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Versions: 00 01 02 03 04 05 06 07 08 RFC 5485

INTERNET DRAFT                                                  R. Housley
Intended Status: Informational                              Vigil Security
Expires: 19 September 2008                                   19 March 2008

             Digital Signatures on Internet-Draft Documents

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   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."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at


   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

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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

      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.
        eContent             OCTET STRING  -- Always absent

3.1.  ContentInfo

   The CMS requires the outer-most encapsulation to be ContentInfo
   [CMS].  The fields of ContentInfo are used as follows:

         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

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         (1.2.840.113549.1.7.2) object identifier MUST be present in
         this field.

         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:

         is the syntax version number, and for this specification, the
         version number MUST be set to 3.

         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.

         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

         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].

         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

         is a collection of per-signer information, and for this
         specification, the collection must contain exactly one
         SignerInfo that represents the IETF Secretariat.  The use of
         the SignerInfo type is discussed further in Section 3.2.1.

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3.2.1.  SignerInfo

   The IETF Secretariat is represented in the SignerInfo type.  The
   fields of SignerInfo are used as follows:

         is the syntax version number.  In this specification, the
         version MUST be set to 3.

         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.

         identifies the one-way hash function, and any associated
         parameters, used by the IETF Secretariat to generate the
         digital signature.

         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.

         identifies the digital signature algorithm, and any associated
         parameters, used by IETF Secretariat to  generate the digital

         is the digital signature value generated by the IETF

         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.

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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:

         is an object identifier that uniquely specifies the content
         type.  It MUST contain id-ct-asciiTextWithCRLF, which is
         defined as:

            id-ct-asciiTextWithCRLF  OBJECT IDENTIFIER  ::= { iso(1)
                  member-body(2) us(840) rsadsi(113549) pkcs(1)
                  pkcs9(9) smime(16) ct(1) 27 }

         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

   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.

Housley                                                         [Page 7]

INTERNET DRAFT    Digital Signatures on Internet-Drafts       March 2008  Content-Type Attribute

   A content-type attribute is required to contain the same object
   identifier as the content type contained in the
   EncapsulatedContentInfo.  The IETF Secretariat MUST include a
   content-type attribute containing id-ct-asciiTextWithCRLF
   (1.2.840.113549.  Section 11.1 of [CMS] defines the
   content-type attribute.  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.  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.  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
   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.  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.

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   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
   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.

5.  References

5.1.  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.

5.2.  Informative References

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   [BinTime]   Housley, R., "BinaryTime: An Alternate Format for
               Representing Date and Time in ASN.1", RFC 4049,
               April 2005.

   [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.

   [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,

6.  Acknowledgements

   The idea for the Internet-Draft signature file came from a discussion
   with Scott Bradner at IETF 69 in Chicago.

7.  IANA Considerations


   {{{ RFC Editor: Please remove this section prior to publication. }}}

Appendix: To Do

   In a future version of this specification, add a section that shows
   how an open source tool can be used to implement the specification.

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Authors' Addresses

   Russell Housley
   Vigil Security, LLC
   918 Spring Knoll Drive
   Herndon, VA 20170

   EMail: housley@vigilsec.com

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Housley                                                        [Page 12]

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