Niet)ondersteund algoritme - Unsupported algoritme

[Koen225142565mmt]

I have a new (Belgian) eID. But the certificates were not longer recognised. So I re-Installed Adobe Acrobat Reader DC. The certificate for signature is now accepted and tested (5 green check marks). But at the end of the process to sign a .pdf document the error appears: (dutch:) “Fout aangetroffen tijdens ondertekenen: Niet-ondersteund algoritme” (free translated:)”Error encountered while signing: Unsupported algorithm”. What can I do? Please answer.

[Senne22794518b6zp]

I'm having the same issue, Did you find a solution yet?

[try67]

Contact the authority that issued this signature profile and ask for exact instructions on how to use it.

It's also recommended to go to the application's Preferences (via the Edit menu on Windows or the Reader app menu on a Mac), then to Trust Manager and click both Update Now buttons there.

[Koen225142565mmt · Accepted solution]

The problem is solved. Solution : configure Acrobat Reader without pkcs#11 module.
If this module has already been linked, the already linked module must therefore be uncoupled again.
After unlinking, restart Acrobat Reader and try signing again.
BELGIUM uses software called eID middleware that communicates with the card for Windows. In the past Acrobat Reader could not use this middleware and Acrobat Reader needed the pkcs11 module to do the communication with the card. Recently, ADOBE has made the necessary changes so that READER can work with the middleware.
The pkcs11 module has now basically become optional within Acrobat Reader.

[chantala61760780]

Thanks, this helped me also, on Belgium eid-website it's mention to couple the pkcs module

[mathias_4680]

After removing the pkcs11 module there is no certificate shown to use to sign? Or am I missing something? I'm on MacOS 15.6.

[shah_2804]

I'm having the same issue. Of course after removing Pkcs11 there is no Root path with the signature. But with pkcs11 the signature can't be placed " unsupported algorithm". Spent 8 hrs on Adobe support, they couldn't help

[danielc29258241]

Romania, mac M4, same problem ....

[AnandSri]

Hello @danielc29258241

I hope you are doing well, and thanks for reaching out.

This error typically occurs when Acrobat Reader encounters a cryptographic algorithm it does not support for digital signatures. Common scenarios:

• Using PKCS#11 modules unnecessarily with eID middleware.
• Certificates or smartcards using algorithms not recognized by Acrobat (e.g., certain ECC or GOST variants).
• Outdated Acrobat version lacking support for newer algorithms.

Please ensure you have the latest version of Acrobat or Reader installed: 25.001.20693 Planned update, Sept 09, 2025. Check for any pending updates from the Menu > help > check for updates, install the updates, restart the app and the machine, and try again.

Remove PKCS#11 Module (if linked): 

Go to Menu > Preferences > Signatures > Identities & Trusted Certificates > More.
If a PKCS#11 module is linked, unlink it.
Restart Acrobat Reader and try signing in again.

Update Trust Manager Settings: In Menu > Preferences > Trust Manager, click Update Now for both options to refresh trusted root certificates.

See these articles for more information:

https://adobe.ly/4gjm4BB

https://adobe.ly/469Sfjz

https://adobe.ly/4mgUR3R

Thanks,

Anand Sri.

[danielc29258241]

10q for your reply, 

if i unlink the PKCS#11 module then with middleware running when i want to sign the popup say Digital ID Configuration Required, -> configure digital id -> use a signature creation device -> continue. ... Adobe Acrobat could not find any new digital IDs. If your digital ID is on a hardware token, please make sure it is plugged in and the token interface is properly configured. Contact your system administrator fo further assistance.

im trying so digitaly sign PDF with new romanian CEI .... 

i have the latest acrobat version

[AnandSri]

Hello @danielc29258241

I hope you are doing well, and thanks for sharing the additional details.

When the PKCS#11 module is unlinked, Acrobat no longer has a way to communicate with your smart card or token. Even if the middleware is running, Acrobat needs the module explicitly configured to access the digital ID. 

• Re-link the PKCS#11 Module:

  • Go to: Menu > Preferences > Signatures > Identities & Trusted Certificates > More...
  • Under PKCS#11 Modules and Tokens, click Attach Module.
  • Browse to the correct .dll (Windows) or .dylib (macOS) file provided by your token vendor (e.g., CEI Romania).
  • Confirm and restart Acrobat.

• Ensure Middleware is Up to Date:

  • Download the latest version of the CEI middleware from the official Romanian eID site.
  • Reinstall it and reboot your system.

See this community dscussion for more details: https://adobe.ly/4mdgWQD

Thanks,

Anand Sri.

[danielc29258241]

With the module relinked and with the middleware app running  after placing signature and hit save signed document i get "unsupported algorithm"

[AnandSri]

Hello @danielc29258241

I hope you are doing well. Thanks for the update and for testing with the module relinked and the middleware app running.

The error message "unsupported algorithm" typically indicates that the cryptographic algorithm used by the signature module or middleware is not recognized or supported by the signing engine.

If possible, please contact the Middleware Vendor Support. Provide them with the “unsupported algorithm” error, the certificate’s algorithm, and see if they have any patches for this.

If the issue persists, please share:

The exact certificate algorithm used.
Whether the signature works in other apps. 
A screenshot of the error message (if possible).

For Adobe Acrobat/Reader: 

1. Use the Adobe Acrobat Diagnostics tool to gather advanced logs.
2. Access the tool through Menu > Help > Troubleshoot Acrobat.
3. Enable advanced logging options, select all the options and start the diagnostics.
3. Share the logs with us and the vendor.

Thanks,

Anand Sri.

[Silviu PERIANU]

Hello Anand,

Same problem for Apple M1, 2020 with latest macOS Tahoe 26.1. 

The algorithm used is ECDSA Signature with SHA-384 ( 1.2.840.10045.4.3.3 ) and it is perfectly working with Acrobat Reader on Windows machines.

Algorithm: Elliptic Curve Public Key ( 1.2.840.10045.2.1 )

Parameters: Elliptic Curve secp384r1 ( 1.3.132.0.34 )

I use Acrobat Reader Version 2025.001.20744 (no diagnostic tool) with AXAGON USB Smart Card/ID Card CRE-SM3TC and Romanian eID.

Thank you!

Silviu

[Silviu PERIANU]

Update on whether the signature works in other apps:

It works with LibreOffice ver.25.8.1.1 (AARCH64). After signing an existing PDF with LibreOffice, the signature validation process is then perfectly working in Acrobat Reader.

Thank you!

Silviu

[AnandSri]

Hello @Silviu PERIANU

I hope you are doing well. Thank you for sharing the details. We will get this test at our end. Could you please install the latest build and try again: 25.001.20756 Planned update, Oct 01, 2025. 

Thanks,

Anand Sri.

[Silviu PERIANU]

Hello Anand,

Following your request, I tested the latest update 20756 with same outcome. Here are the relevant technical details.

When attempting to sign a PDF using Adobe Acrobat Reader on macOS, the qualified certificate from my electronic identity card (CEI) is detected correctly through the PKCS#11 module. Both of the associated PINs (4-digit and 6-digit) are accepted, and the certificate is selectable during the signing flow — even without the IDplugManager middleware running in the background.

To me this confirms that:

- The smart card (CEI) is accessible through the PKCS#11 driver;

- The reader and driver are functioning properly;

- Authentication via PIN works; and

- The certificate is visible and usable up to the signing step.

However, when I proceed to actually sign the document, Acrobat Reader shows this error:

“Error encountered while signing: Unsupported algorithm” sometimes displaying the signature Acrobat logo before deleting it (see attached picture obtained with).

Again, here are the cryptographic parameters of the certificate used:

Public Key Algorithm: Elliptic Curve Public Key

OID: 1.2.840.10045.2.1

Signature Algorithm: ECDSA with SHA-384

OID: 1.2.840.10045.4.3.3

Since the certificate and private key are accessible and the PIN authentication succeeds, it appears to me that the failure is caused by incomplete support for ECDSA with SHA-384 in Adobe Acrobat Reader on macOS, not by the device, driver, or middleware.

Please confirm whether Acrobat Reader for macOS currently supports ECDSA-based qualified electronic signatures (especially with SHA-384), and if not, whether a workaround or update is planned.

Thank you!

Silviu

[AnandSri]

Hello @Silviu PERIANU

Thank you for sharing the details of the extensive testing. 

This issue has been reported to the product team, and they are working on identifying the root cause and a potential solution. We'll update here once the fix is available publicly.

Thanks for your time and cooperation on this.

Regards,

Anand Sri.