csexpr

S-expressions, as specified by RFC 9804. The name is short for canonical S-expression, which is what this format has long been called after the one representation of it that is unique, and by which it is known where it is used, as in SPKI and SDSI.

This package is the Rust crate of the same name, bound to Python. The parsing and the writing happen in Rust, and no Python code stands between them and you.

Install

Python 3.10 or later. One wheel per platform serves every version of it, since the extension is built against the stable ABI.

pip install csexpr

Use

An S-expression is either an octet string or a list of simpler S-expressions, and a Python value stands for one directly. bytes is an octet string, a list is a list, and Atom is an octet string that carries a display hint. Text is accepted wherever octets are, and is taken as UTF-8.

import csexpr

assert csexpr.to_canonical(["issuer", "bob"]) == b"(6:issuer3:bob)"
assert csexpr.parse(b"(6:issuer3:bob)") == [b"issuer", b"bob"]

# The three representations of §6 all carry the same value.
assert csexpr.parse(b"(issuer bob)") == [b"issuer", b"bob"]
assert csexpr.parse(b"{KDY6aXNzdWVyMzpib2Ip}") == [b"issuer", b"bob"]

# A display hint says how an octet string should be shown, and nothing else.
bob = csexpr.Atom(b"bob", hint=b"text/plain")

assert csexpr.to_advanced(["issuer", bob]) == "(issuer [text/plain]bob)"
assert csexpr.parse(b"(issuer [text/plain]bob)") == ["issuer".encode(), bob]

The three representations

Written by Looks like For
to_canonical (§6.2) (6:issuer3:bob) Hashing and signing, since one S-expression has exactly one of these
to_transport (§6.3) {KDY6aXNzdWVyMzpib2Ip} Channels that would disturb raw octets
to_advanced (§6.4) (issuer bob) Being read by a person

parse reads whichever of the three it is given. parse_canonical reads only the first, which is what verifying a signature calls for, since a signature is computed over those octets exactly.

Reading what cannot be trusted

Parser bounds the size of the input and of every octet string in it, and turns off whichever constructs an application has no use for. Each of the restrictions RFC 9804 §8 names is one keyword.

from csexpr import ParseError, Parser

parser = Parser(
    canonical=True,
    max_input_len=64 * 1024,
    max_atom_len=4096,
    max_depth=32,
    allow_display_hints=False,
    allow_empty_lists=False,
)

assert parser.parse(b"(6:issuer3:bob)") == [b"issuer", b"bob"]

try:
    parser.parse(b"(issuer bob)")
except ParseError as error:
    print(error.kind, error.offset)

A ParseError carries offset, the octet the offending construct begins at, and kind, a short name for what was wrong.

Parsing takes no stack in proportion to how deeply the input nests, and no memory on the strength of a length the input states. Lists may nest as deeply as DEFAULT_MAX_DEPTH, and writing refuses a structure nested deeper than that rather than risking the interpreter.

License

MIT, see LICENSE.

Project links

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Credits

Author: AndreKoraleski

GitHub Statistics

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License expression

MIT
View SPDX License List

Requires

Python >=3.10

Tags

sexp s-expression rfc9804 csexp spki

Classifiers

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