DLI Implementation and Reference Guide - Datalogics

15.2 DLI Implementation and Reference Guide
Overview
Concepts and Facilities: Guide to the DL Pager Composition System
PDF files created with DLI may include digital signatures for use in validating a
document's contents. With a digital signature, a certificate is added to a PDF file
containing information about the signer. An encrypted message, and the key to
decrypt this message, are included in the certificate or the signature PDF fields. The
message is generated by a non-reversible mathematical transform of the PDF file's
bytes known as a hash function. A recipient of the PDF file can then decrypt this
message. If the message matches what the recipient calculates using the hash function
for the PDF file, the PDF file has not been altered since it was signed. If it does not
match, the document has been changed.
Public and Private Keys
Digital signatures are based on public-key cryptography. In public-key cryptography,
there are two keys, public and private:
• A private key is used to encrypt data that only the public key can decrypt.
• A public key may encrypt data that only the private key can decrypt.
This allows you, the document creator, to distribute a public key freely, and to encrypt
a message with your private key (which must be kept secret). The reader can decrypt a
message, but cannot re-encrypt the message in the same way that you encrypted it
when you signed it. This prevents the reader from pretending to be the document
author.
Digital signatures also serve the purpose of non-repudiation. Since only the author can
sign a document, a valid signature means that only the author of the document could