Blockchain

On a high level, the idea is to separate the functionality of signing and the integrity away from the pdf, transferring them into the Blockchain. Following you'll find a real life example case. 

• A document is created, and the document ID (along with its hash, and the name of the hashing algorithm) is placed on a public Blockchain. This does not cause any problems, since the hash typically does not allow you to rebuild the document. The hashing algorithm is placed on the chain as well to ensure long term validation.
• As it happens, this document is an invoice. Bob has had some work done on his house, and the renovation company sends him a digital PDF invoice.
• Bob wants to sign the invoice to indicate he agrees with the price and will pay. He uses his private key to encrypt the hash-value of the document. This signature (comprising of the ID of the document, the original hash-value, the signed hash-value, as well as the names of the hashing algorithm and signing algorithm) are stored on the Blockchain.
• Alice, who works for the renovation company wants to check whether Bob has already signed and thereby accepted the invoice . She can easily look up all records on the Blockchain for a given ID (the document ID). In this case, one of these records is be the record Bob created earlier. Since the record contains the original hash, and the names of all the algorithms involved, Alice can verify 2 things
  • It was Bob that signed, using the hash, the signed hash, and his public key.
  • Bob signed the exact document she sent him, using the document ID and the hash.

In short, Bob accepted the invoice as correct and will pay.

Implementation

The interfaces we impose on blockchain implementations are minimal, yet they provide us with the needed abstraction to enable us to build complex applications and workflows on top of them. We abstract a blockchain as a multimap, allowing end-users to store an object (represented by Record, which is HashMap) and tying it to a key (String).

public interface IBlockChain {
 
    /**
     * Put data on the blockchain
     *
     * @param key  the key being used to put the data on the blockchain
     * @param data the data being put on the blockchain
     */
    public boolean put(String key, Record data);
 
    /**
     * Get data from the blockchain
     *
     * @param key the key being queried
     * @return
     */
    public List<Record> get(String key);
 
    /**
     * Get all data from the blockchain
     * @return
     */
    public List<Record> all();
}

Concrete implementation using JSON-RPC and MultiChain

As a proof of concept we have provided an implementation of the interface IBlockchain using JSON-RPC (remote procedure call) and MultiChain.

If you want to learn more about setting up a blockchain instance with MultiChain, check out their website for more resources, in particular the getting started guide here.

In this example, we will show you how to put a document on the blockchain, with signature:

       IBlockChain mc = new MultiChain(
                "http://127.0.0.1",
                4352,
                "chain1",
                "stream1",
                "multichainrpc",
                "BHcXLKwR218R883P6pjiWdBffdMx398im4R8BEwfAxMm");
 
        InputStream keystoreInputStream = BasicFunctionalityTest.class.getClassLoader().getResourceAsStream("ks");
        InputStream inputFileStream = BasicFunctionalityTest.class.getClassLoader().getResourceAsStream("input.pdf");
 
        AbstractExternalSignature sgn = new DefaultExternalSignature(keystoreInputStream, "demo", "password");
 
        PdfChain chain = new PdfChain(mc, sgn);
 
        // put a document on the chain
        boolean wasAdded = chain.put(inputFileStream);
        Assert.assertTrue(wasAdded);
 
        // check whether the chain now contains this value
        boolean isEmpty = chain.get("z�L{�Wd=��\u007F\u0010��G�").isEmpty();

You can retrieve information about a document from the blockchain like this:

  IBlockChain mc = new MultiChain(
                "http://127.0.0.1",
                4352,
                "chain1",
                "stream1",
                "multichainrpc",
                "BHcXLKwR218R883P6pjiWdBffdMx398im4R8BEwfAxMm");
 
        PdfChain chain = new PdfChain(mc);
 
        for(Record record : chain.get(new File("example_file.pdf")))
        {
            for(Map.Entry<String,Object> en : record.entrySet())
            {
                System.out.println(en.getKey() + "\t" + en.getValue().toString());
            }
            System.out.println("");
        }