How to create public RSA key using modulus and exponent for Sign In with Apple?

Oleksii Skorykh :

I am implementing server side part for Sign In with Apple feature that is used in the IOS application.

In order to verify JWT I need to use public key. I stuck at the moment how to create public key from modulus and exponent that I get from Apple.

Oleksii Skorykh :

To generate public key from the exponent and modulus, they need to be transformed to BigInteger, and then KeyFactory from Java security can be used.

For example:

  String modulus = "modulus from Apple";
  String exponent = "exponent from Apple";
  byte[] modulusByte = Base64.getUrlDecoder().decode(modulus);

  BigInteger modulusAsBigInt = new BigInteger(1, modulusByte);
  byte[] exponentByte = Base64.getUrlDecoder().decode(exponent);
  BigInteger exponentAsBigInt = new BigInteger(1, exponentByte);

  RSAPublicKeySpec spec = new RSAPublicKeySpec(modulusAsBigInt, exponentAsBigInt);
  KeyFactory factory = KeyFactory.getInstance("RSA");
  PublicKey pub = factory.generatePublic(spec);