Digital Signature Formats: CAdES, XAdES and Packaging

What are digital signature formats?

Digital signature formats define how a signature and related properties are represented and associated with signed data. They describe technical structures, not simply a visible mark or a promise of legal acceptance.

CAdES, XAdES and PAdES

CAdES builds on Cryptographic Message Syntax (CMS), adding properties for advanced digital-signature applications. ETSI EN 319 122-1 defines its building blocks and baseline requirements.

XAdES extends XML Signature with qualifying properties, as specified in ETSI EN 319 132-1. Its use of XML does not mean that only a visually rendered XML document can be signed.

PAdES addresses signatures within PDF. An ASiC container can instead package data and associated detached signatures together. These structures answer different packaging and interoperability needs.

Attached, detached, enveloped and enveloping

• Attached or encapsulated: the signed content is carried with the signature structure, under the relevant format.
• Detached: signature and signed data are separate objects. They may still be distributed together inside a container.
• Enveloped XML signature: the signature is contained within the signed XML content, with the appropriate processing rules.
• Enveloping XML signature: the signed content is carried within the signature structure.

Preserve the data, references and transformations needed to determine what was signed. A signature file separated from its required payload may not be independently useful.

Format, evidence profile and legal status

A format can include timestamp evidence or information supporting long-term validation. These capabilities do not automatically establish QES, the signer’s authority or compliance with every document formality.

Formats also differ from certificate containers and encodings such as PFX, PEM or DER. Check the intended recipient’s supported format and required evidence rather than infer acceptance from an extension alone.