of the From header field before trying other signatures.
Verifiers MUST NOT attribute ultimate meaning to the order of
multiple DOSETA-Signature Header fields. In particular, there is
reason to believe that some relays will reorder the Header fields in
potentially arbitrary ways.
NOTE: Verifiers might use the order as a clue to signing order in
the absence of any other information. However, other clues as to
the semantics of multiple signatures (such as correlating the
signing host with Received Header fields) might also be
considered.
A verifier SHOULD NOT treat a message that has one or more bad
signatures and no good signatures differently from a message with no
signature at all; such treatment is a matter of local policy and is
beyond the scope of this document.
When a signature successfully verifies, a verifier will either stop
processing or attempt to verify any other signatures, at the
discretion of the implementation. A verifier MAY limit the number of
signatures it tries to avoid denial-of-service attacks.
NOTE: An attacker could send messages with large numbers of faulty
signatures, each of which would require a DNS lookup and
corresponding CPU time to verify the message. This could be an
attack on the domain that receives the message, by slowing down
the verifier by requiring it to do a large number of DNS lookups
and/or signature verifications. It could also be an attack
against the domains listed in the signatures, essentially by
enlisting innocent verifiers in launching an attack against the
DNS servers of the actual victim.
In the following description, text reading "return status
(explanation)" (where "status" is one of "PERMFAIL" or "TEMPFAIL")
means that the verifier MUST immediately cease processing that
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signature. The verifier SHOULD proceed to the next signature, if any
is present, and completely ignore the bad signature. If the status
is "PERMFAIL", the signature failed and SHOULD NOT be reconsidered.
If the status is "TEMPFAIL", the signature could not be verified at
this time but might be tried again later. A verifier MAY either
defer the message for later processing, perhaps by queueing it
locally or issuing a 451/4.7.5 SMTP reply, or try another signature;
if no good signature is found and any of the signatures resulted in a
TEMPFAIL status, the verifier MAY save the message for later
processing. The "(explanation)" is not normative text; it is
provided solely for clarification.
Verifiers SHOULD ignore any DOSETA-Signature Header fields where the
signature does not validate. Verifiers that are prepared to validate
multiple signature Header fields SHOULD proceed to the next signature
header field, if it exists. However, verifiers MAY make note of the
fact that an invalid signature was present for consideration at a
later step.
NOTE: The rationale of this requirement is to permit messages that
have invalid signatures but also a valid signature to work. For
example, a mailing list exploder might opt to leave the original
submitter signature in place even though the exploder knows that
it is modifying the message in some way that will break that
signature, and the exploder inserts its own signature. In this
case, the message ought to succeed even in the presence of the
known-broken signature.
For each signature to be validated, the following steps need to be
performed in such a manner as to produce a result that is
semantically equivalent to performing them in the indicated order.
4.5.2. Validate the Signature Header Field
Implementers MUST meticulously validate the format and values in the
DOSETA-Signature header field; any inconsistency or unexpected values
MUST cause the header field to be completely ignored and the verifier
to return PERMFAIL (signature syntax error). Being "liberal in what
you accept" is definitely a bad strategy in this security context.
Note however that this does not include the existence of unknown tags
in a DOSETA-Signature header field, which are explicitly permitted.
Verifiers MUST ignore DOSETA-Signature Header fields with a "v"
parameter that is inconsistent with this specification and return
PERMFAIL (incompatible version).
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NOTE: An implementation might, of course, choose to also verify
signatures generated by older versions of this specification.
If any tag listed as "required" in Section 4.2 is omitted from the
DOSETA-Signature header field, the verifier MUST ignore the
DOSETA-Signature header field and return PERMFAIL (signature missing
required tag).
NOTE: The tags listed as required in Section 4.2 are "v=", "a=",
"b=", "bh=", "d=", "h=", and "s=". Should there be a conflict
between this note and Section 4.2, is normative.
If the DOSETA-Signature header field does not contain the "i"
parameter, the verifier MUST behave as though the value of that tag
were "@d", where "d" is the value from the "d=" tag.
Verifiers MUST confirm that the domain specified in the "d=" tag is
the same as or a parent domain of the domain part of the "i"
parameter. If not, the DOSETA-Signature header field MUST be ignored
and the verifier SHOULD return PERMFAIL (domain mismatch).
If the "h" parameter does not include the From header field, the
verifier MUST ignore the DOSETA-Signature header field and return
PERMFAIL (From field not signed).
Verifiers MAY ignore the DOSETA-Signature header field and return
PERMFAIL (signature expired) if it contains an "x" parameter and the
signature has expired.
Verifiers MAY ignore the DOSETA-Signature header field if the domain
used by the signer in the "d" parameter is not associated with a
valid signing entity. For example, signatures with "d=" values such
as "com" and "co.uk" might be ignored. The list of unacceptable
domains SHOULD be configurable.
Verifiers MAY ignore the DOSETA-Signature header field and return
PERMFAIL (unacceptable signature header) for any other reason, for
example, if the signature does not sign Header fields that the
verifier views to be essential. As a case in point, if MIME Header
fields are not signed, certain attacks might be possible that the
verifier would prefer to avoid.
4.5.3. Get the Public Key
The public key for a signature is needed to complete the verification
process. The process of retrieving the public key depends on the
query type as defined by the "q" parameter in the DOSETA-Signature
header field. Obviously, a public key need only be retrieved if the
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process of extracting the signature information is completely
successful. Details of key management and representation are
described in Section 3.4. The verifier MUST validate the key record
and MUST ignore any public key records that are malformed.
NOTE: The use of wildcard TXT records in the DNS will produce a
response to a DOSETA query that is unlikely to be valid DOSETA key
record. This problem applies to many other types of queries, and
client software that processes DNS responses needs to take this
problem into account.
When validating a message, a verifier MUST perform the following
steps in a manner that is semantically the same as performing them in
the following order -- in some cases the implementation might
parallelize or reorder these steps, as long as the semantics remain
unchanged:
1. Retrieve the public key as described in Section 3.4 using the
algorithm in the "q=" tag, the domain from the "d" parameter, and
the selector from the "s" parameter.
2. If the query for the public key fails to respond, the verifier
MAY defer acceptance of this data and return TEMPFAIL - key
unavailable. (If verification is occurring during the incoming
SMTP session, this MAY be achieved with a 451/4.7.5 SMTP reply
code.) Alternatively, the verifier MAY store the message in the
local queue for later trial or ignore the signature. Note that
storing a message in the local queue is subject to denial-of-
service attacks.
3. If the query for the public key fails because the corresponding
key record does not exist, the verifier MUST immediately return
PERMFAIL (no key for signature).
4. If the query for the public key returns multiple key records, the
verifier might choose one of the key records or might cycle
through the key records performing the remainder of these steps
on each record at the discretion of the implementer. The order
of the key records is unspecified. If the verifier chooses to
cycle through the key records, then the "return ..." wording in
the remainder of this section means "try the next key record, if
any; if none, return to try another signature in the usual way".
5. If the result returned from the query does not adhere to the
format defined in this specification, the verifier MUST ignore
the key record and return PERMFAIL (key syntax error). Verifiers
are urged to validate the syntax of key records carefully to
avoid attempted attacks. In particular, the verifier MUST ignore
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keys with a version code ("v" parameter) that they do not
implement.
6. If the "h" parameter exists in the public key record and the hash
algorithm implied by the a= tag in the DOSETA-Signature header
field is not included in the contents of the "h" parameter, the
verifier MUST ignore the key record and return PERMFAIL
(inappropriate hash algorithm).
7. If the public key data (the "p" parameter) is empty, then this
key has been revoked and the verifier MUST treat this as a failed
signature check and return PERMFAIL (key revoked). There is no
defined semantic difference between a key that has been revoked
and a key record that has been removed.
8. If the public key data is not suitable for use with the algorithm
and key types defined by the "a=" and "k" parameters in the
DOSETA-Signature header field, the verifier MUST immediately
return PERMFAIL (inappropriate key algorithm).
4.5.4. Compute the Verification
Given a signer and a public key, verifying a signature consists of
actions semantically equivalent to the following steps.
1. Based on the algorithm defined in the "c" parameter, the Content
length specified in the "l" parameter, and the header field names
in the "h" parameter, prepare a canonicalized version of the
Content as is described in Section 4.3 (note that this version
does not actually need to be instantiated). When matching header
field names in the "h" parameter against the actual message
header field, comparisons MUST be case-insensitive.
2. Based on the algorithm indicated in the "a" parameter, compute
the message hashes from the canonical copy as described in
Section 4.3
3. Verify that the hash of the canonicalized Content computed in the
previous step matches the hash value conveyed in the "bh"
parameter. If the hash does not match, the verifier SHOULD
ignore the signature and return PERMFAIL (Content hash did not
verify).
4. Using the signature conveyed in the "b" parameter, verify the
signature against the header hash using the mechanism appropriate
for the public key algorithm described in the "a" parameter. If
the signature does not validate, the verifier SHOULD ignore the
signature and return PERMFAIL (signature did not verify).
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5. Otherwise, the signature has correctly verified.
NOTE: Implementations might wish to initiate the public-key query
in parallel with calculating the hash as the public key is not
needed until the final decryption is calculated. Implementations
might also verify the signature on the message header before
validating that the message hash listed in the "bh" parameter in
the DOSETA-Signature header field matches that of the actual
Content; however, if the Content hash does not match, the entire
signature MUST be considered to have failed.
4.5.5. Communicate Verification Results
Verifiers wishing to communicate the results of verification to other
parts of the data handling system can do so in whatever manner they
see fit. For example, implementations might choose to add a Header
field to the data before passing it on. Any such header field SHOULD
be inserted before any existing DOSETA-Signature or preexisting
verification status Header fields in the header field block. The
Authentication-Results: header field ([RFC5451]) MAY be used for this
purpose.
Patterns intended to search for results Header fields to visibly
mark authenticated data for end users SHOULD verify that the
header field was added by the appropriate verifying domain In
particular, filters SHOULD NOT be influenced by bogus results
header fields added by attackers. To circumvent this attack,
verifiers SHOULD delete existing results Header fields after
verification and before adding a new header field.
4.5.6. Interpret Results/Apply Local Policy
It is beyond the scope of this specification to describe what actions
an Assessment phase will take, but data with a verified DOSETA
signature presents an opportunity to an Assessor that unsigned data
does not. Specifically, signed data creates a predictable identifier
by which other decisions can reliably be managed, such as trust and
reputation. Conversely, unsigned data typically lacks a reliable
identifier that can be used to assign trust and reputation. It is
usually reasonable to treat unsigned data as lacking any trust and
having no positive reputation.
In general, verifiers SHOULD NOT reject data solely on the basis of a
lack of signature or an unverifiable signature; such rejection would
cause severe interoperability problems. However, if the verifier
does opt to reject such data
Temporary failures such as inability to access the key server or
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other external service are the only conditions that SHOULD use a
temporary failure code. In particular, cryptographic signature
verification failures MUST NOT return temporary failure replies.
Once the signature has been verified, that information MUST be
conveyed to the Assessor (such as an explicit allow/whitelist and
reputation system) and/or to the end user. If the DDI is not the
same as the address in the From: header field, the data system SHOULD
take pains to ensure that the actual DDI is clear to the reader.
The verifier MAY treat unsigned Header fields with extreme
skepticism, including marking them as untrusted or even deleting
them.
While the symptoms of a failed verification are obvious -- the
signature doesn't verify -- establishing the exact cause can be more
difficult. If a selector cannot be found, is that because the
selector has been removed, or was the value changed somehow in
transit? If the signature line is missing, is that because it was
never there, or was it removed by an overzealous filter? For
diagnostic purposes, the exact nature of a verification failure
SHOULD be made available to the policy module and possibly recorded
in the system logs. If the data cannot be verified, then it SHOULD
be rendered the same as all unverified data regardless of whether or
not it looks like it was signed.
4.6. Requirements for Tailoring the Signing Service
This generic template requires additional details, to define a
specific service:
D-Signature Association: Specify the means by which a
DOSETA-Signature header field is associated with the data it
signs. For example, DKIM uses the DKIM-Signature email header
field. If the header field is encoded differently than defined
for the DOSETA generic service, such as in XML, then that also
needs to be specified including the algorithm for mapping
between the common encoding provided here and the new encoding.
Semantics Signaling: The means by which a consumer is to know
what semantics apply must be indicated. This is likely to be
the same means by which the consumer knows what specific
service is being used. For example, different service-specific
DOSETA-Signature header fields might be used. "DKIM-Signature"
signals a name-affixing service, while "DKVM-Signature" might
signal a message validation service.
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Semantics: Define the meaning of a signature. Note that exactly
the same algorithms might be used for very different semantics.
One might merely affix an identifier to some data, in a
verifiable fashion, while the same set of mechanisms might
separately be defined as authenticating the validity of that
data.
Header/Content Mapping: The mappings between the template's
generic service and data of a particular service needs to be
defined. For example, with DKIM Header maps to the email
header and Content maps to the email body.
5. Semantics of Multiple Signatures
5.1. Example Scenarios
There are many reasons why a message might have multiple signatures.
For example, a given signer might sign multiple times, perhaps with
different hashing or signing algorithms during a transition phase.
EXAMPLE: Suppose SHA-256 is in the future found to be
insufficiently strong, and DOSETA usage transitions to SHA-1024.
A signer might immediately sign using the newer algorithm, but
continue to sign using the older algorithm for interoperability
with verifiers that had not yet upgraded. The signer would do
this by adding two DOSETA-Signature Header fields, one using each
algorithm. Older verifiers that did not recognize SHA-1024 as an
acceptable algorithm would skip that signature and use the older
algorithm; newer verifiers could use either signature at their
option, and all other things being equal might not even attempt to
verify the other signature.
Similarly, a signer might sign a message including all headers and no
"l" parameter (to satisfy strict verifiers) and a second time with a
limited set of headers and an "l" parameter (in anticipation of
possible message modifications in route to other verifiers).
Verifiers could then choose which signature they preferred.
EXAMPLE: A verifier might receive data with two signatures, one
covering more of the data than the other. If the signature
covering more of the data verified, then the verifier could make
one set of policy decisions; if that signature failed but the
signature covering less of the data verified, the verifier might
make a different set of policy decisions.
Of course, a message might also have multiple signatures because it
passed through multiple signers. A common case is expected to be
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that of a signed message that passes through a mailing list that also
signs all messages. Assuming both of those signatures verify, a
recipient might choose to accept the message if either of those
signatures were known to come from trusted sources.
EXAMPLE: Recipients might choose to whitelist mailing lists to
which they have subscribed and that have acceptable anti- abuse
policies so as to accept messages sent to that list even from
unknown authors. They might also subscribe to less trusted
mailing lists (for example, those without anti-abuse protection)
and be willing to accept all messages from specific authors, but
insist on doing additional abuse scanning for other messages.
Another related example of multiple signers might be forwarding
services, such as those commonly associated with academic alumni
sites.
EXAMPLE: A recipient might have an address at members.example.org,
a site that has anti-abuse protection that is somewhat less
effective than the recipient would prefer. Such a recipient might
have specific authors whose messages would be trusted absolutely,
but messages from unknown authors that had passed the forwarder's
scrutiny would have only medium trust.
5.2. Interpretation
A signer that is adding a signature to a message merely creates a new
DOSETA-Signature header, using the usual semantics of the h= option.
A signer MAY sign previously existing DOSETA-Signature Header fields
using the method described in Section 4.4.3 to sign trace Header
fields.
NOTE: Signers need to be cognizant that signing DOSETA-Signature
Header fields might result in verification failures due to
modifications by intermediaries, such as their reordering
DOSETA-Signature header fields. For this reason, signing existing
DOSETA-Signature Header fields is unadvised, albeit legal.
NOTE: If a header field with multiple instances is signed, those
Header fields are always signed from the "bottom" up (from last to
first). Thus, it is not possible to sign only specific
DOSETA-Signature Header fields. For example, if the message being
signed already contains three DOSETA-Signature header fields A, B,
and C, it is possible to sign all of them, B and C only, or C
only, but not A only, B only, A and B only, or A and C only.
A signer MAY add more than one DOSETA-Signature header field using
different parameters. For example, during a transition period a
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signer might want to produce signatures using two different hash
algorithms.
Signers SHOULD NOT remove any DOSETA-Signature Header fields from
messages they are signing, even if they know that the signatures
cannot be verified.
When evaluating a message with multiple signatures, a verifier SHOULD
evaluate signatures independently and on their own merits. For
example, a verifier that by policy chooses not to accept signatures
with deprecated cryptographic algorithms would consider such
signatures invalid. Verifiers MAY process signatures in any order of
their choice; for example, some verifiers might choose to process
signatures corresponding to the From field in the message header
before other signatures. See Section 4.5.1 for more information
about signature choices.
NOTE: Verifier attempts to correlate valid signatures with invalid
signatures in an attempt to guess why a signature failed are ill-
advised. In particular, there is no general way that a verifier
can determine that an invalid signature was ever valid.
Verifiers SHOULD ignore failed signatures as though they were not
present in the message. Verifiers SHOULD continue to check
signatures until a signature successfully verifies to the
satisfaction of the verifier. To limit potential denial-of-service
attacks, verifiers MAY limit the total number of signatures they will
attempt to verify.
6. DOSETA Claims Registry Definition
A registry entry MUST contain:
Label: Specifies a textual name for claim, to be used in the
"cl=" tag.
Description: Explains the semantics of the claim being asserted.
The registry entries are contained in the IANA DOSETA Claims
Registry, defined in Section 7.1.2
7. Considerations
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7.1. IANA Considerations
7.1.1. DKIM Registries
DOSETA relies on IANA registration data bases specified by DKIM
[DKIMSign]. Services that incorporate DOSETA might need to define
new registries or add to existing ones.
7.1.2. Claims Registry
Per [RFC2434], IANA is requested to establish a DOSETA Claims
Registry, for assertions (claims) that are meant by the presence of
the DOSETA-based signature that contains the claims. See Section 6
for the definition of the columns in the registry table.
+-------------+-----------------------------------------------------+
| LABEL | CLAIM DESCRIPTION |
+-------------+-----------------------------------------------------+
| handled | The signer claims they have had a role in |
| | processing the object. (This claim is |
| | approximately equivalent to the semantics of DKIM.) |
| validauth | If there is a standardized field listing the |
| | purported author of the data, the signer claims |
| | that the value in that field is valid. |
| validdata | The signer claims that all of the data in the |
| | object valid. |
| validfields | The signer claims that the portions of the object |
| | that are covered by the signature hash are valid. |
+-------------+-----------------------------------------------------+
Table 1: DOSETA Claim Registry (with initial values)
7.2. Security Considerations
Any mechanism that attempts to prevent or detect abuse is subject to
intensive attack. DOSETA needs to be carefully scrutinized to
identify potential attack vectors and the vulnerability to each. See
also [RFC4686].
DOSETA core technology derives from DKIM [DKIMSign]. The Security
Considerations of that specification applies equally to DOSETA.
The DOSETA "cl=" claims list provides a list of claimed meanings for
a DOSETA signature. An opportunity for security problems comes from
failing to distinguish between a signer "claim" and claim validity.
Whether to trust claims made by a signer requires a level of
assessment beyond DOSETA.
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8. References
8.1. Normative References
[FIPS-180-2-2002]
U.S. Department of Commerce, "Secure Hash Standard", FIPS
PUB 180-2, August 2002.
[ITU-X660-1997]
"Information Technology - ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER)", 1997.
[RFC1034] Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES",
RFC 1034, November 1987.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and
Examples", RFC 2049, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 2434,
October 1998.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, January 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[RFC5322] Resnick, P., "Internet Message Format", RFC 5322,
October 2008.
[RFC5890] Klensin, J., "Internationalizing Domain Names in
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010.
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8.2. Informative References
[DKIMSign]
Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4871, May 2007.
[RFC1847] Galvin, J., Murphy, S., Crocker, S., and N. Freed,
"Security Multiparts for MIME: Multipart/Signed and
Multipart/Encrypted", RFC 1847, October 1995.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII
Content", RFC 2047, November 1996.
[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For
Public Keys Used For Exchanging Symmetric Keys", BCP 86,
RFC 3766, April 2004.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC4409] Gellens, R. and J. Klensin, "Message Submission for Mail",
RFC 4409, April 2006.
[RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys
Identified Mail (DKIM)", RFC 4686, September 2006.
[RFC4870] Delany, M., "Domain-Based Email Authentication Using
Public Keys Advertised in the DNS (DomainKeys)", RFC 4870,
May 2007.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
"OpenPGP Message Format", RFC 4880, November 2007.
[RFC5451] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 5451, April 2009.
[RFC5672] Crocker, D., Ed., "RFC 4871 DomainKeys Identified Mail
(DKIM) Signatures: Update", RFC 5672, August 2009.
[UTF8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 3629, November 2003.
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Appendix A. Creating a Public Key
The default signature is an RSA signed SHA256 digest of the complete
email. For ease of explanation, the openssl command is used to
describe the mechanism by which keys and signatures are managed. One
way to generate a 1024-bit, unencrypted private key suitable for
DOSETA is to use openssl like this:
$ openssl genrsa -out rsa.private 1024
For increased security, the "-passin" parameter can also be added to
encrypt the private key. Use of this parameter will require entering
a password for several of the following steps. Servers might prefer
to use hardware cryptographic support.
The "genrsa" step results in the file rsa.private containing the key
information similar to this:
-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----
To extract the public-key component from the private key, use openssl
like this:
$ openssl rsa -in rsa.private -out rsa.public -pubout -outform PEM
This results in the file rsa.public containing the key information
similar to this:
-----BEGIN PUBLIC KEY-----
MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkM
oGeLnQg1fWn7/zYtIxN2SnFCjxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v/R
tdC2UzJ1lWT947qR+Rcac2gbto/NMqJ0fzfVjH4OuKhitdY9tf6mcwGjaNBcWToI
MmPSPDdQPNUYckcQ2QIDAQAB
-----END PUBLIC KEY-----
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This public-key data (without the BEGIN and END tags) is placed in
the DNS:
brisbane IN TXT
("v=DKIM1; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQ"
"KBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkMoGeLnQg1fWn7/zYt"
"IxN2SnFCjxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v"
"/RtdC2UzJ1lWT947qR+Rcac2gbto/NMqJ0fzfVjH4OuKhi"
"tdY9tf6mcwGjaNBcWToIMmPSPDdQPNUYckcQ2QIDAQAB")
Appendix B. Acknowledgements
DOSETA is derived from DKIM [DKIMSign]. DKIM is an evolution of
DomainKeys [RFC4870], which was developed by Mark Delany, then of
Yahoo!. In particular, the key management service, based on the DNS,
and the user-INvisible tagging scheme was developed by him.
Appendix C. Example -- DKIM Using DOSETA
This example re-specifies DKIM in terms of DOSETA, while retaining
bit-level compatibility with the existing DKIM specification
[DKIMSign].
NOTE: This section is merely an example. Any use of normative
language in this section is strictly for completness of the
example and has no normative effect on the DOSETA specification.
C.1. Signing and Verification Protocol
The DOSETA template specifies TEMPLATE information that is required
to tailor the signing service:
Signature Association: The DOSETA-Signature data are stored in a
DKIM-Signature header field that is part of the Header of the
message being signed. This contains all of the signature and
key-fetching data, per Section 4.2.
Semantics Signaling: The presence of a DKIM-Signature header
fields signals the use of DKIM.
Semantics: A DKIM signature means that the owner of the signing
domain is taking "some" responsibility for the message. Hence,
the payload, or output, of DKIM is:
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+ A validated domain name, specifically the d= parameter in
the DKIM-Signature header field
+ An indication that its use has been validated
The nature and extent of a DKIM signer's responsibility can
vary widely and is beyond the scope of this specification.
Header/Content Mapping: DKIM maps the DOSETA Header processing
to an email header and the DOSETA Content to an email body, per
[RFC5322]
C.2. Extensions to DOSETA Template
This section contains specifications that are added to the basic
DOSETA H/C Signing Template.
C.2.1. Signature Data Structure
These are DKIM-specific tags:
i= The Agent or User Identifier (AUID) on behalf of which the
SDID is taking responsibility (DOSETA-quoted-printable;
OPTIONAL, default is an empty <local-part> followed by an "@"
followed by the domain from the "d=" tag).
The syntax is a standard email address where the <local-part>
MAY be omitted. The domain part of the address MUST be the
same as, or a subdomain of, the value of the "d=" tag.
Internationalized domain names MUST be converted using the
steps listed in Section 4 of [RFC5890] using the "ToASCII"
function.
ABNF:
sig-i-tag = %x69 [FWS] "=" [FWS]
[ local-part ] "@" domain-name
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The AUID is specified as having the same syntax as an email
address, but is not required to have the same semantics.
Notably, the domain name is not required to be registered in
the DNS -- so it might not resolve in a query -- and the
<local-part> MAY be drawn from a namespace unrelated to any
mailbox. The details of the structure and semantics for the
namespace are determined by the Signer. Any knowledge or use
of those details by verifiers or assessors is outside the scope
of the DOSETA Signing specification. The Signer MAY choose to
use the same namespace for its AUIDs as its users' email
addresses or MAY choose other means of representing its users.
However, the signer SHOULD use the same AUID for each message
intended to be evaluated as being within the same sphere of
responsibility, if it wishes to offer receivers the option of
using the AUID as a stable identifier that is finer grained
than the SDID.
NOTE: The <local-part> of the "i=" tag is optional because in
some cases a signer might not be able to establish a
verified individual identity. In such cases, the signer
might wish to assert that although it is willing to go as
far as signing for the domain, it is unable or unwilling to
commit to an individual user name within their domain. It
can do so by including the domain part but not the <local-
part> of the identity.
NOTE: Absent public standards for the semantics of an AUID,
an assessment based on AUID requires a non-standardized
basis.
NOTE: This specification does not require the value of the
"i=" tag to match the identity in any Header field. This is
considered to be an assessment-time policy issue.
Constraints between the value of the "i=" tag and other
identities in other Header fields might seek to apply basic
authentication into the semantics of trust associated with a
role such as content author. Trust is a broad and complex
topic and trust mechanisms are subject to highly creative
attacks. The real-world efficacy of any but the most basic
bindings between the "i=" value and other identities is not
well established, nor is its vulnerability to subversion by
an attacker. Hence reliance on the use of these options
needs to be strictly limited. In particular, it is not at
all clear to what extent a typical end-user recipient can
rely on any assurances that might be made by successful use
of the "i=" options.
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l= Content length count (plain-text unsigned decimal integer;
OPTIONAL, default is entire Content). This tag informs the
verifier of the number of octets in the Content of the data
after canonicalization included in the cryptographic hash,
starting from 0 immediately following the CRLF preceding the
Content. This value MUST NOT be larger than the actual number
of octets in the canonicalized Content.
ABNF:
sig-l-tag = %x6c [FWS] "=" [FWS]
1*76DIGIT
NOTE: Use of the "l=" tag might allow display of fraudulent
content without appropriate warning to end users. The "l="
tag is intended for increasing signature robustness when
sending to intermediaries that append data to Content, such
as mailing lists that both modify their content and do not
sign their messages. However, using the "l=" tag enables
attacks in which an intermediary with malicious intent
modifies a message to include content that solely benefits
the attacker. It is possible for the appended content to
completely replace the original content in the end
recipient's eyes and to defeat duplicate message detection
algorithms. Examples are described in Security
Considerations Section 7.2. To avoid this attack, signers
need be extremely wary of using this tag, and verifiers
might wish to ignore the tag or remove text that appears
after the specified content length.
NOTE: The value of the "l=" tag is constrained to 76 decimal
digits. This constraint is not intended to predict the size
of future data or to require implementations to use an
integer representation large enough to represent the maximum
possible value, but is intended to remind the implementer to
check the length of this and all other tags during
verification and to test for integer overflow when decoding
the value. Implementers might need to limit the actual
value expressed to a value smaller than 10^76, for example,
to allow a message to fit within the available storage
space.
z= Copied Header fields (DOSETA-quoted-printable, but see
description; OPTIONAL, default is null). A vertical-bar-
separated list of selected Header fields present when the
message was signed, including both the field name and value.
It is not required to include all Header fields present at the
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time of signing. This field need not contain the same Header
fields listed in the "h=" tag. The Header field text itself
MUST encode the vertical bar ("|", %x7C) character. That is,
vertical bars in the "z=" text are meta-characters, and any
actual vertical bar characters in a copied header field MUST be
encoded. Note that all whitespace MUST be encoded, including
whitespace between the colon and the header field value. After
encoding, FWS MAY be added at arbitrary locations in order to
avoid excessively long lines; such whitespace is NOT part of
the value of the header field, and MUST be removed before
decoding.
The Header fields referenced by the "h=" tag refer to the
fields in the [RFC5322] Header, not to any copied fields in the
"z=" tag. Copied header field values are for diagnostic use.
Header fields with characters requiring conversion (perhaps
from legacy MTAs that are not [RFC5322] compliant) SHOULD be
converted as described in MIME Part Three [RFC2047].
ABNF:
sig-z-tag = %x7A [FWS] "=" [FWS]
sig-z-tag-copy
*( "|" [FWS] sig-z-tag-copy )
sig-z-tag-copy = hdr-name [FWS] ":"
qp-hdr-value
EXAMPLE of a signature header field spread across multiple
continuation lines:
DKIM-Signature: v=1; a=rsa-sha256; d=example.net;
s=brisbane; c=simple; q=dns/txt; i=@eng.example.net;
t=1117574938; x=1118006938;
h=from:to:subject:date;
z=From:foo@eng.example.net|To:joe@example.com|
Subject:demo=20run|
Date:July=205,=202005=203:44:08=20PM=20-0700;
bh=MTIzNDU2Nzg5MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTI=;
b=dzdVyOfAKCdLXdJOc9G2q8LoXSlEniSbav+yuU4zGeeruD00lszZVoG4ZHRNiYzR
C.2.1.1. Content Length Limits
A text length count MAY be specified to limit the signature
calculation to an initial prefix of an ASCII text data portion,
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measured in octets. If the Content length count is not specified,
the entire Content is signed.
This capability is provided because it is very common for
intermediate data handling services to add trailers to text (for
example, instructions how to get off a mailing list). Until such
data is signed by the intermediate handler, the text length count can
be a useful tool for the verifier since it can, as a matter of
policy, accept messages having valid signatures that do not cover the
additional data.
NOTE: Using text length limits enables an attack in which an
attacker modifies a message to include content that solely
benefits the attacker. It is possible for the appended content to
completely replace the original content in the end recipient's
eyes and to defeat duplicate message detection algorithms. To
avoid this attack, signers need to be wary of using this tag, and
verifiers might wish to ignore the tag or remove text that appears
after the specified content length, perhaps based on other
criteria.
The text length count allows the signer of text to permit data to be
appended to the end of the text of a signed message. The text length
count MUST be calculated following the canonicalization algorithm;
for example, any whitespace ignored by a canonicalization algorithm
is not included as part of the Content length count. Signers of MIME
messages that include a Content length count SHOULD be sure that the
length extends to the closing MIME boundary string.
NOTE: A creator wishing to ensure that the only acceptable
modifications are to add to a MIME postlude would use a text
length count encompassing the entire final MIME boundary string,
including the final "--CRLF". A signer wishing to allow
additional MIME parts but not modification of existing parts would
use a Content length count extending through the final MIME
boundary string, omitting the final "--CRLF". Note that this only
works for some MIME types, such as, multipart/mixed but not
multipart/signed.
A text length count of zero means that the text is completely
unsigned.
Creators wishing to ensure that no modification of any sort can occur
will specify the "simple" canonicalization algorithm for all data
portions and will and omit the text length counts.
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C.2.1.2. Signature Verification
A Content length specified in the "l=" tag of the signature limits
the number of bytes of the Content passed to the verification
algorithm. All data beyond that limit is not validated by DOSETA.
Hence, verifiers might treat a message that contains bytes beyond the
indicated Content length with suspicion, such as by truncating the
message at the indicated Content length, declaring the signature
invalid (for example, by returning PERMFAIL (unsigned content)), or
conveying the partial verification to the policy module.
NOTE: Verifiers that truncate the Content at the indicated Content
length might pass on a malformed MIME message if the signer used
the "N-4" trick (omitting the final "--CRLF") described in the
informative note in Appendix C.2.1.1. Such verifiers might wish
to check for this case and include a trailing "--CRLF" to avoid
breaking the MIME structure. A simple way to achieve this might
be to append "--CRLF" to any "multipart" message with a Content
length; if the MIME structure is already correctly formed, this
will appear in the postlude and will not be displayed to the end
user.
C.2.2. Stored Key Data
This section defines additions to the DOSETA Library, concerning
stored key data.
g= Granularity of the key (plain-text; OPTIONAL, default is
"*"). This value MUST match the Local-part of the "i=" tag of
the DKIM- Signature header field (or its default value of the
empty string if "i=" is not specified), with a single, optional
"*" character matching a sequence of zero or more arbitrary
characters ("wildcarding"). An email with a signing address
that does not match the value of this tag constitutes a failed
verification. The intent of this tag is to constrain which
signing address can legitimately use this selector, for
example, when delegating a key to a third party that should
only be used for special purposes. Wildcarding allows matching
for addresses such as "user+*" or "*-offer". An empty "g="
value never matches any addresses.
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ABNF:
key-g-tag = %x67 [FWS] "=" [FWS] key-g-tag-lpart
key-g-tag-lpart = [dot-atom-text]
["*" [dot-atom-text] ]
h= Acceptable hash algorithms (plain-text; OPTIONAL, defaults
to allowing all algorithms). A colon-separated list of hash
algorithms that might be used. Signers and Verifiers MUST
support the "sha256" hash algorithm. Verifiers MUST also
support the "sha1" hash algorithm. Unrecognized hash
algorithms MUST be ignored.
ABNF:
key-h-tag = %x68 [FWS] "=" [FWS]
key-h-tag-alg
0*( [FWS] ":" [FWS]
key-h-tag-alg )
key-h-tag-alg = "sha1" / "sha256" /
x-key-h-tag-alg
x-key-h-tag-alg = hyphenated-word
; for future extension
s= Service Type (plain-text; OPTIONAL; default is "*"). A
colon-separated list of service types to which this record
applies. Verifiers for a given service type MUST ignore this
record if the appropriate type is not listed. Unrecognized
service types MUST be ignored. Currently defined service types
are as follows:
* matches all service types
email electronic mail (not necessarily limited to SMTP)
This tag is intended to constrain the use of keys for other
purposes, if use of DOSETA is defined by other services in the
future.
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ABNF:
key-s-tag = %x73 [FWS] "=" [FWS]
key-s-tag-type
0*( [FWS] ":" [FWS]
key-s-tag-type )
key-s-tag-type = "email" / "*" /
x-key-s-tag-type
x-key-s-tag-type = hyphenated-word
; for future extension
t= Flags, represented as a colon-separated list of names
(plain-text; OPTIONAL, default is no flags set). Unrecognized
flags MUST be ignored. The defined flags are as follows:
y This domain is testing DOSETA. Verifiers MUST NOT treat
data from signers in testing mode differently from unsigned
data, even if the signature fails to verify. Verifiers MAY
wish to track testing mode results to assist the signer.
s Any DOSETA-Signature Header fields using the "i=" tag MUST
have the same domain value on the right-hand side of the "@"
in the "i=" tag and the value of the "d=" tag. That is, the
"i=" domain MUST NOT be a subdomain of "d=". Use of this
flag is RECOMMENDED unless subdomaining is required.
ABNF:
key-t-tag = %x74 [FWS] "=" [FWS]
key-t-tag-flag
0*( [FWS] ":" [FWS]
key-t-tag-flag )
key-t-tag-flag = "y" / "s" /
x-key-t-tag-flag
x-key-t-tag-flag = hyphenated-word
; for future extension
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Authors' Addresses
D. Crocker
Brandenburg InternetWorking
675 Spruce Dr.
Sunnyvale
USA
Phone: +1.408.246.8253
Email: dcrocker@bbiw.net
URI: http://bbiw.net
M. Kucherawy
Cloudmark
128 King St., 2nd Floor
San Francisco, CA 94107
USA
Email: msk@cloudmark.com
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