Cannot connect SFTP server: Failed to negotiate key exchange algorithm.

Christoffer's Avatar


22 Feb, 2023 08:25 AM


We are using the FTP provider with the sFTP protocol and got the following error:
Cannot connect SFTP server: Failed to negotiate key exchange algorithm.

The following protocols were enabled on the server:
[email blocked]

After adding these it worked:

My understanding was that you were using SSH.NET as the sFTP client, which according to their documentation should have support for multiple of the algorithms available on the server. Are you using an older version of SSH.NET?

We would not like to have to rely on the sha1 algorithms. Thank you!

  1. 1 Posted by Christoffer on 22 Feb, 2023 08:26 AM

    Christoffer's Avatar

    The email blocked algorithm is:
    curve25519-sha256 AT

  2. Support Staff 2 Posted by Feodor Fitsner on 22 Feb, 2023 04:10 PM

    Feodor Fitsner's Avatar

    Right, AppVeyor doesn't use the latest SSH.NET library.

    Why don't you try updating it during the build, on init stage with the following two commands:

    - nuget install -ExcludeVersion
    - copy /Y SSH.NET\lib\net40\* "C:\Program Files\AppVeyor\BuildAgent"

    If method signatures have not changed it could do the trick. Let me know if that worked.

  3. 3 Posted by Christoffer Bre... on 23 Feb, 2023 06:18 AM

    Christoffer Bredberg's Avatar

    Thanks, I’ll try that. Curious though, what version do you run? Because some of my algorithms should match even in very old versions.

    Från: Feodor Fitsner <[email blocked]>
    Datum: onsdag, 22 februari 2023 17:11
    Till: Christoffer Bredberg <[email blocked]>
    Ämne: Re: Cannot connect SFTP server: Failed to negotiate key exchange algorithm. [Problems #34048]

  4. Support Staff 4 Posted by Feodor Fitsner on 23 Feb, 2023 06:23 PM

    Feodor Fitsner's Avatar

    AppVeyor uses version 2016.0.0.

  5. 5 Posted by Christoffer on 23 Feb, 2023 06:29 PM

    Christoffer's Avatar

    Thank you!

    Even that version should support those algorithms according to that versions readme:

    I might have confused their stated diffie-hellman-group-exchange-sha256 with our accepted diffie-hellman-group14-exchange-sha256 though. Could be a difference there, probably explains it.

    Unfortunately I was not able to run your init suggestion, seems there have been changes since then:

    Could not load file or assembly 'Renci.SshNet, Version=2016.0.0.0, Culture=neutral, PublicKeyToken=1cee9f8bde3db106' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)

    We'll stick with supporting the older algorithms for now, happy to test if you decide to upgrade the SSH.NET version in the future.

    Thank you!

  6. Support Staff 6 Posted by Feodor Fitsner on 24 Feb, 2023 04:05 AM

    Feodor Fitsner's Avatar
  7. Feodor Fitsner closed this discussion on 26 Apr, 2023 09:02 PM.

Comments are currently closed for this discussion. You can start a new one.

Keyboard shortcuts


? Show this help
ESC Blurs the current field

Comment Form

r Focus the comment reply box
^ + ↩ Submit the comment

You can use Command ⌘ instead of Control ^ on Mac