Certificate-based signatures

A certificate-based signature, like a conventional handwritten signature, identifies the person signing a document. Unlike a handwritten signature, a certificate-based signature is difficult to forge because it contains encrypted information that is unique to the signer. It can be easily verified and informs recipients whether the document was modified after the signer initially signed it.

To sign a document with a certificate-based signature, you must obtain a digital ID or create a self-signed digital ID in Acrobat or Acrobat Reader. The digital ID contains a private key and a certificate with a public key, and more. The private key is used to create the certificate-based signature. The certificate is a credential that is automatically applied to the signed document. The signature is verified when recipients open the document.

When you apply a certificate-based 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, certificate details, signature image, and a document version when signed.

Steps to add a certificate-based signature to a PDF

The Use a certificate tool lets you apply two types of certificate-based signatures. You can Certify a document, attest to its content or approve a document with the Digitally sign option.

When you Digitally sign with a certificate, the signature is considered an approval signature.

Certify options provide a higher level of document control than Digitally sign. For documents that require certification, you must certify the documents before others sign them. If a document has already been signed, the Certify options are disabled. When you certify a document, you can control the types of changes other people can make. You can certify with or without displaying a signature.

Signatures made with the Certify or Digitally sign options comply with data protection standards specified by the European Telecommunications Standards Institute (ETSI). In addition, both signature types comply with the PDF Advanced Electronic Signature (PAdES) standard. Acrobat and Acrobat Reader provide an option to change the default signing format to a CAdES format. This option is compliant with Part 3 of the PAdES standard. The timestamp capability and native support for long-term validation of signatures (introduced in Acrobat 9.1) is in compliance with Part 4 of the PAdES standard. The default signing format, when set up accordingly, is compliant with Part 2 of the PAdES standard. You can change the default signing method or format, in the Signatures panel of the Preferences dialog box. Under Creation & Appearance, click More.

Setting up certificate-based signatures

You can expedite the signing process and optimize your results by making the following preparations in advance.

• Get a digital ID from your own organization, buy a digital ID (see the Adobe website for security partners), or create a self-signed one. See Create a self-signed digital ID. You can’t apply a certificate-based signature without a digital id.
  

• Set the default signing method.

• Create an appearance for your certificate-based signature. (See Create the signature appearance.)

• Use the Preview Document mode to suppress any dynamic content that can alter the appearance of the document and mislead you into signing an unsuitable document. For information about using the Preview Document mode, see Sign in Preview Document mode.

• Review all the pages in a document before you sign. Documents can contain signature fields on multiple pages.

• Configure the signing application. Both authors and signers should configure their application environment. (See Set signing preferences).
  For details on the full range of configuration options in enterprise settings, see the Digital Signatures Guide.

• Choose a signature type. Learn about approval and certification signatures to determine the type you should choose to sign your document. (See Signature types.)

Set signing preferences

Signing workflow preferences control what you can see and do when the signing dialog box opens. You can allow certain actions, hide and display data fields, and change how content affects the signing process. Setting signing preferences impacts your ability to see what you are signing. For information on the available signing preferences, see “Signing Workflow Preferences” in the Digital Signature Guide.

Customizing signature workflows using seed values

Seed values offer additional control to document authors by letting them specify which choices signers can make when signing a document. By applying seed values to signature fields in unsigned PDFs, authors can customize options and automate tasks. They can also specify signature requirements for items such as certificates and timestamp servers. For more information about customizing signatures using seed values, see the Digital Signature Guide.

Create the appearance of a certificate-based signature

You determine the look of your certificate-based signature by selecting options in the Signatures panel of the Preferences dialog box. For example, you can include an image of your handwritten signature, a company logo, or a photograph. You can also create different signatures for different purposes. For some, you can provide a greater level of detail.

A signature can also include information that helps others verify your signature , such as the reason for signing, contact information, and more.

Set up a roaming ID account

A roaming ID is a digital ID that is stored on a server and can be accessed by the subscriber. You must have an Internet connection to access a roaming ID and an account from an organization that supplies roaming digital IDs.

Once the roaming ID is added, it can be used for signing or encryption. When you perform a task that uses your roaming ID, you’re automatically logged in to the roaming ID server if your authentication assertion hasn’t expired.

PKCS#12 modules and tokens

You can have multiple digital IDs that you use for different purposes, particularly if you sign documents in different roles or using different certification methods. Digital IDs are usually password protected. They can be stored on your computer in PKCS #12 file format. Digital IDs can also be stored on a smart card, hardware token, or in the Windows certificate store. Roaming IDs can be stored on a server. Acrobat includes a default signature handler that can access digital IDs from various locations. Register the digital ID in Acrobat for it to be available for use.

Store certificates on directory servers

Directory servers are commonly used as centralized repositories of identities within an organization. The server acts as an ideal location to store user certificates in enterprises that use certificate encryption. Directory servers let you locate certificates from network servers, including Lightweight Directory Access Protocol (LDAP) servers. After you locate a certificate, you can add it to your list of trusted identities so that you don’t have to look it up again. By developing a storage area for trusted certificates, you or a member of your workgroup can facilitate the use of encryption in the workgroup.

For more information about directory servers, see the Digital Signature Guide.

Import directory server settings (Windows only)

You import directory server settings using security import/export methodology or a security settings file. Before, you import settings in a file using import/export methodology, ensure that you trust the file provider before opening it.

Export directory server settings (Windows only)

Although it is preferable to export security settings, you can export directory settings as an import/export methodology file. Use the file to configure the directory server on another computer.

Add a timestamp to certificate-based signatures

You can include the date and time you signed the document as part of your certificate-based signature. Timestamps are easier to verify when they are associated with a trusted timestamp authority certificate. A timestamp helps to establish when you signed the document and reduces the chances of an invalid signature. You can obtain a timestamp from a third-party timestamp authority or the certificate authority that issued your digital ID.

Timestamps appear in the signature field and in the Signature Properties dialog box. If a timestamp server is configured, the timestamp appears in the Date/Time tab of the Signature Properties dialog box. If no timestamp server is configured, the signatures field displays the local time of the computer at the moment of signing.

Configure a timestamp server

To configure a timestamp server, you need the server name and the URL, which you can obtain from an administrator or a security settings file.

If you have a security settings file, install it and don’t use the following instructions for configuring a server. Ensure that you obtained the security settings file from a trusted source. Don’t install it without checking with your system administration or IT department.

Set a timestamp server as the default

To be able to use a timestamp server to timestamp signatures, set it as the default server.

Adobe LiveCycle Rights Management (ALCRM) servers

Adobe LiveCycleRights Management (ALCRM) servers let you define centralized policies to control access to documents. The policies are stored on the ALCRM server. You require server access to use them.

ALCRM servers embed user access information in documents. Therefore, specify document recipients in ALCRM policies. Alternatively, let the ALCRM server retrieve the list of recipients from LDAP directories.

Use ALCRM servers to set permissions for separate document tasks, for example opening, editing, and printing. You can also define document auditing policies on ALCRM servers.