Work with JSON Web Signatures

This short guide shows how to create a JSON Web Signature (JWS) when working with our Open Banking API.

JSON Web Signatures (JWSs) are required in several steps when working with all the Payment initiation endpoints. They are used to validate authenticity and integrity of the content of an API request or response.

JWSs are similar to JSON Web Tokens (JWTs), as they both consist of a header, a payload and a cryptographic signature, separated by a dot ..

The JWSs in Open Banking are a special case, because the payload section is omitted. This is because it is already present in the payload of the API request or response.

Prerequisites​

To work with JWSs, you must first prepare:

• A signing key and signing certificate pair
• A JWKs URL at which your JSON Web Key (JWK) is publicly available
• A JWS library to cryptographically create a JSON Web Signature

To learn how to obtain them for testing purposes, see Get Started: Prepare your Sandbox environment.

Header​

The header section of the JWS contains mandatory parameters to validate the signature of payment requests.

You must provide the kid parameter of your signing certificate and the root domain of the URL where your JWK is hosted.

This is a template JWS header that you can copy and fill with your own kid value and JWKs URL root domain:

{
    "typ": "JOSE",
    "alg": "PS256",
    "kid": "<kid parameter of your signing certificate>",
    "crit": ["http://openbanking.org.uk/tan"],
    "http://openbanking.org.uk/tan": "<root domain of your JWKs URL>"
}

Payload​

In the case of JWS, the payload will be the JSON payload of the API request.

For demonstration purposes, in this guide we are using this sample JSON payload:

{
  "test": "payload"
}

Signature​

To compute the signature, you can use any library that is compatible with your environment.

For example, in JavaScript, a library such as jsrsasign can be used.

private_key = "---abc---";

header = {
  "typ": "JOSE",
  "alg": "PS256",
  "kid": "abc123",
  "crit": ["http://openbanking.org.uk/tan"],
  "http://openbanking.org.uk/tan": "example.com"
};

payload = {
  "test": "payload"
};

sJWS = KJUR.jws.JWS.sign("PS256", header, payload, private_key);

Full Request with JWS​

The final JWS for the above header and payload would result in:

The payload section above was bolded to highlight the three sections composing a JWS and facilitate reading.

To avoid sending redundant data, the payload is then removed from the JWS, leaving only the header and signature with an empty payload section:

ewogICJ0eXAiOiAiSk9TRSIsCiAgImFsZyI6ICJQUzI1NiIsCiAgImtpZCI6ICJhYmMxMjMiLAogICJjcml0IjogWyJodHRwOi8vb3BlbmJhbmtpbmcub3JnLnVrL3RhbiJdLAogICJodHRwOi8vb3BlbmJhbmtpbmcub3JnLnVrL3RhbiI6ICJleGFtcGxlLmNvbSIKfQ..M-tx9BIPmHO8_VpGVmO3yDq--------dFtS6VgiM1kmQuK6E_3S8tKh2BKV_W69Q

For Open Banking API requests that require a JSON Web signature, this string must be included in the x-jws-signature header, as in the example below:

curl --location 'https://oba.revolut.com/domestic-payments' \
--header 'x-fapi-financial-id: 001580000103UAvAAM' \
--header 'Authorization: Bearer oa_prod_pwDmU5mA2QOFvxcWszVP06uhI2nIcOAsgC4iuwlzTiw' \
--header 'Content-Type: application/json' \
--header 'x-jws-signature: ewogICJ0eXAiOiAiSk9TRSIsCiAgImFsZyI6ICJQUzI1NiIsCiAgImtpZCI6ICJhYmMxMjMiLAogICJjcml0IjogWyJodHRwOi8vb3BlbmJhbmtpbmcub3JnLnVrL3RhbiJdLAogICJodHRwOi8vb3BlbmJhbmtpbmcub3JnLnVrL3RhbiI6ICJleGFtcGxlLmNvbSIKfQ..M-tx9BIPmHO8_VpGVmO3yDq--------dFtS6VgiM1kmQuK6E_3S8tKh2BKV_W69Q' \
--header 'x-idempotency-key: 048d46a1-f381-4f2b-a79e-a43ee7d1ff90' \
--data '{
  "test": "payload"
}'

What's next​

• Initiate your first payment
• Test getting a payment consent with Postman
• See the API reference for Domestic payments