The XML signature standard specifies the format of the XML signature, but it does not specify how the XML document containing the signature should be structured. In addition to the low-level CreaSign:XmlSignature class, CreaSign also helps you with structuring electronic documents by providing a simple, yet powerful CreaDoc format that supports XML data, visualisations, attachments and multiple signatures.
The document in the CreaDoc format consists of the following parts:
- XML representation of the data (element <cd:Data>). This part is used by applications which need to parse and process the data in the XML format.
- Visualisations of data (element cd:Visualisations). A visualisation contains the data in a format suitable for displaying to the user (for example, a GIF image, a Microsoft Word document, a PDF document,...). The visualisation should be displayed to the signer and the verifier of the document. Note that the data stored into <cd:Data> is not suitable for displaying to the user - a user is not capable of understanding the meaning of the XML tags.
- Attachments. Several attachments can be added to the original document.
- Signatures. The document can contain multiple signatures with timestamps.
The following figure shows the schema of the document's root node:
The document is represented by the CreaDoc class.
Data
The <cd:Data>, <cd:Visualisations> and <cd:Attachments> nodes contain data with the following structure:
The <cd:DataFormat> node describes the format of data:
- <cd:Identifier> is an URI used to identify the data
- <cd:MimeType> describes the mime type of the data (for example image/gif)
- <cd:Encoding> specified encoding of the data (for example base64)
- <cd:FileName> optionally specifies the name of the file that was used as the source for this data node.
The <cd:DataFormat> node can be used by the receiver of the document to select the appropriate viewer for displaying the data to the user. The node is represented by the DataFormat class.
The <cd:Description> node contains a human readable description of the data (for example: "The photography of Alice's house").
The <cd:Content> node contains either the data itself (<cd:EmbeddedData>) or the reference to the data <cd:ReferencedData>. The reference to the data is defined by:
- <cd:DigestMethod> the digest method, used to calculate the digest of the data being referenced,
- <cd:DigestValue> the digest of the data,
- <cd:URI> an identifier used to identify or access the data. This can also be an URL which stores the data.
- <cd:Size> size (in bytes) of the referenced data. This can be used by the receiver application to estimate the time needed to download the referenced data.
The reference to the data is represented by the ReferencedData class.
*Requirements: Requires CreaSign Pro (not supported in CreaSign Lite).
Signatures
Digital signatures are stored inside the <cd:Signatures> node. The detached signatures are used. Each of the signatures should either sign another signature or be a root signature (see Signature types). Each root signature must sign the following:
- the cd:Document/cd:Data node,
- each of the cd:Document/cd:Attachment/cd:Attachments nodes,
- each of the cd:Document/cd:Visualisations/cd:Visualisation nodes.
CreaSign takes care to verify whether all of the important data in the document is signed. If a signature does not cover all of the data specified above, the Verify method reports an error.
*Requirements: Requires CreaSign Pro (not supported by CreaSign Lite).
(c) Crea 2002-2006