RSA -- signing/verifying data

Question

assuming i have the public and private keys already....
class RSA{
     Signature sign;
... //sets up keys and stuff
     private void setupRSA(){
          sign = Signature.getInstance("RSA");
          sign.initSign(privateKey);
          sign.initVerify(publicKey);
     public void sign(final String plaintext) throws SignatureException{
          byte [] plaintextByte = plaintext.getBytes();
          sign.update(plaintextByte);
          byte [] signedData = sign.sign();
          return new sun.misc.BASE64Encoder().encode(signedData);
     public boolean verify(final String signed) throws IOException, SignatureException{
          byte [] signedByte = new sun.misc.BASE64Decoder().decodeBuffer(signed);
          sign.update(signedByte);
          return sign.verify(signedByte);
     public static void main(String [] args){
          String signed = rsa.sign("Signature");
          boolean verify = rsa.verify(signed);
     }i get the following exception:
java.security.NoSuchAlgorithmException: RSA Signature not available
     at java.security.Signature.getInstance(Signature.java:193)
     at RSA.setupRSA(RSA.java:62)
     at RSA.<init>(RSA.java:18)
     at Test.main(Test.java:7)
java.lang.NullPointerException
what should i do to fix this problem?
over CodeProject, i see the example using "SHA1withRSA". when i use that, i get only the NullPointerException.
is there a way to use RSA only... and how do i get rid of the NullPointerException?

Answer 1

1) Normally, a signature is the encryption of the digest of the data to be signed so you need to specify the digest method. "RSA" on it's own does not specify the digest method since there is no default digest method.
2) A Signature cannot be initialised for for both 'signing' and 'verification' at the same time. You can re-initialize whenever you like so you just have to initSign() just before signing and initVerify() just before verifying.