Check signatures

Verify the events that Fanspay sends to your webhook endpoints.

Fanspay can optionally sign the webhook events it sends to your endpoints by including a signature in each event’s Fanspay-Signature header. This allows you to verify that the events were sent by Fanspay, not by a third party. You can verify signatures manually using your own solution.

Fanspay generates a unique secret key for each account.

Verifying signatures manually

The Fanspay-Signature header included in each signed event contains a timestamp and one or more signatures. The timestamp is prefixed by t=, and each signature is prefixed by a scheme. Schemes start with v, followed by an integer. Currently, the only valid live signature scheme is v1.

Fanspay-Signature: t=1492774577, v1=5257a869e7ecebeda32affa62cdca3fa51cad7e77a0e56ff536d0ce8e108d8bd

Note that newlines have been added for clarity, but a real Fanspay-Signature header is on a single line.

Fanspay generates signatures using a hash-based message authentication code (HMAC) with SHA-256. To prevent downgrade attacks, you should ignore all schemes that are not v1.

You can create a solution to verify webhook event signatures by following these steps:

Step 1: Extract the timestamp and signatures from the header

Split the header, using the , character as the separator, to get a list of elements. Then split each element, using the = character as the separator, to get a prefix and value pair.

The value for the prefix t corresponds to the timestamp, and v1 corresponds to the signature (or signatures). You can discard all other elements.

Step 2: Prepare the signed_payload string

The signed_payload string is created by concatenating:

  • The timestamp (as a string)
  • The character .
  • The actual JSON payload (that is, the request body)

Step 3: Determine the expected signature

Compute an HMAC with the SHA256 hash function. Use the endpoint’s signing secret as the key, and use the signed_payload string as the message.

Step 4: Compare the signatures

Compare the signature (or signatures) in the header to the expected signature. For an equality match, compute the difference between the current timestamp and the received timestamp, then decide if the difference is within your tolerance.

To protect against timing attacks, use a constant-time string comparison to compare the expected signature to each of the received signatures.

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