Secure electronic documents Flashcards
Characteristics that a secure electronic document must have
- human-readable
- long term archival/verification
- unforgeable
Parallel and sequential signatures
Parallel:
- the order does not matter
- the co-signers sign the same input
Sequential:
- the signatures order matters cause it is related to an hierarchy
- each signer counter-signs the doc + the signatures already made on it
Digital signature in a PDF file
The PDF format allows to leave a space for the signature that will be made on the document. This space has to be enough large so that it can contain also the structures (certificates, crls) to verify the signature. The more the info contained, the more it is a sel-contained secure signature.
It allows to sign any type of content.
The signature on a document is done this way:
- the document is converted in a byte stream and a specific place is reserved for the signature
- the digest is computed on data from %PDF to %PDF + offset1 (till <Contents) and from > to %%EOF
- the obtained digest is encrypted with the signer’s private key -> signature
- the signature is encoded as a PKCS#7 signature
- the hex encoding of it is placed in the space reserved for the signature and filled with zeros if needed
Adobe Acrobat signature formats and algorithms
The default signature format of Adoba Acrobat is adbe.pkcs7.detached but other formats can be used.
The main algorithms are:
- digest: SHA-1, SHA-256
- encryption or signature: DSA, RSA, ECDSA
Adoba Acrobat multiple signatures
Adoba Acrobat shows the final version of a signed document but it allows to view the previous versions too.
Multiple signatures are performed as incremental updates.
Electronic Signatures in EU
The EU decided to have a common policy about electronic signatures to permit interoperability across the EU.
ES
Electronic Signature: data in electronic form which are attached to or logically associated with other electornic data and which serve as a method of authentication.
Beware: a scanned signature is a ES according to this definition
AES
Advanced Electronic Signature: it is a subset of ES.
An AES is a ES which meets the following requirements:
- uniquely linked to the signatory
- identifies the singatory (≠ authN)
- created using meaans that the signatory can maintain under his sole control. The signatory must demonstrate that it is the only one in control of them
- linked to the data to whch it relates in such a manner that any subsequent change of the data is detectable
QC
A Qualified Certificate is a PKC certifying the identity of a person and containing:
- indication that it was issued as a QC
- the name of the signatory or a pseudonym
- specific attributes of the signatory if relevant
- possible limitations on the scope of the PKC
- possible limitations on the value of the transactions for which it can be used
QES
A Qualified Electronic Signature is a AES based on QC and created by a secure-signature-creation device meaning that to have a QES the whole creation of the signature must be secure (≠ AES).
A QES satisfies the legal requirements of a signature in relation to data in electronic form.
ETSI standards for electronic signature
CAdES (CMS Advanced Electronic Signatures) is a standard for electronic signatures based on the Cryptographic Message Syntax (CMS) format.
CAdES formats
CAdES can implement 3 basic formats:
- ES: digital signature + other signed attributes + signature policy ID
- ES-T: ES + timestamp over the digital signature -> useful to verify the validity
- ES-C: ES-T + complete certificates and revocation references -> issue: if they are checked years later, are the references still valid?
Also ES-X was defined.
ES-X
It is the suggested format if CA certificates may be compromised.
There are two possible ways to have it:
- ES-C + timestamp over the whole ES-C -> it is useful when OCSP is used
- ES-C + timestamp over just the references -> useful when CRL is used
TSL
The Trust Service Status List (TSL) is an important component of the eIDAS regulation in the European Union.
It is essentially a signed list that contains information about the status of Trust Service Providers (TSPs) and their services, such as electronic signature services, certificate authorities (CAs), and timestamping services.
The status of each TSP can be:
- supervised
- suspended
- revoked
- others…
Each TSP also has the history of its status (useful for verifying signatures) and the the schema according to which it is trusted.
ASiC
Associated Signature Containers (ASiC) specifies the use of container structures to bind together one or more signed objects into one single digital container.
A container is based on the ZIP format and is the structure used to associate e-documents with detached signatures (CAdES or XAdES) and or timestamps.
The ZIP contains the objects to be signed and a signature or timestamp.
