Network Working Group
Internet Engineering Task Force (IETF) R. Rivest
Internet-Draft
Request for Comments: 9804 MIT CSAIL
Intended status:
Category: Informational D. Eastlake
Expires: 13 July 2025
ISSN: 2070-1721 Independent
9 January
June 2025
Simple Public Key Infrastructure (SPKI) S-Expressions
draft-rivest-sexp-13
Abstract
This memo specifies the data structure representation that was
devised to support Simple Public Key Infrastructure (SPKI, (SPKI)
certificates, as detailed in RFC 2692)
certificates and 2692, with the intent that it be
more widely applicable. It has been and is being used elsewhere.
There are multiple implementations in a variety of programming
languages. Uses of this representation are referred to in this
document as "S-expressions". This memo makes precise the encodings
of these SPKI S-expressions: it It gives a "canonical form" for them,
describes two "transport" representations, and also describes an
"advanced" format for display to people.
Status of This Memo
This Internet-Draft document is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents valid
approved by the IESG are candidates for a maximum any level of six months Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained 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 13 July 2025.
https://www.rfc-editor.org/info/rfc9804.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Uses of S-Expressions . . . . . . . . . . . . . . . . . . 4
1.2. Formalization . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Historical Note . . . . . . . . . . . . . . . . . . . . . 5
1.4. Conventions Used in This Document . . . . . . . . . . . . 5
2. S-expressions -- informal introduction . . . . . . . . . . . 5 Informal Introduction
3. Character set . . . . . . . . . . . . . . . . . . . . . . . . 6 Set
4. Octet-string representation types . . . . . . . . . . . . . . 7 Octet-String Representation Types
4.1. Verbatim representation . . . . . . . . . . . . . . . . . 7 Representation
4.2. Quoted-string representation . . . . . . . . . . . . . . 8 Quoted-String Representation
4.3. Token representation . . . . . . . . . . . . . . . . . . 9 Representation
4.4. Hexadecimal representation . . . . . . . . . . . . . . . 10 Representation
4.5. Base-64 representation Representation of octet-strings . . . . . . . . . 10 Octet-Strings
4.6. Display-Hints and Internationalization . . . . . . . . . 11
4.7. Comparison of octet-strings . . . . . . . . . . . . . . . 12 Octet-Strings
5. Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6. S-expression representation types . . . . . . . . . . . . . . 13 S-Expression Representation Types
6.1. Base-64 representation Representation of S-expressions . . . . . . . . . 14 S-Expressions
6.2. Canonical representation . . . . . . . . . . . . . . . . 14 Representation
6.3. Basic transport representation . . . . . . . . . . . . . 15 Transport Representation
6.4. Advanced transport representation . . . . . . . . . . . . 15 Transport Representation
7. ABNF of the syntax . . . . . . . . . . . . . . . . . . . . . 15 Syntax
7.1. ABNF for advanced transport . . . . . . . . . . . . . . . 16 Advanced Transport
7.2. ABNF for canonical . . . . . . . . . . . . . . . . . . . 17 Canonical
7.3. ABNF for basic transport . . . . . . . . . . . . . . . . 17 Basic Transport
8. Restricted S-expressions . . . . . . . . . . . . . . . . . . 17 S-Expressions
9. In-memory representations . . . . . . . . . . . . . . . . . . 18 In-Memory Representations
9.1. List-structure memory representation . . . . . . . . . . 18 List-Structure Memory Representation
9.2. Array-layout memory representation . . . . . . . . . . . 18 Array-Layout Memory Representation
9.2.1. Octet-string . . . . . . . . . . . . . . . . . . . . 19 Octet-String
9.2.2. Octet-string Octet-String with display-hint . . . . . . . . . . . 19 Display-Hint
9.2.3. List . . . . . . . . . . . . . . . . . . . . . . . . 19
10. Security Considerations . . . . . . . . . . . . . . . . . . . 20
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
12. References
12.1. Normative References . . . . . . . . . . . . . . . . . . . . 20
13.
12.2. Informative References . . . . . . . . . . . . . . . . . . . 21
Appendix A. Implementations . . . . . . . . . . . . . . . . . . 23
Appendix B. Change History . . . . . . . . . . . . . . . . . . . 24
B.1. -00 Changes . . . . . . . . . . . . . . . . . . . . . . . 24
B.2. Changes from -00 to -01 . . . . . . . . . . . . . . . . . 24
B.3. Changes from -01 to -02 . . . . . . . . . . . . . . . . . 25
B.4. Changes from -02 to -03 . . . . . . . . . . . . . . . . . 25
B.5. Changes from -03 to -04 . . . . . . . . . . . . . . . . . 25
B.6. Changes from -04 to -05 . . . . . . . . . . . . . . . . . 25
B.7. Changes from -05 to -06 . . . . . . . . . . . . . . . . . 25
B.8. Changes from -06 to -07 . . . . . . . . . . . . . . . . . 26
B.9. Changes from -07 to -08 . . . . . . . . . . . . . . . . . 26
B.10. Changes from -08 to -09 . . . . . . . . . . . . . . . . . 27
B.11. Changes from -09 to -10 . . . . . . . . . . . . . . . . . 27
B.12. Changes from -10 to -12 . . . . . . . . . . . . . . . . . 27
B.13. Changes from -12 to -13 . . . . . . . . . . . . . . . . . 27
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 27
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
1. Introduction
This memo specifies the data structure representation that was
devised to support Simple Public Key Infrastructure (SPKI) [RFC2692]
certificates and [RFC2692], with the intent that it be more widely
applicable (see Section 1.3, History). "Historical Note"). It is suitable for
representing arbitrary, complex data structures and has been and is
being used elsewhere. Uses of this representation herein are
referred to as "S-expressions".
This memo makes precise the encodings of these SPKI S-expressions: it It
gives a "canonical form" for them, describes two "transport"
representations, and also describe describes an "advanced" format for display
to people. There are multiple implementations of S-expressions in a
variety of programming languages including Python, Ruby, and C (see
Appendix A).
These S-expressions are either byte-strings ("octet-strings") or
lists of simpler S-expressions. Here is a sample S-expression:
(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 The hexadecimal constant #03#
(which represents a one octet long one-octet-long octet-string with the value of
that octet being 0x03) and the base-64 constant |YWJj| (which
represents the same octet-string as "abc")
This document specifies how to construct and use these S-expressions.
The design goals for S-expressions were as follows:
generality:
* Generality: S-expressions should be good at representing arbitrary
data.
readability:
* Readability: It should be easy for someone to examine and
understand the structure of an S-expression.
economy:
* Economy: S-expressions should represent data compactly.
transportability:
* Transportability: S-expressions should be easy to transport over
communication media (such as email) that are known to be less than
perfect.
flexibility:
* Flexibility: S-expressions should make it relatively simple to
modify and extend data structures.
canonicalization:
* Canonicalization: It should be easy to produce a unique
"canonical" form of an S-expression, for digital signature
purposes.
efficiency:
* Efficiency: S-expressions should admit in-memory representations
that allow efficient processing.
For implementors of new applications and protocols other technologies
also worthy of consideration include the following: XML [XML], CBOR
[RFC8949], and JSON [RFC7159]. [RFC8259].
1.1. Uses of S-Expressions
The S-expressions specified herein are in active use today between
GnuPG [GnuPG] and Ribose's RNP [Ribose]. Ribose has implemented C++
software to compose and parse these S-expressions [RNPGP_SEXPP]. The
GNU software is here [Libgcrypt] the Libgcrypt library [Libgcrypt], and there are
other implementations (see Appendix A).
They are used or referenced in the following RFCs:
* [RFC2693] for [SPKI]
* [RFC3275] XML-Signature Syntax and Processing
In addition, S-Expressions S-expressions are the inspiration for the encodings in
other protocols. For example, [RFC3259] or Section 6 of
[CDDLfreezer].
[CDDL-freezer].
1.2. Formalization
An Internet Draft [formal] has been posted showing
[Formal] is an Internet-Draft that shows a formal model of SPKI S-Expressions
S-expressions and which formally demonstrates that the examples and ABNF in
this document are correct.
1.3. Historical Note
The S-expressions described here were originally developed for "SDSI"
(the Simple Distributed Security Infrastructure by Lampson and Rivest
[SDSI]) in 1996, although their origins date back to McCarthy's
[LISP] programming language. They were further refined and improved
during the merger of SDSI and SPKI [SPKI] [RFC2692] [RFC2693] during
the first half of 1997. S-expressions are more readable and flexible
than,
than Bernstein's "net-strings" "netstrings" [BERN], which were developed
contemporaneously.
| Although a specification was 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. This document is a
| clarified and modernized version of that document.
1.4. 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 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. An octet-
string may be zero length. 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
|YWJj| -- as a base-64 encoding of the octet-string
"abc"
The above encodings are all equivalent in that they all denote the
same octet-string. Details of these encodings are given below, and
how to give a "display type" to a simple-string is also described in
Section 4.6.
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 can be seen, there is variability possible in the encoding of an
S-expression. In some applications, 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 these S-expressions
aim to handle:
* a "transport" or "basic" encoding for transporting the
S-expression between computers. computers
* a "canonical" encoding, used when signing the S-expression. S-expression
* an "advanced" encoding used for input/output to people. people
* an "in-memory" encoding used for processing the S-expression in
the computer. computer
In this document, related encoding techniques for each of these uses
are provided.
3. Character set Set
This document specifies encodings of S-expressions. Except when
giving "verbatim" encodings, the character set used is limited to the
following characters in ASCII [RFC0020]:
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 are called "pseudo-
alphabetic" in this document:
- hyphen or minus
. period
/ slash
_ underscore
: colon
* asterisk
+ plus
= equal
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
' single-quote (apostrophe)
, comma
< less than
> greater than
? question mark
4. Octet-string representation types Octet-String Representation Types
This section describes in detail the ways in which an octet-string
may be represented.
Recall that an octet-string is any finite sequence of octets, octets and that
an octet-string may have length zero.
4.1. Verbatim representation 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. zeros
* a colon ":"
* the octet-string itself, verbatim. 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.
Here are some sample verbatim encodings:
3:abc
7:subject
4:::":
12:hello world!
10:abcdefghij
0:
4.2. Quoted-string representation 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 the "C" C programming language [C] escape
conventions (\n, etc.)
* a final double-quote (")
The specified length is the length of the resulting string after any
backslash escape sequences have been converted to the octet value
they denote. The string does not have any "terminating NULL" that
[C] includes, and the length does not count such an octet.
The length is optional.
The escape conventions within the quoted string are as follows (these
follow the "C" [C] C programming language [C] conventions, with an extension
for ignoring line terminators of just CR, LF, CRLF, or LFCR and more
restrictive octal and hexadecimal value formats):
\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 line-feed 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"
"\xFE is the same octet as \376"
3"\n\n\n"
"This has\n two lines."
"This has \
one line."
""
4.3. Token representation 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 following eight "pseudo-alphabetic"
punctuation marks:
- . / _ : * + =
* it is length 1 or greater.
Note: Upper and lower case are not equivalent. A token may begin
with punctuation, including ":".
Here are some examples of token representations:
subject
not-before
:=..
class-of-1997
//example.net/names/smith
*
4.4. Hexadecimal representation 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 final 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"
## -- represents the zero-length string
4.5. Base-64 representation Representation of octet-strings Octet-Strings
An octet-string may be represented in a base-64 encoding [RFC4648]
consisting of:
* an (optional) decimal length of the octet-string
* a vertical bar "|"
* the base-64 [RFC4648] encoding of the octet-string.
* a final vertical bar "|"
Base-64 encoding produces four characters of output for each three
octets of input. If the length of the input divided by three leaves
a remainder of one or two, it produces an output block of length four
ending in two or one equals signs, respectively. These equals signs
MUST be included on output output, but input routines MAY accept inputs
where one or two equals signs are dropped.
Whitespace inserted in the midst of the base-64 encoding 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"
|| -- represents the zero-length string
Note the difference between this base-64 encoding of an octet-string
using vertical bars ("| |") and the base-64 encoding of an
S-expression using curly braces ("{ }") in Section 6.1.
4.6. Display-Hints and Internationalization
An octet-string can contain any type of data representable by a
finite octet-string, for example e.g., text, a fixed or variable length variable-length integer,
or an image. Normally Normally, the application producing / and/or consuming
S-expressions will understand their structure and structure, the data type type, and the
encoding of the octet-strings within the S-expressions used by that
application. If the octet-string consists of text, use of UTF-8
encoding is RECOMMENDED [RFC2130] [RFC3629].
The purposes purpose of a display-hint is to provide information on how to
display an 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 representation surrounded by square
brackets. There may be whitespace separating the display hint octet-
string from the surrounding brackets. Any of the legal octet-string
representations may be used for the display-hint string string, but a
display-hint may not be applied to a display-hint string, string -- that is,
display-hints may not be nested.
A display-hint that can be used for UTF-8 encoded UTF-8-encoded text is shown in
the following example where the octet-string is text saying "bob", "böb☺", i.e., "bob"
with an umlaut over the central "o", followed by a smilie emoji. WHITE SMILING FACE (U+263A).
["text/plain; charset=utf-8"]"b\xC3\xB7b\xE2\x98\xBA"
Every octet-string representation is either preceded by a single
display-hint or not so preceded. There may be whitespace between the
close square bracket and the octet-string to which the hint applies.
Here are some other examples of display-hints:
[image/gif]
[charset=unicode-1-1]
[ text/richtext ]
["text/plain; charset=iso-8859-1"]
[application/postscript]
[audio/basic]
["http://example.com/display-types/funky.html"]
An octet-string that has no display-hint may be considered to have a
MIME [RFC2046] type specified by the application or use. In the
absence of such a specification, the default is as follows:
[application/octet-stream]
When an S-Expression S-expression is being encoded in one of the representations
described in Section 6, any display-hint present is included. If a
display-hint is the default, it is not suppressed nor is the default
display-hint included in the representation for an octet-string
without a display-hint.
4.7. Comparison of octet-strings Octet-Strings
It is RECOMMENDED that two octet-strings be considered equivalent for
most computational and algorithmic purposes if and only if they have
the same display-hint and the same data octet-strings. However, a
particular application might need a different criterion. For
example, it might ignore the display hint on comparisons.
Note that octet-strings are "case-sensitive"; the octet-string "abc"
is not equal to the octet-string "ABC".
An octet-string without a display-hint may be compared to another
octet-string (with or without a display hint) by considering it as an
octet-string with the default display-hint specified for the
applications or, in the absence of such specification, the general
default display-hint specified in Section 4.6 .
5. Lists
Just as with octet-strings, there are variations in representing 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. To be precise, an octet-string represented as a token
(Section 4.3) MUST be separated by whitespace from a following token,
verbatim representation, or any of the following if they are prefixed
with a length: quoted-string, hexadecimal, or base-64 representation.
Also, whitespace may follow the initial left parenthesis, parenthesis or precede
the final right parenthesis of a list.
Here are some examples of encodings of lists:
(a bob c)
( a ( bob c ) ( ( d e ) ( e f ) ) )
(11:certificate(6:issuer3:bob)(7:subject5:alice))
(|ODpFeGFtcGxlIQ==| "1997" murphy 3:XC+)
()
6. S-expression representation types S-Expression Representation Types
There are three "types" of representation:
* canonical
* basic transport
* advanced transport
The first two MUST be supported by any implementation; the last is
OPTIONAL. As part of basic representation, the base-64 [RFC4648]
representation of an S-expression may be used as described in
Section 6.1.
6.1. Base-64 representation Representation of S-expressions S-Expressions
An S-expression may be represented in a base-64 encoding [RFC4648]
consisting of:
* an opening curly brace "{"
* the base-64 [RFC4648] encoding of the S-expression. S-expression
* a final closing curly brace "}"
Base-64 encoding produces four characters of output for each three
octets of input. If the length of the input divided by three leaves
a remainder of one or two, it produces an output block of length four
ending in two or one equals signs, respectively. These equals signs
MUST be included on output output, but input routines MAY accept inputs
where one or two equals signs are dropped.
Whitespace inserted in the midst of the base-64 encoding, after the
opening curly brace, or before the closing curly brace is ignored.
It is an error to have characters other than whitespace and base-64
characters.
Note the difference between this base-64 encoding of an S-expression
using curly braces ("{ }") and the base-64 encoding of an octet-
string using vertical bars ("| |") in Section 4.5.
6.2. Canonical representation Representation
This canonical representation is used for digital signature purposes
and transport over channels not sensitive to specific octet values.
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. (See [CANON].)
See [CANON1] and [CANON2].
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 (see parentheses. See
also Section 7.2). 7.2.
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))
10:foo)]}>bar
0:
6.3. Basic transport representation Transport Representation
There are two forms of the "basic transport" representation:
* the
1. The canonical representation
* an [RFC4648]
2. A base-64 [RFC4648] representation of the canonical
representation, surrounded by braces (see Section 6.1). 6.1)
The basic transport representations (see Section 7.3) are intended to
provide a universal means of representing S-expressions for transport
from one machine to another. The base-64 encoding would be
appropriate if the channel over which the S-expression is being sent
might be sensitive to octets of some special values, such as an octet
of all zero bits (NULL) or an octet of all one bits (DEL), or if the
channel is sensitive to "line length" such that occasional line
terminating whitespace is needed.
Here are two examples of an S-expression represented in basic
transport mode:
(1:a1:b1:c)
{KDE6YTE6YjE
6Yyk= }
The second example above is the same S-expression as the first
encoded in base-64.
6.4. Advanced transport representation 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 octet-string
representation forms described above in Section 4: quoted strings,
base-64, hexadecimal, tokens, representations of strings with omitted
lengths, and so on. (See See Section 7.1). 7.1.
7. ABNF of the syntax Syntax
ABNF is the Augmented Backus-Naur Form for syntax specifications as
defined in [RFC5234]. The ABNF for advanced representation of
S-expressions is given first first, and the basic and canonical forms are
derived therefrom. The rule names below in all capital letters are
defined in Appendix B.1 of [RFC5234].
7.1. ABNF for advanced transport 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] simple-string
display = "[" *whitespace simple-string *whitespace "]"
*whitespace
simple-string = verbatim / quoted-string / token / hexadecimal /
base-64
verbatim = decimal ":" *OCTET
; the length followed by a colon and the exact
; number of OCTETs indicated by the length
decimal = %x30 / (%x31-39 *DIGIT)
quoted-string = [decimal] DQUOTE *(printable / escaped) DQUOTE
printable = %x20-21 / %x23-5B / %x5D-7E
; All US-ASCII printable but double-quote and
; backslash
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
quote = %x27 ; single quote
token = (ALPHA / simple-punc) *(ALPHA / DIGIT /
simple-punc)
simple-punc = "-" / "." / "/" / "_" / ":" / "*" / "+" / "="
hexadecimal = [decimal] "#" *whitespace *hexadecimals "#"
hexadecimals = 2(HEXDIG *whitespace)
base-64 = [decimal] "|" *whitespace *base-64-chars
[base-64-end] "|"
base-64-chars = 4(base-64-char *whitespace)
base-64-char = ALPHA / DIGIT / "+" / "/"
base-64-end = base-64-chars /
3(base-64-char *whitespace) ["=" *whitespace] /
2(base-64-char *whitespace) *2("=" *whitespace)
7.2. ABNF for canonical Canonical
c-sexp = c-string / ("(" *c-sexp ")")
c-string = [ "[" verbatim "]" ] verbatim
7.3. ABNF for basic transport Basic Transport
b-sexp = c-sexp / b-base-64
b-base-64 = "{" *whitespace *base-64-chars base-64-end "}"
; encodes a c-sexp, which has a minimum
; length of 2
8. Restricted S-expressions S-Expressions
This document has described S-expressions in general form.
Applications may wish to restrict their use of S-expressions in
various ways as well as to specify a different default display-hint.
Here are some possible restrictions that might be considered:
* no advanced representations (only canonical and basic)
* 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
* no base-64 or hexadecimal encodings
* fixed limits on the size of octet-strings
As provided in Section 6, conformant implementations will support
canonical and basic representation representation, but support for advanced
representation is not generally required. Thus Thus, advanced
representation can only be used in applications which that mandate its
support or where a capability discovery mechanism indicates support.
9. In-memory representations 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. This document suggests two alternatives:
* "list-structure"
* "array-layout"
These are only sketched here, as they are only suggestive. The
[SexpCode] code
in [SexpCode] illustrates these styles in more detail.
9.1. List-structure memory representation List-Structure Memory Representation
Here there are separate records for simple-strings, strings, and
lists or list nodes. An S-expression of the form ("abc" "de") could
be encoded as two records for the simple-strings, two for the
strings, and two for the list elements, elements where a record is a relatively
small block of memory and, except for simple-string, might have
pointers in it to other records. This is a fairly conventional
representation as discussed in Section 4 of [LISP2].
9.2. Array-layout memory representation Array-Layout Memory Representation
Here each S-expression is represented as a contiguous array of
octets. The first octet codes the "type" of the S-expression:
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-octet integer
indicating the size (in octets) of the following representation.
Here
Here, k is an integer that depends on the implementation, it implementation. It might
be anywhere from 2 to 8, but it 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
right parenthesis, you know how long the list representation was, was and
where to go back to fill in the missing length.
9.2.1. Octet-string Octet-String
This is represented as follows:
01 <length> <octet-string>
For example (here (here, k = 2) 2):
01 0003 a b c
9.2.2. Octet-string Octet-String with display-hint 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 S-expression:
[gif] #61626364#
would be represented as (with k = 2) 2):
02 000d
01 0003 g i f
01 0004 61 62 63 64
9.2.3. List
This is represented as as:
03 <length> <item1> <item2> <item3> ... <item> 00
For example, the list (abc [d]ef (g)) is represented in memory as
(with k = 2) 2):
03 001b
01 0003 a b c
02 0009
01 0001 d
01 0002 e f
03 0005
01 0001 g
00
00
10. Security Considerations
As a pure data representation format, there are few security
considerations to S-expressions. A canonical form is required for
the consistent creation and verification of digital signatures. This
is provided in Section 6.2.
The default display-hint (see Section 4.6) can be specified for an
application. Note that if S-expressions containing untyped octet-
strings represented for that application are processed by a different
application, those untyped octet-string may be treated as if they had
a different display-hint.
11. IANA Considerations
This document requires has no IANA actions.
12. References
12.1. Normative References
[C] Kernighan, B. and D. Ritchie, "The C Programming
Language", ISBN 0-13-110370-9, 1988.
[RFC0020] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC0020, October 1969,
<https://www.rfc-editor.org/info/rfc20>.
[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>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.
[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>.
13.
12.2. Informative References
[BERN] Bernstein, D., D. J., "Netstrings", Work in progress, Progress,
Internet-Draft, draft-bernstein-netstrings-02, 1 February January
1997, <https://www.ietf.org/archive/id/draft-bernstein-
netstrings-02.txt>.
[CANON] <https://datatracker.ietf.org/doc/html/draft-
bernstein-netstrings-02>.
[CANON1] Wikipedia, "Canonical S-expressions",
<https://en.wikipedia.org/wiki/Canonical_S-expressions>.
[CANON2] Grinberg, R., "Csexp - Canonical S-expressions", 24 March
2023, <https://github.com/ocaml-dune/csexp>.
[CDDLfreezer]
[CDDL-freezer]
Bormann, C., "A feature freezer for the Concise Data
Definition Language (CDDL)", work Work in progress, 12
September 2023, <https://datatracker.ietf.org/doc/draft-
bormann-cbor-cddl-freezer/>.
[formal] Progress, Internet-
Draft, draft-bormann-cbor-cddl-freezer-15, 28 February
2025, <https://datatracker.ietf.org/doc/html/draft-
bormann-cbor-cddl-freezer-15>.
[Formal] Petit-Huguenin, M., "A Formalization of Symbolic
Expressions", work Work in progress, 24 Progress, Internet-Draft, draft-
petithuguenin-ufmrg-formal-sexpr-06, 4 May 2024, 2025,
<https://datatracker.ietf.org/doc/html/draft-
petithuguenin-ufmrg-formal-sexpr-04>.
petithuguenin-ufmrg-formal-sexpr-06>.
[GnuPG] Free Software Foundation, Inc., GnuPG, "The GNU Privacy Guard", <https://www.gnupg.org/>.
[Inferno] Uriel, "Inferno S-expressions",
<http://man.cat-v.org/inferno/6/sexprs>. Inferno Manual Page,
<https://man.cat-v.org/inferno/6/sexprs>.
[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, J., Abrahams, P. W., Edwards, D. J., Hart, T.
P., and M. Levin, "LISP 1.5 Programmer's Manual",
ISBN-13 978-0-262-12011-0, ISBN-10 0262130114, 15 August
1962.
[LISP2] McCarthy, J., "Recursive Functions of Symbolic Expressions
and Their Computation by Machine, Part I", April 1960,
<https://people.cs.umass.edu/~emery/classes/cmpsci691st/
readings/PL/LISP.pdf>.
[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>.
[RFC2130] Weider, C., Preston, C., Simonsen, K., Alvestrand, H.,
Atkinson, R., Crispin, M., and P. Svanberg, "The Report of
the IAB Character Set Workshop held 29 February - 1 March,
1996", RFC 2130, DOI 10.17487/RFC2130, April 1997,
<https://www.rfc-editor.org/info/rfc2130>.
[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>.
[RFC7159]
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 7159, 8259,
DOI 10.17487/RFC7159, March
2014, <https://www.rfc-editor.org/info/rfc7159>. 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.
[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, Version 0.9.2, commit 249c6e3, 22 March 2025,
<https://github.com/rnpgp/sexpp>.
[SDSI] Rivest, R. and B. Lampson, "A Simple Distributed Security
Architecture", working document, Working document for 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)", commit 4aa7c36, 10 June 2015,
<https://github.com/jpmalkiewicz/rivest-sexp>.
[SEXPP] Davis, R., "SexpProcessor", 10 June 2015, commit a90f90f, 11 April 2025,
<https://github.com/seattlerb/sexp_processor>.
[SFEXP] Sottile, M., "Small Fast X-Expression Library", commit b7d3bea, 24
March 2023, <https://github.com/mjsottile/sfsexp>.
[SPKI] Rivest, R., "SPKI/SDSI 2.0 A Simple Distributed Security
Infrastructure",
<https://people.csail.mit.edu/rivest/pubs/RL96.slides-
maryland.pdf>.
[XML] Bray, T., Paoli, J., Sperberg-McQueen, C.M., Maler, E.,
and F. Yergeau, "Extensible Markup Language (XML) 1.0",
W3C Recommendation, 26 November 2008,
<https://www.w3.org/TR/2008/REC-xml-20081126/>. Latest
version available at <https://www.w3.org/TR/REC-xml/>.
Appendix A. Implementations
At this time there are multiple implementations, many open source,
available that are 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]. [Libgcrypt]
* Ribose's RNP [RNPGP_SEXPP] in C++. C++
* Github project of J. P. Malkiewicz [SexpCode] in C. C
* The Inferno implementation [Inferno]. [Inferno]
* Small Fast X-Expression Library [SFEXP]. [SFEXP]
* S-expression Processor [SEXPP] in Ruby. Ruby
* Canonical S-expressions [CANON] (OCAML).
Appendix B. Change History
RFC Editor Note: Please delete this section before publication.
B.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].
6. Add a couple of sentences to the "Historical note" section about
the history of -00 versions of the draft.
B.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 Appendix A 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.
B.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.
B.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.
B.5. Changes from -03 to -04
Trivial keep-alive update.
B.6. Changes from -04 to -05
1. Add reference to [Inferno] implementation.
2. Eliminate remaining references to being a "proposal".
3. Emphasize that a particular application can specify a different
default display-hint.
4. Add reference to [RFC0020] for ASCII.
5. Minor editorial improvements.
B.7. Changes from -05 to -06
1. Move implementations list to Appendix A. Add numerous
implementations.
2. Change default display-hint to "application/octet-stream".
3. Expand Abstract and include most of Abstract in the Introduction.
4. Use different tokens for the top-level rule in the three ABNF
encodings so that the rules would not collide if all were used.
Fix ABNF for "printable".
5. Add an illustration of list-structure memory representation.
6. Editorial improvements.
B.8. Changes from -06 to -07
1. Re-order some top-level sections.
2. Replace "list-structure" memory figure with explanation and
[LISP2] reference.
3. Re-organize ABNF to give full ABNF for advanced transport first
and then mostly derive canonical and basic from advanced.
4. Correct reference to [RFC5234] to be to Appendix B.1, not
Appendix A.
5. Attempt to clarify the difference between canonicalization and
equality.
6. Add the explicit Section 6.1 on base-64 representation of
S-expressions.
7. Globally hyphenate "octet-string" and "display-hint", generally
replace "byte" with "octet".
8. Add some more examples here and there.
9. Fix typos. Other editorial improvements.
B.9. Changes from -07 to -08
1. A variety of minor fixes and more precise wording.
2. Give exact circumstances under which a space is needed to
separate successive octet-string representations in a list.
3. Additional editorial improvements.
B.10. Changes from -08 to -09
1. Add mention of and reference to [formal].
2. Add mention in the text that whitespace can appear just after the
opening curly brace and before just before the closing curly
brace of base-64 encoding (the ABNF was correct).
3. Minor editorial improvements.
B.11. Changes from -09 to -10
1. Revert default display hint to more closely follow the original
SPKI S-expressions.
2. Editorial improvements.
B.12. Changes from -10 to -12
Minor ABNF fixes and editorial changes.
B.13. Changes from -12 to -13
Added recommendation and references for using UTF-8 to support
Interntionalization for text octet-strings. Minor other updates
based on IESG reviews. [CANON2] (OCAML)
Acknowledgements
Special thanks to Daniel K. Gillmore for his extensive comments.
The comments and suggestions of the following are gratefully
acknowledged: John Klensin and Caleb Malchik.
Contributors
Special thanks to Marc Petit-Huguenin, particularly for his extensive
work and advice on the ABNF and on locating and fixing unclear parts
of earlier draft versions of this document:
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
Independent
2386 Panoramic Circle
Apopka, Florida 32703
United States of America
Phone: +1-508-333-2270
Email: d3e3e3@gmail.com