BinaryTime: An Alternate Format for Representing Date and Time in ASN.1

Network Working Group                                        R. Housley
Request for Comments: 4049                               Vigil Security
Category: Experimental                                       April 2005


                              BinaryTime:
      An Alternate Format for Representing Date and Time in ASN.1

Status of This Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   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.

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 date and time in a
   manner that is also an integer.  Although some conversion may be
   necessary due to the selection of a 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
   from 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 to 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 which both memory and processor cycles can be
   saved.

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 in order to
   obtain 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 (0..MAX)
   The integer value is the number of seconds, excluding leap 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 millennia.

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

   When encoding an integer value that consists of more than one octet,
   which includes almost all 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 value.

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.

   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
   attribute:

      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:

      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 (CMS)", RFC
              3852, July 2004.

   [STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

4.2.  Informative References

   [TSP]      Adams, C., Cain, P., Pinkas, D., 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 when 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.

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].

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

   DEFINITIONS IMPLICIT TAGS ::=
   BEGIN


   -- BinaryTime Definition

   BinaryTime ::= INTEGER (0..MAX)

   -- 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

   END

Author's Address

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

   EMail: housley@vigilsec.com

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