iCalendar parameter escapes: Parameter Value Encoding in iCalendar and vCard
A way to put double quotes and line breaks inside iCalendar and vCard parameter values.
Daboo, C., "Parameter Value Encoding in iCalendar and vCard", RFC 6868, DOI 10.17487/RFC6868, February 2013, <https://www.rfc-editor.org/info/rfc6868>.
@misc{rfc6868,
series = {Request for Comments},
number = 6868,
howpublished = {RFC 6868},
publisher = {RFC Editor},
doi = {10.17487/RFC6868},
url = {https://www.rfc-editor.org/info/rfc6868},
author = {C. Daboo},
title = {{Parameter Value Encoding in iCalendar and vCard}},
pagetotal = 7,
year = 2013,
month = feb,
} RFC text
Contents
Internet Engineering Task Force (IETF) C. Daboo
Request for Comments: 6868 Apple
Updates: 5545, 6321, 6350, 6351 February 2013
Category: Standards Track
ISSN: 2070-1721
Parameter Value Encoding in iCalendar and vCard
Abstract
This specification updates the data formats for iCalendar (RFC 5545)
and vCard (RFC 6350) to allow parameter values to include certain
characters forbidden by the existing specifications.
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/rfc6868.
Copyright Notice
Copyright (c) 2013 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.
Table of Contents
1. Introduction ....................................................2
2. Conventions Used in This Document ...............................2
3. Parameter Value Encoding Scheme .................................3
3.1. iCalendar Example ..........................................4
3.2. vCard Example ..............................................4
4. Security Considerations .........................................4
5. Acknowledgments .................................................4
6. Normative References ............................................5
Appendix A. Choice of Quoting Mechanism ............................6
1. Introduction
The iCalendar [RFC5545] specification defines a standard way to
describe calendar data. The vCard [RFC6350] specification defines a
standard way to describe contact data. Both of these use a similar
text-based data format. Each iCalendar and vCard data object can
include "properties" that have "parameters" and a "value". The value
of a "parameter" is typically a token or URI value, but a "generic"
text value is also allowed. However, the syntax rules for both
iCalendar and vCard prevent the use of a double-quote character or
control characters in such values, though double-quote characters and
some subset of control characters are allowed in the actual property
values.
As more and more extensions are being developed for these data
formats, there is a need to allow at least double-quotes and line
feeds to be included in parameter values. The \-escaping mechanism
used for property text values is not defined for use with parameter
values and cannot be easily used in a backwards-compatible manner.
This specification defines a new character escaping mechanism,
compatible with existing parsers and chosen to minimize any impact on
existing data.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
3. Parameter Value Encoding Scheme
This specification defines the ^ character (U+005E -- Circumflex
Accent) as an escape character in parameter values whose value type
is defined using the "param-value" syntax element (Section 3.1 of
iCalendar [RFC5545] and Section 3.3 of vCard [RFC6350]). The
^-escaping mechanism can be used when the value is either unquoted or
quoted (i.e., whether or not the value is surrounded by double-
quotes).
When generating iCalendar or vCard parameter values, the following
apply:
o formatted text line breaks are encoded into ^n (U+005E, U+006E)
o the ^ character (U+005E) is encoded into ^^ (U+005E, U+005E)
o the " character (U+0022) is encoded into ^' (U+005E, U+0027)
When parsing iCalendar or vCard parameter values, the following
apply:
o the character sequence ^n (U+005E, U+006E) is decoded into an
appropriate formatted line break according to the type of system
being used
o the character sequence ^^ (U+005E, U+005E) is decoded into the ^
character (U+005E)
o the character sequence ^' (U+005E, U+0027) is decoded into the "
character (U+0022)
o if a ^ (U+005E) character is followed by any character other than
the ones above, parsers MUST leave both the ^ and the following
character in place
When converting between iCalendar and vCard text-based data formats
and alternative data-format representations such as XML (as described
in [RFC6321] and [RFC6351], respectively), implementations MUST
ensure that parameter value escape sequences are generated correctly
in the text-based format and are decoded when the parameter values
appear in the alternate data formats.
3.1. iCalendar Example
The following example is an "ATTENDEE" property with a "CN" parameter
whose value includes two double-quote characters. The parameter
value is not quoted, as there are no characters in the value that
would trigger quoting as required by iCalendar.
ATTENDEE;CN=George Herman ^'Babe^' Ruth:mailto:babe@example.com
The unescaped parameter value is
George Herman "Babe" Ruth
3.2. vCard Example
The following example is a "GEO" property with an "X-ADDRESS"
parameter whose value includes several line feed characters. The
parameter value is also quoted, since it contains a comma, which
triggers quoting as required by vCard.
GEO;X-ADDRESS="Pittsburgh Pirates^n115 Federal St^nPitt
sburgh, PA 15212":geo:40.446816,-80.00566
The unescaped parameter value (where each line is terminated by a
line break character sequence) is
Pittsburgh Pirates
115 Federal St
Pittsburgh, PA 15212
4. Security Considerations
There are no additional security issues beyond those of iCalendar
[RFC5545] and vCard [RFC6350].
5. Acknowledgments
Thanks to Michael Angstadt, Tim Bray, Mike Douglass, Barry Leiba,
Simon Perreault, and Pete Resnick for feedback on this specification.
6. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5545] Desruisseaux, B., "Internet Calendaring and Scheduling
Core Object Specification (iCalendar)", RFC 5545,
September 2009.
[RFC6321] Daboo, C., Douglass, M., and S. Lees, "xCal: The XML
Format for iCalendar", RFC 6321, August 2011.
[RFC6350] Perreault, S., "vCard Format Specification", RFC 6350,
August 2011.
[RFC6351] Perreault, S., "xCard: vCard XML Representation",
RFC 6351, August 2011.
Appendix A. Choice of Quoting Mechanism
Having recognized the need for escaping parameter values, the
question is what mechanism to use? One obvious choice would be to
adopt the \-escaping used for property values. However, that could
not be used as-is, because it escapes a double-quote as the sequence
of \ followed by double-quote. Consider what the example in
Section 3.1 might look like using \-escaping:
ATTENDEE;CN="George Herman \"Babe\" Ruth":mailto:babe@example.com
Existing iCalendar/vCard parsers know nothing about escape sequences
in parameters. So they would parse the parameter value as:
George Herman \
i.e., the text between the first and second occurrence of a double-
quote. However, the text after the second double-quote ought to be
either a : or a ; (to delimit the parameter value from the following
parameter or property) but is not, so the parser could legitimately
throw an error at that point because the data is syntactically
invalid. Thus, for backwards-compatibility reasons, a double-quote
cannot be escaped using a sequence that itself includes a double-
quote, and hence the choice of using a single-quote in this
specification.
Another option would be to use a form of \-escaping modified for use
in parameter values only. However, some incorrect, non-interoperable
use of \ in parameter values has been observed, and thus it is best
to steer clear of that to achieve guaranteed, reliable
interoperability. Also, given that double-quote gets changed to
single-quote in the escape sequence for a parameter, but not for a
value, it is better to not give the impression that the same escape
mechanism (and thus code) can be used for both (which could lead to
other issues, such as an implementation incorrectly escaping a ; as
\; as opposed to quoting the parameter value).
The choice of ^ as the escape character was made based on the
requirement that an ASCII symbol (non-alphanumeric character) be
used, and it ought to be one least likely to be found in existing
data.
Author's Address
Cyrus Daboo
Apple Inc.
1 Infinite Loop
Cupertino, CA 95014
USA
EMail: cyrus@daboo.name
URI: http://www.apple.com/
ISO Plain Language
On this page
§1Introduction
The iCalendar specification of RFC 5545 defines a standard way to describe calendar data, and the vCard specification of RFC 6350 defines a standard way to describe contact data. Both use a similar text-based data format. Each iCalendar and vCard data object can include properties that have parameters and a value. The value of a parameter is typically a token or URI value, but a generic text value is also allowed.
The syntax rules for both formats prevent the use of a double-quote character or control characters in such values, though double-quote characters and some subset of control characters are allowed in the actual property values.
As more and more extensions are developed for these data formats, there is a need to allow at least double quotes and line feeds in parameter values. The backslash-escaping mechanism used for property text values is not defined for use with parameter values and cannot be easily used in a backward-compatible manner. This specification defines a new character escaping mechanism, compatible with existing parsers and chosen to minimize any impact on existing data.
A property line looks like a name, then some extra words in brackets that qualify it, then the value itself. The extra words could not contain a quote or a line break. Extensions need to put those in, so a way of writing them is needed.
§3The Parameter Value Encoding Scheme
The specification defines the caret character, U+005E, as an escape character in parameter values whose value type is defined using the param-value syntax element, which is Section 3.1 of RFC 5545 and Section 3.3 of RFC 6350. The escaping mechanism can be used whether the value is unquoted or quoted, that is, whether or not the value is surrounded by double quotes.
When generating parameter values, a formatted text line break is encoded into the two characters ^n, the caret and the letter n. The caret character itself is encoded into ^^. The double-quote character is encoded into ^', a caret followed by a single quote.
When parsing parameter values, the sequence ^n is decoded into an appropriate formatted line break according to the type of system being used. The sequence ^^ is decoded into the caret character. The sequence ^' is decoded into the double-quote character. If a caret is followed by any character other than those, parsers must leave both the caret and the following character in place.
When converting between the iCalendar and vCard text-based data formats and alternative representations such as XML, as described in RFC 6321 and RFC 6351, implementations must ensure that parameter value escape sequences are generated correctly in the text-based format and are decoded when the parameter values appear in the alternate formats.
Three sequences to learn: a caret and n for a line break, two carets for a literal caret, and a caret and an apostrophe for a double quote. A caret followed by anything else is left exactly as it is, so a stray caret does not corrupt the value.
- ^nLine break — Generated for a formatted text line break. Decoded into an appropriate formatted line break according to the type of system being used.
- ^^Circumflex accent — Generated for the caret character itself. Decoded into the caret character.
- ^'Double quote — Generated for the double-quote character. Decoded into the double-quote character.
- ^xAnything else — If a caret is followed by any other character, parsers must leave both the caret and the following character in place.
§3.1Examples in iCalendar and vCard
The first example is an ATTENDEE property with a CN parameter whose value includes two double-quote characters. The parameter value is not quoted, because there are no characters in the value that would trigger quoting as required by iCalendar:
The unescaped parameter value is the text George Herman "Babe" Ruth.
The second example is a GEO property with an X-ADDRESS parameter whose value includes several line feed characters. The parameter value is quoted, because it contains a comma, which triggers quoting as required by vCard:
The unescaped parameter value, where each line is terminated by a line break character sequence, is the three lines Pittsburgh Pirates, 115 Federal St, and Pittsburgh, PA 15212.
The first example puts a nickname in double quotes inside the display name of an attendee. The second example puts a three-line street address inside a quoted parameter, using the escape sequence for the line breaks.
ATTENDEE;CN=George Herman ^'Babe^' Ruth:mailto:babe@example.comGEO;X-ADDRESS="Pittsburgh Pirates^n115 Federal St^nPitt
sburgh, PA 15212":geo:40.446816,-80.00566§4Security Considerations
There are no additional security issues beyond those of iCalendar in RFC 5545 and vCard in RFC 6350.
Nothing new. The security rules of the two formats it changes apply unchanged.
§5Choice of the Escaping Mechanism
The appendix of the RFC explains why the caret was chosen, and why the backslash was not.
Adopting the backslash escaping used for property values was the obvious first choice, but it could not be used as it stands, because it escapes a double quote as a backslash followed by a double quote. The Section 3.1 example would then look like this. Existing iCalendar and vCard parsers know nothing about escape sequences in parameters, so they would parse the parameter value as the text between the first and second occurrence of a double quote, that is, the text up to the backslash. The text after the second double quote ought to be a colon or a semicolon, to delimit the parameter value from the following parameter or property, but it is not, so the parser could legitimately throw an error at that point because the data is syntactically invalid. For backward-compatibility reasons a double quote cannot be escaped using a sequence that itself includes a double quote, and hence the choice of using a single quote in this specification.
Another option would have been a form of backslash escaping modified for use in parameter values only. However, some incorrect, non-interoperable use of the backslash in parameter values has been observed, so it is best to steer clear of it to achieve guaranteed, reliable interoperability. Also, because the double quote changes to a single quote in the escape sequence for a parameter but not for a value, it is better not to give the impression that the same escape mechanism, and thus the same code, can be used for both. That impression could lead to other issues, such as an implementation incorrectly escaping a semicolon as a backslash-semicolon instead of quoting the parameter value.
The choice of the caret as the escape character was made on the requirement that an ASCII symbol, a non-alphanumeric character, be used, and that it be one least likely to be found in existing data.
The backslash was rejected for a concrete reason: the old escape for a quote would itself contain a quote, and a parser that does not know about escapes would then stop at the wrong place and reject the file. The caret was picked because it is unlikely to appear in real data already.
| Option | Why it was not chosen |
|---|---|
| Backslash escaping as used for property values | It escapes a double quote as a backslash followed by a double quote. A parser that knows nothing about escapes would stop at the wrong double quote and then hit a syntax error, because the text after it is not a colon or a semicolon. |
| A modified backslash escaping for parameters only | Incorrect, non-interoperable use of the backslash in parameter values has been observed, and a separate mechanism risks the impression that one piece of code can escape both parameters and values. |
| The caret THIS RFC | An ASCII symbol that is least likely to be found in existing data. |