Public key authentication fails with JSch but work with OpenSSH with the same key

Issue

I'm trying to establish an SSH connection using Kotlin + JSch, but it fails with

com.jcraft.jsch.jSchException: Auth fail

Steps taken:

1. Generate SSH key pair using ssh-keygen -t rsa -m PEM (OpenSSH version: OpenSSH_8.2p1)
2. Append contents of generated id_rsa.pub to /home/username/.ssh/authorized_keys file on the server
3. Test connection in the terminal: ssh -i /path/to/id_rsa [email protected] – works fine
4. Execute the following Kotlin code:

import com.jcraft.jsch.JSch

const val USER = "username"
const val HOST = "example.host.com"
const val IDENTITY = "/path/to/id_rsa"


fun main() {
    val jsch = JSch().apply {
        addIdentity(IDENTITY)
        setKnownHosts("/path/to/known_hosts")
    }

    jsch.getSession(USER, HOST)
        .connect()
}

...which fails with an exception:

Exception in thread "main" com.jcraft.jsch.JSchException: Auth fail
    at com.jcraft.jsch.Session.connect(Session.java:519)
    at com.jcraft.jsch.Session.connect(Session.java:183)
    at MainKt.main(Main.kt:18)
    at MainKt.main(Main.kt)

What can be the problem here?

JSch log output:

INFO: Connecting to example.host.com port 22
INFO: Connection established
INFO: Remote version string: SSH-2.0-OpenSSH_8.9p1 Ubuntu-3
INFO: Local version string: SSH-2.0-JSCH-0.1.54
INFO: CheckCiphers: aes256-ctr,aes192-ctr,aes128-ctr,aes256-cbc,aes192-cbc,aes128-cbc,3des-ctr,arcfour,arcfour128,arcfour256
INFO: CheckKexes: diffie-hellman-group14-sha1,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521
INFO: CheckSignatures: ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521
INFO: SSH_MSG_KEXINIT sent
INFO: SSH_MSG_KEXINIT received
INFO: kex: server: curve25519-sha256,[email protected],ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,[email protected],diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256
INFO: kex: server: rsa-sha2-512,rsa-sha2-256,ecdsa-sha2-nistp256,ssh-ed25519
INFO: kex: server: [email protected],aes128-ctr,aes192-ctr,aes256-ctr,[email protected],[email protected]
INFO: kex: server: [email protected],aes128-ctr,aes192-ctr,aes256-ctr,[email protected],[email protected]
INFO: kex: server: [email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],hmac-sha2-256,hmac-sha2-512,hmac-sha1
INFO: kex: server: [email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],hmac-sha2-256,hmac-sha2-512,hmac-sha1
INFO: kex: server: none,[email protected]
INFO: kex: server: none,[email protected]
INFO: kex: server: 
INFO: kex: server: 
INFO: kex: client: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group14-sha1,diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group1-sha1
INFO: kex: client: ssh-rsa,ssh-dss,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521
INFO: kex: client: aes128-ctr,aes128-cbc,3des-ctr,3des-cbc,blowfish-cbc,aes192-ctr,aes192-cbc,aes256-ctr,aes256-cbc
INFO: kex: client: aes128-ctr,aes128-cbc,3des-ctr,3des-cbc,blowfish-cbc,aes192-ctr,aes192-cbc,aes256-ctr,aes256-cbc
INFO: kex: client: hmac-md5,hmac-sha1,hmac-sha2-256,hmac-sha1-96,hmac-md5-96
INFO: kex: client: hmac-md5,hmac-sha1,hmac-sha2-256,hmac-sha1-96,hmac-md5-96
INFO: kex: client: none
INFO: kex: client: none
INFO: kex: client: 
INFO: kex: client: 
INFO: kex: server->client aes128-ctr hmac-sha1 none
INFO: kex: client->server aes128-ctr hmac-sha1 none
INFO: SSH_MSG_KEX_ECDH_INIT sent
INFO: expecting SSH_MSG_KEX_ECDH_REPLY
INFO: Host 'example.host.com' is known and matches the ECDSA host key
INFO: SSH_MSG_NEWKEYS sent
INFO: SSH_MSG_NEWKEYS received
INFO: SSH_MSG_SERVICE_REQUEST sent
INFO: SSH_MSG_SERVICE_ACCEPT received
INFO: Authentications that can continue: publickey,keyboard-interactive,password
INFO: Next authentication method: publickey
INFO: Disconnecting from example.host.com port 22
Exception in thread "main" com.jcraft.jsch.JSchException: Auth fail
    at com.jcraft.jsch.Session.connect(Session.java:519)
    at com.jcraft.jsch.Session.connect(Session.java:183)
    at MainKt.main(Main.kt:37)
    at MainKt.main(Main.kt)

ssh -vvv output:

OpenSSH_8.2p1 Ubuntu-4, OpenSSL 1.1.1f  31 Mar 2020
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 19: include /etc/ssh/ssh_config.d/*.conf matched no files
debug1: /etc/ssh/ssh_config line 21: Applying options for *
debug2: resolving "example.host.com" port 22
debug2: ssh_connect_direct
debug1: Connecting to example.host.com [51.83.250.141] port 22.
debug1: Connection established.
debug1: identity file /path/to/id_rsa type 0
debug1: identity file /path/to/id_rsa-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_8.2p1 Ubuntu-4
debug1: Remote protocol version 2.0, remote software version OpenSSH_8.9p1 Ubuntu-3
debug1: match: OpenSSH_8.9p1 Ubuntu-3 pat OpenSSH* compat 0x04000000
debug2: fd 3 setting O_NONBLOCK
debug1: Authenticating to dev.sages.pl:22 as 'sages-devops'
debug3: hostkeys_foreach: reading file "/path/to/known_hosts"
debug3: record_hostkey: found key type ECDSA in file /path/to/known_hosts:5
debug3: load_hostkeys: loaded 1 keys from example.host.com
debug3: order_hostkeyalgs: prefer hostkeyalgs: [email protected],[email protected],[email protected],ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521
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: [email protected],[email protected],[email protected],ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],ssh-ed25519,[email protected],rsa-sha2-512,rsa-sha2-256,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-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,[email protected],diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256
debug2: host key algorithms: rsa-sha2-512,rsa-sha2-256,ecdsa-sha2-nistp256,ssh-ed25519
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]
debug2: compression stoc: none,[email protected]
debug2: languages ctos:
debug2: languages stoc:
debug2: first_kex_follows 0
debug2: reserved 0
debug1: kex: algorithm: curve25519-sha256
debug1: kex: host key algorithm: ecdsa-sha2-nistp256
debug1: kex: server->client cipher: [email protected] MAC: <implicit> compression: none
debug1: kex: client->server cipher: [email protected] MAC: <implicit> compression: none
debug3: send packet: type 30
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug3: receive packet: type 31
debug1: Server host key: ecdsa-sha2-nistp256 SHA256:QcwhazMtZl/qgVMu6FLP+vLtUEmo6yDULBaK8+4SHnI
debug3: hostkeys_foreach: reading file "/path/to/known_hosts"
debug3: record_hostkey: found key type ECDSA in file /path/to/known_hosts:5
debug3: load_hostkeys: loaded 1 keys from dev.sages.pl
debug3: hostkeys_foreach: reading file "/path/to/known_hosts"
debug3: record_hostkey: found key type ECDSA in file /path/to/known_hosts:6
debug3: load_hostkeys: loaded 1 keys from 51.83.250.141
debug1: Host 'example.host.com' is known and matches the ECDSA host key.
debug1: Found key in /path/to/known_hosts:5
debug3: send packet: type 21
debug2: set_newkeys: mode 1
debug1: rekey out after 134217728 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 134217728 blocks
debug1: Will attempt key: /path/to/id_rsa RSA SHA256:FxKSoi+/DPu0D6JZczCPMdydLyWuuKFFPeMEzdsiFkU explicit
debug2: pubkey_prepare: done
debug3: send packet: type 5
debug3: receive packet: type 7
debug1: SSH2_MSG_EXT_INFO received
debug1: kex_input_ext_info: server-sig-algs=<ssh-ed25519,[email protected],ssh-rsa,rsa-sha2-256,rsa-sha2-512,ssh-dss,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,[email protected],[email protected]>
debug1: kex_input_ext_info: [email protected] (unrecognised)
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 gssapi-with-mic,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: /path/to/id_rsa RSA SHA256:FxKSoi+/DPu0D6JZczCPMdydLyWuuKFFPeMEzdsiFkU explicit
debug3: send packet: type 50
debug2: we sent a publickey packet, wait for reply
debug3: receive packet: type 60
debug1: Server accepts key: /path/to/id_rsa RSA SHA256:FxKSoi+/DPu0D6JZczCPMdydLyWuuKFFPeMEzdsiFkU explicit
debug3: sign_and_send_pubkey: RSA SHA256:FxKSoi+/DPu0D6JZczCPMdydLyWuuKFFPeMEzdsiFkU
debug3: sign_and_send_pubkey: signing using rsa-sha2-512 SHA256:FxKSoi+/DPu0D6JZczCPMdydLyWuuKFFPeMEzdsiFkU
debug3: send packet: type 50
debug3: receive packet: type 52
debug1: Authentication succeeded (publickey).
Authenticated to example.host.com ([51.83.250.141]:22).
debug1: channel 0: new [client-session]
debug3: ssh_session2_open: channel_new: 0
debug2: channel 0: send open
debug3: send packet: type 90
debug1: Requesting [email protected]
debug3: send packet: type 80
debug1: Entering interactive session.
debug1: pledge: network
debug3: receive packet: type 80
debug1: client_input_global_request: rtype [email protected] want_reply 0
debug3: receive packet: type 4
debug1: Remote: /home/username/.ssh/authorized_keys:1: key options: agent-forwarding port-forwarding pty user-rc x11-forwarding
debug3: receive packet: type 4
debug1: Remote: /home/username/.ssh/authorized_keys:1: key options: agent-forwarding port-forwarding pty user-rc x11-forwarding
debug3: receive packet: type 91
debug2: channel_input_open_confirmation: channel 0: callback start
debug2: fd 3 setting TCP_NODELAY
debug3: ssh_packet_set_tos: set IP_TOS 0x10
debug2: client_session2_setup: id 0
debug2: channel 0: request pty-req confirm 1
debug3: send packet: type 98
debug1: Sending environment.
debug3: Ignored env SHELL
debug3: Ignored env WSL_DISTRO_NAME
debug3: Ignored env JAVA_HOME
debug3: Ignored env SDKMAN_CANDIDATES_DIR
debug3: Ignored env NAME
debug3: Ignored env PWD
debug3: Ignored env LOGNAME
debug3: Ignored env HOME
debug1: Sending env LANG = C.UTF-8
debug2: channel 0: request env confirm 0
debug3: send packet: type 98
debug3: Ignored env WSL_INTEROP
debug3: Ignored env LS_COLORS
debug3: Ignored env WAYLAND_DISPLAY
debug3: Ignored env SDKMAN_VERSION
debug3: Ignored env LESSCLOSE
debug3: Ignored env TERM
debug3: Ignored env LESSOPEN
debug3: Ignored env USER
debug3: Ignored env SDKMAN_DIR
debug3: Ignored env DISPLAY
debug3: Ignored env SHLVL
debug3: Ignored env SDKMAN_CANDIDATES_API
debug3: Ignored env XDG_RUNTIME_DIR
debug3: Ignored env WSLENV
debug3: Ignored env XDG_DATA_DIRS
debug3: Ignored env PATH
debug3: Ignored env SDKMAN_PLATFORM
debug3: Ignored env HOSTTYPE
debug3: Ignored env PULSE_SERVER
debug3: Ignored env _
debug2: channel 0: request shell confirm 1
debug3: send packet: type 98
debug2: channel_input_open_confirmation: channel 0: callback done
debug2: channel 0: open confirm rwindow 0 rmax 32768
debug3: receive packet: type 99
debug2: channel_input_status_confirm: type 99 id 0
debug2: PTY allocation request accepted on channel 0
debug2: channel 0: rcvd adjust 2097152
debug3: receive packet: type 99
debug2: channel_input_status_confirm: type 99 id 0
debug2: shell request accepted on channel 0

Solution

The ssh is using rsa-sha2-512 key signature. While that does not prove that your server requires it, it's quite probable that it does.

---

JSch does not support rsa-sha2. And as JSch seems not to be updated anymore, it quite likely never will.

There's a fork of JSch that does though:
https://github.com/mwiede/jsch
At least test it, to verify that this is indeed the issue.

---

Another (less secure obviously) option is to reconfigure the server not to require the rsa-sha2 by adding deprecated ssh-rsa to PubkeyAcceptedAlgorithms.

Answered By - Martin Prikryl