BinaryTime: An Alternate Format for Representing Date and Time in ASN.1
A time format that is just an integer: seconds since 1970, plus a CMS signing-time attribute that uses it.
Housley, R., "BinaryTime: An Alternate Format for Representing Date and Time in ASN.1", RFC 6019, DOI 10.17487/RFC6019, September 2010, <https://www.rfc-editor.org/info/rfc6019>.
@misc{rfc6019,
series = {Request for Comments},
number = 6019,
howpublished = {RFC 6019},
publisher = {RFC Editor},
doi = {10.17487/RFC6019},
url = {https://www.rfc-editor.org/info/rfc6019},
author = {R. Housley},
title = {{BinaryTime: An Alternate Format for Representing Date and Time in ASN.1}},
pagetotal = 6,
year = 2010,
month = sep,
} RFC text
Contents
Internet Engineering Task Force (IETF) R. Housley
Request for Comments: 6019 Vigil Security
Obsoletes: 4049 September 2010
Category: Standards Track
ISSN: 2070-1721
BinaryTime:
An Alternate Format for Representing Date and Time in ASN.1
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 5652.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6019.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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] 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 in 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
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [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 representation of time is
sometimes called "UNIX time" [POSIX]. 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. 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.
5. References
5.1. Normative References
[ASN1] CCITT. Recommendation X.208: Specification of Abstract
Syntax Notation One (ASN.1). 1988.
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009.
[STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
5.2. Informative References
[POSIX] Institute of Electrical and Electronics Engineers. IEEE
P1003.1, Information Technology Portable Operating System
Interface (POSIX) Part 1: System Application Program
Interface (API) [C Language], 1990.
[TSP] Adams, C., Cain, P., Pinkas, D., and R. Zuccherato,
"Internet X.509 Public Key Infrastructure Time-Stamp
Protocol (TSP)", RFC 3161, August 2001.
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
AI ELI5
On this page
§1Why Another Time Format
RFC 6019 defines BinaryTime, a new ASN.1 type for date and time values. It also defines an alternative to the signing-time attribute that the Cryptographic Message Syntax (CMS) uses with its SignedData and AuthenticatedData content types. The older attribute is defined in RFC 5652.
The RFC gives three reasons to prefer an integer. A BinaryTime value is smaller than either a UTCTime or a GeneralizedTime value. Many operating systems already hold time as an integer, so conversion needs only simple arithmetic: addition or subtraction for a different epoch, multiplication or division for a different granularity, and some byte shuffling or padding for endianness and fixed lengths. And comparing two integers is easy, while comparing UTCTime or GeneralizedTime values given in different time zones can be complex.
The RFC calls this a rare case where both memory and processor cycles can be saved.
Computers already like to keep time as one counting number. Writing time that way means less to store and an easy way to tell which of two times came first.
§2The BinaryTime Type
BinaryTime represents an absolute time and date based on coordinated universal time (UTC). Its syntax is a single line of ASN.1: BinaryTime ::= INTEGER (0..MAX).
The integer is the number of seconds, excluding leap seconds, after midnight UTC, January 1, 1970. The RFC notes this is sometimes called UNIX time. The consequence is that the format cannot represent any time before January 1, 1970.
The upper end is not fixed. The latest time that fits in a four-octet integer is 03:14:07 on January 19, 2038, which is the hexadecimal value 7FFFFFFF. Later times simply use integers longer than four octets, and a five-octet integer is enough to cover the next seventeen millennia.
The clock starts counting seconds on a day long ago, January 1, 1970. If a counter runs out of room, you just give it more digits, so the clock never has to stop.
§2.1Variable-Length Integer Encoding
BinaryTime uses the variable-length encoding of INTEGER, which lets any time after midnight UTC, January 1, 1970 be represented. Implementations therefore cannot assume a fixed length for the value.
When an integer takes more than one octet, which covers almost every time value of interest, the bits of the first octet and bit 8 of the second octet MUST NOT all be ones or all be zeros. This rule guarantees that the value uses the smallest possible number of octets.
Always write the number in the shortest way you can, with no wasted leading digits. Because the length can change, a reader must not guess how long it is.
§3The binary-signing-time Attribute
The binary-signing-time attribute states the time at which the signer (purportedly) performed the signing process. It is meant for CMS SignedData, and may also be used with AuthenticatedData. It is identified by id-aa-binarySigningTime, which is arc 46 under smime(16) aa(2), and its values have type BinarySigningTime ::= BinaryTime.
Placement is strict. The attribute MUST be a signed or authenticated attribute, and MUST NOT be unsigned, unauthenticated or unprotected. It MUST have exactly one attribute value, even though CMS defines the syntax as a SET OF AttributeValue, so zero or multiple values are forbidden. SignedAttributes in a signerInfo and AuthAttributes in an AuthenticatedData MUST NOT include more than one instance of it.
The RFC imposes no requirement that the signing time be correct. Accepting a purported signing time is up to the recipient, although it is expected that some signers, such as time-stamp servers, will be trusted implicitly.
A signer can stamp a note saying when they signed, as one number. There must be exactly one such stamp, and it must be covered by the signature so nobody can swap it later.
§4Security Considerations
Using the attribute does not necessarily give confidence about when the signature was produced, so accepting a signing time is the recipient's choice. For a trustworthy time, the RFC points to the time-stamp protocol in RFC 3161, which obtains time stamps from a trusted entity.
The original signing-time attribute has the same semantics as binary-signing-time. For that reason only one of them SHOULD be present in the signedAttrs of a SignerInfo or the authAttrs of an AuthenticatedData. If both are present, they MUST give the same date and time.
Anyone can write any time on a note they sign, so the note alone is not proof. If you need proof, ask a trusted clock-keeper to stamp it. If a message carries two time notes, they must agree.
§6The ASN.1 Module
Appendix A gives the ASN.1 module needed to implement the specification, meant to be used together with the ASN.1 modules in CMS. It uses IMPLICIT TAGS and contains just three definitions: BinaryTime, the attribute identifier id-aa-binarySigningTime and BinarySigningTime.
The appendix is the formal recipe card that computers read, repeating the same three definitions given in the text.
BinaryTime ::= INTEGER (0..MAX)
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