The term PAdES stands for PDF Advanced Electronic Signatures. It refers to a group of extensions and restrictions that are used with PDF and ISO 32000-1. They allow for advanced electronic signatures that adhere to the eIDAS Regulation, which has evolved from the European Union Directive 1999/93/EC.
The ISO 32000-1 standard specifies the Portable document format, commonly known as PDF. Currently this standard allows for:
By using the PAdES standard, also known as ETSI Technical Specification (TS) 1-2 778, there is an assurance that electronically signed documents will remain valid for longer periods of time regardless if the cryptographic algorithms used are broken. This will allow these digitally-signed documents to be archived for many years. At any given time, it is possible to confirm that the signature was valid through a concept referred to as Long-Term Validation (LTV).
With PAdES, different PDF signature types utilize additional document signature functions. Certification signatures rely on modification permissions that are referenced under clause 12.8.4 of ISO 32000-1. This allows the document to be modified as in allowing comments or the filling in of forms, but will still allow the original signature to be read as valid. The usage rights signature type referenced under clause 188.8.131.52 of ISO 32000-1, allows documents to be enhanced with additional privileges and rights during the course of workflow, but uses the signature to confirm that the document and rights have not been tampered with.
Multiple implementations for using Cryptographic Message Syntax (CMS) - based digital signatures within a PDF document are defined in ISO 32000-1. Each is defined with a pair of values from the signature dictionary:
ETSI Technical Specification (TS) 102778-6 states that the signature appearance will represent elements of the AdES signature, in addition to other attributes, such as the time of signing or location. When creating the signature appearance, the signature handler should include information certifying the identity of the signer and time of signing.
For PDF documents that require multiple signatures, serial signatures allow for additional signatures despite the limitation of only a single signing certificate within the PDF. This is done by creating multiple signature dictionaries that each have their own associated Byte Range for each signature.
The PAdES Baseline Profile specified in ETSI TS 103 172 V2.2.2 defines four levels of conformance for digital signatures:
When exchanging digitally signed information between parties, the signatures should conform to the level that will allow the parties to trust the signature when the exchange occurs.
A verifier shall be able to accept a signature that has properties or elements that conform to PAdES. Compliance requirements are grouped into four categories that each have their own corresponding identifier:
Cover image: courtesy of Highways England (HA0478-029), Flickr