Digital Signatures

• 1. Sender creates a key pair (private, public) since digital signatures rely on asymmetric key cryptography

private KeyPair generateKeyPair()throws NoSuchAlgorithmException{
    KeyPairGenerator keyPairGenerator=KeyPairGenerator.getInstance("RSA");
    keyPairGenerator.initialize(2048);
    return keyPairGenerator.generateKeyPair();
    }

• 2. Document contents are hashed to create a digest a.k.a byte [] documentHash for example using sha256

    private byte[] calculateHash(String data)throws NoSuchAlgorithmException{
        MessageDigest digest=MessageDigest.getInstance("SHA-256");
        return digest.digest(data.getBytes());
        }

• 3. Signature aka((byte [] signatureByte)) is created by encrypting the digest(byte [] documentHash) with the
     sender's private key.

private byte[] signDocument(byte[] documentHash, PrivateKey privateKey) throws Exception {
    Signature signature = Signature.getInstance("SHA256withRSA");
    signature.initSign(privateKey);
    signature.update(documentHash);
    return signature.sign();
}

• 4. Digest is embedded in the document which is then sent

Receiver;

• Recipient decrypts the digest using the sender's public key.

• Recipient calculates the hash from the documents content

• If the recalculated digest matches the decrypted digest, the document has not been tampered with since it was sent

• Digital certificate issued by a certification authority guarantees sender's identity.

• Digital certificate contains a public key along with other information about the sender

• a digest is a fixed-size representation of the original document's content that is generated using a hash function
  like SHA-256.