Trying to ssh into my wireless router from my Raspberry PI – debug1: send_pubkey_test: no mutual signature algorithm

I have an Asus router: RT-AC1300G PLUS

In its settings I set a ssh public key that I generated on my Windows PC using ssh-keygen.

I can connect from my PC without a problem.

I can’t however connect to my router from my Raspberry PI. I get Permission denied (publickey) with the debug showing “debug1: send_pubkey_test: no mutual signature algorithm”.

I also tried moving the private/public key pair generated on the PC (which works on the PC, as mentioned earlier), to the PI and tried to connect with the same key that works on PC, the result is the same Permission denied (publickey) with “no mutual signature algorithm”.

The public keys are present in the router’s web interface and also in the router’s authorized_keys.

This is from the debug (ssh user@host -vvv) when I connect from the PC:

debug1: Offering public key: D:\id_rsa_pi4router_pcgen RSA SHA256:toeuBlcfX2juW8AjDCntufrSvx9U1MRFcKNTmDBszEQ explicit
debug3: send packet: type 50
debug2: we sent a publickey packet, wait for reply
debug3: receive packet: type 60
debug1: Server accepts key: D:\id_rsa_pi4router_pcgen RSA SHA256:toeuBlcfX2juW8AjDCntufrSvx9U1MRFcKNTmDBszEQ explicit

When connecting with the same key from the PI: (the output is similar when the key is generated on the PI, rather than copied from the PC)

debug1: Offering public key: /home/ndr/.ssh/id_rsa_pi4router_pcgen RSA SHA256:toeuBlcfX2juW8AjDCntufrSvx9U1MRFcKNTmDBszEQ explicit
debug1: send_pubkey_test: no mutual signature algorithm
debug2: we did not send a packet, disable method
debug1: No more authentication methods to try.

I also tried with an ed25519 key, but the router does not seem to accept it. This is from the debug of ssh:

debug1: Offering public key: /home/ndr/.ssh/id_ed25519 ED25519 SHA256:LzXxVd/VRoB/QKE5OXjKk+pR69B2HT+x0nS/EfIRL3U explicit
debug3: send packet: type 50
debug2: we sent a publickey packet, wait for reply
debug3: receive packet: type 51
debug1: Authentications that can continue: publickey
debug2: we did not send a packet, disable method
debug1: No more authentication methods to try.

EDIT: This makes sense since Dropbear only got support for ed25519 keys in a later version

ssh -V on PC returns “OpenSSH_for_Windows_8.1p1, LibreSSL 3.0.2”.

ssh -V on PI returns “OpenSSH_9.2p1 Debian-2+deb12u1, OpenSSL 3.0.11 19 Sep 2023”

ssh -V on the router returns “Dropbear v2019.78”

Any help is appreciated, thank you.