Implement custom encryption with Metadata signatures

GroupDocs.Signature provides ability to embed into Metadata signature custom objects. This feature is implemented over object serialization to string and further encryption. By default library uses json format serialization and symmetric encryption but allows to provide custom encryption. This procedure requires implementation of interface IDataEncryption with two methods to encrypt and decrypt data.

Here are the steps to embed into Metadata values with custom encryption with GroupDocs.Signature: 

  • Define custom data encryption class that implements IDataEncryption interface. By default GroupDocs.Signature has several encryption implementation you can use but allows user to customize it.

  • Implement if needed custom data serialization class that implement IDataSerializer interface. By default GroupDocs.Signature uses embedded json format serialization but allows user to customize it.

  • Implement class with properties and specify if needed class attributes (like custom serialization attribute, custom encryption attribute), specify attributes for properties like  FormatAttribute to specify serialization name and display format, same as SkipSerializationAttribute to mark property of class as not serialize.

  • Create new instance of Signature class and pass source document path as a constructor parameter.

  • Create one or several objects of MetadataSignature object with  MetadataSignature.Value as object instance. Setup MetadataSignature.DataEncryption property or use same for MetadataSignOptions.DataEncryption if all signatures should be encrypted.

  • Instantiate the MetadataSignOptions object according to your requirements, add all metadata signatures to its collection and setup if needed DataEncryption property.

  • Call Sign method of Signature class instance and pass MetadataSignOptions to it.

This example shows how to specify custom serialization class. This class could be implemented also as Attribute (optional) to specify as class attribute.

// Define class that implements IDataEncryption interface
private class CustomXOREncryption : IDataEncryption
{
    /// <summary>
    /// Gets or sets non empty key for encryption (at least one character)
    /// </summary>
    public int Key { get; set; }
    /// <summary>
    /// Encode method to encrypt string.
    /// </summary>
    /// <param name="source">Source string to encode.</param>
    /// <returns>Returns enccrypted string</returns>
    public string Encode(string source)
    {
        return Process(source);
    }
    /// <summary>
    /// Decode method to obtain decrypted string.
    /// </summary>
    /// <param name="source">Source string to decode.</param>
    /// <returns>Returns decrypted string</returns>
    public string Decode(string source)
    {
        return Process(source);
    }
    /// <summary>
    /// Using XOR operation get encoded / decoded string
    /// </summary>
    /// <param name="source"></param>
    /// <returns></returns>
    private string Process(string source)
    {
        StringBuilder src = new StringBuilder(source);
        StringBuilder dst = new StringBuilder(src.Length);
        char chTmp;
        for (int index = 0; index < src.Length; ++index)
        {
            chTmp = src[index];
            chTmp = (char)(chTmp ^ this.Key);
            dst.Append(chTmp);
        }
        return dst.ToString();
    }
}

Definition of class

This example shows how to define custom class with serialization and encryption properties and setup Format attributes for properties. 

public class DocumentSignatureData
{
    [Format("SignID")]
    public string ID { get; set; }
    [Format("SAuth")]
    public string Author { get; set; }
    [Format("SDate", "yyyy-MM-dd")]
    public DateTime Signed { get; set; }
    [Format("SDFact", "N2")]
    public decimal DataFactor { get; set; }
    [SkipSerialization]
    public string Comments { get; set; }
}

Implementation of embedding custom object into Metadata signature

This example shows how to embed custom object into Metadata signature.

 using (Signature signature = new Signature("sample.docx"))
{
    // create data encryption
    IDataEncryption encryption = new CustomXOREncryption();
    // setup options with text of signature
    MetadataSignOptions options = new MetadataSignOptions()
    {
        // set encryption for all metadata signatures for this options
        // if you need separate encryption use own MetadataSignature.DataEncryption property
        DataEncryption = encryption
    };
    // create custom object
    DocumentSignatureData documentSignatureData = new DocumentSignatureData()
    {
        ID = Guid.NewGuid().ToString(),
        Author = Environment.UserName,
        Signed = DateTime.Now,
        DataFactor = 11.22M
    };
    // setup signature metadata
    WordProcessingMetadataSignature mdSignature = new WordProcessingMetadataSignature("Signature", documentSignatureData);
    // setup signature metadata
    WordProcessingMetadataSignature mdAuthor = new WordProcessingMetadataSignature("Author", "Mr.Scherlock Holmes");
    // setup data of document id
    WordProcessingMetadataSignature mdDocId = new WordProcessingMetadataSignature("DocumentId", Guid.NewGuid().ToString());
    // add signatures to options
    options.Signatures.Add(mdSignature);
    options.Signatures.Add(mdAuthor);
    options.Signatures.Add(mdDocId);
    // sign document to file
    signature.Sign("MetadataCustomEncryptionObject.docx", options);
}

More resources

GitHub Examples

You may easily run the code above and see the feature in action in our GitHub examples:

Free Online App 

Along with full-featured .NET library we provide simple, but powerful free Apps.
You are welcome to eSign PDF, Word, Excel, PowerPoint documents with free to use online GroupDocs Signature App.