sbp-client

SBP (Session Bridge Protocol) end-to-end encryption core. Implements the wire format from sbp-spec/spec/SBP-0.4-e2ee.md.

The relay (sbp-dispatcher) never holds a secret: confidentiality comes from crypto_box sealing to the pinned recipient box public key, and integrity from an Ed25519 signature over tag || eph || cipher. Routing is blinded with HMAC(rk, scope), so the relay cannot even tell which session a frame belongs to.

Install

pip install sbp4ai-client

Quick start

from sbp_client import KeyPair, KeyRing, derive_rk, E2EELink

rk = derive_rk("your-routing-passphrase")          # 32-byte shared routing key
me = KeyPair.generate()                            # your box+sign keypair
ring = KeyRing()
ring.add("peer", peer_box_pk_b64, peer_sign_pk_b64)  # pin the other side's publics

link = E2EELink(me, ring, rk, my_key_id="cli", device_did="dev_abc", role="client")
hello = link.hello()                               # send to relay first
link.subscribe(link.scope.device())                # ask relay to route device-tag frames
envelope = link.send(inner_frame, link.scope.session("sess1"), peer_key_id="dev")
# ... ship `envelope` over WebSocket; on the other side:
inner = link.recv(envelope)                        # decrypted + signature-checked

Key formats

KeyPair.to_strings() exports sign_sk in the 64-byte libsodium form, so the same string can be pasted straight into a JS client (libsodium / tweetnacl both want 64 bytes).

Wire format

envelope = { v:"sbp/0.3e", tag, kid, eph, sig, cipher }
  tag    = base64url(HMAC-SHA256(rk, scope))       scope: "dev|{did}" | "sid|{did}|{sid}"
  eph    = base64url(ephemeral X25519 public key)
  cipher = base64url(nonce[24] || crypto_box(inner))
  sig    = base64url(Ed25519.sign(sign_sk, tag || eph || cipher))

Every frame uses a fresh ephemeral keypair, so each crypto_box key is independent (forward secrecy within a session). The relay verifies sig with sign_pk from its public-key directory and routes by tag — it can do neither without rk / box_sk.

Layout

• sbp_client/crypto.py — KeyPair, KeyRing, derive_rk, compute_tag, wrap/unwrap, sign_challenge/verify_challenge, verify_sig.
• sbp_client/transport.py — E2EELink / Scope: transport-agnostic envelope wrap/unwrap + subscription control, shared by sbp-agents and client apps.

Related

• sbp-dispatcher — the relay (public-key directory + opaque-tag pub/sub).
• sbp-agents — the device/panel that reports local tmux / CLI sessions.