A digital signature, like
a conventional handwritten signature, identifies the person signing
a document. Unlike a handwritten signature, a digital signature
is difficult to forge because it contains encrypted information
that is unique to the signer and easily verified.
Most digital signatures are referred to as approval signatures.
Signatures that certify an Adobe® PDF are
called certifying signatures. Only the first person
to sign a PDF (most often, the author) can add a certifying signature.
A certifying signature attests to the contents of the document.
It also allows the signer to specify the types of changes allowed
for the document to remain certified. Changes to the document are
shown in the Signatures panel.
To sign a document, you must obtain a digital ID or create a
self-signed digital ID in Adobe Acrobat®.
The digital ID contains a private key and a certificate (or public
key). The private key is used to add the digital signature, and
the certificate that you share with people who use it to validate
your signature.
When you apply a digital signature, Acrobat uses
a hashing algorithm to generate a message digest, which it encrypts
using your private key. Acrobat embeds the encrypted
message digest in the PDF, along with details from your certificate,
an image of your signature, and a version of the document at the
time it was signed.
Note: For the latest information about digital signatures, choose
Help > Online Support > Knowledgebase to open the Adobe Acrobat
support page on the Adobe website. Then search for “digital signatures.”
For articles on digital signatures, see these resources:
