I tried to generate the SSH key using the following code

        KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance("RSA");
         keyPairGenerator.initialize(2048);
         KeyPair keyPair=keyPairGenerator.generateKeyPair();

         RSAPublicKey publicKey=(RSAPublicKey)keyPair.getPublic();
         RSAPrivateKey privateKey=(RSAPrivateKey)keyPair.getPrivate();

         String base64PubKey = Base64.encodeBase64String(publicKey.getEncoded());
         ByteArrayOutputStream byteOs = new ByteArrayOutputStream();
         DataOutputStream dos = new DataOutputStream(byteOs);
         dos.writeInt("ssh-rsa".getBytes().length);
         dos.write("ssh-rsa".getBytes());
         dos.writeInt(publicKey.getPublicExponent().toByteArray().length);
         dos.write(publicKey.getPublicExponent().toByteArray());
         dos.writeInt(publicKey.getModulus().toByteArray().length);
         dos.write(publicKey.getModulus().toByteArray());
         String publicKeyEncoded = new String(
                                    Base64.encodeBase64(byteOs.toByteArray()));
         String key =  "ssh-rsa " + publicKeyEncoded + " ";
         System.out.println("Public Key ------");
         System.out.println(key);

         System.out.println("------------------------------");
         System.out.println("Private key");
         System.out.println(Base64.encodeBase64(privateKey.getEncoded()));

Now, When I store the content of private key in a file and trying to validate it with putty it says invalid format of private key.

Could you guys help me out on this, some how I missing the private key format so putty is not recognizing it.

Putty is unable to recognize OpenSSH formatted keys -- they must first be converted into the Putty style using Puttygen.

There is a Debian/Ubuntu package for this conversion:

apt-get install putty-tools
puttygen openssh_formattted_key -o putty_formatted_key.ppk

You might be able to investigate this and figure out how the keys are converted, or run the command as a process from inside your code.

On Windows, the Puttygen GUI is available for this:

https://devops.profitbricks.com/tutorials/use-ssh-keys-with-putty-on-windows/

Download Link: https://the.earth.li/~sgtatham/putty/latest/x86/puttygen.exe

If you want to use this key generated directly, I would suggest you use something like MINGW32/MINGW64, Cygwin, etc. which allow you to use the ssh command in Command Prompt or some other terminal-like window.

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