Validating digital signatures
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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.
Setting up digital signature validation
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.
Open the Preferences dialog box.
Under Categories, select Signatures.
For Verification, click More.
To automatically validate all signatures in a PDF when you open the document, select Verify Signatures When The Document Is Opened. This option is selected by default.
Select verification options as needed and click OK.
These options specify methods that determine which plug-in to choose when verifying a signature. The appropriate plug-in is often selected automatically. Contact your system administrator about specific plug-in requirements for validating signatures.
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.
Use Expired Timestamps
Uses the secure time provided by the timestamp or embedded in the signature, even if the signature’s certificate has expired. This option is selected by default. Deselecting this option allows discarding of expired timestamps.
Verify Signatures Using
Select an option to specify how to check the digital signature for validity. By default, you can check the time based on when the signature was created. Alternatively, check based on the current time or the time set by a timestamp server when the document was signed.
Specifies whether to add verification information to the signed PDF. Default is to alert user when verification information is too large.
Trust All root certificates in the Windows Certificate Store for
Specify whether to trust all root certificates in the Windows Certificates Store for:
- Validating signatures: Certificates are trusted for approval signature validation.
- Validating certified documents: Certificates are trusted for certification signature validation.
Selecting these options can compromise security.Caution:
It is not recommended to trust all root certificates in the Windows Certificate Store. 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.
Open the Preferences dialog box.
Under Categories, select Signatures.
For Identities & Trusted Certificates, click More.
Select Trusted Certificates on the left.
Select a certificate from the list, and click Edit Trust.
In the Trust tab, select any of the following items to trust this certificate:
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.
Signed Documents Or Data
Acknowledges the identity of the signer.
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:
Allows movies, sound, and other dynamic elements to play in a certified document.
Privileged System Operations
Allows Internet connections, cross domain scripting, silent printing, external-object references, and import/export methodology operations on certified documents.Note:
Click OK, close the Digital ID and Trusted Certificate Settings dialog box, and then click OK in the Preferences dialog box.
For more information, see the Digital Signature Guide at www.adobe.com/go/acrodigsig.
Signatures panel for digital signatures
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.
Choose View > Show/Hide > Navigation Panes > Signatures, or click the Signature Panel button in the document message bar.
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.
Sign in Preview Document mode
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.
Open the Preferences dialog box.
Under Categories, select Signatures.
For Creation & Appearance, click More.
For When Signing, select View Documents In Preview Mode, and click OK.
In the PDF, click the signature field and choose Sign Document.
The document message bar appears with the compliance status and options.
(Optional) Click View Report in the document message bar (if available) and select each item in the list to show details. When you’re done, close the PDF Signature Report dialog box.
If you’re satisfied with the compliance status of the document, click Sign Document in the document message bar, and add your digital signature.
Save the PDF using a different name than the original, and close the document without making any further changes.
Certify a PDF
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.
Choose Tools > Certificates to open the panel.
Click one of the following options:
Certify (Visible Signature)
Places a certified signature in either an exiting digital signature field (if available) or in the location you designate.
Certify (Invisible Signature)
Certifies the document, but your signature appears only in the Signatures panel.
Follow the onscreen instructions to place the signature (if applicable), specify a digital ID, and set an option for Permitted Actions After Certifying.Note:
If you enabled the When Signing: View Documents In Preview Mode in the Signature preferences, click Sign Document in the document message bar.
Save the PDF using a different filename than the original file, and then close the document without making additional changes. It is a good idea to save it as a different file so that you can retain the original unsigned document.
Timestamp a document
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. (See Configure a timestamp server.) 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.
For more information on PAdES, see blogs.adobe.com/security/2009/09/eliminating_the_penone_step_at.html.
Open the document to which you want to add a timestamp.
Choose Tools > Certificates > Time Stamp.
In the Choose Default Timestamp Server dialog box, select a default timestamp server from the list, or add a new default timestamp server.
Click Next, and then save the document with the timestamp.
Validate a digital signature
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.
For more information about signature warnings and valid and invalid signatures, see the Digital Signature Guide at www.adobe.com/go/acrodigsig.
You assess the validity of a Digital Signature and Timestamp by checking the Signature Properties.
Set your signature verification preferences. For more information, see Set signature verification preferences.
Open the PDF containing the signature, then click the signature. The Signature Validation Status dialog box describes the validity of the signature.
For more information about the Signature and Timestamp, click 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.
For details about the signer’s certificate, such as trust settings or legal restrictions of the signature, click Show Signer’s Certificate in the Signature Properties dialog box.
If the document was modified after it was signed, check the signed version of the document and compare it to the current version.
Remove a digital signature
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.)
View previous versions of a digitally signed document
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.
In the Signatures panel, select and expand the signature, and choose View Signed Version from the Option menu .
The previous version opens in a new PDF, with the version information and the name of the signer in the title bar.
To return to the original document, choose the document name from the Window menu.
Compare versions of a signed document
After a document is signed, you can display a list of the changes made to the document after the last version.
In the Signatures panel, select the signature.
Choose Compare Signed Version To Current Version from the Option menu .
When you’re done, close the temporary document.
Trust a signer’s certificate
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.
Digital signatures that were added using a self-signed certificate cannot be automatically validated by Adobe as the certificate is not in the list of Trusted Identities that Adobe uses to validate signatures. A self-signed certificate is a certificate that you have generated yourself using a third-party application. You won’t be able to manually validate the signature until the certificate is trusted by Adobe. If you open such a PDF, you will see a warning At least one signature has problems.
For security reasons, Adobe does not recommend adding a self-signed certificate, or any random certificate to the Adobe's list of Trusted Identities.
To add the certificate that was used to apply the digital signature into Adobe’s list of Trusted Identities, do the following:
Click the Signatures button in the left-pane.
Right-click the signature, and then click Show Signature Properties.
In the Signature Properties dialog box, click Show Signer's Certificate.
In the Certificate Viewer dialog box, click the Trust tab, and then click Add To Trusted Certificates.
Click OK in the trust settings pop-up dialog, and then click OK.
PDF Portfolios and digital 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.
Digital signatures on attachments to component PDFs
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.
Signed and certified PDF Portfolios
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.
For a list and explanation of each warning, see the DigSig Admin Guide at www.adobe.com/go/acrodigsig.
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.
Establish long-term signature validation
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.
Add verification information at signing
Make sure that your computer can connect to the appropriate network resources.
Ensure that the preference Include Signature’s Revocation Status is still selected (Preferences > Signatures > Creation & Appearances: More). This preference is selected by default.
Sign the PDF.
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.
Add verification information after signing
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.
Ensure that your computer can connect to the appropriate network resources, and then right-click the signature in the PDF.
Choose Add Verification 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. For more information, see blogs.adobe.com/security/2009/09/eliminating_the_penone_step_at.html. 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.