How Can I Recover an RSA Public Key from a Byte Array?

How can I recover an RSA public key from a byte array in Java?

byte[] keyBytes = publicKey.getEncoded();

Recovering an RSA public key from a byte array is a common task in Java, especially when dealing with cryptographic operations. The process involves converting the byte array back into a PublicKey object using the appropriate KeyFactory and specifications.

try {
    byte[] keyBytes = publicKey.getEncoded(); // Obtain the byte array
    KeyFactory keyFactory = KeyFactory.getInstance("RSA"); // Get RSA KeyFactory instance
    X509EncodedKeySpec keySpec = new X509EncodedKeySpec(keyBytes); // Create key specification
    PublicKey recoveredPublicKey = keyFactory.generatePublic(keySpec); // Generate key from spec
} catch (NoSuchAlgorithmException | InvalidKeySpecException e) {
    e.printStackTrace();
}

Causes

• The RSA public key is often stored or transmitted as a byte array for ease of use.
• Converting a public key to a byte array may be needed for storage in a database.
• The byte array may come from external sources or libraries that handle keys.

Solutions

• Use the KeyFactory class to reconstruct the RSA public key from the byte array.
• Specify the X.509 encoding format for the public key, as RSA keys are typically encoded in this format.

Mistake: Not using the correct encoding format when reconstructing the key.

Solution: Ensure you apply X.509 format when creating the X509EncodedKeySpec.

Mistake: Assuming all byte arrays can be converted without validation.

Solution: Validate the byte array to ensure it is a valid RSA public key before attempting to recover it.