data.cs

// this method validates a document with the last block on the blockchain
[System.Web.Http.HttpPost]
public bool ValidateDocument([FromBody] string Document, [FromBody] string UniqueId)
{
    if (Document == null || UniqueId == null)
        return false;

    // calculate the MD5 of the uploaded document
    string sChecksum = Checksum.CalculateMD5(Convert.FromBase64String(Document));

    Blockchain newDocument;

    // load the associated blockchain
    if (System.IO.File.Exists(
        Server.MapPath("~/App_Data/Blockchains/" + UniqueId + ".bc")))
    {
        string bc = System.IO.File.ReadAllText(
            Server.MapPath("~/App_Data/Blockchains/" + UniqueId + ".bc"));

        newDocument = JsonConvert.DeserializeObject<Blockchain>(bc);

        // get the SignedDocument object from the block
        SignedDocument signedDocument = 
            JsonConvert.DeserializeObject<SignedDocument>(newDocument.GetCurrentBlock().Data);

        // compare the checksum in the stored block
        // with the checksum of the uploaded document
        if (signedDocument.Checksum == sChecksum)
            return true; // document valid
        else
            return false; // not valid
    }
    else
        return false; // blockchain doesn't exist
}