要約

When attempting to use an SSH key generated using the ssh-rsa sha-1 hash algorithm, the SSH key isn't accepted (the user receives a 'Permission denied' message), and the following message is displayed when the verbose SSH output is reviewed:

• debug1: send_pubkey_test: no mutual signature algorithm

環境

• Bitbucket Data Center/Server
• Known operating systems impacted:
  • Fedora 33+
  • Any given system running OpenSSH 8.8 or newer as this release disables RSA signatures using the SHA-1 hash algorithm by default

診断

When running the command ssh -vvvv, the following output can be seen:

debug3: authmethod_is_enabled publickey
debug1: Next authentication method: publickey
debug1: Offering public key: /home/user/.ssh/id_rsa RSA ... agent
debug1: send_pubkey_test: no mutual signature algorithm <-- ssh-rsa is not enabled 
debug1: No more authentication methods to try.
user@hostname: Permission denied (publickey).

GIT_SSH_COMMAND="ssh -vvvv" git clone NAME_OF_REPOSITORY

原因

The RSA SHA-1 hash algorithm is being quickly deprecated across operating systems and SSH clients because of various security vulnerabilities, with many of these technologies now outright denying the use of this algorithm.

•  For example - here is the announcement from OpenSSH regarding their upcoming deprecation of the ssh-rsa algorithm.

In the event that you are using an operating system or SSH client whose version has this algorithm disabled, it's possible that any SSH keys previously generated using this algorithm will no longer be accepted by these technologies. 

ソリューション

Our team has the following recommendations for helping to resolve the problem being experienced, in both the form of a workaround and resolution:

Workaround for re-enabling RSA

(!) This workaround should only be taken if it is impossible to regenerate SSH keys with ECDSA and ED25519 algorithms. If you can generate new ECDSA and ED25519 based keys, please proceed to the Resolution using ECDSA and ED25519 algorithms right below.

In order to re-enable ssh-rsa support, inserting the following line into the affected SSH client's config file can re-enable this algorithm:

PubkeyAcceptedKeyTypes +ssh-rsa

Resolution using ECDSA and ED25519 algorithms

To fully resolve this issue, our team recommends that these deprecated keys be re-generated using a supported and more secure algorithm such as ECDSA and ED25519:

debug3: authmethod_is_enabled publickey
debug1: Next authentication method: publickey
debug1: Offering public key: /home/user/.ssh/id_rsa RSA ... agent
debug1: send_pubkey_test: no mutual signature algorithm <-- ssh-rsa is not enabled
debug1: Offering public key: /home/user/.ssh/id_ed25519 ED25519 ... agent
debug3: send packet: type 50
debug2: we sent a publickey packet, wait for reply
debug3: receive packet: type 60
debug1: Server accepts key: /home/user/.ssh/id_ed25519 ED25519 ... agent <- The Ed25519 key is accepted without issue

To generate a key using this updated algorithm, it's recommended that you consult the documentation for the usual software that your team uses for SSH key generation.

For OpenSSH's ssh-keygen command in particular, the full list of algorithms for this command can be found on SSH.com: choosing an algorithm. In addition, here is an example command that creates a new SSH key using the ED25519 algorithm:

ssh-keygen -t ed25519 -C "your_email@example.com"