DKIM D. Crocker, Ed.
Internet-Draft Brandenburg InternetWorking
Intended status: Standards Track M. Kucherawy, Ed.
Expires: July 18, 2011 Cloudmark
January 14, 2011
DomainKeys Security Tagging (DOSETA)
draft-crocker-dkim-doseta-00
Abstract
DomainKeys Security Tagging (DOSETA) is a component mechanism that
enables development of a security-related service, such as
authentication or encryption, with keys based on domain names; the
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
retrieval of self-certified keys. The DOSETA Signing Template
affixes a signature to data that is in a "header/content" form.
Defining the meaning of the signature is the responsibility of the
service that incorporates DOSETA. The signature is validated through
a cryptographic signature and querying the signer's domain directly,
to retrieve the appropriate public key.
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 July 18, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
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
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. DOSETA Features . . . . . . . . . . . . . . . . . . . . . 5
1.2. DOSETA Architecture . . . . . . . . . . . . . . . . . . . 6
2. Terminology and Definitions {rfc4871bis-02 2, subset} . . . . 6
2.1. Identity . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2. Actors . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. DOSETA Library . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1. Canonicalization {rfc4871bis-02 3.4} . . . . . . . . . . . 11
3.2. Tag=Value Parameters {rfc4871bis-02 3.2} . . . . . . . . . 15
3.3. Key Management {rfc4871bis-02 3.6} . . . . . . . . . . . . 16
3.4. Selectors for Keys {rfc4871bis-02 3.1} . . . . . . . . . . 17
3.5. DNS Binding for Key Retrieval {rfc4871bis-02 3.6.2} . . . 19
3.6. Stored Key Data {rfc4871bis-02 3.6.1, subset, with
preface} . . . . . . . . . . . . . . . . . . . . . . . . . 20
4. DOSETA H/C Signing Template . . . . . . . . . . . . . . . . . 22
4.1. Cryptographic Algorithms {rfc4871bis-02 3.3, subset} . . . 22
4.2. Signature Data Structure {rfc4871bis-02 3.5, subset} . . . 24
4.3. Signature Calculation {rfc4871bis-02 3.7, reorganized
and clarified} . . . . . . . . . . . . . . . . . . . . . . 30
4.4. Signer Actions {rfc4871bis-02 5} . . . . . . . . . . . . . 33
4.5. Verifier Actions {rfc4871bis-02 6} . . . . . . . . . . . . 37
4.6. Requirements for Tailoring the Signing Service {added} . . 44
4.7. Signing by Parent Domains {rfc4871bis-02 3.8} . . . . . . 44
5. Semantics of Multiple Signatures {rfc4871bis-02 4} . . . . . . 45
5.1. Example Scenarios {rfc4871bis-02 4.1} . . . . . . . . . . 45
5.2. Interpretation {rfc4871bis-02 4.2} . . . . . . . . . . . . 46
6. Considerations . . . . . . . . . . . . . . . . . . . . . . . . 47
6.1. IANA Considerations {rfc4871bis-02 7, subset} . . . . . . 47
6.2. Security Considerations {rfc4871bis-02 8, subset} . . . . 50
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 56
7.1. Normative References . . . . . . . . . . . . . . . . . . . 56
7.2. Informative References . . . . . . . . . . . . . . . . . . 57
Appendix A. Creating a Public Key {rfc4871bis-02 C} . . . . . . . 58
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 59
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 59
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1. Introduction
DomainKeys Security Tagging (DOSETA) is a component mechanism that
enables development of a security-related service, such as
authentication or encryption, with keys based on domain names
[RFC1034]; the 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 retrieval of self-certified keys. The DOSETA Signing Template
affixes a signature to data that is in a "header/content" form.
Defining the meaning of the signature is the responsibility of the
service that incorporates DOSETA. The signature is validated through
a cryptographic signature and querying the signer's domain directly,
to retrieve the appropriate public key.
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 signature-related content
appearing in the Content;
o there is no dependency on 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 no attempt is made to include encryption as part of the mechanism;
DOSETA:
o enables compatibility with the existing data handling
infrastructure and transparent to the fullest extent possible;
o requires minimal new infrastructure;
o can be implemented independently of data clients in order to
reduce deployment time;
o can be deployed incrementally;
o allows delegation of signing to third parties.
DOSETA is taken directly from the original DKIM Signing specification
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[RFC4871], [RFC5672]. It has merely extracted the core portions of
the specification, 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.
NOTE: Much of the text for this draft is taken from the DKIM
working group draft-ietf-DKIM-rfc4871bis-02 revision. Sections in
this document cross reference their source with the notation:
{rfc4871bis-02 xx}
where "xx" indicates the section number. It might also indicate
that a subset is taken, such as when a portion is applied to the
DKIM-over-DOSETA draft and a portion to the DOSETA draft. In some
cases the base text also has been enhanced.
1.1. DOSETA Features
DOSETA features include:
Identity: DOSETA separates the question of the identity of the
DOSETA Creator from any other associated identifier, such as
the data's purported author. In particular, a DOSETA header
includes the identity of the signer. DOSETA Consumers can use
the DOSETA identity information to decide how they want to
process the data. That identity can be included in the
attributes of the data or recorded elsewhere.
NOTE: DOSETA does not, iiself, require that the identity it
uses be 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 service that incorporates
DNS might choose to add constraints on the choice of
identifier, such as having it match another identifier
associated with the data.
Scalability: DOSETA is designed to support the extreme
scalability 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 in the domain of the claimed signer
directly rather than from a third party. Hence DOSETA provides
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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.
Data Integrity: A DOSETA signature associates its identifier
with the computed hash of some or all of the data in order to
prevent the re-use of the signature with different data.
Verifying the signature asserts that the hashed data has not
changed since it was signed, and asserts nothing else about
"protecting" the end-to-end integrity of the data.
Assessment: For identity and authentication related processes,
this phase integrates the validation output for determing
further action.
1.2. 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:
+--------+ +-----------------+
| DKIM | | Message Privacy |
+---+----+ +--------+--------+
| |
+-----V---------------------------+ |
| Header/Content Signing Template | |
+----------------+----------------+ |
| |
...................V.................................V.............
. D O S E T A L I B R A R Y .
.+------------------+ +------------+ +-------------+ +-----------+.
.| | | Key | | Common | | Signature |.
.| Canonicalization | | Management | | Parameters | | Header |.
.| | | (DNS) | | (tab=value) | | |.
.+------------------+ +------------+ +-------------+ +-----------+.
...................................................................
2. Terminology and Definitions {rfc4871bis-02 2, subset}
This section defines terms used in the rest of the document.
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Within the specification, the label "TEMPLATE" is used to indicate
actions that are required to incorporate DOSETA into a service.
Syntax descriptions use Augmented BNF (ABNF) [RFC5234].
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].
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.
Signing Domain Identifier (SDID): A single domain name that refers
to the identity owning the key to be used for DOSETA processing.
The name has only basic domain name semantics; any possible owner-
specific semantics are outside the scope of DOSETA. It is
specified in Section 4.2.
2.2. Actors
Creator: An element in the data handling system that produces a
cryptographic encoding, on behalf of a domain, is referred to as a
Creator.
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.
Signer: An element in the data handling system that signs messages
on behalf of a domain is referred to as a signer. This element
specifies the organization acting as a type of DOSETA Creator. It
might be a client, server or other agent such as a reputation
service. The key issue is that the data MUST be signed before it
leaves the administrative domain of the signer.
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 (Creator) of
the data or some consuming agent such as a data processing
intermediary.
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2.3. Syntax
2.3.1. Whitespace
There are three forms of whitespace:
WSP: represents simple whitespace, that is, a space or a tab
character (formal definition in [RFC5234]).
LWSP: is linear whitespace, defined as WSP plus CRLF (formal
definition in [RFC5234]).
FWS: is folding whitespace. It allows multiple lines separated
by CRLF followed by at least one whitespace, to be joined.
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
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2.3.3. Imported ABNF Tokens
The following tokens are imported from other RFCs as noted. Those
RFCs SHOULD be considered definitive.
From [RFC5321]:
<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
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(%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
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 is distinguished by a DNS-based, self-certifying public key
mechanism, common canonicalization algorithms, and a common parameter
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encoding mechanism.
3.1. Canonicalization {rfc4871bis-02 3.4}
Some data handling systems modify the original data, potentially
invalidating a cryptographic function, such as with a signature. For
most signers, mild modification of data is immaterial to validation
of the DOSETA domain name's use. For such signers, a
canonicalization algorithm that survives modest handling modification
is preferred.
Other signers demand that any modification of the data, however
minor, result in a signature verification failure. These signers
prefer a canonicalization algorithm that does not tolerate any in-
transit modification of the signed email.
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 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 an Internet Mail
header fields
Content: Lines of ASCII text
Some DOSETA Creators might be willing to accept modifications to some
portions of the data, but not other portions. For DOSETA, a Creator
MAY specify either algorithm for one portion of the data and another
for a different portion when signing the data.
If no canonicalization algorithm is specified, the "simple" algorithm
defaults for all of the data. DOSETA Creators MUST implement both
canonicalization algorithms. Because further canonicalization
algorithms might be defined in the future, Creators MUST ignore any
signatures that use unrecognized canonicalization algorithms.
Canonicalization simply prepares the data for presentation to the
signing or verification algorithm. It MUST NOT change the
transmitted data in any way. Canonicalization of distinct data
portions is described below.
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NOTE: This section assumes that data is already in "network normal"
format (text is ASCII encoded, lines are separated with CRLF
characters, etc.). See also Section 4.4.3 for information about
normalizing the message.
3.1.1. Header Canonicalization Algorithms
This section specifies initial entry for the IANA registry defined in
Table 2.
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 signing or
verification algorithm exactly as they are in the message being
signed or verified. 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. It MUST apply the following steps in order:
* 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.
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3.1.2. Content Canonicalization Algorithms
This section specifies initial entry for the IANA registry defined in
Table 3.
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=
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 3.4.3. 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=
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NOTE: It ought to be noted that the relaxed Content
canonicalization algorithm might enable certain types of
extremely crude "ASCII Art" attacks where a message might be
conveyed by adjusting the spacing between words. If this is a
concern, the "simple" Content canonicalization algorithm SHOULD
be used instead.
3.1.3. Canonicalization Examples (3.4.6}
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.
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>
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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>
3.2. Tag=Value Parameters {rfc4871bis-02 3.2}
DOSETA uses a simple "tag=value" syntax in several contexts,
including to represent associated cryptographic data and domain
cryptographic key records.
Values are a series of strings containing either plain text, "base64"
text (as defined in [RFC2045], Section 6.8), "qp-section" (ibid,
Section 6.7), or "D-quoted-printable" (as defined in Section 2.6).
The name of the tag will determine the encoding of each value.
Unencoded semicolon (";") characters MUST NOT occur in the tag value,
since that separates tag-specs.
NOTE: Although the "plain text" defined below (as "tag-value") only
includes 7-bit characters, an implementation that wished to
anticipate future standards would be advised not to preclude the
use of UTF8-encoded text in tag=value lists.
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 that 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 be interpreted in a case-sensitive manner. Values MUST be
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processed as case sensitive unless the specific tag description of
semantics specifies case insensitivity.
Tags with 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.
3.3. Key Management {rfc4871bis-02 3.6}
Applications require some level of assurance that a cited public key
is associated with the Creator using it. Many applications achieve
this by using public key certificates issued by a trusted third
party. For applications with modest certification requirements,
DOSETA achieves a sufficient level of security, with significantly
enhanced scalability, by simply having the Consumer query the
purported Creator's DNS entry (or a supported equivalent) in order to
retrieve the public key.
DOSETA keys might be stored in multiple types of key servers and in
multiple formats. The storage and format of keys are irrelevant to
the remainder of the DOSETA algorithm.
The 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=" tag
(Section 4.2)
d-val: The domain of the signature, as specified in the "d=" tag
(Section 4.2)
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s-val: The selector of the lookup as specified in the "s=" tag
(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, using DNS TXT records to
distribute the keys, per Section 3.5. Other bindings might be
defined in the future.
3.4. Selectors for Keys {rfc4871bis-02 3.1}
To support 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.
Selectors are needed to support some important use cases. For
example:
o 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.
o Domains that want to allow frequent travelers to generate signed
data locally without the need to connect to a particular server.
o "Affinity" domains (such as, college alumni associations) that
provide data forwarding, but that do not operate a data
origination agent for outgoing data.
Periods are allowed in selectors and are component separators. 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 corresponding selectors for each domain
is determined by the domain owner. Many domain owners will be
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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.
Beyond administrative convenience, 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 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, signers are ill-advised to
reuse selectors for new keys. A better strategy is to assign
new keys to new selectors.
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3.5. DNS Binding for Key Retrieval {rfc4871bis-02 3.6.2}
This section defines a binding using DNS TXT records as a key
service. All implementations MUST support this binding.
3.5.1. Namespace
A DOSETA key is stored in a subdomain named:
ABNF:
dns-record = s "._domainkey." d
where:
s: is the selector of the lookup as specified in the "s=" tag
(Section 4.2)
d: is the domain of the signature, as specified in the "d=" tag
(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, in 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.
Given a DOSETA-Signature field with a "d=" tag of "example.com" and
an "s=" tag 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 this context and SHOULD not be used. Note also
that wildcards within domains (for example,
s._domainkey.*.example.com) are not supported by the DNS.
3.5.2. Resource Record Types for Key Storage
The DNS Resource Record type used is specified by an option to the
query-type ("q=") tag. The only option defined in this base
specification is "txt", indicating the use of a TXT Resource Record
(RR). 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
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selector name; that is, if there are multiple records in an RRset,
the results are undefined.
TXT RRs are encoded as described in Section 3.6.
3.6. Stored Key Data {rfc4871bis-02 3.6.1, subset, with preface}
This section defines a syntax for encoding stored key data within an
unstructured environment such as simple text.
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 3.5.
The overall syntax is a tag-list as described in Section 3.2. The
current valid tags are 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=" tag 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.
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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 3.4 and A.1.1) is
being used in the "p=" tag. (Note: the "p=" tag further
encodes the value using the base64 algorithm.) Unrecognized
key types MUST be ignored.
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=" tag.
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.
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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.
4. DOSETA H/C Signing Template
This section specifies the basic components of a signing mechanism,
which 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 with DKIM it 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.
TEMPLATE: (Header/Content Mapping) The service incorporating
this mechanism MUST define the precise mappings onto the
template provided in this section. (Note that 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 {rfc4871bis-02 3.3, subset}
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
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than the native binary form before being signed. The signing
algorithm SHOULD use a public exponent of 65537.
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:
o The practical constraint that large (for example, 4096 bit) keys
might not fit within a 512-byte DNS UDP response packet
o The security constraint that keys smaller than 1024 bits are
subject to off-line attacks
o Larger keys impose higher CPU costs to verify and sign data
o Keys can be replaced on a regular basis, thus their lifetime can
be relatively short
o 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 {rfc4871bis-02 3.5, subset}
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 3.2.
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 is part of a sequence of structures -- such
as being added to an email header -- it SHOULD NOT be reordered and
SHOULD be prepended 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 a 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=" tag (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= Message 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 IANA registry defined in
Table 2
sig-c-content: A value from IANA registry defined in
Table 3
These algorithms are described in Section 3.1. 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
d= The SDID doing the signing (plain-text; REQUIRED). Hence,
the SDID value is used to form the query for the public key.
The SDID 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 SDID 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
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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;
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 4.4.4 for a discussion of
choosing Header fields to sign.
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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".
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).
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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=" tag, 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 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=" tag MUST be greater than the value of the "t=" tag
if both are present.
NOTE: The "x=" tag 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.
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ABNF:
sig-x-tag = %x78 [FWS] "=" [FWS]
1*12DIGIT
4.3. Signature Calculation {rfc4871bis-02 3.7, reorganized and
clarified}
Hashing and Cryptographic algorithms are combined into a procedure
for computing a digital signature.
Creators will choose appropriate parameters for the signing process;
Creators will use the tags that are then passed as an associated
DOSETA Header field. Section 4.2 defines the contents of the Header
field.
In the following discussion, the names of the tags in the Header
field that either exist (when consuming) or will be created (when
creating) are used.
NOTE: Canonicalization (see Section 3.1) prepares a separate
representation of the data for additional processing; it does
not affect the transmitted data in any way.
Creators MUST compute hashes in the order defined. Consumers MAY
compute them in any order convenient to the Creator, provided that
the result is semantically identical to the semantics that would be
the case had they been computed in this order.
The combined hashing algorithm is:
Step 1: The Creator/Creator hashes the Content portion,
canonicalized using the Content canonicalization algorithm
specified in the "c=" tag and then truncated to the length
specified in the "l=" tag. That hash value is then converted
to base64 form. Signers then insert it into the "bh=" tag of
the DOSETA-Signature field; verifiers compare their hash with
the value in the "bh=" tag.
Step 2: Pass the following to a second hash algorithm in the
indicated order:
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+ Complete attribute:value Header fields specified by the "h="
tag, in the order specified in that tag, and canonicalized
using the Header canonicalization algorithm specified in the
"c=" tag. Each field MUST be terminated with a single CRLF.
+ The DOSETA-Signature field that exists (verifying), or will
be inserted (signing), in the Header, with the value of the
"b=" signature data tag (including all surrounding
whitespace) deleted (that is, treated as the empty string),
canonicalized using the Header canonicalization algorithm
specified in the "c=" tag, and without a trailing CRLF.
+ The hash produced in Step 1.
When calculating or verifying a signature, the value of the "b=" tag
(signature value) of that DOSETA-Signature structure MUST be treated
as though it were an empty string. Unknown tags in the
DOSETA-Signature header field MUST be included in the signature
calculation but MUST be otherwise ignored by verifiers. Other
DOSETA-Signature tags that are included in the signature SHOULD be
treated as normal tags; in particular, the "b=" tag is not treated
specially.
All tags and their values in the DOSETA-Signature field are included
in the cryptographic hash with the sole exception of the value
portion of the "b=" (signature) tag, which MUST be treated as the
null string. All tags MUST be included even if they might not be
understood by the verifier. The DOSETA-Signature field MUST be
presented to the hash algorithm after the Content. rather than with
the rest of the Header fields and MUST be canonicalized as specified
in the "c=" (canonicalization) tag. The DOSETA-Signature header
structure MUST NOT be included in its own h= tag, although other
DOSETA-Signature Header fields MAY be signed (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 3.1, the DOSETA signing process treats the Content as simply
a string of octets. DOSETA Content MAY be either in plain-text or in
MIME format; no special treatment is afforded to MIME content.
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More formally, the algorithm for the signature is as follows:
ABNF:
content-hash = bh-hash-alg (canon-content, l-param)
data-hash = a-hash-alg (h-headers, D-SIG, content-hash)
signature = sig-alg (d-domain, selector, data-hash)
where:
content-hash: is the output of a function to hash the data
Content.
bh-hash-alg: is the hashing algorithm specified in the "bh"
parameter.
canon-body: is a canonicalized representation of the body.
l-param: is the value of the l= length parameter.
data-hash: is the output from hashing the header, the
DOSETA-Signature header, and the Content hash.
a-hash-alg: is the hash algorithm specified by the "a=" tag,
h-headers: is the list of headers to be signed, as specified in
the h= parameter.
D-SIG: is the canonicalized DOSETA-Signature field sans the
signature value itself.
canon-content: is the canonicalized data Content(respectively)
as defined in Section 3.1 (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=" tag,
d-domain: is the domain name specified in the d= parameter.
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selector: is the value of the s= selector parameter
NOTE: Many digital signature APIs provide both hashing and
application of the RSA private key using last two steps in the
algorithm would probably be combined into a single call that would
perform both the "a-hash-alg" and the "sig-alg".
4.4. Signer Actions {rfc4871bis-02 5}
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
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interval before being removed from the key server.
4.4.3. Normalize the Message to Prevent Transport Conversions
Some messages, particularly those using 8-bit characters, are subject
to modification during transit, notably from conversion to 7-bit
form. Such conversions will break DOSETA signatures. In order to
minimize the chances of such breakage, signers SHOULD convert the
data to a suitable encoding, such as quoted-printable or base64, as
described in [RFC2045] before signing. Such conversion is outside
the scope of DOSETA; the data SHOULD be converted to 7-bit prior to
presentation to the DOSETA signing procedure.
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.
If the data is submitted to the signer with any local encoding that
will be modified before transmission, that modification to a
canonical form MUST be done before signing. 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, the signer MUST sign the data as it is expected to be
received by the verifier rather than in some local or internal form.
4.4.4. 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.
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The DOSETA-Signature header field is always implicitly signed and
MUST NOT be included in the "h=" tag 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=" tag
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=" tag 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=" tag.
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
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.
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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.
4.4.5. 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.
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.
4.4.6. 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=" tag MUST
be the appropriately signed hash computed in the previous step,
signed using the algorithm specified in the "a=" tag of the
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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 {rfc4871bis-02 6}
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
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.
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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
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.
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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=" tag
that is inconsistent with this specification and return PERMFAIL
(incompatible version).
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=" tag,
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=" tag.
If not, the DOSETA-Signature header field MUST be ignored and the
verifier SHOULD return PERMFAIL (domain mismatch).
If the "h=" tag does not include the From header field, the verifier
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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=" tag and the
signature has expired.
Verifiers MAY ignore the DOSETA-Signature header field if the domain
used by the signer in the "d=" tag 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=" tag in the DOSETA-Signature header
field. Obviously, a public key need only be retrieved if the process
of extracting the signature information is completely successful.
Details of key management and representation are described in
Section 3.3. 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 order indicated -- 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 3.3 using the
algorithm in the "q=" tag, the domain from the "d=" tag, and the
selector from the "s=" tag.
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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
keys with a version code ("v=" tag) that they do not implement.
6. If the "h=" tag 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=" tag, the
verifier MUST ignore the key record and return PERMFAIL
(inappropriate hash algorithm).
7. If the public key data (the "p=" tag) 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=" tags in the
DOSETA-Signature header field, the verifier MUST immediately
return PERMFAIL (inappropriate key algorithm).
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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=" tag, the Content
length specified in the "l=" tag, and the header field names in
the "h=" tag, 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=" tag against the actual message header field,
comparisons MUST be case-insensitive.
2. Based on the algorithm indicated in the "a=" tag, 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=" tag.
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=" tag, verify the
signature against the header hash using the mechanism appropriate
for the public key algorithm described in the "a=" tag. If the
signature does not validate, the verifier SHOULD ignore the
signature and return PERMFAIL (signature did not verify).
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=" tag 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
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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
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 SDID is not the
same as the address in the From: header field, the data system SHOULD
take pains to ensure that the actual SDID 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
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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 {added}
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.
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 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.
4.7. Signing by Parent Domains {rfc4871bis-02 3.8}
In some circumstances, it is desirable for a domain to apply a
signature on behalf of any of its subdomains without the need to
maintain separate selectors (key records) in each subdomain. By
default, private keys corresponding to key records can be used to
sign messages for any subdomain of the domain in which they reside;
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for example, a key record for the domain example.com can be used to
verify messages where the AUID ("i=" tag of the signature) is
sub.example.com, or even sub1.sub2.example.com. In order to limit
the capability of such keys when this is not intended, the "s" flag
MAY be set in the "t=" tag of the key record, to constrain the
validity of the domain of the AUID. If the referenced key record
contains the "s" flag as part of the "t=" tag, the domain of the AUID
("i=" flag) MUST be the same as that of the SDID (d=) domain. If
this flag is absent, the domain of the AUID MUST be the same as, or a
subdomain of, the SDID.
5. Semantics of Multiple Signatures {rfc4871bis-02 4}
5.1. Example Scenarios {rfc4871bis-02 4.1}
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=" tag (to satisfy strict verifiers) and a second time with a
limited set of headers and an "l=" tag (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
that of a signed message that passes through a mailing list that also
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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 {rfc4871bis-02 4.2}
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 4.4.4 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
signer might want to produce signatures using two different hash
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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. Considerations
6.1. IANA Considerations {rfc4871bis-02 7, subset}
This section carries forward the IANA registrations made by DKIM
[RFC4871]. The original DKIM language has been retained, including
the use of "DKIM" in the name of these registries, in order to
provide continuity from the original specification.
In all cases, new values are assigned only for values that have been
documented in a published RFC that has IETF Consensus [RFC5226].
6.1.1. DKIM-Signature Tag Specifications
NOTE: The content of a DOSETA-Signature structure match those in a
DKIM-Signature header field.
A DKIM-Signature provides for a list of tag specifications. IANA has
established the DKIM-Signature Tag Specification Registry for tag
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specifications that can be used in DKIM-Signature fields.
The initial entries in the registry comprise:
+------+------------------------------+
| TYPE | REFERENCE |
+------+------------------------------+
| v | (this document, Section 4.2) |
| a | (this document, Section 4.2) |
| b | (this document, Section 4.2) |
| bh | (this document, Section 4.2) |
| c | (this document, Section 4.2) |
| d | (this document, Section 4.2) |
| h | (this document, Section 4.2) |
| q | (this document, Section 4.2) |
| s | (this document, Section 4.2) |
| t | (this document, Section 4.2) |
| x | (this document, Section 4.2) |
+------+------------------------------+
Table 1: DKIM-Signature Tag Specification Registry Initial Values
6.1.2. DKIM-Signature Query Method Registry
The "q=" tag-spec (specified in Section 4.2) provides for a list of
query methods.
IANA has established the DKIM-Signature Query Method Registry for
mechanisms that can be used to retrieve the key that will permit
validation processing of a message signed using DKIM.
The initial entry in the registry comprises:
+------+--------+------------------------------------+
| TYPE | OPTION | REFERENCE |
+------+--------+------------------------------------+
| dns | txt | (this document, Section 4.2, "q=") |
+------+--------+------------------------------------+
DKIM&nbhy;Signature Query Method Registry Initial Values
6.1.3. DKIM-Signature Canonicalization Registry
The "c=" tag-spec (specified in Section 4.2) provides for a specifier
for canonicalization algorithms for the header and Content.
IANA has established the DKIM-Signature Canonicalization Algorithm
Registry for algorithms for converting a message into a canonical
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form before signing or verifying using DKIM.
The initial entries in the Header registry comprise:
+---------+--------------------------------+
| TYPE | REFERENCE |
+---------+--------------------------------+
| simple | (this document, Section 3.1.1) |
| relaxed | (this document, Section 3.1.1) |
+---------+--------------------------------+
Table 2: DKIM-Signature Header Canonicalization Algorithm Registry
Initial Values
The initial entries in the Content (Body) registry comprise:
+---------+--------------------------------+
| TYPE | REFERENCE |
+---------+--------------------------------+
| simple | (this document, Section 3.1.2) |
| relaxed | (this document, Section 3.1.2) |
+---------+--------------------------------+
Table 3: DKIM-Signature Body Canonicalization Algorithm Registry
Initial Values
6.1.4. _domainkey DNS TXT Record Tag Specifications
A _domainkey DNS TXT record provides for a list of tag
specifications. IANA has established the DKIM _domainkey DNS TXT Tag
Specification Registry for tag specifications that can be used in DNS
TXT Records.
The initial entries in the registry comprise:
+------+------------------------------+
| TYPE | REFERENCE |
+------+------------------------------+
| v | (this document, Section 3.6) |
| k | (this document, Section 3.6) |
| n | (this document, Section 3.6) |
| p | (this document, Section 3.6) |
+------+------------------------------+
DOSETA _domainkey DNS TXT Record Tag Specification Registry
Initial Values
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6.1.5. DKIM Key Type Registry
The "k=" <key-k-tag> (specified in Section 3.6) and the "a=" <sig-
a-tag-k> (specified in Section 4.2) tags provide for a list of
mechanisms that can be used to decode a DKIM signature.
IANA has established the DKIM Key Type Registry for such mechanisms.
The initial entry in the registry comprises:
+------+-----------+
| TYPE | REFERENCE |
+------+-----------+
| rsa | [RFC3447] |
+------+-----------+
DKIM Key Type Initial Values
6.1.6. DKIM Hash Algorithms Registry
The "h=" <key-h-tag> (specified in Section 3.6) and the "a=" <sig-
a-tag-h> (specified in Section 4.2) tags provide for a list of
mechanisms that can be used to produce a digest of message data.
IANA has established the DKIM Hash Algorithms Registry for such
mechanisms.
The initial entries in the registry comprise:
+--------+-------------------+
| TYPE | REFERENCE |
+--------+-------------------+
| sha1 | [FIPS-180-2-2002] |
| sha256 | [FIPS-180-2-2002] |
+--------+-------------------+
DKIM Hash Algorithms Initial Values
6.2. Security Considerations {rfc4871bis-02 8, subset}
It has been observed that any mechanism that is introduced that
attempts to stem the flow of spam is subject to intensive attack.
DOSETA needs to be carefully scrutinized to identify potential attack
vectors and the vulnerability to each. See also [RFC4686].
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6.2.1. Misappropriated Private Key
If the private key for a user is resident on their computer and is
not protected by an appropriately secure mechanism, it is possible
for malware to send mail as that user and any other user sharing the
same private key. The malware would not, however, be able to
generate signed spoofs of other signers' addresses, which would aid
in identification of the infected user and would limit the
possibilities for certain types of attacks involving socially
engineered messages. This threat applies mainly to MUA-based
implementations; protection of private keys on servers can be easily
achieved through the use of specialized cryptographic hardware.
A larger problem occurs if malware on many users' computers obtains
the private keys for those users and transmits them via a covert
channel to a site where they can be shared. The compromised users
would likely not know of the misappropriation until they receive
"bounce" messages from messages they are purported to have sent.
Many users might not understand the significance of these bounce
messages and would not take action.
One countermeasure is to use a user-entered passphrase to encrypt the
private key, although users tend to choose weak passphrases and often
reuse them for different purposes, possibly allowing an attack
against DOSETA to be extended into other domains. Nevertheless, the
decoded private key might be briefly available to compromise by
malware when it is entered, or might be discovered via keystroke
logging. The added complexity of entering a passphrase each time one
sends a message would also tend to discourage the use of a secure
passphrase.
A somewhat more effective countermeasure is to send messages through
an outgoing MTA that can authenticate the submitter using existing
techniques (for example, SMTP Authentication), possibly validate the
message itself (for example, verify that the header is legitimate and
that the content passes a spam content check), and sign the message
using a key appropriate for the submitter address. Such an MTA can
also apply controls on the volume of outgoing mail each user is
permitted to originate in order to further limit the ability of
malware to generate bulk email.
6.2.2. Key Server Denial-of-Service Attacks
Since the key servers are distributed (potentially separate for each
domain), the number of servers that would need to be attacked to
defeat this mechanism on an Internet-wide basis is very large.
Nevertheless, key servers for individual domains could be attacked,
impeding the verification of messages from that domain. This is not
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significantly different from the ability of an attacker to deny
service to the mail exchangers for a given domain, although it
affects outgoing, not incoming, mail.
A variation on this attack is that if a very large amount of mail
were to be sent using spoofed addresses from a given domain, the key
servers for that domain could be overwhelmed with requests. However,
given the low overhead of verification compared with handling of the
email message itself, such an attack would be difficult to mount.
6.2.3. Attacks Against the DNS
Since the DNS is a required binding for key services, specific
attacks against the DNS need to be considered.
While the DNS is currently insecure [RFC3833], these security
problems are the motivation behind DNS Security (DNSSEC) [RFC4033],
and all users of the DNS will reap the benefit of that work.
DOSETA is only intended as a "sufficient" method of proving
authenticity. It is not intended to provide strong cryptographic
proof about authorship or contents. Other technologies such as
OpenPGP [RFC4880] and S/MIME [RFC5751] address those requirements.
A second security issue related to the DNS revolves around the
increased DNS traffic as a consequence of fetching selector-based
data as well as fetching signing domain policy. Widespread
deployment of DOSETA will result in a significant increase in DNS
queries to the claimed signing domain. In the case of forgeries on a
large scale, DNS servers could see a substantial increase in queries.
A specific DNS security issue that ought to be considered by DOSETA
verifiers is the name chaining attack described in Section 2.3 of
[RFC3833]. A DOSETA verifier, while verifying a DOSETA-Signature
header field, could be prompted to retrieve a key record of an
attacker's choosing. This threat can be minimized by ensuring that
name servers, including recursive name servers, used by the verifier
enforce strict checking of "glue" and other additional information in
DNS responses and are therefore not vulnerable to this attack.
6.2.4. Replay Attacks
In this attack, a spammer sends a message to be spammed to an
accomplice, which results in the message being signed by the
originating MTA. The accomplice resends the message, including the
original signature, to a large number of recipients, possibly by
sending the message to many compromised machines that act as MTAs.
The messages, not having been modified by the accomplice, have valid
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signatures.
Partial solutions to this problem involve the use of reputation
services to convey the fact that the specific email address is being
used for spam and that messages from that signer are likely to be
spam. This requires a real-time detection mechanism in order to
react quickly enough. However, such measures might be prone to
abuse, if for example an attacker resent a large number of messages
received from a victim in order to make them appear to be a spammer.
Large verifiers might be able to detect unusually large volumes of
mails with the same signature in a short time period. Smaller
verifiers can get substantially the same volume of information via
existing collaborative systems.
6.2.5. Limits on Revoking Keys
When a large domain detects undesirable behavior on the part of one
of its users, it might wish to revoke the key used to sign that
user's messages in order to disavow responsibility for messages that
have not yet been verified or that are the subject of a replay
attack. However, the ability of the domain to do so can be limited
if the same key, for scalability reasons, is used to sign messages
for many other users. Mechanisms for explicitly revoking keys on a
per-address basis have been proposed but require further study as to
their utility and the DNS load they represent.
6.2.6. Intentionally Malformed Key Records
It is possible for an attacker to publish key records in DNS that are
intentionally malformed, with the intent of causing a denial-of-
service attack on a non-robust verifier implementation. The attacker
could then cause a verifier to read the malformed key record by
sending a message to one of its users referencing the malformed
record in a (not necessarily valid) signature. Verifiers MUST
thoroughly verify all key records retrieved from the DNS and be
robust against intentionally as well as unintentionally malformed key
records.
6.2.7. Intentionally Malformed DOSETA-Signature Header Fields
Verifiers MUST be prepared to receive messages with malformed
DOSETA-Signature Header fields, and thoroughly verify the header
field before depending on any of its contents.
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6.2.8. Information Leakage
An attacker could determine when a particular signature was verified
by using a per-message selector and then monitoring their DNS traffic
for the key lookup. This would act as the equivalent of a "web bug"
for verification time rather than when the message was read.
6.2.9. Remote Timing Attacks
In some cases it might be possible to extract private keys using a
remote timing attack [BONEH03]. Implementations SHOULD obfuscate the
timing to prevent such attacks.
6.2.10. Reordered Header Fields
Existing standards allow intermediate MTAs to reorder Header fields.
If a signer signs two or more Header fields of the same name, this
can cause spurious verification errors on otherwise legitimate
messages. In particular, signers that sign any existing
DOSETA-Signature fields run the risk of having messages incorrectly
fail to verify.
6.2.11. RSA Attacks
An attacker could create a large RSA signing key with a small
exponent, thus requiring that the verification key have a large
exponent. This will force verifiers to use considerable computing
resources to verify the signature. Verifiers might avoid this attack
by refusing to verify signatures that reference selectors with public
keys having unreasonable exponents.
In general, an attacker might try to overwhelm a verifier by flooding
it with data requiring verification. This is similar to other server
denial-of-service attacks and needs to be dealt with in a similar
fashion.
6.2.12. Inappropriate Signing by Parent Domains
The trust relationship described in Section 4.7 could conceivably be
used by a parent domain to sign messages with identities in a
subdomain not administratively related to the parent. For example,
the ".com" registry could create messages with signatures using an
"i=" value in the example.com domain. There is no general solution
to this problem, since the administrative cut could occur anywhere in
the domain name. For example, in the domain "example.podunk.ca.us"
there are three administrative cuts (podunk.ca.us, ca.us, and us),
any of which could create messages with an identity in the full
domain.
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NOTE: This is considered an acceptable risk for the same reason
that it is acceptable for domain delegation. For example, in the
example above any of the domains could potentially simply delegate
"example.podunk.ca.us" to a server of their choice and completely
replace all DNS-served information. Note that a verifier MAY
ignore signatures that come from an unlikely domain such as
".com", as discussed in Section 4.5.2.
6.2.13. Attacks Involving Addition of Header Fields
Many email implementations do not enforce [RFC5322] with strictness.
As discussed in Section 4.4.3 DOSETA processing is predicated on a
valid mail message as its input. However, DOSETA implementers need
to be aware of the potential effect of having loose enforcement by
email components interacting with DOSETA modules.
For example, a message with multiple From: Header fields violates
Section 3.6 of [RFC5322]. With the intent of providing a better user
experience, many agents tolerate these violations and deliver the
message anyway. An MUA then might elect to render to the user the
value of the last, or "top", From: field. This might also be done
simply out of the expectation that there is only one, where a "find
first" algorithm would have the same result. Such code in an MUA can
be exploited to fool the user if it is also known that the other
From: field is the one checked by arriving message filters. Such is
the case with DOSETA; although the From: field MUST be signed, a
malformed message bearing more than one From: field might only have
the first ("bottom") one signed, in an attempt to show the message
with some "DOSETA passed" annotation while also rendering the From:
field that was not authenticated. (This can also be taken as a
demonstration that DOSETA is not designed to support author
validation.)
To resist this specific attack the signed header field list can
include an additional reference for each field that was present at
signing. For example, a proper message with one From: field could be
signed using "h=From:From:..." Due to the way header fields are
canonicalized for input to the hash function, the extra field
references will prevent instances of the cited fields from being
added after signing, as doing so would render the signature invalid.
The From: field is used above to illustrate this issue, but it is
only one of > several fields that Section 3.6 of [RFC5322] constrains
in this way. In reality any agent that forgives malformations, or is
careless about identifying which parts of a message were
authenticated, is open to exploitation.
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7. References
7.1. 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.
[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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7.2. Informative References
[BONEH03] "Remote Timing Attacks are Practical", Proceedings 12th
USENIX Security Symposium, 2003.
[RFC1847] Galvin, J., Murphy, S., Crocker, S., and N. Freed,
"Security Multiparts for MIME: Multipart/Signed and
Multipart/Encrypted", RFC 1847, October 1995.
[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For
Public Keys Used For Exchanging Symmetric Keys", BCP 86,
RFC 3766, April 2004.
[RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain
Name System (DNS)", RFC 3833, August 2004.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 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.
[RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4871, May 2007.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
"OpenPGP Message Format", RFC 4880, November 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[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.
[RFC5751] Ramsdell, B., "Secure/Multipurpose Internet Mail
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Extensions (S/MIME) Version 3.1 Message Specification",
RFC 5751, January 2010.
Appendix A. Creating a Public Key {rfc4871bis-02 C}
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-----
MIICXwIBAAKBgQDwIRP/UC3SBsEmGqZ9ZJW3/DkMoGeLnQg1fWn7/zYtIxN2SnFC
jxOCKG9v3b4jYfcTNh5ijSsq631uBItLa7od+v/RtdC2UzJ1lWT947qR+Rcac2gb
to/NMqJ0fzfVjH4OuKhitdY9tf6mcwGjaNBcWToIMmPSPDdQPNUYckcQ2QIDAQAB
AoGBALmn+XwWk7akvkUlqb+dOxyLB9i5VBVfje89Teolwc9YJT36BGN/l4e0l6QX
/1//6DWUTB3KI6wFcm7TWJcxbS0tcKZX7FsJvUz1SbQnkS54DJck1EZO/BLa5ckJ
gAYIaqlA9C0ZwM6i58lLlPadX/rtHb7pWzeNcZHjKrjM461ZAkEA+itss2nRlmyO
n1/5yDyCluST4dQfO8kAB3toSEVc7DeFeDhnC1mZdjASZNvdHS4gbLIA1hUGEF9m
3hKsGUMMPwJBAPW5v/U+AWTADFCS22t72NUurgzeAbzb1HWMqO4y4+9Hpjk5wvL/
eVYizyuce3/fGke7aRYw/ADKygMJdW8H/OcCQQDz5OQb4j2QDpPZc0Nc4QlbvMsj
7p7otWRO5xRa6SzXqqV3+F0VpqvDmshEBkoCydaYwc2o6WQ5EBmExeV8124XAkEA
qZzGsIxVP+sEVRWZmW6KNFSdVUpk3qzK0Tz/WjQMe5z0UunY9Ax9/4PVhp/j61bf
eAYXunajbBSOLlx4D+TunwJBANkPI5S9iylsbLs6NkaMHV6k5ioHBBmgCak95JGX
GMot/L2x0IYyMLAz6oLWh2hm7zwtb0CgOrPo1ke44hFYnfc=
-----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 [RFC4871].
Authors' Addresses
D. Crocker (editor)
Brandenburg InternetWorking
675 Spruce Dr.
Sunnyvale
USA
Phone: +1.408.246.8253
Email: dcrocker@bbiw.net
URI: http://bbiw.net
M. Kucherawy (editor)
Cloudmark
128 King St., 2nd Floor
San Francisco, CA 94107
USA
Email: msk@cloudmark.com
Crocker & Kucherawy Expires July 18, 2011 [Page 59]