INTERNET DRAFT R. Housley
Intended Status: Informational Vigil Security
Expires: 10 October 2008 10 April 2008
Digital Signatures on Internet-Draft Documents
<draft-housley-internet-draft-sig-file-03.txt>
Status of this Memo
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Internet-Drafts are draft documents valid for a maximum of six months
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The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html
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Abstract
This document specifies the conventions for digital signatures on
Internet-Drafts. The Cryptographic Message Syntax (CMS) is used to
create a detached signature, which is stored in a separate companion
file so that no existing utilities are impacted by the addition of
the digital signature.
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1. Introduction
This document specifies the conventions for storing a digital
signature on Internet-Drafts. The Cryptographic Message Syntax (CMS)
[CMS] is used to create a detached signature. The signature is
stored in a separate companion file so that no existing utilities are
impacted by the addition of the digital signature.
At the time the IETF Secretariat posts the Internet-Draft in the
repository, the digital signature is generated and posted as a
companion file in the same repository. The digital signature allows
anyone to confirm that the contents of the Internet-Draft have not
been altered since the time that the document was posted in the
repository.
1.1. Terminology
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 RFC 2119 [STDWORDS].
1.2. ASN.1
The CMS uses Abstract Syntax Notation One (ASN.1) [X.680]. ASN.1 is
a formal notation used for describing data protocols, regardless of
the programming language used by the implementation. Encoding rules
describe how the values defined in ASN.1 will be represented for
transmission. The Basic Encoding Rules (BER) [X.690] are the most
widely employed rule set, but they offer more than one way to
represent data structures. For example, definite length encoding and
indefinite length encoding are supported. This flexibility is not
desirable when digital signatures are used. As a result, the
Distinguished Encoding Rules (DER) [X.690] were invented. DER is a
subset of BER that ensures a single way to represent a given value.
For example, DER always employs definite length encoding.
2. Internet-Draft Signature File
All Internet-Draft file names begin with "draft-". The next portion
of the file name depends on the source of the document. For example,
documents from IETF working groups will have "ietf-" followed by the
working group abbreviation, and this is followed by a string that
helps people figure out the subject of the document.
All Internet-Draft file names end with a hyphen followed by a two
digit version number and a suffix. The suffix indicates the type of
file. A plain text file with a suffix of ".txt" is required. Other
formats may also be provided, and they employ the appropriate suffix
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for the file format.
The companion signature file has exactly the same file name as the
Internet-Draft, except that ".p7s" is added to the end. This file
name suffix conforms to the conventions in [MSG]. Here are a few
example names:
Internet-Draft: draft-ietf-example-widgets-03.txt
Signature File: draft-ietf-example-widgets-03.txt.p7s
Internet-Draft: draft-ietf-example-widgets-03.ps
Signature File: draft-ietf-example-widgets-03.ps.p7s
Internet-Draft: draft-housley-internet-draft-sig-file-00.txt
Signature File: draft-housley-internet-draft-sig-file-00.txt.p7s
The IETF Secretariat will post the signature file in the repository
at the same time that the Internet-Draft is posted.
3.1. Need for Canonicalization of Text Files
In general, the content of the Internet-Draft is treated like a
single octet string for the generation of the digital signature.
Unfortunately, the plain text file requires canonicalization to avoid
signature validation problems. The primary concern is the manner in
which different operating systems indicate the end of a line of text.
Some systems use a single new-line character, and other systems use
the combination of the carriage-return character followed by a line-
feed character. For the digital signature to validate properly, a
single convention must be employed.
3.2. Canonicalization
The canonicalization procedure follows the conventions used for text
files in the File Transfer Protocol (FTP) [FTP]. Such files must be
supported by FTP implementations, so code reuse seems likely.
The canonicalization procedure converts the data from its internal
character representation to the standard 8-bit NVT-ASCII
representation (see TELNET [TELNET]). In accordance with the NVT
standard, the <CRLF> sequence MUST be used to denote the end of a
line of text. Using the standard NVT-ASCII representation means that
data MUST be interpreted as 8-bit bytes.
Other nonprintable characters, such as tab, form-feed, and backspace,
do occur in Internet-Drafts, and these characters MUST NOT be changed
in any way.
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3. CMS Profile
CMS is used to construct the detached signature of the Internet-
Draft. The CMS ContentInfo content type MUST always be present, and
it MUST encapsulate the CMS SignedData content type. Since a
detached signature is being created, the CMS SignedData content type
MUST NOT encapsulate the Internet-Draft. The CMS detached signature
is summarized by:
ContentInfo {
contentType id-signedData, -- (1.2.840.113549.1.7.2)
content SignedData
}
SignedData {
version CMSVersion, -- Always set to 3
digestAlgorithms DigestAlgorithmIdentifiers,
encapContentInfo EncapsulatedContentInfo,
certificates CertificateSet, -- Secretariat certificate
crls CertificateRevocationLists,
signerInfos SET OF SignerInfo -- Only one signer
}
SignerInfo {
version CMSVersion, -- Always set to 3
sid SignerIdentifier,
digestAlgorithm DigestAlgorithmIdentifier,
signedAttrs SignedAttributes, -- Always present
signatureAlgorithm SignatureAlgorithmIdentifier,
signature SignatureValue,
unsignedAttrs UnsignedAttributes -- Optional
}
EncapsulatedContentInfo {
eContentType id-ct-asciiTextWithCRLF,
-- (1.2.840.113549.1.9.16.1.27)
eContent OCTET STRING -- Always absent
}
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3.1. ContentInfo
The CMS requires the outer-most encapsulation to be ContentInfo
[CMS]. The fields of ContentInfo are used as follows:
contentType
indicates the type of the associated content, and for the
detached Internet-Draft signature file, the encapsulated type
is always SignedData, so the id-signedData
(1.2.840.113549.1.7.2) object identifier MUST be present in
this field.
content
holds the content, and for the detached Internet-Draft
signature file, the content is always a SignedData content.
3.2. SignedData
The SignedData content type [CMS] contains the signature of the
Internet-Draft and information to aid in the validation of that
signature. The fields of SignedData are used as follows:
version
is the syntax version number, and for this specification, the
version number MUST be set to 3.
digestAlgorithms
is a collection of one-way hash function identifiers. It MUST
contain the identifier used by the IETF Secretariat to generate
the digital signature. See the discussion of digestAlgorithm
in Section 3.2.1.
encapContentInfo
is the signed content, including a content type identifier.
Since a detached signature is being created, it does not
encapsulate the Internet-Draft. The use of the
EncapsulatedContentInfo type is discussed further in Section
3.2.2.
certificates
is an optional collection of certificates. It SHOULD include
the X.509 certificate needed to validate the digital signature
value. Certification Authority (CA) certificates and end
entity certificates MUST conform to the certificate profile
specified in [PKIX1].
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crls
is an optional collection of certificate revocation lists
(CRLs). It SHOULD NOT include any CRLs; however, any CRLs that
are present MUST conform to the CRL profile specified in
[PKIX1].
signerInfos
is a collection of per-signer information, and for this
specification, each item in the collection must represent the
IETF Secretariat. More than one SignerInfo MAY appear to
facilitate transitions between keys or algorithms. The use of
the SignerInfo type is discussed further in Section 3.2.1.
3.2.1. SignerInfo
The IETF Secretariat is represented in the SignerInfo type. The
fields of SignerInfo are used as follows:
version
is the syntax version number. In this specification, the
version MUST be set to 3.
sid
identifies the IETF Secretariat's public key. In this
specification, the subjectKeyIdentifier alternative is always
used, which identifies the public key directly. This
identifier MUST match the value included in the
subjectKeyIdentifier certificate extension in the IETF
Secretariat's X.509 certificate.
digestAlgorithm
identifies the one-way hash function, and any associated
parameters, used by the IETF Secretariat to generate the
digital signature.
signedAttrs
is an optional set of attributes that are signed along with the
content. The signedAttrs are optional in the CMS, but
signedAttrs is required for in this specification. The SET OF
Attribute must be encoded with the distinguished encoding rules
(DER) [X.690]. Section 2.2.3 of this document lists the signed
attributes that MUST be included in the collection. Other
signed attributes MAY also be included.
signatureAlgorithm
identifies the digital signature algorithm, and any associated
parameters, used by IETF Secretariat to generate the digital
signature.
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signature
is the digital signature value generated by the IETF
Secretariat.
unsignedAttrs
is an optional set of attributes that are not signed. Unsigned
attributes are usually omitted; however, the unsigned
attributes MAY hold a trusted timestamp generated in accordance
with [TSP]. Section 2.2.4 of [TSP] provides more information
about this unsigned attribute.
3.2.2. EncapsulatedContentInfo
The EncapsulatedContentInfo structure contains a content type
identifier. Since a detached signature is being created, it does not
encapsulate the Internet-Draft. The fields of
EncapsulatedContentInfo are used as follows:
eContentType
is an object identifier that uniquely specifies the content
type. The content type associated with the plain text file
MUST be id-ct-asciiTextWithCRLF. Other file formats may also
be posted, and the appropriate content type for each format is
discussed in Section 4. Additional file formats can be added
if the Internet community chooses.
eContent
is optional. When an encapsulated signature is generated, the
content to be signed is carried in this field. Since a
detached signature is being created, eContent MUST be absent.
3.2.3. Signed Attributes
The IETF Secretariat MUST digitally sign a collection of attributes
along with the Internet-Draft. Each attribute in the collection MUST
be DER-encoded. The syntax for attributes is defined in [X.501], and
the X.500 Directory provides a rich attribute syntax. A very simple
subset of this syntax is used extensively in [CMS], where
ATTRIBUTE.&Type and ATTRIBUTE.&id are the only parts of the ATTRIBUTE
class that are employed.
Each of the attributes used with this CMS profile has a single
attribute value. Even though the syntax is defined as a SET OF
AttributeValue, there MUST be exactly one instance of AttributeValue
present.
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The SignedAttributes syntax within signerInfo is defined as a SET OF
Attribute. The SignedAttributes MUST include only one instance of
any particular attribute.
The IETF Secretariat MUST include the content-type, message-digest,
and signing-time attributes. The IETF Secretariat MAY also include
the binary-signing-time signed attribute as well as any other
attribute that is deemed appropriate. The intent is to allow
additional signed attributes to be included if a future need is
identified. This does not cause an interoperability concern because
unrecognized signed attributes are ignored at verification.
3.2.3.1. Content-Type Attribute
A content-type attribute is required to contain the same object
identifier as the content type contained in the
EncapsulatedContentInfo. The appropriate content type for each
format is discussed in Section 4. The IETF Secretariat MUST include
a content-type attribute containing the appropriate content type.
Section 11.1 of [CMS] defines the content-type attribute.
3.2.3.2. Message-Digest Attribute
The IETF Secretariat MUST include a message-digest attribute, having
as its value the output of a one-way hash function computed on the
Internet-Draft that is being signed. Section 11.2 of [CMS] defines
the message-digest attribute.
3.2.3.3. Signing-Time Attribute
The IETF Secretariat MUST include signing-time attribute, specifying
the time, based on the local system clock, at which the digital
signature was applied to the Internet-Draft. Section 11.3 of [CMS]
defines the content-type attribute.
3.2.3.4. Binary-Signing-Time Attribute
The IETF Secretariat MAY include a binary-signing-time attribute,
specifying the time at which the digital signature was applied to the
Internet-Draft. If present, the time that is represented MUST match
the time represented in the signing-time attribute. The binary-
signing-time attribute is defined in [BinTime].
3.2.4. Unsigned Attributes
Unsigned attributes are usually omitted. However, an unsigned
attribute MAY hold a trusted timestamp generated in accordance with
[TSP]. The idea is to time-stamp the IETF Secretariat digital
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signature to prove that it was created before a given time. If the
IETF Secretariat's certificate is revoked the time stamp allows a
verifier to know whether the signature was created before or after
the revocation date. Appendix A of [TSP] defines the signature time-
stamp attribute that can be used to time-stamp a digital signature.
4. Content Types
This section lists the content types that are used in this
specification. The eContentType field as described in Section 3.2.2
contains a content type identifier, and the same value appears in the
content-type attribute as described in Section 3.2.3.1.
The following table lists the file formats and the associated content
type.
File Format Content Type
----------- ------------
Plain text id-ct-asciiTextWithCRLF
Extensible Markup Language (XML) id-ct-xml
Portable Document Format (PDF) id-ct-pdf
PostScript id-ct-postscript
The object identifiers associated with the content types listed in
the above table are:
id-ct OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) 1 }
id-ct-asciiTextWithCRLF OBJECT IDENTIFIER ::= { id-ct 27 }
id-ct-xml OBJECT IDENTIFIER ::= { id-ct 28 }
id-ct-pdf OBJECT IDENTIFIER ::= { id-ct 29 }
id-ct-postscript OBJECT IDENTIFIER ::= { id-ct 30 }
5. Security Considerations
The Secretariat MUST protect its private key. The use of a hardware
security module is RECOMMENDED because compromise of the
Secretariat's private key permits masquerade.
The generation of a public/private key pair for signature operations
relies on random number generation. The use of an inadequate pseudo-
random number generator (PRNG) can result in little or no security.
An attacker may find it much easier to reproduce the PRNG environment
that produced the key pair, searching the resulting small set of
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possibilities, rather than brute force searching the whole private
key space. The generation of quality random numbers is difficult,
but [RANDOM] offers important guidance in this area.
The Secretariat should be aware that cryptographic algorithms become
weaker with time. As new cryptoanalysis techniques are developed and
computing performance improves, the work factor to break a particular
digital signature algorithm or one-way hash function will be reduced.
Therefore, it SHOULD be possible to migrate these algorithms. That
is, Secretariat SHOULD be prepared for the supported algorithms to
change over time.
The Secretariat must take care to use the correct time in signing-
time and binary-signing-time attributes. The inclusion of a
compatible date within the Internet-Draft by the authors and the
signing time attributes by the Secretariat provides confidence about
the date that the Internet-Draft was posted to the repository.
6. References
6.1. Normative References
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)",
RFC 3852, July 2004.
[PKIX1] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[STDWORDS] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[X.680] ITU-T Recommendation X.680: Information Technology -
Abstract Syntax Notation One, 1997.
[X.690] ITU-T Recommendation X.690 Information Technology -
ASN.1 encoding rules: Specification of Basic Encoding
Rules (BER), Canonical Encoding Rules (CER) and
Distinguished Encoding Rules (DER), 1997.
6.2. Informative References
[BinTime] Housley, R., "BinaryTime: An Alternate Format for
Representing Date and Time in ASN.1", RFC 4049,
April 2005.
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[FTP] Postel, J. and J. Reynolds, "File Transfer Protocol",
STD 9, RFC 959, October 1985.
[MSG] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, July 2004.
[OpenSSL] http://www.openssl.org/
[RANDOM] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Recommendations for Security", RFC 4086, June 2005.
[TELNET] Postel, J. and J. Reynolds, "Telnet Protocol
Specification", STD 8, RFC 854, May 1983.
[TSP] Adams, C., Cain, P., Pinkas, D., and R. Zuccherato,
"Internet X.509 Public Key Infrastructure Time-Stamp
Protocol (TSP)", RFC 3161, August 2001.
[X.501] ITU-T Recommendation X.501: Information Technology -
Open Systems Interconnection - The Directory: Models,
1993.
7. Acknowledgements
The idea for the Internet-Draft signature file came from a discussion
with Scott Bradner at IETF 69 in Chicago. Many helpful suggestions
came from Jim Schaad.
8. IANA Considerations
None.
{{{ RFC Editor: Please remove this section prior to publication. }}}
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Appendix: A
OpenSSL 0.9.9 [OpenSSL] includes an implementation of CMS. The
following command line can be used to verify an Internet-Draft
signature:
openssl cms -verify -CAfile <cert-file> -content <internet-draft> /
-inform DER -in <p7s-file> -out /dev/null
The arguments need to be provided as follows:
<cert-file>
the file name of the trust anchor. This will typically be the
self-signed certificate of the Certification Authority (CA)
that issued a certificate to the IETF Secretariat.
<internet-draft>
the file name of the signed Internet-Draft.
<p7s-file>
the file name of the detached signature that was generated in
accordance with this specification.
Authors' Addresses
Russell Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
USA
EMail: housley@vigilsec.com
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