How to sign stuff and then verify it using PKCS7

You'd think security is about obfuscation when trying to find resources on how to do stuff. We end up with people not really knowing how to implement security and implement it wrongly.
I might be one of those people so be warned, but if you need to a resource to get started at least, you can look here. So let's begin.

What we're doing is creating a pair of X509 keys; a private key and a public key. Both keys will be stored in a file called a keystore file. This file exists on your computer and is unlocked by a password. Once the file is unlocked, you need to unlock the key as well, so two passwords are needed, unless you use the same password for both.

You can use the private key to sign data and public key to verify the signature. Here's what I think happens during the process.

1. Sign some data using the private.
2. Optionally include the public key and data in the signature so that it can be verified.
3. Send the signature to the recipient.
4. Recipient verifies the data in the signature
5. Recipient verifies the signature with the certificate containing the Public key shared earlier.
6. If both 4 and 5 are verified correct, then the data can be considered to the signed.

Here's some code using the free bouncy castle library for Java.

Here we open the keyfile and unlock it using the keyfile password.

    KeyStore keystore = KeyStore.getInstance("jks");
    InputStream input = new FileInputStream("c:\\keyfile.jks");
    try {
  keystore.load(input, password);
    } catch (IOException e) {
    }

Now we get the private key for the key pair that we have in the keystore. The keypair is identified by an alias. Again we use the password to unlock the private key.

    PrivateKey privateKey = (PrivateKey) keystore.getKey(alias, password.toCharArray());

Next we get the Certificate for the key pair to include in the signature. The certificate is not protected by a password. This means its OK to share it with others. It's not ok to share the Private key as it will enable others to sign stuff on your behalf.

    Certificate cert = keystore.getCertificate(alias);

Here is the signing code. First we add the provider to the Security Manager. It will then be reference using the provider code "BC". We create a data generator, initialize it with a signer, wrap our data in a content wrapper and finally generate the signature based on the content, and the "BC" security provider.

    // Create the provider
    Security.addProvider(new BouncyCastleProvider());
    // Create a generator
    CMSSignedDataGenerator generator = new CMSSignedDataGenerator();
    // initialize the signer
    generator.addSigner(privateKey, (X509Certificate) cert, CMSSignedDataGenerator.DIGEST_SHA1);
    // turn our data into content
    byte[] data = "ABC123".getBytes();
    CMSProcessable content = new CMSProcessableByteArray(data);
    // sign the data
    CMSSignedData signedData = generator.generate(content, true, "BC")

The results gets put into bytes and sent out. You probably want to encode it somewhat using BASE64. I'll leave that to you.

    byte[] result = signedData.getEncoded();

That's it for signing. Now what about verification of the signature. Remember that we will share the certificate containing the Public key with the recipient - probably ahead of time. If not you need to verify the credentials within the certificate. This will probably involve a Certificate authority. If the certificate was exchanged securely the CA should not be an issue.

Next comes the data and the signature. We receive the signature as an array of bytes and we wrap it.

    CMSSignedData s = new CMSSignedData(result);

We extract the public key from the keystore in the usual way. But this would be the keystore of the recipient. It would not have the private key.

    PublicKey publicKey = cert.getPublicKey();

We can extract the content from the signature in this way. Notice its an array of bytes so, re-encode it in ASCII or other format. This covers step 4 above. I will not go into details of how to verify it. For me, it was sufficient to print it out and verify it was indeed what I signed in the first place.

    byte[] signedContent = (byte[]) s.getSignedContent().getContent();

Now we verify. We iterate the list of signers, and ensure one of the signers is the party we expect. If verified returns true for at least one signer, that's ok for us. Your rules for verification may be different. For example, it may need to be signed by three people or at least two out of three.

    java.util.Iterator i = signers.getSigners().iterator();
    while ( i.hasNext()) {
        SignerInformation signer = (SignerInformation) i.next();
        verified = signer.verify(publicKey, "BC");
    }

That's it. Take it for what you paid for it. Nothing.