Network Working Group                                        R. Housley
Internet Draft                                           Vigil Security
expires in six months                                    September 2004

      An alternate format for representing date and time in ASN.1


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   This document specifies a new ASN.1 type for representing time:
   BinaryTime.  This document also specifies an alternate to the
   signing-time attribute for use with the Cryptographic Message Syntax
   (CMS) SignedData and AuthenticatedData content types; the binary-
   signing-time attribute uses BinaryTime.  CMS and the signing-time
   attribute are defined in RFC 3852.

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1  Introduction

   This document specifies a new ASN.1 [ASN1] type for representing
   time: BinaryTime.  This ASN.1 type can be used to represent date and
   time values.

   This document also specifies an alternative to the signing-time
   attribute used with the Cryptographic Message Syntax (CMS) [CMS]
   SignedData and AuthenticatedData content types, allowing the
   BinaryTime type to be used instead of the traditional UTCTime and
   GeneralizedTime types.

1.1  BinaryTime

   Many operating systems represent date and time as an integer.  This
   document specifies an ASN.1 type for representing a date and time in
   a manner that is also an integer.  While some conversion may be
   necessary due to the selection of different epoch or a different
   granularity, an integer representation has several advantages over
   the UTCTime and GeneralizedTime types.

   First, a BinaryTime value is smaller than either a UTCTime or a
   GeneralizedTime value.

   Second, in some operating systems, the value can be used with little
   or no conversion.  Conversion, when it is needed, requires only
   straightforward computation.  If the endian ordering is different
   than the ASN.1 representation of an INTEGER, then straightforward
   manipulation is needed to obtain an equivalent integer value.  If the
   epoch is different than the one chosen for BinaryTime, addition or
   subtraction is needed to compensate.  If the granularity is something
   other than seconds, then multiplication or division is needed to
   compensate.  Also, padding may be needed convert the variable length
   ASN.1 encoding of INTEGER to a fixed length value used in the
   operating system.

   Third, date comparison is very easy with BinaryTime.  Integer
   comparison is easy, even when multi-precision integers are involved.
   Date comparison with UTCTime or GeneralizedTime can be complex when
   the two values to be compared are provided in different time zones.

   This is a rare instance where both memory and processor cycles can be

1.2  Binary Signing Time Attribute

   The signing-time attribute is defined in [CMS].  The alternative
   binary-signing-time attribute is defined in this document to obtain

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   the benefits of the BinaryTime type.

1.3  Terminology

   In this document, the key words MUST, MUST NOT, REQUIRED, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL are to be interpreted as
   described in [STDWORDS].

2  BinaryTime Definition

   The BinaryTime ASN.1 type is used to represent an absolute time and
   date.  A positive integer value is used to represent time values
   based on coordinated universal time (UTC), which is also called
   Greenwich Mean Time (GMT) and ZULU clock time.

   The syntax for BinaryTime is:

      BinaryTime ::= INTEGER

   The integer value is the number of seconds after midnight UTC,
   January 1, 1970.  This time format cannot represent time values prior
   to January 1, 1970.  The latest UTC time value that can be
   represented by a four-octet integer value is 03:14:07 on January 19,
   2038, which is represented by the hexadecimal value 7FFFFFFF.  Time
   values beyond 03:14:07 on January 19, 2038 are represented by integer
   values that are longer than four octets, and a five-octet integer
   value is sufficient to represent dates covering the next seventeen

   This specification uses a variable length encoding of INTEGER.  This
   permits any time value after midnight UTC, January 1, 1970 to be

   When encoding of an integer value that consists of more than one
   octet, which includes almost all of the time values of interest, the
   bits of the first octet and bit 8 of the second octet MUST NOT all be
   ones or all zeros.  This rule ensures that an integer value is always
   encoded in the smallest possible number of octets.  However, it means
   that implementations cannot assume a fixed length for the integer

3  Binary Signing Time Attribute Definition

   The binary-signing-time attribute type specifies the time at which
   the signer (purportedly) performed the signing process.  The binary-
   signing-time attribute type is intended for use in the CMS SignedData
   content type; however, the attribute can also be used with the
   AuthenticatedData content type.

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   The binary-signing-time attribute MUST be a signed attribute or an
   authenticated attribute; it MUST NOT be an unsigned attribute,
   unauthenticated attribute, or unprotected attribute.

   The following object identifier identifies the binary-signing-time

      id-aa-binarySigningTime OBJECT IDENTIFIER ::= { iso(1)
          member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
          smime(16) aa(2) 46 }

   The binary-signing-time attribute values have ASN.1 type

      BinarySigningTime ::= BinaryTime

   In [CMS], the SignedAttributes syntax and the AuthAttributes syntax
   are each defined as a SET OF Attributes.  However, the binary-
   signing-time attribute MUST have a single attribute value, even
   though the syntax is defined as a SET OF AttributeValue.  There MUST
   NOT be zero or multiple instances of AttributeValue present.

   The SignedAttributes contained in the signerInfo structure within
   SignedData MUST NOT include multiple instances of the binary-signing-
   time attribute.  Similarly, the AuthAttributes in an
   AuthenticatedData MUST NOT include multiple instances of the binary-
   signing-time attribute.

   No requirement is imposed concerning the correctness of the signing
   time itself, and acceptance of a purported signing time is a matter
   of a recipient's discretion.  It is expected, however, that some
   signers, such as time-stamp servers, will be trusted implicitly.

4  References

   This section provides normative and informative references.

4.1  Normative References

   ASN1       CCITT.  Recommendation X.208: Specification of Abstract
              Syntax Notation One (ASN.1).  1988.

   CMS        Housley, R.  Cryptographic Message Syntax.  RFC 3852.
              July 2004.

   STDWORDS   Bradner, S.  Key Words for Use in RFCs to Indicate
              Requirement Levels.  RFC 2119.  March 1997.

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4.2  Informative References

   TSP        Adams, C., P. Cain, D. Pinkas, and R. Zuccherato.
              Internet X.509 Public Key Infrastructure Time-Stamp
              Protocol (TSP).  RFC 3161.  August 2001.

5  Security Considerations

   Use of the binary-signing-time attribute does not necessarily provide
   confidence in the time that the signature value was produced.
   Therefore, acceptance of a purported signing time is a matter of a
   recipient's discretion.  RFC 3161 [TSP] specifies a protocol for
   obtaining time stamps from a trusted entity.

   The original signing-time attribute defined in [CMS] has the same
   semantics as the binary-signing-time attribute specified in this
   document.  Therefore, only one of these attributes SHOULD be present
   in the signedAttrs of a SignerInfo object or in the authAttrs of an
   AuthenticatedData object.  However, if both of these attributes are
   present, they MUST provide the same date and time.

6  IANA Considerations

   No IANA actions are needed.

7  IPR Considerations

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   or will be disclosed, and any of which I become aware will be
   disclosed, in accordance with RFC 3668.

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   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at

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   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-

8  Author's Address

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

Appendix A:  ASN.1 Module

   The ASN.1 module contained in this appendix defines the structures
   that are needed to implement this specification.  It is expected to
   be used in conjunction with the ASN.1 modules in [CMS].

     { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
       pkcs-9(9) smime(16) modules(0) 27 }


   -- BinaryTime Definition

   BinaryTime ::= INTEGER

   -- Signing Binary Time Attribute

   id-aa-binarySigningTime OBJECT IDENTIFIER ::= { iso(1)
       member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
       smime(16) aa(2) 46 }

   BinarySigningTime ::= BinaryTime


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