Introduction

In the Fabasoft Cloud, documents can be signed digitally with a certificate, resulting in an PDF signature according the PDF standard ISO 32000-1:2008. This document describes the process of how the signatures are applied to documents, the administrative use cases to configure this functionality, and the technical details of these signatures.

For digital signatures, the following criteria are of special interest:

• Subsequent change of the document is detectable
  Digital signatures in the Fabasoft Cloud make use of PDF signatures that are based upon an asymmetric cryptographic system using signing certificates with public/private key pairs. Due to modern hashing and signature algorithms (SHA512, RSA-4096), subsequent changes in the document can be detected when validating the signature in an appropriate PDF viewer.
• High level of confidence in the signature creation system and in the protection of the signature creation keys
  The Fabasoft Cloud uses Fabasoft Secomo to generate the digital signature during the signing process. In Fabasoft Secomo, the private keys used for creating the signatures are stored in a highly secure manner, utilizing a hardware security module (HSM) that meets the requirements of FIPS 140-2 Level 4 physical security certification.
• Linking the signature to the signatory
  Each PDF signature is linked to exactly one signatory. These signatories are able to use their visual signature images when applying a digital signature. In that image, a hash of the globally unique ID of the user in the Fabasoft Cloud environment is generated (signatory identification), which will allow to verify whether the signature was applied by exactly that particular user.
  Moreover, the names of the signatories, the name of their organizations, the login e-mail address and the signatory identification are added to the so-called “signature reason” of the PDF signature so that the signatory can easily be verified directly in an appropriate PDF viewer.
  In addition to the identification data in the “signature reason” of the PDF signature, the underlying signature structure will also contain the login e-mail addresses and the user IDs of the signatories as well as the ID of the signed document in the Fabasoft Cloud. Although this additional identification data is not directly displayed in the PDF viewer, it can be checked by using special analysis tools as explained below.
• Authentication and identification of the signatory
  In the Fabasoft Cloud, signatories must authenticate themselves using various strong authentication methods (including 2-factor authentication). Hence, it is not possible to sign documents just because of receiving documents via e-mail.
  As mentioned above, the login e-mail address as well as the signatory identification are added to the “signature reason” so that you can see directly the identification data of the signatory when checking the signature data in an appropriate PDF viewer.