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Trail: Security Features in Java SE
Lesson: Generating and Verifying Signatures
Section: Verifying a Digital Signature
Generating and Verifying Signatures
Generating a Digital Signature
Prepare Initial Program Structure
Generate Public and Private Keys
Sign the Data
Save the Signature and the Public Key in Files
Compile and Run the Program
Verifying a Digital Signature
Prepare Initial Program Structure
Input and Convert the Encoded Public Key Bytes
Input the Signature Bytes
Verify the Signature
Compile and Run the Program
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Verify the Signature

> You've added code to the `VerSig` program to

> - Input the encoded key bytes and converted them to a `PublicKey` named `pubKey`

> - Input the signature bytes into a byte array named `sigToVerify`
> You can now proceed to do the verification.

> Initialize the Signature Object for Verification

> > As with signature generation, a signature is verified by using an instance of the `Signature` class. You need to create a `Signature` object that uses the same signature algorithm as was used to generate the signature. The algorithm used by the `GenSig` program was the SHA1withDSA algorithm from the SUN provider.

> > >     Signature sig = Signature.getInstance("SHA1withDSA", "SUN");

> > Next, you need to initialize the `Signature` object. The initialization method for verification requires the public key.

> > >     sig.initVerify(pubKey);

> Supply the Signature Object With the Data to be Verified

> > You now need to supply the `Signature` object with the data for which a signature was generated. This data is in the file whose name was specified as the third command line argument. As you did when signing, read in the data one buffer at a time, and supply it to the `Signature` object by calling the `update` method.

> > >     FileInputStream datafis = new FileInputStream(args[2]);
> > >     BufferedInputStream bufin = new BufferedInputStream(datafis);

> > >     byte[] buffer = new byte[1024];
> > >     int len;
> > >     while (bufin.available() != 0) {
> > >         len = bufin.read(buffer);
> > >         sig.update(buffer, 0, len);
> > >     };

> > >     bufin.close();

> Verify the Signature

> > Once you have supplied all of the data to the `Signature` object, you can verify the digital signature of that data and report the result. Recall that the alleged signature was read into a byte array called `sigToVerify`.

> > >     boolean verifies = sig.verify(sigToVerify);

> > >     System.out.println("signature verifies: " + verifies);

> > The `verifies` value will be `true` if the alleged signature (`sigToVerify`) is the actual signature of the specified data file generated by the private key corresponding to the public key `pubKey`.

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