Network Working Group R. Housley
Internet Draft Vigil Security
expires in six months September 2004
BinaryTime:
An alternate format for representing date and time in ASN.1
<draft-housley-binarytime-02.txt>
Status of this Memo
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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.
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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
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 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
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 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
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.
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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
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. 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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8 Author's Address
Russell Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
USA
housley@vigilsec.com
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
-- 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
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