Network Working Group R. Rivest Internet-Draft MIT CSAIL Intended status: Informational D. Eastlake Expires: 15 February 2024 Futurewei Technologies 14 August 2023 S-Expressions draft-rivest-sexp-03 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 15 February 2024. Copyright Notice Copyright (c) 2023 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 15 February 2024 [Page 1] Internet-Draft S-Expressions August 2023 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 15 February 2024 [Page 2] Internet-Draft S-Expressions August 2023 (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 15 February 2024 [Page 3] Internet-Draft S-Expressions August 2023 * 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 15 February 2024 [Page 4] Internet-Draft S-Expressions August 2023 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 15 February 2024 [Page 5] Internet-Draft S-Expressions August 2023 * 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 15 February 2024 [Page 6] Internet-Draft S-Expressions August 2023 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 15 February 2024 [Page 7] Internet-Draft S-Expressions August 2023 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 15 February 2024 [Page 8] Internet-Draft S-Expressions August 2023 \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) \ -- causes carriage-return to be ignored. \ -- causes linefeed to be ignored. \ -- causes CRLF to be ignored. \ -- 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 15 February 2024 [Page 9] Internet-Draft S-Expressions August 2023 (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 15 February 2024 [Page 10] Internet-Draft S-Expressions August 2023 * 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 15 February 2024 [Page 11] Internet-Draft S-Expressions August 2023 [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 15 February 2024 [Page 12] Internet-Draft S-Expressions August 2023 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 15 February 2024 [Page 13] Internet-Draft S-Expressions August 2023 (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 15 February 2024 [Page 14] Internet-Draft S-Expressions August 2023 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 15 February 2024 [Page 15] Internet-Draft S-Expressions August 2023 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 15 February 2024 [Page 16] Internet-Draft S-Expressions August 2023 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 For example (here k = 2) 01 0003 a b c 8.2.2. Octet-string with display-hint This is represented as follows: 02 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 Rivest & Eastlake Expires 15 February 2024 [Page 17] Internet-Draft S-Expressions August 2023 8.2.3. List This is represented as 03 ... 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 15 February 2024 [Page 18] Internet-Draft S-Expressions August 2023 * 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, . [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, . [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . 14. Informative References [BERN] Bernstein, D., "Netstrings", Work in progress, 1 February 1997, . Rivest & Eastlake Expires 15 February 2024 [Page 19] Internet-Draft S-Expressions August 2023 [CDDLfreezer] Borman, C., "A feature freezer for the Concise Data Definition Language (CDDL)", work in progress, 12 September 2023, . [GnuPG] Free Software Foundation, Inc., "The GNU Privacy Guard", . [Libgcrypt] GnuPG, "The Libgcrypt Library", Libgcrypt version 1.10.2, 6 April 2023, . [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, . [RFC1203] Rice, J., "Interactive Mail Access Protocol: Version 3", RFC 1203, DOI 10.17487/RFC1203, February 1991, . [RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, DOI 10.17487/RFC2046, November 1996, . [RFC2692] Ellison, C., "SPKI Requirements", RFC 2692, DOI 10.17487/RFC2692, September 1999, . [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, . [RFC3259] Ott, J., Perkins, C., and D. Kutscher, "A Message Bus for Local Coordination", RFC 3259, DOI 10.17487/RFC3259, April 2002, . [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, . Rivest & Eastlake Expires 15 February 2024 [Page 20] Internet-Draft S-Expressions August 2023 [RFC9208] Melnikov, A., "IMAP QUOTA Extension", RFC 9208, DOI 10.17487/RFC9208, March 2022, . [Ribose] Ribose Group Inc., "Open-source projects for developers and designers", 13 April 2023, . [RNPGP_SEXPP] RNP, R., "S-Expressions parser and generator library in C++ (SEXP in C++)", version 0.8.7, 28 June 2023, . [SDSI] Rivest, R. and B. Lampson, "A Simple Distributed Security Architecture", working document, SDSI version 1.1, 2 October 1996, . [SexpCode] Malkiewicz, J., "SEXP---(S-expressions)", 10 June 2015, . [SPKI] "SPKI--A Simple Public Key Infrastructure", . 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 15 February 2024 [Page 21] Internet-Draft S-Expressions August 2023 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 15 February 2024 [Page 22] Internet-Draft S-Expressions August 2023 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 15 February 2024 [Page 23]