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Futurewei Technologies

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Applications and Real Time Network Working Group Sexp 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.

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: (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# (i.e., 0x03) 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.
• 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.

Historical Notes The S-expression technology described here was originally developed for "SDSI" (the Simple Distributed Security Infrastructure by Lampson and Rivest ) in 1996, although the origins clearly date back to McCarthy's programming language. It was further refined and improved during the merger of SDSI and SPKI during the first half of 1997. S-expressions are similar to, but more readable and flexible than, Bernstein's "net-strings" . 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 (see ) and, when it has not been otherwise constrained by the application, changing the default character set in to UTF-8 .

Uses of S-Expressions S-expressions are in active use today between GnuPG and Ribose's RNP . Ribose has implemented C++ software to compose and parse S-expressions and, in fact, references this document. The GNU software is here and there are other implementations (see . S-expressions are used/referenced in the following RFCs:

• IMAP QUOTA Extension
• XML-Signature Syntax and Processing
• for
• IMAP v3
• IMAP v2

In addition, S-Expressions are the inspiration for the encodings in other protocols. For example, Section 6 of or .

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 when, and only when, they appear in all capitals, as shown here.

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 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 we aim to handle:

• 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; as specified herein, the canonical encoding is the same as the transport encoding, for example. In this document we provide (related) encoding techniques for each of these uses.

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 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

The following graphics characters, which are "reserved punctuation":

The following characters are unused and unavailable, except in "verbatim" and "quoted string" encodings:

greater than ? question mark ]]>

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.

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. Here are some sample verbatim encodings: 3:abc 7:subject 4::::: 12:hello world! 10:abcdefghij 0:

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" 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 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" programming language conventions, with an extension for ignoring line terminators of just 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 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." ""

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:

- . / _ : * + = (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 *

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"

Base-64 representation An octet-string may be represented in a base-64 coding consisting of:

• an (optional) decimal length of the octet string
• a vertical bar "|"
• the base-64 encoding of the octet string.
• 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"

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 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: [image/gif] [URI] [charset=unicode-1-1] [text/richtext] ["text/plain; charset=iso-8859-1"] [application/postscript] [application/octet-stream] [audio/basic] ["http://abc.com/display-types/funky.html"] Unless some other type is specified for the application of use, an octet-string that is untyped may be considered to have a pre-specified "default" MIME typeas follows: "text/plain; charset=utf-8"

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 type specified for the applications or, in the absence of such specificaion, the default type specified in .

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+)

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.

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))

Basic transport representation There are two forms of the "basic transport" representation:

• the canonical representation
• an 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. The base-64 encoding would be appropriate if the channel over which the S-expression is being sent might be sensitive to bytes of some special values such as a byte of all zero bits (NULL) or a byte of all one bits (DEL). Here are two examples of an S-expression represented in basic transport mode: (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 single octet-string.

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.

ABNF for syntax ABNF is the Augmented Backus-Naur Form for syntax specifications as defined in . 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 .

ABNF for canonical transport

ABNF for basic transport

ABNF for advanced transport

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 code illustrates these styles in more detail.

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.

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: 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.

Octet string This is represented as follows: ]]> For example (here k = 2) 01 0003 a b c

Octet-string with display-hint This is represented as follows: 01 /* for display-type */ 01 /* 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

List This is represented as ... 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

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 .
• Ribose's RNP .
• Github project of J. P. Malkiewicz .
• The Inferno implementation .

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 as well as to specify a different default display hint. 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
• no base-64 or hexadecimal encodings
• fixed limits on the size of octet-strings

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 .

IANA Considerations This document requires no IANA actions.

Normative References Informative References Universität Bremen TZI Free Software Foundation, Inc. Random Contrarian Insurgent Organization GnuPG The Computer Center and Research Laboratory of Electronics, Massachusetts Institute of Technology Ribose Group Inc.

Change History RFC Editor Note: Please delete this section before publication.

-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 and .
5. Divide references into Normative and Informational and update base-64 reference to be to .
6. Add a couple of sentences to the "Historical note" section about the history of -00 versions of the draft.

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 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 , , and .
7. Add simple security considerations.
8. Minor editorial fixes/improvements.

Changes from -01 to -02

1. Change default MIME Type in to have charset=utf-8 .
2. Change BNF to ABNF and add reference to .
3. Move Marc Petit-Huguenin to a Contributors section for his work on the ABNF.

Changes from -02 to -03

1. Add current S-expression usage Section 1.2.
2. Add the white book as a reference.
3. Add reference to the Ribose RNP code .
4. Minor editorial improvements.

Changes from -03 to -04 Trivial keep-alive update.

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 RFC 0020 for ASCII.
5. Minor editorial improvements.

Acknowledgements The comments and suggestions of the following are gratefully acknowledged: John Klensin and Caleb Malchik.

Contributors Special thanks to the following contributor: Impedance Mismatch LLC

marc@petit-huguenin.org