How to validate a public and private key pair in Java

You can verify if a key pair matches by

• creating a challenge (random byte sequence of sufficient length)
• signing the challenge with the private key
• verifying the signature using the public key

This gives you a sufficiently high confidence (almost certainity) that a key pair matches if the signature verification is ok, and an absolute certainity that a key pair does not match otherwise.

Example code:

KeyPairGenerator keyGen = KeyPairGenerator.getInstance("RSA");
keyGen.initialize(2048);

KeyPair keyPair = keyGen.generateKeyPair();
PublicKey publicKey = keyPair.getPublic();
PrivateKey privateKey = keyPair.getPrivate();

// create a challenge
byte[] challenge = new byte[10000];
ThreadLocalRandom.current().nextBytes(challenge);

// sign using the private key
Signature sig = Signature.getInstance("SHA256withRSA");
sig.initSign(privateKey);
sig.update(challenge);
byte[] signature = sig.sign();

// verify signature using the public key
sig.initVerify(publicKey);
sig.update(challenge);

boolean keyPairMatches = sig.verify(signature);