No matching host key type found. their offer ssh-rsa

After start of using NixOS as a new package management system, I get the following error when using git within Azure DevOps repositories and rsa ssh key:

jaroslavbezdek@mac> git pull
Unable to negotiate with 40.74.28.9 port 22: no matching host key type found. Their offer: ssh-rsa
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

What can I do with that, please?

asked Nov 7, 2021 at 18:45

Jaroslav BezděkJaroslav Bezděk

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1

With SSH, there are several different types of keys and RSA keys (the ssh-rsa) kind can support multiple kinds of signatures. The signature type ssh-rsa refers to RSA with SHA-1, whereas the signature type rsa-sha2-256 is RSA with SHA-256 and rsa-sha2-512 is RSA with SHA-512.

In the case of Azure DevOps, it only supports the kind of RSA with SHA-1, and SHA-1 is considered very weak. This essentially means that there are no secure ways to connect to it over SSH, and until they fix that, you're better off using HTTPS or a different hosting service. GitHub, GitLab, and Bitbucket all support secure methods of authentication.

If you really need to use SSH with Azure DevOps at the moment, you can add an entry to your ~/.ssh/config file to work around this:

Host ssh.dev.azure.com
    User git
    PubkeyAcceptedAlgorithms +ssh-rsa
    HostkeyAlgorithms +ssh-rsa

However, be aware that this is a workaround and it's known to be insecure, so you should contact Azure DevOps about this problem and switch to HTTPS until they do, or move elsewhere.

answered Nov 7, 2021 at 19:21

8

OpenSSH will report the error no matching host key type found. Their offer: ssh-rsa if the server it's connecting to is offering to authenticate over ssh-rsa ( RSA/SHA1).

Azure Devops (TFS) is offering to authenticate over ssh-rsa. As noted in the answer by bk2204, this algorithm is not considered cryptographically secure.

Since it's considered weak, OpenSSH deprecated using SHA-1 in 8.2 in 2020-02-14.

It is now possible[1] to perform chosen-prefix attacks against the SHA-1 hash algorithm for less than USD$50K. For this reason, we will be disabling the "ssh-rsa" public key signature algorithm that depends on SHA-1 by default in a near-future release.

Azure Devops Services subsequently announced a patch to allow SHA-2

On may 5 2021, the Azure DevOps documentation was updated to mention using RSA 3072.

Q: Is this true?

¯\_(ツ)_/¯

Q: Which algorithms are supported?

Doesn't say anywhere. Probably only ssh-rsa.

Q: How do I use a cryptographically unsafe algorithm

Add this

  HostkeyAlgorithms +ssh-rsa
  PubkeyAcceptedAlgorithms +ssh-rsa

To your ~/.ssh/config

Host your-azure-devops-domain
  IdentityFile ~/.ssh/id_rsa
  IdentitiesOnly yes
  HostkeyAlgorithms +ssh-rsa
  PubkeyAcceptedAlgorithms +ssh-rsa

Q: Is Microsoft aware that this is a problem?

Yes they are.

Q: Do they care?

No it's a feature

answered Dec 21, 2021 at 11:10

CervEdCervEd

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2

According to this post, you can add ssh.dev.azure.com host config to your ~/.ssh/config file:

Final ~/.ssh/config that worked for me:

Host ssh.dev.azure.com
    HostName ssh.dev.azure.com
    User git
    IdentityFile ~/.ssh/id_rsa
    IdentitiesOnly yes
    PubkeyAcceptedAlgorithms +ssh-rsa
    HostkeyAlgorithms +ssh-rsa

answered Nov 7, 2021 at 18:45

Jaroslav BezděkJaroslav Bezděk

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3

scp or ssh could used this

ssh -o HostKeyAlgorithms=+ssh-rsa -o PubkeyAcceptedKeyTypes=+ssh-rsa  user@myhost -p 22

answered May 24 at 11:00

张馆长张馆长

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1

I also got this problem, this worked for me:

cd ~/.ssh/
vim config

Host [Hostname]
User [User]
PubkeyAcceptedAlgorithms +ssh-rsa
HostkeyAlgorithms +ssh-rsa

I got this problem for a few hostnames so now i have several of those configurations in my ssh config file.

answered May 9 at 6:24

1

With NixOS 21.11 openSSH got updated to 8.8p1 ( see Changelog ). OpenSSH deprecated ssh-rsa along with a couple of other insecure ciphers.

If i understood correctly, you are only using nix as package manager and not NixOS. If that is the case you can follow the guides in the remaining answers (edit ~/.ssh/config).

However, when you are using NixOS to configure your server you can re-enable ssh-rsa for the ssh client, by adding to your configuration.nix:

programs.ssh.extraConfig = ''
  PubkeyAcceptedAlgorithms +ssh-rsa
  HostkeyAlgorithms +ssh-rsa
''

To re-enable the insecure ssh-rsa cipher for your openssh server (e.g. when legacy clients connect to the server), you can simply add the following lines to your configuration.nix:

services.openssh.extraConfig = ''
  PubkeyAcceptedAlgorithms +ssh-rsa
  HostkeyAlgorithms +ssh-rsa
'';

answered Dec 30, 2021 at 18:53

makefumakefu

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2

Correction for the posted answer. I had the same issue and I fixed it with the following snippet from above with a tiny fix:

Host YOUR-DOMAIN
Hostname YOUR-DOMAIN
IdentityFile ~/.ssh/id_rsa
IdentitiesOnly yes
HostKeyAlgorithms=+ssh-rsa
PubkeyAcceptedAlgorithms=+ssh-rsa

Dont forget to replace YOUR-DOMAIN with the domain you are using on AzureDevOps.

answered Mar 28 at 15:54

NekoMisakiNekoMisaki

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For those using Azure DevOps, you should use the following ~/.ssh/config, as Azure has a thing with varying what url it returns in its Clone Repository:

Host ssh.dev.azure.com
  PubkeyAcceptedAlgorithms +ssh-rsa
  HostkeyAlgorithms +ssh-rsa

Host vs-ssh.visualstudio.com
  PubkeyAcceptedAlgorithms +ssh-rsa
  HostkeyAlgorithms +ssh-rsa

answered Aug 1 at 13:53

Esben EickhardtEsben Eickhardt

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In your ~/.ssh/config file, add these lines.

Host *.drush.in
    HostkeyAlgorithms +ssh-rsa
    PubkeyAcceptedAlgorithms +ssh-rsa

answered Aug 8 at 10:33

I also faced this issue on my windows machine while setting up the SSH key for bitbucket

Initially, the config file was not created when I generated the public and private key files using the ssh-keygen command, so I used GitBash to create the config file and wrote the below content on it.

To create the file

touch config

To open and update the created file

nano config

Content added to the config file

Host [Hostname]
   HostName [Hostname]
   IdentityFile ~/.ssh/id_rsa
   IdentitiesOnly yes
   PubkeyAcceptedAlgorithms +ssh-rsa
   HostkeyAlgorithms +ssh-rsa

Note: If you are using your organization's bitbucket account, the hostname will be different or else by default it will be bitbucket.org

answered Jul 26 at 14:04

The format of the workaround wasn't working for me for windows 10 and git version 2.32.0. This snippet worked for me

Host = Hostname.com
IdentityFile = ~/.ssh/id_rsa
IdentitiesOnly = yes
HostkeyAlgorithms = +ssh-rsa
PubkeyAcceptedAlgorithms = +ssh-rsa

answered Apr 19 at 16:46

I googled a lot a bout this mistake: I have Ubuntu 22.04 and here all my configuration.
I hope it will help someone.

linux@linux:~$ cat /home/username/.ssh/config

Host *

KexAlgorithms +diffie-hellman-group1-sha1,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512
Ciphers aes128-cbc,3des-cbc,aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr

User username # it depends on your login; this one only for understanding

PubkeyAcceptedAlgorithms +ssh-rsa
HostkeyAlgorithms +ssh-rsa

And:

/etc/ssh/sshd_config

# Ciphers and keying

Ciphers             aes128-cbc,3des-cbc
KexAlgorithms +diffie-hellman-group1-sha1,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512

HostkeyAlgorithms ssh-dss,ssh-rsa
KexAlgorithms diffie-hellman-group1-sha1

VonC

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answered Jun 1 at 6:12

I had this issue and it turned out to be because my computer was referencing the wrong ssh.exe file.

Run the command:

which ssh.exe

If this does not return OpenSSH/ssh.exe then this is likely your issue.

Take the return value and rename the ssh.exe file as ssh.exe.org

Run

which ssh.exe

again and it should now show the OpenSSH/ssh.exe file.

answered Jul 20 at 16:27

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