How can I generate a RSA key for use with com.auth0 java-jwt?

Question

States that setting up the algorithm for JWT should be as simple as

//RSA
RSAPublicKey publicKey = //Get the key instance
RSAPrivateKey privateKey = //Get the key instance
Algorithm algorithmRS = Algorithm.RSA256(publicKey, privateKey);

The problem is I can't work out how to create an RSAPublicKey and RSAPrivateKey instance without touching the filesystem.

It should be secure.
It shouldn't create the key on the file system, as I plan on storing it via another method.

Normally this is the sort of thing I'd guess at until I get right, but considering it's cryptography I want to do the right thing.

keygen = KeyPairGenerator.getInstance("RSA");
        RSAKeyGenParameterSpec spec = new RSAKeyGenParameterSpec(2048, RSAKeyGenParameterSpec.F4); //What does F4 mean vs F0?
            keygen.initialize(spec);
            KeyPair keypair = keygen.generateKeyPair();
            PublicKey pub = keypair.getPublic(); //Wrong type, need RSAPublicKey
            PrivateKey priv = keypair.getPrivate(); //Wrong type, need RSAPrivateKey

pedrofb · Accepted Answer · 2017-10-16 08:17:51Z

4

You can directly cast the public and private keys to RSAPublicKey and RSAPrivateKey because you are using a RSA KeyPairGenerator

RSAPublicKey rsaPublicKey = (RSAPublicKey) keypair.getPublic();
RSAPrivateKey rsaPrivateKey = (RSAPrivateKey) keypair.getPrivate();

You can get the key content using key.getEncoded(); (no cast needed) and store it as a byte array any way you like

answered Oct 16, 2017 at 8:17

pedrofb

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2 Comments

Ryan Leach Over a year ago

Do you know what the F0 vs F4 exponents are for?

pedrofb Over a year ago

I think F0 (3) is the minimum value for public exponent used by RSAKeyPairGenerator and F4(65537) is the recommended value by RFC. See crypto.stackexchange.com/questions/3110/…

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