Network Working Group R. Housley
Internet Draft Vigil Security
expires in six months August 2004
Binary Signing Time:
A Signed Attribute for use with the Cryptographic Message Syntax (CMS)
<draft-housley-binarytime-00.txt>
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Abstract
This document specifies a new ASN.1 type for representing time:
BinaryTime. This document also specifies the binary-signing-time
attribute for use with the Cryptographic Message Syntax (CMS)
SignedData content type. CMS is defined in RFC 3852.
1 Introduction
This document specifies a new ASN.1 [ASN1] type for representing
time: BinaryTime. This document also specifies the binary-signing-
time attribute for use with the Cryptographic Message Syntax (CMS)
[CMS] SignedData content type that makes use of the BinaryTime type.
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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 compatible with these operating systems. This
approach 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 many operating systems, the value can be used without
conversion.
This is a rare instance where both memory and processor cycles are
saved.
1.2 Binary Signing Time Attribute
The signing-time attribute is defined in [CMS]. The binary-signing-
time attribute is defined in this document 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
The integer value is the number of seconds after midnight, 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.
This specification uses a variable length encoding of INTEGER. This
permits any time value after midnight, January 1, 1970 to be
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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. 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
By using the BinaryTime type, BinarySigningTime values MUST be
expressed in Coordinated Universal Time (formerly known as Greenwich
Mean Time (GMT) and Zulu clock time), and the granularity of the time
is seconds. It is not possible to represent a finer granularity.
In [CMS], the SignedAttributes syntax and the AuthAttributes syntax
are each defined as a SET OF Attributes. However, a 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
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AuthenticatedData MUST NOT include multiple instances of the binary-
signing-time attribute.
No requirement is imposed concerning the correctness of the signing
time, 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.
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
This specification does not introduce any new security considerations
beyond those already discussed in [CMS].
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.
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,
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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
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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
-- 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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