Sign your requests

Learn how to sign requests to your Payment API.

All modification requests (for example, POST, DELETE) to our Payments APIs, authenticated with a client_credentials token generated by TrueLayer's authentication server, must be signed.

Signing provides a second layer of security on top of the authorisation bearer token and guarantees that the payload has not been tampered with.

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Supported algorithms

We only support ES512 signing algorithm for modification requests to our Payments APIs.

ES512 belongs to the family of Elliptic Curve Digital Signature Algorithms (ECDSA).

To sign an HTTP request using ECDSA you will need to generate an Elliptic Curve (EC) key pair. You will need:

  • a public key, to be uploaded in the new Payments Settings page in our Console.
  • a private key, to be used for signing requests, which you should not share with anyone outside of your organisation. ES512, in particular, requires a key pair that use the P-521 family of elliptic curves (also known as secpt521r1).

To sign your requests to the Payments API, you'll need to:

  1. Generate signing key pair
  2. Sign your requests

Step 1: Generate signing key pair

You can generate a key pair using openssl.

To generate the private key, run:

docker run --rm -v ${PWD}:/out -w /out -it alpine/openssl ecparam -genkey -name secp521r1 -noout -out ec512-private-key.pem

You can then obtain the public key by running:

docker run --rm -v ${PWD}:/out -w /out -it alpine/openssl ec -in ec512-private-key.pem -pubout -out ec512-public-key.pem

ec512-public-key.pem is the file you should upload in the new Payments Settings page in our Console.

Step 2: Sign a request

You need to specify a Tl-Signature header in your HTTP request. The header value is a JWS with detached content, signed using the ES512 algorithm.

A JWS with detached content has the following structure:

<Base64URLSafeEncoding(JOSEHeader)>..<Base64URLSafeEncoding(signature)>

The payload segment is omitted in the JWS with detached content to generate the signature.

The JOSE header must contain the following parameters and values:

  • alg: ES512
  • kid: The id of the key used for signing as value. This is the UUID value shown in the new Payments Settings in Console next to your uploaded public key.
  • tl_version: 2
  • tl_headers: An ordered comma separated list of headers to include in signing (must at least include Idempotency-Key) as value.

The following example shows what a JOSE header looks like:

{
  "alg": "ES512",
  "kid": "9f2b7bd6-c055-40b5-b616-120ccfd33c49",
  "tl_version": "2",
  "tl_headers": "Idempotency-Key"
}

The JWS payload should be built using the following in order:

  • The HTTP VERB (capitalized), followed by a space, then the absolute path (without trailing slashes), for example: POST /payouts, followed by a newline character \n
  • For each header specified in tl_headers, in the same order (and with the same casing):
  • The header name, followed by a colon, a space, then the header value, for example: Idempotency-Key: 619410b3-b00c-406e-bb1b-2982f97edb8b, followed by a newline character \n
  • The serialized HTTP request body (if sending a body)
POST /payouts
Idempotency-Key: 619410b3-b00c-406e-bb1b-2982f97edb8b
{
  "merchant_account_id": "ee69a988-5e1b-4dab-a1f9-94593f6a132e",
  "amount_in_minor": 100,
  "currency": "GBP",
  "beneficiary": {
    "type": "external_account",
    "name": "Benny Fishery",
    "scheme_identifier": {
      "type": "sort_code_account_number",
      "sort_code": "123456",
      "account_number": "12345678"
    },
    "reference": "myref"
  }
 }

Once you build your payload, you generate a signature with your private key and place it on the right side of your JWS.

Signing libraries

In this public repository we maintain libraries in a range of languages, which provide methods for signing and verifying according to the scheme described above. Our backend libraries also make use of these signing libraries for the requests that they create to our APIs.


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