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A Canonical S-expression (or csexp) is a binary encoding form of a subset of general S-expression. It was designed for use in SPKI - to retain the power of S-expressions, while achieving the compactness of a binary form and maximizing the speed of parsing.
An S-expression is composed of atoms, which are byte strings, and parentheses used to delimit lists or sub-lists. S-expressions are fully recursive. Typically, S-expressions are encoded as text, with spaces delimiting atoms and quotation marks used to surround atoms that contain spaces.
Atoms in a Canonical S-expression are encoded as length-prefixed byte strings. The length of the following byte string is expressed as an ASCII decimal number followed by a ":".
(this "Canonical S-expression" has 5 atoms)
becomes the csexp
There is no white space to be normalized - thus the adjective canonical - and the atoms can be any binary string. So, a cryptographic hash value or a public key modulus that would have to be encoded in base64 or some other printable encoding can be expressed in csexp as its binary bytes.
Binary byte strings can represent various encodings. A csexp includes a non-S-expression construct for indicating the encoding of a string, when that encoding is not obvious. Any atom in csexp can be prefixed by a single atom in square brackets - such as "[4:JPEG]" or "[7:UNICODE]".
Finally, an csexp as used in SPKI has one limitation compared to a full S-expression - that every list must start with an atom - and therefore there can be no empty lists.
Typically, a list's first atom is treated as one treats an element name in XML.
Comparisons to other encodings
There are other encodings in common use:
Generally, csexp has a parser one or two decimal orders of magnitude smaller than that of either XML or ASN.1. This small size and corresponding speed give csexp its main advantage. In addition to the parsing advantage, there are other differences.
csexp vs. XML
A csexp is roughly as expressive as XML. This is not surprising, since XML is described as an ASCII form for S-expressions. However, csexp does not have a concept like XML attributes (within an element). When encoding in csexp, one must plan on not using such attributes.
A csexp, like a general sexp, is fully recursive. XML, however, has limitations on recursive use of element names.
The first atom in a csexp list - the equivalent of an XML element name - can be any atom in any encoding (e.g., a JPEG, a UNICODE string, a WAV file, ...). XML element names are constrained to use a subset of the printable character set.
XML merges a sequence of strings within one element into a single string, while csexp allows a sequence of atoms within a list and those atoms remain separate from one another.
Finally, csexp is inherently binary while XML is printable - so binary quantities in XML must be encoded, for example using base64.
csexp vs. ASN.1
ASN.1 is a popular binary encoding form. However, it expresses only syntax (data types), not semantics. Two different structures - each a SEQUENCE of two INTEGERS - have identical representations on the wire (barring special tag choices to distinguish them). To parse an ASN.1 structure, one must tell the parser what set of structures one is expecting and the parser must match the data type being parsed against the structure options. This adds to the complexity of an ASN.1 parser.
A csexp structure, like an XML document, carries its own semantics (encoded in element names), and the parser for a csexp structure does not care what structure is being parsed. Once a wire-format expression has been parsed into an internal tree form (similar to XML's DOM), the consumer of that structure can examine it for conformance to what was expected.