Set your verification preferences in advance. This helps ensure that Digital Signatures are valid when you open a PDF and verification details appear with the signature. See Set signature verification preferences for details.
When Digital Signatures are validated, an icon appears in the document message bar to indicate the signature status. Additional status details appear in the Signatures panel and in the Signature Properties dialog box.
When you receive a signed document, you may want to validate its signature(s) to verify the signer and the signed content. Depending on how you have configured your application, validation may occur automatically. Signature validity is determined by checking the authenticity of the signature’s digital ID certificate status and document integrity:
- Authenticity verification confirms that the signer’s certificate or its parent certificates exist in the validator’s list of trusted identities. It also confirms whether the signing certificate is valid based on the user’s Acrobat or Reader configuration.
- Document integrity verification confirms whether the signed content changed after it was signed. If content changes, document integrity verification confirms whether the content changed in a manner permitted by the signer.
Require Certificate Revocation Checking To Succeed Whenever Possible …
Checks certificates against a list of excluded certificates during validation. This option is selected by default. If you deselect this option, the revocation status for approval signatures is ignored. The revocation status is always checked for certifying signatures.
Specifies whether to add verification information to the signed PDF. Default is to alert user when verification information is too large.
specify whether to trust all root certificates in the Windows Certificates feature when validating signatures and certified documents. Selecting these options can compromise security.
It is not recommended to trust all root certificates in the Windows Certificate feature. Many certificates that are distributed with Windows are designed for purposes other than establishing trusted identities.
In Acrobat or Reader, the signature of a certified or signed document is valid if you and the signer have a trust relationship. The trust level of the certificate indicates the actions for which you trust the signer.
Use This Certificate As A Trusted Root
A root certificate is the originating authority in a chain of certificate authorities that issued the certificate. By trusting the root certificate, you trust all certificates issued by that certificate authority.
Trusts documents in which the author has certified the document with a signature. You trust the signer for certifying documents, and you accept actions that the certified document takes.
When this option is selected, the following options are available:
The Signatures panel displays information about each digital signature in the current document and the change history of the document since the first digital signature. Each digital signature has an icon identifying its verification status. Verification details are listed beneath each signature and can be viewed by expanding the signature. The Signatures panel also provides information about the time the document was signed, and trust and signer details.
You can right-click a signature field in the Signatures panel to do most signature-related tasks, including adding, clearing, and validating signatures. In some cases, however, the signature field becomes locked after you sign it.
When document integrity is critical for your signature workflow, use the Preview Document feature to sign documents. This feature analyzes the document for content that may alter the appearance of the document. It then suppresses that content, allowing you to view and sign the document in a static and secure state.
You can also use Preview Document mode outside a signing workflow to check the integrity of a document.
When you certify a PDF, you indicate that you approve of its contents. You also specify the types of changes that are permitted for the document to remain certified. For example, suppose that a government agency creates a form with signature fields. When the form is complete, the agency certifies the document, allowing users to change only form fields and sign the document. Users can fill the form and sign the document. However, if they remove pages or add comments, the document doesn’t retain its certified status.
You can apply a certifying signature only if the PDF doesn’t already contain any other signatures. Certifying signatures can be visible or invisible. A blue ribbon icon in the Signatures panel indicates a valid certifying signature. A digital ID is required to add the certifying digital signature.
Follow the onscreen instructions to place the signature (if applicable), specify a digital ID, and set an option for Permitted Actions After Certifying.
If you enabled the When Signing: View Documents In Preview Mode in the Signature preferences, click Sign Document in the document message bar.
Acrobat provides users with the capability to add a document timestamp to a PDF without also requiring an identity-based signature. A timestamp server is required to timestamp a PDF. A timestamp assures the authenticity and existence of a document at a particular time. These timestamps are compliant with the timestamp and revocation features described in Part 4 of ETSI 102 778 PDF Advanced Electronic Signatures (PAdES) standard. Users of Reader X (and later) can also timestamp a document if the document includes appropriate Reader Enabling features.
If the signature status is unknown or unverified, validate the signature manually to determine the problem and possible solution. If the signature status is invalid, contact the signer about the problem.
You assess the validity of a Digital Signature and Timestamp by checking the Signature Properties.
Review the Validity Summary in the Signature Properties dialog box. The summary might display one of the following messages:
Signature date/time are from the clock on the signer’s computer
The time is based on the local time on the signer’s computer.
Signature is timestamped
The signer used a Timestamp Server and your settings indicate that you have a trust relationship with that timestamp server.
Signature is timestamped but the timestamp could not be verified
Timestamp verification requires obtaining the timestamp server’s certificate to your list of trusted identities. Check with your system administrator.
Signature is timestamped but the timestamp has expired
Acrobat and Reader validate a timestamp based on the current time. This message is displayed if the timestamp signer’s certificate expires before the current time. To let Acrobat or Reader accept an expired timestamp, select Use Expired Timestamps in the Signature Verification Preferences dialog box (Preferences > Signatures > Verification: More). Acrobat and Reader display an alert message when validating signatures with expired timestamp.
You cannot remove a digital signature unless you are the one who placed it and you have the digital ID for signing it installed.
Do one of the following:
- To remove a digital signature, right-click the signature field and choose Clear Signature.
- To remove all digital signatures in a PDF, choose Clear All Signature Fields from the options menu in the Signatures panel. (To open the Signatures panel, choose View > Show/Hide > Navigation Panes > Signatures.)
Each time a document is signed using a certificate, a signed version of the PDF at that time is saved with the PDF. Each version is saved as append-only and the original cannot be modified. All digital signatures and their corresponding versions can be accessed from the Signatures panel.
After a document is signed, you can display a list of the changes made to the document after the last version.
Trusting a certificate involves adding it to the user’s trusted identity list in the Trusted Identity Manager and manually setting its trust level. End users often exchange certificates as needed when using certificate security. Alternatively, they add certificates directly from signatures in signed documents and then set trust levels. However, enterprises often require employees to validate the signatures of others without performing any manual task. Acrobat trusts all certificates for signing and certifying that chain up to a trust anchor. Therefore, administrators should preconfigure client installations or let their end users add a trust anchor or anchors. For more information on trusting certificates, see About certificate-based signatures.
You can sign component PDFs within a PDF Portfolio, or sign the PDF Portfolio as a whole. Signing a component PDF locks the PDF for editing and secures its content. After signing all the component PDFs, you can sign the entire PDF Portfolio to finalize it. Alternatively, you can sign the PDF Portfolio as a whole to lock the content of all component PDFs simultaneously.
- To sign a component PDF, see Signing PDFs. The signed PDF is automatically saved to the PDF Portfolio.
- To sign a PDF Portfolio as a whole, sign the cover sheet (View > Portfolio > Cover Sheet). Once you sign the PDF Portfolio as a whole, you cannot add signatures to the component documents. However, you can add more signatures to the cover sheet.
You can add signatures to attachments before signing the cover sheet. To apply signatures to attached PDFs, open the PDF in a separate window. Right-click the attachment, and choose Open File from the context menu. To view signatures on the PDF Portfolio, navigate to the cover sheet to view the document message bar and signatures pane.
A properly signed or certified PDF Portfolio has one or more signatures that approve or certify the PDF Portfolio. The most significant signature appears in a Signature Badge in the toolbar. Details of all signatures appear in the cover sheet.
- To view the name of the organization or person that signed the PDF Portfolio, hover the pointer over the Signature Badge.
- To view details about the signature that appears in the Signature Badge, click the Signature Badge. The cover sheet and the Signatures pane on the left open with details.
If the PDF Portfolio approval or certification is invalid or has a problem, the Signature Badge shows a warning icon. To view an explanation of the problem, hover the pointer over a Signature Badge with a warning icon. Different warning icons appear for different situations.
XML data signatures
Acrobat and Reader support XML data signatures that are used to sign data in XML Forms Architectures (XFA) forms. The form author provides XML signing, validating, or clearing instructions for form events, such as button click, file save, or submit.
XML data signatures conform to the W3C XML-Signature standard. Like PDF digital signatures, XML digital signatures ensure integrity, authentication, and non-repudiation in documents.
However, PDF signatures have multiple data verification states. Some states are called when a user alters the PDF-signed content. In contrast, XML signatures only have two data verification states, valid and invalid. The invalid state is called when a user alters the XML-signed content.
Long-term signature validation allows you to check the validity of a signature long after the document was signed. To achieve long-term validation, all the required elements for signature validation must be embedded in the signed PDF. Embedding these elements can occur when the document is signed, or after signature creation.
Without certain information added to the PDF, a signature can be validated for only a limited time. This limitation occurs because certificates related to the signature eventually expire or are revoked. Once a certificate expires, the issuing authority is no longer responsible for providing revocation status on that certificate. Without conforming revocation status, the signature cannot be validated.
The required elements for establishing the validity of a signature include the signing certificate chain, certificate revocation status, and possibly a timestamp. If the required elements are available and embedded during signing, the signature can be validated requiring external resources for validation. Acrobat and Reader can embed the required elements, if the elements are available. The PDF creator must enable usage rights for Reader users (File > Save As Other > Reader Extended PDF).
Embedding timestamp information requires an appropriately configured timestamp server. In addition, the signature validation time must be set to Secure Time (Preferences > Security >Advanced Preferences > Verification tab). CDS certificates can add verification information, such as revocation and timestamp into the document without requiring any configuration from the signer. However, the signer must be online to fetch the appropriate information.
If all the elements of the certificate chain are available, the information is added to the PDF automatically. If a timestamp server has been configured, the timestamp is also added.
In some workflows, signature validation information is unavailable at signing, but can be obtained later. For example, a company official may sign a contract using a laptop while traveling by air. The computer cannot communicate with the Internet to obtain timestamping and revocation information to add to the signature. When Internet access is available later, anyone who validates the signature can add this information to the PDF. All subsequent signature validations can also use this information.
Information and methods used to include this long term validation (LTV) information in the PDF comply with Part 4 of the ETSI 102 778 PDF Advanced Electronic Signatures (PAdES) standard. The command is unavailable if the signature is invalid, or is signed with a self-signed certificate. The command is also unavailable in case the verification time equals the current time.