rnp-sexp

Pure-Rust S-expression parser and serializer for OpenPGP key formats.

A faithful port of librnp's sexpp C++ library, which implements the S-expression syntax from RFC 2693 (SPKI/SDSI) and the GnuPG extended key format.

Features

• Three printing modes:
  • Canonical — binary-safe length:bytes form for hashing/transmission
  • Advanced — human-readable, pretty-printed, line-wrapped at 75 columns
  • Base64 — canonical form base64-encoded and wrapped in {...}
• Five simple-string encodings on input: verbatim (5:bytes), token (abc), quoted ("hi"), hex (#6162#), and base64 (|YWJj|)
• Full GnuPG 2.3+ extended private key format: Name: value header pairs with case-insensitive names, multi-valued fields, single-line continuation via a leading space, blank-continuation encodes a newline, #-prefixed comment lines
• Configurable nesting-depth limit (default 1024) enforced on both parser and serializer
• Position-precise error messages matching the upstream library, so test fixtures and diagnostics from the C++ version port directly

Quick start

use rnp_sexp::Sexp;

// Parse advanced (human-readable) form
let input = "(11:test-key (1:a 3:bcd))";
let sexp = Sexp::parse_advanced(input).unwrap();

// Round-trip back to advanced form
assert_eq!(sexp.to_advanced(), input);

// Canonical bytes for hashing/transmission
let canonical: Vec<u8> = sexp.to_canonical();

For the GnuPG extended private key format:

use rnp_sexp::parse_extended;

let key = parse_extended(b"Created: 20221130T160847\n\
                          Key: (private-key (rsa (n #00AB...#) (e #010001#)))\n")?;

assert_eq!(key.find("Created").unwrap(), "20221130T160847");
let key_sexp = &key.key;  // SexpList

For finer-grained control over parsing or serialization (custom max-depth, incremental reads, base64 print mode, custom column width), use SexpInputStream and SexpOutputStream directly.

Compatibility with upstream

The module layout mirrors ../sexp/src/ file-by-file (chars.rs, error.rs, depth.rs, input.rs, output.rs, ext_key.rs), and behavior is byte-for-byte identical. Error message text matches the C++ version exactly because the ported test fixtures assert on the formatted what() strings.

The tests/samples/ directory is a verbatim copy of sexp/samples/ from the upstream repository.

License

MIT, matching the upstream sexpp project.