Create a signature to validate a subscription offer using your private key.
Before you can create a signature on your server, you must complete the one-time setup to generate a private key in App Store Connect, as described in Setting Up Subscription Offers. Always use a secure connection when sending data, including the signature, between your app and server. For more information on ensuring your data’s security, see Preventing Insecure Network Connections.
To create the signature, you will need parameters that identify the product and offer, parameters generated by the server, and your private key. To generate the signature, you combine required parameters, then sign and encode the resulting string.
Combine the Parameters
In the first step of generating the signature, you need the following parameters, most of which you also supply for
The app bundle identifier.
A string that identifies the private key you use to generate the signature. You can find this identifier in App Store Connect Users and Access > Keys, in the KEY ID column for the subscription key you generated.
The subscription product identifier,
product. The app can provide this value.
The subscription discount identifier,
identifier. The app can provide this value.
An optional string value that you define; may be an empty string. The app can provide this value and uses it in
UUIDvalue that your server defines. The string representation of the
nonceused in the signature must be lowercase.
A timestamp your server generates in UNIX epoch time format, in milliseconds; the timestamp keeps the offer active for 24 hours.
Combine the parameters into a UTF-8 string with an invisible separator (
'\u2063') between them, in the order shown:
Sign the Combined String
Sign the combined UTF-8 string with the following key and algorithm:
The PKCS#8 private key (downloaded from App Store Connect) that corresponds to the
keyin the UTF-8 string.
The Elliptic Curve Digital Signature Algorithm (ECDSA) with a SHA-256 hash.
The result should be a Digital Encoding Rules (DER)-formatted binary value, which is the signature.
Validate Locally and Encode the Signature
Consider validating your signatures locally to ensure your signing process works as designed. You can create a public key derivative of your private key to test against. One way to create this key is by running the OpenSSL command from the terminal with:
Base64-encode the binary signature you generated to get the final signature string to send to the App Store for validation. The signature string will look similar to:
Respond to the Request
Respond to the app’s request for the signature over a secure connection, providing the encoded signature string, the
timestamp, and the
key. Keep in mind that each payload, signature, and
nonce is only valid for one buy request, even if the buy fails.
See Create a Signature for information about the app’s request and how it uses the signature.