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I've create a RSA key pair with ssh-keygen. In /etc/dropbear-initramfs/authorized_keys I have the public key, and on my client machine I have the keypair in the /home/<name>/.ssh/ directory. After running update-initramfs -u, I still am not able to connect with Dropbear.

Here is how I have it set up on the server:

/etc/dropbear-initramfs/authorized_keys:

ssh-rsa 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 root@ubuntu

ls -la /etc/dropbear-initramfs shows:

drwxr-xr-x  2 root root 4096 Apr 17 23:11 .
drwxr-xr-x 99 root root 4096 Apr 17 21:33 ..
-rw-------  1 root root  565 Apr 17 23:11 authorized_keys
-rw-r--r--  1 root root  579 Apr 17 22:48 config
-rw-------  1 root root  458 Apr 17 21:33 dropbear_dss_host_key
-rw-------  1 root root  140 Apr 17 21:33 dropbear_ecdsa_host_key
-rw-------  1 root root  805 Apr 17 21:33 dropbear_rsa_host_key
-rw-------  1 root root 2590 Apr 17 23:11 id_rsa
-rw-r--r--  1 root root  565 Apr 17 23:11 id_rsa.pub

After seeing "Begin: Starting dropbear ..." on my server, and then running the following command on my client machine from a different network: ssh -o "HostKeyAlgorithms ssh-rsa" -p <port> root@<ip> -vvv, I get the following output:

OpenSSH_8.8p1, OpenSSL 1.1.1m  14 Dec 2021
debug1: Reading configuration data /etc/ssh/ssh_config
debug2: resolve_canonicalize: hostname <ip> is address
debug3: expanded UserKnownHostsFile '~/.ssh/known_hosts' -> '/home/<name>/.ssh/known_hosts'
debug3: expanded UserKnownHostsFile '~/.ssh/known_hosts2' -> '/home/<name>/.ssh/known_hosts2'
debug3: ssh_connect_direct: entering
debug1: Connecting to <ip> [<ip>] port <port>.
debug3: set_sock_tos: set socket 3 IP_TOS 0x48
debug1: Connection established.
debug1: identity file /home/<name>/.ssh/id_rsa type 0
debug1: identity file /home/<name>/.ssh/id_rsa-cert type -1
debug1: identity file /home/<name>/.ssh/id_dsa type -1
debug1: identity file /home/<name>/.ssh/id_dsa-cert type -1
debug1: identity file /home/<name>/.ssh/id_ecdsa type -1
debug1: identity file /home/<name>/.ssh/id_ecdsa-cert type -1
debug1: identity file /home/<name>/.ssh/id_ecdsa_sk type -1
debug1: identity file /home/<name>/.ssh/id_ecdsa_sk-cert type -1
debug1: identity file /home/<name>/.ssh/id_ed25519 type -1
debug1: identity file /home/<name>/.ssh/id_ed25519-cert type -1
debug1: identity file /home/<name>/.ssh/id_ed25519_sk type -1
debug1: identity file /home/<name>/.ssh/id_ed25519_sk-cert type -1
debug1: identity file /home/<name>/.ssh/id_xmss type -1
debug1: identity file /home/<name>/.ssh/id_xmss-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_8.8
debug1: Remote protocol version 2.0, remote software version dropbear_2019.78
debug1: compat_banner: no match: dropbear_2019.78
debug2: fd 3 setting O_NONBLOCK
debug1: Authenticating to <ip>:<port> as 'root'
debug3: send packet: type 20
debug1: SSH2_MSG_KEXINIT sent
debug3: receive packet: type 20
debug1: SSH2_MSG_KEXINIT received
debug2: local client KEXINIT proposal
debug2: KEX algorithms: curve25519-sha256,[email protected],ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256,ext-info-c
debug2: host key algorithms: ssh-rsa
debug2: ciphers ctos: [email protected],aes128-ctr,aes192-ctr,aes256-ctr,[email protected],[email protected]
debug2: ciphers stoc: [email protected],aes128-ctr,aes192-ctr,aes256-ctr,[email protected],[email protected]
debug2: MACs ctos: [email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],hmac-sha2-256,hmac-sha2-512,hmac-sha1
debug2: MACs stoc: [email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],hmac-sha2-256,hmac-sha2-512,hmac-sha1
debug2: compression ctos: none,[email protected],zlib
debug2: compression stoc: none,[email protected],zlib
debug2: languages ctos: 
debug2: languages stoc: 
debug2: first_kex_follows 0 
debug2: reserved 0 
debug2: peer server KEXINIT proposal
debug2: KEX algorithms: curve25519-sha256,[email protected],ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1,[email protected]
debug2: host key algorithms: ecdsa-sha2-nistp256,ssh-rsa,ssh-dss
debug2: ciphers ctos: aes128-ctr,aes256-ctr,aes128-cbc,aes256-cbc,3des-ctr,3des-cbc
debug2: ciphers stoc: aes128-ctr,aes256-ctr,aes128-cbc,aes256-cbc,3des-ctr,3des-cbc
debug2: MACs ctos: hmac-sha1-96,hmac-sha1,hmac-sha2-256
debug2: MACs stoc: hmac-sha1-96,hmac-sha1,hmac-sha2-256
debug2: compression ctos: [email protected],none
debug2: compression stoc: [email protected],none
debug2: languages ctos: 
debug2: languages stoc: 
debug2: first_kex_follows 0 
debug2: reserved 0 
debug1: kex: algorithm: curve25519-sha256
debug1: kex: host key algorithm: ssh-rsa
debug1: kex: server->client cipher: aes128-ctr MAC: hmac-sha2-256 compression: none
debug1: kex: client->server cipher: aes128-ctr MAC: hmac-sha2-256 compression: none
debug3: send packet: type 30
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug3: receive packet: type 31
debug1: SSH2_MSG_KEX_ECDH_REPLY received
debug1: Server host key: ssh-rsa SHA256:k5mkg88bVoGfNcqRVQ4dO+A3bUDOsVvKH7QBLuUud5A
debug3: put_host_port: [<ip>]:<port>
debug3: put_host_port: [<ip>]:<port>
debug3: record_hostkey: found key type RSA in file /home/<name>/.ssh/known_hosts:1
debug3: load_hostkeys_file: loaded 1 keys from [<ip>]:<port>
debug1: load_hostkeys: fopen /home/<name>/.ssh/known_hosts2: No such file or directory
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts: No such file or directory
debug1: load_hostkeys: fopen /etc/ssh/ssh_known_hosts2: No such file or directory
debug1: Host '[<ip>]:<port>' is known and matches the RSA host key.
debug1: Found key in /home/<name>/.ssh/known_hosts:1
debug3: send packet: type 21
debug2: set_newkeys: mode 1
debug1: rekey out after 4294967296 blocks
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug3: receive packet: type 21
debug1: SSH2_MSG_NEWKEYS received
debug2: set_newkeys: mode 0
debug1: rekey in after 4294967296 blocks
debug1: Will attempt key: /home/<name>/.ssh/id_rsa RSA SHA256:uibL6T72PUCtFFh9FUMwgvTng4YjrQyoR1pido1PKa8
debug1: Will attempt key: /home/<name>/.ssh/id_dsa 
debug1: Will attempt key: /home/<name>/.ssh/id_ecdsa 
debug1: Will attempt key: /home/<name>/.ssh/id_ecdsa_sk 
debug1: Will attempt key: /home/<name>/.ssh/id_ed25519 
debug1: Will attempt key: /home/<name>/.ssh/id_ed25519_sk 
debug1: Will attempt key: /home/<name>/.ssh/id_xmss 
debug2: pubkey_prepare: done
debug3: send packet: type 5
debug3: receive packet: type 6
debug2: service_accept: ssh-userauth
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug3: send packet: type 50
debug3: receive packet: type 51
debug1: Authentications that can continue: publickey
debug3: start over, passed a different list publickey
debug3: preferred publickey,keyboard-interactive,password
debug3: authmethod_lookup publickey
debug3: remaining preferred: keyboard-interactive,password
debug3: authmethod_is_enabled publickey
debug1: Next authentication method: publickey
debug1: Offering public key: /home/<name>/.ssh/id_rsa RSA SHA256:uibL6T72PUCtFFh9FUMwgvTng4YjrQyoR1pido1PKa8
debug1: send_pubkey_test: no mutual signature algorithm
debug1: Trying private key: /home/<name>/.ssh/id_dsa
debug3: no such identity: /home/<name>/.ssh/id_dsa: No such file or directory
debug1: Trying private key: /home/<name>/.ssh/id_ecdsa
debug3: no such identity: /home/<name>/.ssh/id_ecdsa: No such file or directory
debug1: Trying private key: /home/<name>/.ssh/id_ecdsa_sk
debug3: no such identity: /home/<name>/.ssh/id_ecdsa_sk: No such file or directory
debug1: Trying private key: /home/<name>/.ssh/id_ed25519
debug3: no such identity: /home/<name>/.ssh/id_ed25519: No such file or directory
debug1: Trying private key: /home/<name>/.ssh/id_ed25519_sk
debug3: no such identity: /home/<name>/.ssh/id_ed25519_sk: No such file or directory
debug1: Trying private key: /home/<name>/.ssh/id_xmss
debug3: no such identity: /home/<name>/.ssh/id_xmss: No such file or directory
debug2: we did not send a packet, disable method
debug1: No more authentication methods to try.
root@<ip>: Permission denied (publickey).

Been struggling with this for a while now...

Any ideas? Thanks!

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1 Answer 1

Reset to default
8

The SSH client is OpenSSH 8.8. Its release notes tell:

Potentially-incompatible changes

This release disables RSA signatures using the SHA-1 hash algorithm by default.
[...]
Incompatibility is more likely when connecting to older SSH implementations that have not been upgraded or have not closely tracked improvements in the SSH protocol.
[...]
For example, the following stanza in ~/.ssh/config will enable RSA/SHA1 for host and user authentication for a single destination host:

Host old-host
    HostkeyAlgorithms +ssh-rsa
    PubkeyAcceptedAlgorithms +ssh-rsa

or its equivalent command line option (actually only the 2nd entry should be needed for this case):

ssh -o PubkeyAcceptedAlgorithms=+ssh-rsa ...

which should prevent this to happen:

debug1: send_pubkey_test: no mutual signature algorithm

Reproduced failing and fixed with Dropbear version 2018.76 (and tested not needed with 2020.81). OP's 2019.78 isn't good enough either. Dropbear's documentation tells the required adjustment was done after version 2020.79:

  • Support using rsa-sha2 signatures. No changes are needed to hostkeys/authorized_keys entries, existing RSA keys can be used with the new signature format (signatures are ephemeral within a session). Old ssh-rsa signatures will no longer be supported by OpenSSH in future so upgrading is recommended.

Switching to a non-RSA private/public key pair (and updating authorized_keys) should also be an alternative to lowering client's default security settings or to upgrading Dropbear, but again, one must ensure compatible choices are used.

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5
  • So I created an unencrypted ecdsa key pair, and that seemed to have fixed it. Hopefully this is still kinda secure. Thanks!
    – lasttry
    Apr 18, 2022 at 12:54
  • I hope you did also see in my answer that you picked the wrong of the two options: ssh -o "HostKeyAlgorithms ssh-rsa" (won't work) instead of ssh -o PubkeyAcceptedAlgorithms=+ssh-rsa (will work with current keys).
    – A.B
    Apr 18, 2022 at 12:57
  • I understood it, based on your last paragraph, that switching to a different key type would be better to not have to "lower clients default security settings (...)"? Is using ecdsa a good solution?
    – lasttry
    Apr 18, 2022 at 13:00
  • should be: security.stackexchange.com/questions/50878/…
    – A.B
    Apr 18, 2022 at 13:05
  • I'd started to have this issue after Updating to MacOS Ventura 13.0 at my laptop. It has bundled OpenSSH 9.0
    – Andrey Zentavr
    Oct 29, 2022 at 0:23

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