Internet DRAFT - draft-rivest-sexp
draft-rivest-sexp
Network Working Group R. Rivest
Internet-Draft MIT CSAIL
Intended status: Informational D. Eastlake
Expires: 14 August 2024 Futurewei Technologies
11 February 2024
S-Expressions
draft-rivest-sexp-04
Abstract
This memo describes a data structure called "S-expressions" that are
suitable for representing arbitrary, complex data structures. We
make precise the encodings of S-expressions: we give a "canonical
form" for S-expressions, described two "transport" representations,
and also describe an "advanced" format for display to people.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 14 August 2024.
Copyright Notice
Copyright (c) 2024 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 (https://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 Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Rivest & Eastlake Expires 14 August 2024 [Page 1]
Internet-Draft S-Expressions February 2024
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Historical Notes . . . . . . . . . . . . . . . . . . . . 4
1.2. Uses of S-Expressions . . . . . . . . . . . . . . . . . . 4
1.3. Conventions Used in This Document . . . . . . . . . . . . 5
2. S-expressions -- informal introduction . . . . . . . . . . . 5
3. Character set . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Octet string representations . . . . . . . . . . . . . . . . 7
4.1. Verbatim representation . . . . . . . . . . . . . . . . . 7
4.2. Quoted-string representation . . . . . . . . . . . . . . 8
4.3. Token representation . . . . . . . . . . . . . . . . . . 9
4.4. Hexadecimal representation . . . . . . . . . . . . . . . 10
4.5. Base-64 representation . . . . . . . . . . . . . . . . . 10
4.6. Display hint . . . . . . . . . . . . . . . . . . . . . . 11
4.7. Equality of octet-strings . . . . . . . . . . . . . . . . 12
5. Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6. Representation types . . . . . . . . . . . . . . . . . . . . 13
6.1. Canonical representation . . . . . . . . . . . . . . . . 13
6.2. Basic transport representation . . . . . . . . . . . . . 13
6.3. Advanced transport representation . . . . . . . . . . . . 14
7. ABNF for syntax . . . . . . . . . . . . . . . . . . . . . . . 14
8. In-memory representations . . . . . . . . . . . . . . . . . . 16
8.1. List-structure memory representation . . . . . . . . . . 16
8.2. Array-layout memory representation . . . . . . . . . . . 16
8.2.1. Octet string . . . . . . . . . . . . . . . . . . . . 17
8.2.2. Octet-string with display-hint . . . . . . . . . . . 17
8.2.3. List . . . . . . . . . . . . . . . . . . . . . . . . 18
9. Implementations . . . . . . . . . . . . . . . . . . . . . . . 18
10. Restricted S-expressions . . . . . . . . . . . . . . . . . . 18
11. Security Considerations . . . . . . . . . . . . . . . . . . . 19
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
13. Normative References . . . . . . . . . . . . . . . . . . . . 19
14. Informative References . . . . . . . . . . . . . . . . . . . 19
Appendix A. Change History . . . . . . . . . . . . . . . . . . . 21
A.1. -00 Changes . . . . . . . . . . . . . . . . . . . . . . . 21
A.2. Changes from -00 to -01 . . . . . . . . . . . . . . . . . 22
A.3. Changes from -01 to -02 . . . . . . . . . . . . . . . . . 22
A.4. Changes from -02 to -03 . . . . . . . . . . . . . . . . . 22
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 23
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
S-expressions are data structures for representing complex data.
They are either byte-strings ("octet-strings") or lists of simpler
S-expressions. Here is a sample S-expression:
Rivest & Eastlake Expires 14 August 2024 [Page 2]
Internet-Draft S-Expressions February 2024
(snicker "abc" (#03# |YWJj|))
It is a list of length three containing the following:
* the octet-string "snicker"
* the octet-string "abc"
* a sub-list containing two elements: the hexadecimal constant #03#
and the base-64 constant |YWJj| (which is the same as "abc")
This document gives a specific proposal for constructing and
utilizing S-expressions. The proposal is independent of any
particular application.
Here are the design goals for S-expressions:
generality: S-expressions should be good at representing arbitrary
data.
readability: It should be easy for someone to examine and understand
the structure of an S-expression.
economy: S-expressions should represent data compactly.
tranportability: S-expressions should be easy to transport over
communication media (such as email) that are known to be less than
perfect.
flexibility: S-expressions should make it relatively simple to
modify and extend data structures.
canonicalization: It should be easy to produce a unique "canonical"
form of an S-expression, for digital signature purposes.
efficiency: S-expressions should admit in-memory representations
that allow efficient processing.
Section 1.1 below has notes of this history of this document.
Section 1.2 describes some current uses.
Subsequent Sections of this document cover material as follows:
* Section 2 gives an introduction to S-expressions.
* Section 3 discusses the character sets used.
* Section 4 presents the various representations of octet-strings.
* Section 5 describes how to represent lists.
Rivest & Eastlake Expires 14 August 2024 [Page 3]
Internet-Draft S-Expressions February 2024
* Section 6 discusses how S-expressions are represented for various
uses.
* Section 7 gives an ABNF syntax for S-expressions.
* Section 8 talks about how S-expressions might be represented in
memory.
* Section 9 briefly describes implementations for handling
S-expressions.
* Section 10 discusses how applications might utilize restricted
S-expressions.
* Section 11 briefly describes Security Considerations.
1.1. Historical Notes
The S-expression technology described here was originally developed
for "SDSI" (the Simple Distributed Security Infrastructure by Lampson
and Rivest [SDSI]) in 1996, although the origins clearly date back to
McCarthy's [LISP] programming language. It was further refined and
improved during the merger of SDSI and SPKI [SPKI] [RFC2692]
[RFC2693] during the first half of 1997. S-expressions are similar
to, but more readable and flexible than, Bernstein's "net-strings"
[BERN].
Although made publicly available as a file named draft-rivest-sexp-
00.txt on 4 May 1997, that file was never actually submitted to the
IETF. Version -00 of this document was a modernized version of that
file. Version -01 had some further polishing and corrections. The
further significant changes made in this document were changing the
original BNF notation to ABNF [RFC5234] (see Section 7) and changing
the default character set in Section 4.6 to UTF-8 [RFC4648].
1.2. Uses of S-Expressions
S-expressions are in active use today between GnuPG [GnuPG] and
Ribose's RNP [Ribose]. Ribose recently implemented C++ software to
compose and parse S-expressions [RNPGP_SEXPP] and, in fact,
references this document. The GNU software is here [Libgcrypt].
S-expressions are used/referenced in the following RFCs:
* [RFC9208] IMAP QUOTA Extension
* [RFC3275] XML-Signature Syntax and Processing
* [RFC2693] for [SPKI]
* [RFC1203] IMAP v3
* [RFC1176] IMAP v2
Rivest & Eastlake Expires 14 August 2024 [Page 4]
Internet-Draft S-Expressions February 2024
In addition, S-Expressions are the inspiration for the encodings in
other protocols. For example, Section 6 of [CDDLfreezer] or
[RFC3259].
1.3. 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 BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. S-expressions -- informal introduction
Informally, an S-expression is either:
* an octet-string, or
* a finite list of simpler S-expressions.
An octet-string is a finite sequence of eight-bit octets. There may
be many different but equivalent ways of representing an octet-string
abc -- as a token
"abc" -- as a quoted string
#616263# -- as a hexadecimal string
3:abc -- as a length-prefixed "verbatim" encoding
{MzphYmM=} -- as a base-64 encoding of the verbatim
encoding (that is, an encoding of "3:abc")
|YWJj| -- as a base-64 encoding of the octet-string
"abc"
The above encodings are all equivalent; they all denote the same
octet string.
We will give details of these encodings later on, and also describe
how to give a "display type" to a simple string.
A list is a finite sequence of zero or more simpler S-expressions. A
list is represented by using parentheses to surround the sequence of
encodings of its elements, as in:
(abc (de #6667#) "ghi jkl")
As we see, there is variability possible in the encoding of an
S-expression. In some cases, it is desirable to standardize or
restrict the encodings; in other cases, it is desirable to have no
restrictions. The following are the target cases we aim to handle:
Rivest & Eastlake Expires 14 August 2024 [Page 5]
Internet-Draft S-Expressions February 2024
* a "transport" or "basic" encoding for transporting the
S-expression between computers.
* a "canonical" encoding, used when signing the S-expression.
* an "advanced" encoding used for input/output to people.
* an "in-memory" encoding used for processing the S-expression in
the computer.
These need not be different; in this proposal the canonical encoding
is the same as the transport encoding, for example. In this document
we propose (related) encoding techniques for each of these uses.
3. Character set
We will be describing encodings of S-expressions. Except when giving
"verbatim" encodings, the character set used is limited to the
following characters in US-ASCII:
Alphabetic:
A B ... Z a b ... z
Numeric:
0 1 ... 9
Whitespace:
space, horizontal tab, vertical tab, form-feed
carriage-return, line-feed
The following graphics characters, which we call "pseudo-
alphabetic":
- hyphen or minus
. period
/ slash
_ underscore
: colon
* asterisk
+ plus
= equal
Rivest & Eastlake Expires 14 August 2024 [Page 6]
Internet-Draft S-Expressions February 2024
The following graphics characters, which are "reserved
punctuation":
( left parenthesis
) right parenthesis
[ left bracket
] right bracket
{ left brace
} right brace
| vertical bar
# number sign
" double quote
& ampersand
\ backslash
The following characters are unused and unavailable, except in
"verbatim" and "quoted string" encodings:
! exclamation point
% percent
^ circumflex
~ tilde
; semicolon
' apostrophe
, comma
< less than
> greater than
? question mark
4. Octet string representations
This section describes in detail the ways in which an octet-string
may be represented.
We recall that an octet-string is any finite sequence of octets, and
that the octet-string may have length zero.
4.1. Verbatim representation
A verbatim encoding of an octet string consists of three parts:
* the length (number of octets) of the octet-string, given in
decimal, most significant digit first, with no leading zeros.
* a colon ":"
* the octet string itself, verbatim.
There are no blanks or whitespace separating the parts. No "escape
sequences" are interpreted in the octet string. This encoding is
also called a "binary" or "raw" encoding.
Rivest & Eastlake Expires 14 August 2024 [Page 7]
Internet-Draft S-Expressions February 2024
Here are some sample verbatim encodings:
3:abc
7:subject
4:::::
12:hello world!
10:abcdefghij
0:
4.2. Quoted-string representation
The quoted-string representation of an octet-string consists of:
* an optional decimal length field
* an initial double-quote (")
* the octet string with "C" [C] escape conventions (\n, etc)
* a final double-quote (")
The specified length is the length of the resulting string after any
escape sequences have been handled. The string does not have any
"terminating NULL" that [C] includes, and the length does not count
such a character.
The length is optional.
The escape conventions within the quoted string are as follows (these
follow the "C" [C] programming language conventions, with an
extension for ignoring line terminators of just LF, CRLF, or LFCR and
more restrictive octal and hexadecimal value formats):
Rivest & Eastlake Expires 14 August 2024 [Page 8]
Internet-Draft S-Expressions February 2024
\a -- audible alert (bell)
\b -- backspace
\t -- horizontal tab
\v -- vertical tab
\n -- new-line
\f -- form-feed
\r -- carriage-return
\" -- double-quote
\' -- single-quote
\? -- question mark
\\ -- back-slash
\ooo -- character with octal value ooo (all three
digits MUST be present)
\xhh -- character with hexadecimal value hh (both
digits MUST be present)
\<carriage-return> -- causes carriage-return
to be ignored.
\<line-feed> -- causes linefeed to be
ignored.
\<carriage-return><line-feed> -- causes
CRLF to be ignored.
\<line-feed><carriage-return> -- causes
LFCR to be ignored.
Here are some examples of quoted-string encodings:
"subject"
"hi there"
7"subject"
3"\n\n\n"
"This has\n two lines."
"This has \
one."
""
4.3. Token representation
An octet string that meets the following conditions may be given
directly as a "token".
* it does not begin with a digit
* it contains only characters that are: alphabetic (upper or lower
case); numeric; or one of the eight "pseudo-alphabetic"
punctuation marks:
- . / _ : * + =
Rivest & Eastlake Expires 14 August 2024 [Page 9]
Internet-Draft S-Expressions February 2024
(Note: upper and lower case are not equivalent.)
(Note: A token may begin with punctuation, including ":").
Here are some examples of token representations:
subject
not-before
class-of-1997
//microsoft.com/names/smith
*
4.4. Hexadecimal representation
An octet-string may be represented with a hexadecimal encoding
consisting of:
* an (optional) decimal length of the octet string
* a sharp-sign "#"
* a hexadecimal encoding of the octet string, with each octet
represented with two hexadecimal digits, most significant digit
first. There MUST be an even number of such digits.
* a sharp-sign "#"
There may be whitespace inserted in the midst of the hexadecimal
encoding arbitrarily; it is ignored. It is an error to have
characters other than whitespace and hexadecimal digits.
Here are some examples of hexadecimal encodings:
#616263# -- represents "abc"
3#616263# -- also represents "abc"
# 616
263 # -- also represents "abc"
4.5. Base-64 representation
An octet-string may be represented in a base-64 coding [RFC4648]
consisting of:
* an (optional) decimal length of the octet string
* a vertical bar "|"
* the base-64 [RFC4648] encoding of the octet string.
Rivest & Eastlake Expires 14 August 2024 [Page 10]
Internet-Draft S-Expressions February 2024
* a final vertical bar "|"
The base-64 encoding uses only the characters
A-Z a-z 0-9 + / =
It produces four characters of output for each three octets of input.
If the input has one or two left-over octets of input, it produces an
output block of length four ending in two or one equals signs,
respectively. Output routines compliant with this standard MUST
output the equals signs as specified. Input routines MAY accept
inputs where the equals signs are dropped.
There may be whitespace inserted in the midst of the base-64 encoding
arbitrarily; it is ignored. It is an error to have characters other
than whitespace and base-64 characters.
Here are some examples of base-64 encodings:
|YWJj| -- represents "abc"
| Y W
J j | -- also represents "abc"
3|YWJj| -- also represents "abc"
|YWJjZA==| -- represents "abcd"
|YWJjZA| -- also represents "abcd"
4.6. Display hint
Any octet string may be preceded by a single "display hint".
The purposes of the display hint is to provide information on how to
display the octet string to a user. It has no other function. Many
of the MIME [RFC2046] types work here.
A display-hint is an octet string surrounded by square brackets.
There may be whitespace separating the octet string from the
surrounding brackets. Any of the legal formats may be used for the
octet string.
Here are some examples of display-hints:
Rivest & Eastlake Expires 14 August 2024 [Page 11]
Internet-Draft S-Expressions February 2024
[image/gif]
[URI]
[charset=unicode-1-1]
[text/richtext]
["text/plain; charset=iso-8859-1"]
[application/postscript]
[audio/basic]
["http://abc.com/display-types/funky.html"]
In applications an octet-string that is untyped may be considered to
have a pre-specified "default" MIME [RFC2046] type. The MIME type
"text/plain; charset=utf-8"
is the standard default.
4.7. Equality of octet-strings
Two octet strings are considered to be "equal" if and only if they
have the same display hint and the same data octet strings.
Note that octet-strings are "case-sensitive"; the octet-string "abc"
is not equal to the octet-string "ABC".
An untyped octet-string can be compared to another octet-string
(typed or not) by considering it as a typed octet-string with the
default mime-type specified in Section 4.6 .
5. Lists
Just as with octet-strings, there are several ways to represent a
list. Whitespace may be used to separate list elements, but they are
only required to separate two octet strings when otherwise the two
octet strings might be interpreted as one, as when one token follows
another. Also, whitespace may follow the initial left parenthesis,
or precede the final right parenthesis.
Here are some examples of encodings of lists:
(a b c)
( a ( b c ) ( ( d e ) ( e f ) ) )
(11:certificate(6:issuer3:bob)(7:subject5:alice))
({ODpFeGFtcGxlIQ==} "1997" murphy 3:XC+)
Rivest & Eastlake Expires 14 August 2024 [Page 12]
Internet-Draft S-Expressions February 2024
6. Representation types
There are three "types" of representations:
* canonical
* basic transport
* advanced transport
The first two MUST be supported by any implementation; the last is
OPTIONAL.
6.1. Canonical representation
This canonical representation is used for digital signature purposes,
transmission, etc. It is uniquely defined for each S-expression. It
is not particularly readable, but that is not the point. It is
intended to be very easy to parse, to be reasonably economical, and
to be unique for any S-expression.
The "canonical" form of an S-expression represents each octet-string
in verbatim mode, and represents each list with no blanks separating
elements from each other or from the surrounding parentheses.
Here are some examples of canonical representations of S-expressions:
(6:issuer3:bob)
(4:icon[12:image/bitmap]9:xxxxxxxxx)
(7:subject(3:ref5:alice6:mother))
6.2. Basic transport representation
There are two forms of the "basic transport" representation:
* the canonical representation
* an [RFC4648] base-64 representation of the canonical
representation, surrounded by braces.
The transport mechanism is intended to provide a universal means of
representing S-expressions for transport from one machine to another.
Here are some examples of an S-expression represented in basic
transport mode:
Rivest & Eastlake Expires 14 August 2024 [Page 13]
Internet-Draft S-Expressions February 2024
(1:a1:b1:c)
{KDE6YTE6YjE6YykK}
The second example above is the same S-expression as the first
encoded in base-64.
There is a difference between the brace notation for base-64 used
here and the || notation for base-64'd octet-strings described above.
Here the base-64 contents are converted to octets, and then re-
scanned as if they were given originally as octets. With the ||
notation, the contents are just turned into an octet-string.
6.3. Advanced transport representation
The "advanced transport" representation is intended to provide more
flexible and readable notations for documentation, design, debugging,
and (in some cases) user interface.
The advanced transport representation allows all of the
representation forms described above in Section 4, include quoted
strings, base-64 and hexadecimal representation of strings, tokens,
representations of strings with omitted lengths, and so on.
7. ABNF for syntax
ABNF is the Augmented Backus-Naur Form for syntax specifications as
defined in [RFC5234]. We give separate ABNF's for canonical, basic,
and advanced forms of S-expressions. The rules below in all caps are
defined in Appendix A of [RFC5234].
For canonical transport:
sexp = raw / ("(" *sexp ")")
raw = decimal ":" *OCTET
; the length followed by a colon and the exact
; number of OCTET indicated by the length
decimal = %x30 / (%x31-39 *DIGIT)
For basic transport:
Rivest & Eastlake Expires 14 August 2024 [Page 14]
Internet-Draft S-Expressions February 2024
sexp = canonical / base-64-raw
canonical = raw / ("(" *canonical ")")
raw = decimal ":" *OCTET
; the length followed by a colon and the exact
; number of OCTET indicated by the length
decimal = %x30 / (%x31-39 *DIGIT)
base-64-raw = "{" 3*base-64-char "}"
base-64-char = ALPHA / DIGIT / "+" / "/" / "="
For advanced transport:
sexp = *whitespace value *whitespace
whitespace = SP / HTAB / vtab / CR / LF / ff
vtab = %x0B ; vertical tab
ff = %x0C ; form feed
value = string / ("(" *(value / whitespace) ")")
string = [display] *whitespace simple-string
display = "[" *whitespace simple-string *whitespace "]"
simple-string = raw / token / base-64 / base-64-raw /
hexadecimal / quoted-string
raw = decimal ":" *OCTET
; the length followed by a colon and the exact
; number of OCTET indicated by the length
decimal = %x30 / (%x31-39 *DIGIT)
token = (ALPHA / simple-punc) *(ALPHA / DIGIT /
simple-punc)
simple-punc = "-" / "." / "/" / "_" / ":" / "*" / "+" / "="
base-64 = [decimal] "|" *(base-64-char / whitespace) "|"
base-64-char = ALPHA / DIGIT / "+" / "/" / "="
Rivest & Eastlake Expires 14 August 2024 [Page 15]
Internet-Draft S-Expressions February 2024
base-64-raw = [decimal] "{" 1*(base-64-char / whitespace) "}"
; at least 3 base-64-char
hexadecimal = [decimal] "#" *(HEXDIG / whitespace) "#"
; even number of hexadecimal digits
quoted-string = [decimal] DQUOTE *(printable / escaped) DQUOTE
escaped = backslash (%x3F / %x61 / %x62 / %x66 / %x6E /
%x72 / %x74 / %x76 / DQUOTE / quote / backslash /
3(%x30-37) / (%x78 2HEXDIG) / CR / LF /
(CR LF) / (LF CR))
backslash = %x5C
printable = %x21-26 / %x28-7E
quote = %x27 ; single quote
8. In-memory representations
For processing, the S-expression would typically be parsed and
represented in memory in a way that is more amenable to efficient
processing. We suggest two alternatives:
* "list-structure"
* "array-layout"
We only sketch these here, as they are only suggestive. The
[SexpCode] code illustrates these styles in more detail.
8.1. List-structure memory representation
Here there are separate records for simple-strings, strings, and
lists. An S-expression of the form ("abc" "de") would require two
records for the simple strings, two for the strings, and two for the
list elements. This is a fairly conventional representation, and
details are omitted here.
8.2. Array-layout memory representation
Here each S-expression is represented as a contiguous array of bytes.
The first byte codes the "type" of the S-expression:
Rivest & Eastlake Expires 14 August 2024 [Page 16]
Internet-Draft S-Expressions February 2024
01 octet-string
02 octet-string with display-hint
03 beginning of list (and 00 is used for "end of list")
Each of the three types is immediately followed by a k-byte integer
indicating the size (in bytes) of the following representation. Here
k is an integer that depends on the implementation, it might be
anywhere from 2 to 8, but would be fixed for a given implementation;
it determines the size of the objects that can be handled. The
transport and canonical representations are independent of the choice
of k made by the implementation.
Although the lengths of lists are not given in the usual S-expression
notations, it is easy to fill them in when parsing; when you reach a
right-parenthesis you know how long the list representation was, and
where to go back to fill in the missing length.
8.2.1. Octet string
This is represented as follows:
01 <length> <octet-string>
For example (here k = 2)
01 0003 a b c
8.2.2. Octet-string with display-hint
This is represented as follows:
02 <length>
01 <length> <octet-string> /* for display-type */
01 <length> <octet-string> /* for octet-string */
For example, the S-expression
[gif] #61626364#
would be represented as (with k = 2)
02 000d
01 0003 g i f
01 0004 61 62 63 64
Rivest & Eastlake Expires 14 August 2024 [Page 17]
Internet-Draft S-Expressions February 2024
8.2.3. List
This is represented as
03 <length> <item1> <item2> <item3> ... <itemn> 00
For example, the list (abc [d]ef (g)) is represented in memory as
(with k=2)
03 001b
01 0003 a b c
02 0009
01 0001 d
01 0002 e f
03 0005
01 0001 g
00
00
9. Implementations
At this time there is code available that is intended to read and
parse some or all of the various S-expression formats specified here.
In particular, see the following likely incomplete list:
* Project GNU's [Libgcrypt].
* Ribose's RNP [RNPGP_SEXPP].
* Github project of J. P. Malkiewicz [SexpCode].
10. Restricted S-expressions
This document has described S-expressions in general form.
Application writers may wish to restrict their use of S-expressions
in various ways. Here are some possible restrictions that might be
considered:
* no display-hints
* no lengths on hexadecimal, quoted-strings, or base-64 encodings
* no empty lists
* no empty octet-strings
* no lists having another list as its first element
Rivest & Eastlake Expires 14 August 2024 [Page 18]
Internet-Draft S-Expressions February 2024
* no base-64 or hexadecimal encodings
* fixed limits on the size of octet-strings
11. Security Considerations
As a pure data representation format, there are few security
considerations to S-expressions. A canonical form is required for
the reliable verification of digital signatures. This is provided in
Section 6.1.
12. IANA Considerations
This document requires no IANA actions.
13. Normative References
[C] Kernighan, B. and D. Ritchie, "The C Programming
Language", ISBN 0-13-110370-9, 1988.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
14. Informative References
[BERN] Bernstein, D., "Netstrings", Work in progress, 1 February
1997, <https://www.ietf.org/archive/id/draft-bernstein-
netstrings-02.txt>.
Rivest & Eastlake Expires 14 August 2024 [Page 19]
Internet-Draft S-Expressions February 2024
[CDDLfreezer]
Borman, C., "A feature freezer for the Concise Data
Definition Language (CDDL)", work in progress, 12
September 2023, <https://datatracker.ietf.org/doc/draft-
bormann-cbor-cddl-freezer/>.
[GnuPG] Free Software Foundation, Inc., "The GNU Privacy Guard",
<https://www.gnupg.org/>.
[Libgcrypt]
GnuPG, "The Libgcrypt Library", Libgcrypt version 1.10.2,
6 April 2023,
<https://www.gnupg.org/documentation/manuals/gcrypt/>.
[LISP] Levin, M. and J. McCarthy, "LISP 1.5 Programmer's Manual",
ISBN-13 978-0-262-12011-0, ISBN-10 0262130114, 15 August
1962.
[RFC1176] Crispin, M., "Interactive Mail Access Protocol: Version
2", RFC 1176, DOI 10.17487/RFC1176, August 1990,
<https://www.rfc-editor.org/info/rfc1176>.
[RFC1203] Rice, J., "Interactive Mail Access Protocol: Version 3",
RFC 1203, DOI 10.17487/RFC1203, February 1991,
<https://www.rfc-editor.org/info/rfc1203>.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
DOI 10.17487/RFC2046, November 1996,
<https://www.rfc-editor.org/info/rfc2046>.
[RFC2692] Ellison, C., "SPKI Requirements", RFC 2692,
DOI 10.17487/RFC2692, September 1999,
<https://www.rfc-editor.org/info/rfc2692>.
[RFC2693] Ellison, C., Frantz, B., Lampson, B., Rivest, R., Thomas,
B., and T. Ylonen, "SPKI Certificate Theory", RFC 2693,
DOI 10.17487/RFC2693, September 1999,
<https://www.rfc-editor.org/info/rfc2693>.
[RFC3259] Ott, J., Perkins, C., and D. Kutscher, "A Message Bus for
Local Coordination", RFC 3259, DOI 10.17487/RFC3259, April
2002, <https://www.rfc-editor.org/info/rfc3259>.
[RFC3275] Eastlake 3rd, D., Reagle, J., and D. Solo, "(Extensible
Markup Language) XML-Signature Syntax and Processing",
RFC 3275, DOI 10.17487/RFC3275, March 2002,
<https://www.rfc-editor.org/info/rfc3275>.
Rivest & Eastlake Expires 14 August 2024 [Page 20]
Internet-Draft S-Expressions February 2024
[RFC9208] Melnikov, A., "IMAP QUOTA Extension", RFC 9208,
DOI 10.17487/RFC9208, March 2022,
<https://www.rfc-editor.org/info/rfc9208>.
[Ribose] Ribose Group Inc., "Open-source projects for developers
and designers", 13 April 2023, <https://open.ribose.com/>.
[RNPGP_SEXPP]
RNP, R., "S-Expressions parser and generator library in
C++ (SEXP in C++)", version 0.8.7, 28 June 2023,
<https://github.com/rnpgp/sexpp>.
[SDSI] Rivest, R. and B. Lampson, "A Simple Distributed Security
Architecture", working document, SDSI version 1.1, 2
October 1996, <https://people.csail.mit.edu/rivest/pubs/
RL96.ver-1.1.html>.
[SexpCode] Malkiewicz, J., "SEXP---(S-expressions)", 10 June 2015,
<https://github.com/jpmalkiewicz/rivest-sexp>.
[SPKI] "SPKI--A Simple Public Key Infrastructure",
<http://www.clark.net/pub/cme/html/spki.html>.
Appendix A. Change History
RFC Editor Note: Please delete this section before publication.
A.1. -00 Changes
This sub-section summarizes significant changes between the original
1997 -00 version of this document and the 2023 -00 version submitted
to the IETF.
1. Convert to XML v3.
2. Update Ron Rivest author information and, with his permission,
add Donald Eastlake as an author.
3. Add minimal "IANA Considerations" and "Security Considerations"
sections.
4. Since implementation requirements terminology is used, add the
usual paragraph about it as a sub-section of Section 1 and add
references to [RFC2119] and [RFC8174].
5. Divide references into Normative and Informational and update
base-64 reference to be to [RFC4648].
Rivest & Eastlake Expires 14 August 2024 [Page 21]
Internet-Draft S-Expressions February 2024
6. Add a couple of sentences to the "Historical note" section about
the history of -00 versions of the draft.
A.2. Changes from -00 to -01
1. Fix glitches and errors in the BNF.
2. Add Acknowledgements section to list Marc Petit-Huguenin (who
provided BNF improvements) and John Klensin.
3. Update code references in Section 9 and add to Informative
References section. Note: The code in the Malkiewicz github
repository may be the code that was originally at
http://theory.lcs.mit.edu/~rivest/sexp.html
4. Add this Change History Appendix.
5. Move "Historical Notes" which were formerly a separate section at
the end of the document up to be a sub-section of Section 1.
6. Add references to [LISP], [RFC2692], and [RFC2693].
7. Add simple security considerations.
8. Minor editorial fixes/improvements.
A.3. Changes from -01 to -02
1. Change default MIME Type in Section 4.6 to have charset=utf-8
[RFC4648].
2. Change BNF to ABNF and add reference to [RFC5234].
3. Move Marc Petit-Huguenin to a Contributors section for his work
on the ABNF.
A.4. Changes from -02 to -03
1. Add current S-expression usage Section 1.2.
2. Add the white book [C] as a reference.
3. Add reference to the Ribose RNP code [RNPGP_SEXPP].
4. Minor editorial improvements.
Rivest & Eastlake Expires 14 August 2024 [Page 22]
Internet-Draft S-Expressions February 2024
Acknowledgements
The comments and suggestions of the following are gratefully
acknowledged: John Klensin.
Contributors
Special thanks to the following contributor:
Marc Petit-Huguenin
Impedance Mismatch LLC
Email: marc@petit-huguenin.org
Authors' Addresses
Ronald L. Rivest
MIT CSAIL
32 Vassar Street, Room 32-G692
Cambridge, Massachusetts 02139
United States of America
Email: rivest@mit.edu
URI: https://www.csail.mit.edu/person/ronald-l-rivest
Donald E. Eastlake 3rd
Futurewei Technologies
2386 Panoramic Circle
Apopka, Florida 32703
United States of America
Phone: +1-508-333-2270
Email: d3e3e3@gmail.com
Rivest & Eastlake Expires 14 August 2024 [Page 23]
