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Grain signs all webhook payloads with an HMAC signature so you can verify they originated from Grain and haven’t been tampered with.

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Headers

Each webhook request includes two security headers:
HeaderDescription
X-Grain-SignatureHMAC-SHA256 signature in format v1={signature}
X-Grain-TimestampUnix timestamp (seconds) when the request was signed

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Verification Steps

  1. Extract the timestamp and signature from the request headers.
  2. Check the timestamp is within an acceptable window (recommended: 5 minutes). This protects against replay attacks.
  3. Compute the expected signature using your webhook secret.
  4. Compare signatures using a timing-safe comparison function.

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Computing the Signature

The signature is computed over the string {timestamp}.{raw_request_body}: 
signed_payload = "{timestamp}.{raw_json_body}"
expected_signature = "v1=" + HMAC-SHA256(signed_payload, your_webhook_secret)

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Python Example

import hmac
import hashlib
import time

TOLERANCE_SECONDS = 300  # 5 minutes

def verify_webhook_signature(
    payload: str,      # raw request body
    signature: str,    # X-Grain-Signature header
    timestamp: str,    # X-Grain-Timestamp header
    secret: str        # your webhook secret
) -> bool:
    timestamp_int = int(timestamp)
    now = int(time.time())

    # Reject if timestamp is too old
    if now - timestamp_int > TOLERANCE_SECONDS:
        return False

    # Compute expected signature
    signed_payload = f"{timestamp}.{payload}"
    expected_sig = "v1=" + hmac.new(
        secret.encode(),
        signed_payload.encode(),
        hashlib.sha256
    ).hexdigest()

    # Timing-safe comparison
    return hmac.compare_digest(signature, expected_sig)

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Important Notes

  • Always use the raw request body — do not parse and re-serialize the JSON, as this may change the payload.
  • Use timing-safe comparison — standard string comparison is vulnerable to timing attacks.
  • Validate the timestamp — this protects against replay attacks where an attacker resends a captured request.
  • Store your secret securely — treat it like a password; never commit it to source control.