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.
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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)
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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
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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.
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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.
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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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RFC 4049 BinaryTime April 2005
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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