A small integration, with the bank in control.

# Your backend, after your own login
import hashlib, hmac, json, secrets, time, httpx

def signed(method, path, payload):
    body = json.dumps(payload).encode()
    ts, nonce = str(int(time.time())), secrets.token_hex(12)
    msg = f"{ts}\n{nonce}\n{method}\n{path}\n" + hashlib.sha256(body).hexdigest()
    sig = hmac.new(PARTNER_SECRET, msg.encode(), hashlib.sha256).hexdigest()
    return httpx.request(method, HATHPAY + path, content=body, headers={
        "X-HathPay-Bank": BANK_ID, "X-HathPay-Timestamp": ts,
        "X-HathPay-Nonce": nonce, "X-HathPay-Signature": sig})

r = signed("POST", "/v1/partner/sessions", {
    "customer_ref": customer.ref,
    "accounts": [{"account_token": a.token, "label": a.label}
                 for a in customer.accounts],
    "purpose": "activate",
})
session_token = r.json()["session_token"]   # hand to the in-app module

# Your adapter: HathPay asks, the bank decides
@app.post("/hathpay/authorize-debit")
def authorize_debit(req):
    verify_hathpay(req, ADAPTER_SECRET)          # signature + window + nonce
    p = req.json()
    if seen(p["idempotency_key"]):
        return previous_result(p["idempotency_key"])   # safe retries

    account = accounts.by_token(p["account_token"])
    if account.balance < p["amount_minor"]:
        return {"status": "declined", "reason": "insufficient_funds"}

    ref = ledger.debit(account, p["amount_minor"], merchant=p["merchant_id"])
    return {"status": "approved", "bank_txn_ref": ref}

@app.post("/hathpay/reverse")
def reverse(req): ...

# Every event is signed. Check it, and refuse a nonce you have seen.
def verify_hathpay(req, secret):
    ts    = req.headers["X-HathPay-Timestamp"]
    nonce = req.headers["X-HathPay-Nonce"]
    sig   = req.headers["X-HathPay-Signature"]
    if abs(time.time() - int(ts)) > 300:
        raise Unauthorized("stale")
    msg = f"{ts}\n{nonce}\n{req.method}\n{req.path}\n" + sha256_hex(req.body)
    expected = hmac.new(secret, msg.encode(), hashlib.sha256).hexdigest()
    if not hmac.compare_digest(expected, sig):
        raise Unauthorized("bad signature")
    if nonce_cache.seen(nonce):               # a replayed event
        raise Unauthorized("replayed")
    nonce_cache.add(nonce, ttl=305)