Need help with implementing Digital Signatures

Question

Hello,
Here's an excerpt from a security book:
To create a digital signature, a sender first takes the original plaintext message and runs it through a hash function.
The Secure Hash Algorithm (SHA-1) is the standard for hash functions.
The hash value is also known as a message digest.
Next, the sender uses the its private key to encrypt the message digest. This step creates
a digital signature and authenticates the sender, since only the owner of that private key could
encrypt the message. The sender encrypts the original message with the receiver’s public key
and sends the encrypted message and the digital signature to the receiver.
The receiver uses the sender’s public key to decipher the original digital signature and reveal the message
digest. The receiver then uses his or her own private key to decipher the original message.
Finally, the receiver applies the agreed upon hash function (e.g. SHA-1 or MD5) to the original
message. If the hash value of the original message matches the message digest included
in the signature, there is message integrity-the message has not been altered in transmission.
I would like to implement the sender's part and create an encrypted message and the digital signature.
Suppose I have in hand a message (let's call it myMessage), my private key (let's call it myPrivateKey) and the receiver's public key (let's call it receiverPublicKey).
In order to encrypt the original message with the receiver’s public key I've implemented this piece of code:
Cipher cipher = Cipher.getInstance("RSA");
cipher.init(Cipher.ENCRYPT_MODE,receiverPublicKey);
byte[] binaryCryptData = cipher.doFinal(myMessage.getBytes());
String encPayload = Base64.byteArrayToBase64(binaryCryptData);
return encPayload;In order to create the signature I've implemented this piece of code:
Signature RSA = Signature.getInstance("SHA1withRSA");
RSA.initSign(myPrivateKey);
byte bt[] = message.getBytes();
RSA.update(bt);
byte[] signature = RSA.sign();My assumption is that I don't follow the exact guidlines in the book for the sender.
Can someone please let me know what I'm missing and help me in adjusting my code with the books standards?
Thank you in advance,
Roy

Answer 1

it's your homework, not ours.
Congratulations on typing in everything BUT the most important part that it's all about, but that's not enough.