make MessageDigest and Cipher equivalent to java.security.Signature

Question

I want to implement my own signature function using RSA as a cipher algorithm and SHA-1 as a hash function, to do so I implemented those two function:

public byte[] mySign(byte[] aMessage){
    try{
        // get an instance of a cipher with RSA with ENCRYPT_MODE
        // Init the signature with the private key
          Cipher cipher = Cipher.getInstance("RSA");
          cipher.init(Cipher.ENCRYPT_MODE, this.thePrivateKey);

        // get an instance of the java.security.MessageDigest with sha1
             MessageDigest meassDs = MessageDigest.getInstance("SHA-1");

        // process the digest
             meassDs.update(aMessage);
             byte[] digest = meassDs.digest();

            byte [] signature = cipher.doFinal(digest);

        // return the encrypted digest
        return signature;

    }catch(Exception e){
        System.out.println(e.getMessage()+"Signature error");
        e.printStackTrace();
        return null;
    }

}



public boolean myCheckSignature(byte[] aMessage, byte[] aSignature, PublicKey aPK){
    try{
        // get an instance of a cipher with RSA with DECRYPT_MODE
        // Init the signature with the public key
          Cipher cipher = Cipher.getInstance("RSA");
          cipher.init(Cipher.DECRYPT_MODE, aPK);

        // decrypt the signature
             byte [] digest1 = cipher.doFinal(aSignature);

        // get an instance of the java.security.MessageDigest with sha1
             MessageDigest meassDs = MessageDigest.getInstance("SHA-1");

        // process the digest
             meassDs.update(aMessage);
             byte[] digest2 = meassDs.digest();

        // check if digest1 == digest2
        if (digest1 == digest2)
        return true;
        else
            return false;

    }catch(Exception e){
        System.out.println("Verify signature error");
        e.printStackTrace();
        return false;
    }
}   

Then when I use those function, I always obtain false as result which means that my functions are not working right :

byte[] message = "hello world".getBytes();
byte[] signature;

signature = mySign(message );
boolean bool = myCheckSignature(message , signature, thePublicKey);
System.out.println(bool);

Answer 1

While your requirement

to implement a method to create a signature using an alternative to java.security.Signature

as a whole is questionable (as mentioned in the comments to your question), your solution can be improved.

In particular there is an error in your myCheckSignature code:

// check if digest1 == digest2
if (digest1 == digest2)
    return true;
else
    return false;

(digest1 == digest2) checks whether you have the identical array object, not whether you have two arrays with equal contents.

What you really want to do is

if (Arrays.equals(digest1, digest2))
    return true;
else
    return false;

or more compactly

return Arrays.equals(digest1, digest2);

Arrays is a utility class in the package java.util.

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By the way, you do

byte[] message = "hello world".getBytes();

getBytes without explicitly selecting a character encoding may result in different results on different platforms or different Java versions. Something that should not happen in this context!