No Mutual Signature Supported

TLDR - Quick Summary

Problem: "sign_and_send_pubkey: no mutual signature supported" error when connecting via SFTP

Cause: Local SSH client deprecated ssh-rsa algorithm

Fix 1: Add PubkeyAcceptedKeyTypes +ssh-rsa to local SSH config

Fix 2: Use -o PubkeyAcceptedKeyTypes=ssh-rsa flag in sftp command

Overview

In SFTP Gateway 3.x, when connecting via SFTP over the cli/terminal, some users may run into this error message:

sign_and_send_pubkey: no mutual signature supported

The cause of the error message is due to the SSH client on your local machine deprecating the use of the ssh-rsa algorithm.

Workarounds

To get past the error message and connect to server, there are a couple solutions.

The first being to edit the SSH config file on your local machine and apend this line:

PubkeyAcceptedKeyTypes +ssh-rsa

Another option is to add the flag -o PubkeyAcceptedKeyTypes=ssh-rsa in the sftp command to allow ssh-rsa:

sftp -o PubkeyAcceptedKeyTypes=ssh-rsa -i Bryce.key SFTPuser@20.4.128.165

After specifying ssh-rsa as an accepted key type, you will then be able to connect to the SFTP server.