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ART Sexp Sexpression S-expression This memo specifies the data structure representation that was devised to support Simple Public Key Infrastructure (SPKI) certificates, as detailed in RFC 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 gives a "canonical form" for them, describes two "transport" representations, and also describes an "advanced" format for display to people.

Introduction This memo specifies the data structure representation that was devised to support Simple Public Key Infrastructure (SPKI) certificates , with the intent that it be more widely applicable (see , "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 gives a "canonical form" for them, describes two "transport" representations, and also 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 ). These S-expressions are either octet-strings or lists of simpler S-expressions. Here is a sample S-expression: 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# (which represents a 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: 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.
• Transportability: 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.

For implementors of new applications and protocols other technologies also worthy of consideration include the following: XML , CBOR , and JSON .

Uses of S-Expressions The S-expressions specified herein are in active use today between GnuPG and Ribose's RNP . Ribose has implemented C++ software to compose and parse these S-expressions . The GNU software is the Libgcrypt library , and there are other implementations (see ). S-expressions are also used or referenced in the following RFCs:

• for
• XML-Signature Syntax and Processing

In addition, S-expressions are the inspiration for the encodings in other protocols. For example, or .

Formalization is an Internet-Draft that shows a formal model of SPKI S-expressions and formally demonstrates that the examples and ABNF in this document are correct.

Historical Note The S-expressions described here were originally developed for "SDSI" (the Simple Distributed Security Infrastructure by Lampson and Rivest ) in 1996, although their origins date back to McCarthy's programming language. They were further refined and improved during the merger of SDSI and SPKI during the first half of 1997. S-expressions are more readable and flexible than Bernstein's "netstrings" , which were developed contemporaneously.

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. An octet-string may be zero length. There may be many different but equivalent ways of representing an octet-string 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 . 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: 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
• 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

In this document, related encoding techniques for each of these uses are provided.

Character 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 : Alphabetic: Numeric: Whitespace: The following graphics characters, which are called "pseudo-alphabetic" in this document: 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 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 and that an 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:

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 programming language 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 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 programming language conventions, with an extension for ignoring line terminators of just CR, LF, CRLF, or LFCR and more restrictive octal and hexadecimal value formats): -- causes carriage-return to be ignored. \ -- causes line-feed to be ignored. \ -- causes CRLF to be ignored. \ -- causes LFCR to be ignored. ]]> Here are some examples of quoted-string encodings:

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 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:

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 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:

Base-64 Representation of Octet-Strings An octet-string may be represented in a base-64 encoding 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 "|"

Base-64 encoding produces four characters of output for each three octets of input. When the length of the input is divided by three:

• if the remainder is one, it produces an output block of length four ending in two equals signs.
• if the remainder is two, it produces an output block of length four ending in one equals sign.

These equals signs MUST be included on 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: 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 .

Display-Hints and Internationalization An octet-string can contain any type of data representable by a finite octet-string, e.g., text, a fixed or variable-length integer, or an image. Normally, the application producing and/or consuming S-expressions will understand their structure, the data 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 . The 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 media 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, but a display-hint may not be applied to a display-hint string -- that is, display-hints may not be nested. A display-hint that can be used for UTF-8-encoded text is shown in the following example where the octet-string represents "böb☺", that is, "bob" with an umlaut over the "o", followed by the Unicode character WHITE SMILING FACE (U+263A). Every octet-string representation is or is not preceded by a single display-hint. 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: An octet-string that has no display-hint may be considered to have a media type specified by the application or use. In the absence of such a specification, the default is as follows: When an S-expression is being encoded in one of the representations described in , 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.

Comparison of 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 .

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 () 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 or precede the final right parenthesis of a list. Here are some examples of encodings of lists:

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 representation of an S-expression may be used as described in .

Base-64 Representation of S-Expressions An S-expression may be represented in a base-64 encoding consisting of:

• an opening curly brace "{"
• the base-64 encoding of the 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, 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 .

Canonical 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, reasonably economical, and unique for any S-expression. See and . 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 also . Here are some examples of canonical representations of S-expressions: bar 0: ]]>

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

1. The canonical representation
2. A base-64 representation of the canonical representation, surrounded by braces (see )

The basic transport representations (see ) 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: The second example above is the same S-expression as the first encoded in base-64.

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 octet-string representation forms described above in : quoted strings, base-64, hexadecimal, tokens, representations of strings with omitted lengths, and so on. See .

ABNF of the Syntax ABNF is the Augmented Backus-Naur Form for syntax specifications as defined in . The ABNF for advanced representation of S-expressions is given first, and the basic and canonical forms are derived therefrom. The rule names below in all capital letters are defined in .

ABNF for Advanced Transport

ABNF for Canonical

ABNF for Basic Transport

Restricted 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 , conformant implementations will support canonical and basic representation, but support for advanced representation is not generally required. Thus, advanced representation can only be used in applications that mandate its support or where a capability discovery mechanism indicates support.

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 code in illustrates these styles in more detail.

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 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 .

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: Each of the three types is immediately followed by a k-octet integer indicating the size (in octets) of the following representation. Here, k is an integer that depends on the 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, 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):

Octet-String with Display-Hint This is represented as follows: 01 /* for display-type */ 01 /* for octet-string */ ]]> For example, the S-expression: would be represented as (with k = 2):

List This is represented as: ... 00 ]]> For example, the list (abc [d]ef (g)) is represented in memory as (with k = 2):

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 . The default display-hint (see ) 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.

IANA Considerations This document has no IANA actions.

References Normative References Informative References GnuPG Inferno Manual Page GnuPG Libgcrypt version 1.10.2 The Computer Center and Research Laboratory of Electronics, Massachusetts Institute of Technology Massachusetts Institute of Technology Ribose Group Inc. Version 0.9.2, commit 249c6e3 Working document for SDSI version 1.1 commit 4aa7c36 commit a90f90f commit b7d3bea MIT Lab for Computer Science Textuality and Netscape Microsoft W3C Sun Microsystems W3C Recommendation Latest version available at . The Unicode Consortium

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
• Ribose's RNP in C++
• Github project of J. P. Malkiewicz in C
• The Inferno implementation
• Small Fast X-Expression Library
• S-expression Processor in Ruby
• Canonical S-expressions (OCAML)

Acknowledgements Special thanks to for his extensive comments. The comments and suggestions of the following are gratefully acknowledged: and .

Contributors Special thanks to , particularly for his extensive work and advice on the ABNF and on locating and fixing unclear parts of earlier draft versions of this document: Impedance Mismatch LLC

marc@petit-huguenin.org