trumpsignal-backend/app/services/signed_request.py

"""
Signed-request verification: EIP-191 signature that binds {action, wallet, timestamp, body}.

Message format (human-readable — what the user sees in MetaMask):

    TrumpSignal · {ACTION}
    wallet: 0x...
    timestamp: 1713724800000
    body: <sha256_hex_of_canonical_json, or "-" for empty>

Server verifies:
  1. recovered signer == wallet
  2. |now - timestamp| <= MAX_SKEW_SECONDS (5 min)
  3. sha256(canonical_json(body)) matches the hash in the message
  4. signature not seen before (replay cache, TTL = MAX_SKEW)

Same scheme is used for writes (with body) and reads (body=None → "-").
"""
import hashlib
import json
import logging
import time
from typing import Any, Optional

from eth_account import Account
from eth_account.messages import encode_defunct
from fastapi import HTTPException

logger = logging.getLogger(__name__)

MAX_SKEW_SECONDS = 300  # 5 minutes


def canonical_body_hash(body: Optional[Any]) -> str:
    """Stable sha256 of a Python dict/primitive. `None` → '-'."""
    if body is None:
        return "-"
    payload = json.dumps(body, sort_keys=True, separators=(",", ":"), default=str)
    return hashlib.sha256(payload.encode("utf-8")).hexdigest()


def build_message(action: str, wallet: str, timestamp_ms: int, body_hash: str) -> str:
    return (
        f"TrumpSignal · {action}\n"
        f"wallet: {wallet.lower()}\n"
        f"timestamp: {timestamp_ms}\n"
        f"body: {body_hash}"
    )


# ── Replay cache ──────────────────────────────────────────────────────────
# In-memory; fine for a single-process deploy. For multi-worker, move to Redis.
_seen: dict[str, float] = {}  # sha256(signature) → expiry epoch seconds

# Purge when the dict exceeds this size. At 5 minutes TTL and ~10 req/s peak
# that's ~3000 entries at saturation; 1000 gives comfortable headroom while
# keeping purge cost tiny (<1 ms for 1000-entry linear scan).
_SEEN_PURGE_THRESHOLD = 1000


def _cache_put(sig: str, ttl: float) -> bool:
    """Return True if sig was new, False if replay."""
    now = time.time()
    # Lazy purge: only scan when we've crossed the threshold, not every call.
    # Scanning 1000 vs 5000 entries keeps individual request latency negligible
    # even under burst traffic.
    if len(_seen) >= _SEEN_PURGE_THRESHOLD:
        expired = [k for k, v in _seen.items() if v < now]
        for k in expired:
            del _seen[k]
    key = hashlib.sha256(sig.encode("utf-8")).hexdigest()
    if key in _seen and _seen[key] > now:
        return False
    _seen[key] = now + ttl
    return True


# ── Public API ────────────────────────────────────────────────────────────
def verify_signed_request(
    *,
    action: str,
    wallet: str,
    timestamp_ms: int,
    signature: str,
    body: Optional[Any],
    allow_replay: bool = False,
) -> None:
    """Raise HTTPException(401/422) if invalid. Silent on success.

    allow_replay=True skips the nonce cache — use only for idempotent reads
    (e.g. GET /user). Timestamp expiry still bounds the window to MAX_SKEW.
    """
    wallet = wallet.lower().strip()

    # 1. Freshness
    now_ms = int(time.time() * 1000)
    if abs(now_ms - timestamp_ms) > MAX_SKEW_SECONDS * 1000:
        raise HTTPException(401, "Signed request expired or clock skew too large")

    # 2. Recover signer
    body_hash = canonical_body_hash(body)
    message = build_message(action, wallet, timestamp_ms, body_hash)
    try:
        recovered = Account.recover_message(encode_defunct(text=message), signature=signature).lower()
    except Exception as exc:
        logger.warning("Signature recovery failed (%s): %s", action, exc)
        raise HTTPException(401, "Signature verification failed")
    if recovered != wallet:
        raise HTTPException(401, "Signature does not match wallet")

    # 3. Replay guard (writes only)
    if not allow_replay:
        if not _cache_put(signature, ttl=MAX_SKEW_SECONDS):
            raise HTTPException(401, "Signature already used (replay blocked)")


def verify_signed_request_any(
    *,
    actions: list[str],
    wallet: str,
    timestamp_ms: int,
    signature: str,
    body: Optional[Any],
    allow_replay: bool = False,
) -> None:
    """Accept any one of several signed read-actions.

    Useful for wallet-owned read endpoints where the frontend may already hold
    a fresh cached envelope for a sibling read action such as `view_user`.
    """
    last_exc: Optional[HTTPException] = None
    for action in actions:
        try:
            verify_signed_request(
                action=action,
                wallet=wallet,
                timestamp_ms=timestamp_ms,
                signature=signature,
                body=body,
                allow_replay=allow_replay,
            )
            return
        except HTTPException as exc:
            last_exc = exc
    if last_exc is not None:
        raise last_exc