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Cryptographic primitives

LXMF builds on Reticulum primitives for all hashing, signing, and encryption. An implementation MUST use primitives that produce byte-identical results to these, because their outputs are signed, hashed, and exchanged on the wire.

Hashing

  • full_hash(x) is SHA-256 over x, 32 bytes (RNS.Identity.HASHLENGTH = 256 bits). LXMF uses it for the message hash and message-id (LXMessage.py:365-366), the transient-id (LXMessage.py:431), and the stamp digest (LXStamper.py:34,44).
  • truncated_hash(x) is the leading 16 bytes of full_hash(x) (RNS.Identity.TRUNCATED_HASHLENGTH = 128 bits). LXMF uses it for the ticket stamp shortcut (LXMessage.py:274,297).

Signing

  • Identity.sign(m) is Ed25519 over m, producing a 64-byte signature (RNS.Identity.SIGLENGTH = 512 bits). Ed25519 is deterministic (RFC 8032), so a given key and message always yield the same signature; [VEC-MSG-1] pins one.
  • Identity.validate(sig, m) verifies an Ed25519 signature. The inbound path calls it as source.identity.validate(signature, signed_part) (LXMessage.py:794).

Encryption

  • Destination.encrypt(plaintext) encrypts to a SINGLE destination using Reticulum’s ECDH scheme: a fresh ephemeral X25519 key per call, an HKDF-derived AES-128-CBC key, and an HMAC token, optionally keyed by the destination’s current ratchet. Because the ephemeral key is fresh per call, the ciphertext is not reproducible across runs; LXMF vectors that involve encryption ([VEC-PROP-ENVELOPE], [VEC-PAPER-URI]) are proven by a decrypt round trip, not by frozen ciphertext.
  • LXMF calls encrypt for the propagated and paper forms over the message tail packed[16:] (LXMessage.py:427,446), and Destination.decrypt on receipt.
  • The encryption description strings "AES-128" / "Curve25519" / "Unencrypted" (LXMessage.py:97-99) are local labels only, not on the wire.

Key derivation

  • Cryptography.hkdf(length, derive_from, salt, context) is HKDF-SHA-256. LXMF uses it only to build the stamp workblock (LXStamper.py:22-25); see Stamps and proof-of-work.

Identities

An Identity carries an X25519 key pair (encryption) and an Ed25519 key pair (signing). The reference constructs deterministic identities for the vectors from fixed 64-byte private material X25519(32) || Ed25519(32) (gen_vectors.py, recorded in vectors.json meta.src_identity_prv_hex / meta.dst_identity_prv_hex). An identity’s 16-byte hash is truncated_hash of its concatenated public keys; LXMF treats the hash as opaque and obtains it from the Reticulum Destination.