ssh rsa key - Bad server host key: Invalid key length

[Carlos Martinez]

After updating to Fedora 37 beta, I was not able to login to a device that uses rsa to
authenticate anymore.

The message shown is: "Bad server host key: Invalid key length"

[jcm@fedora ~]$ ssh sysadmin(a)192.168.225.130 -vvvv
OpenSSH_8.8p1, OpenSSL 3.0.5 5 Jul 2022
debug1: Reading configuration data /etc/ssh/ssh_config
debug3: /etc/ssh/ssh_config line 55: Including file /etc/ssh/ssh_config.d/50-redhat.conf
depth 0
debug1: Reading configuration data /etc/ssh/ssh_config.d/50-redhat.conf
debug2: checking match for 'final all' host 192.168.225.130 originally
192.168.225.130
debug3: /etc/ssh/ssh_config.d/50-redhat.conf line 3: not matched 'final'
debug2: match not found
debug3: /etc/ssh/ssh_config.d/50-redhat.conf line 5: Including file
/etc/crypto-policies/back-ends/openssh.config depth 1 (parse only)
debug1: Reading configuration data /etc/crypto-policies/back-ends/openssh.config
debug3: gss kex names ok:
[gss-curve25519-sha256-,gss-nistp256-sha256-,gss-group14-sha256-,gss-group16-sha512-]
debug3: kex names ok:
[curve25519-sha256,[e-mail],ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512]
debug1: configuration requests final Match pass
debug2: resolve_canonicalize: hostname 192.168.225.130 is address
debug1: re-parsing configuration
debug1: Reading configuration data /etc/ssh/ssh_config
debug3: /etc/ssh/ssh_config line 55: Including file /etc/ssh/ssh_config.d/50-redhat.conf
depth 0
debug1: Reading configuration data /etc/ssh/ssh_config.d/50-redhat.conf
debug2: checking match for 'final all' host 192.168.225.130 originally
192.168.225.130
debug3: /etc/ssh/ssh_config.d/50-redhat.conf line 3: matched 'final'
debug2: match found
debug3: /etc/ssh/ssh_config.d/50-redhat.conf line 5: Including file
/etc/crypto-policies/back-ends/openssh.config depth 1
debug1: Reading configuration data /etc/crypto-policies/back-ends/openssh.config
debug3: gss kex names ok:
[gss-curve25519-sha256-,gss-nistp256-sha256-,gss-group14-sha256-,gss-group16-sha512-]
debug3: kex names ok:
[curve25519-sha256,[e-mail],ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512]
debug3: expanded UserKnownHostsFile '~/.ssh/known_hosts' ->
'/home/jcm/.ssh/known_hosts'
debug3: expanded UserKnownHostsFile '~/.ssh/known_hosts2' ->
'/home/jcm/.ssh/known_hosts2'
debug3: ssh_connect_direct: entering
debug1: Connecting to 192.168.225.130 [192.168.225.130] port 22.
debug3: set_sock_tos: set socket 3 IP_TOS 0x48
debug1: Connection established.
debug1: identity file /home/jcm/.ssh/id_rsa type -1
debug1: identity file /home/jcm/.ssh/id_rsa-cert type -1
debug1: identity file /home/jcm/.ssh/id_dsa type -1
debug1: identity file /home/jcm/.ssh/id_dsa-cert type -1
debug1: identity file /home/jcm/.ssh/id_ecdsa type -1
debug1: identity file /home/jcm/.ssh/id_ecdsa-cert type -1
debug1: identity file /home/jcm/.ssh/id_ecdsa_sk type -1
debug1: identity file /home/jcm/.ssh/id_ecdsa_sk-cert type -1
debug1: identity file /home/jcm/.ssh/id_ed25519 type -1
debug1: identity file /home/jcm/.ssh/id_ed25519-cert type -1
debug1: identity file /home/jcm/.ssh/id_ed25519_sk type -1
debug1: identity file /home/jcm/.ssh/id_ed25519_sk-cert type -1
debug1: identity file /home/jcm/.ssh/id_xmss type -1
debug1: identity file /home/jcm/.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_2022.82
debug1: compat_banner: no match: dropbear_2022.82
debug2: fd 3 setting O_NONBLOCK
debug1: Authenticating to 192.168.225.130:22 as 'sysadmin'
debug1: load_hostkeys: fopen /home/jcm/.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
debug3: order_hostkeyalgs: no algorithms matched; accept original
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,[e-mail],ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,ext-info-c
debug2: host key algorithms:
[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],ssh-ed25519,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,[e-mail],sk-ecdsa-sha2-nistp256(a)openssh.com,rsa-sha2-512,rsa-sha2-256
debug2: ciphers ctos:
[e-mail],[e-mail],aes256-ctr,aes128-gcm(a)openssh.com,aes128-ctr
debug2: ciphers stoc:
[e-mail],[e-mail],aes256-ctr,aes128-gcm(a)openssh.com,aes128-ctr
debug2: MACs ctos:
[e-mail],[e-mail],[e-mail],[e-mail],hmac-sha2-256,hmac-sha1,umac-128(a)openssh.com,hmac-sha2-512
debug2: MACs stoc:
[e-mail],[e-mail],[e-mail],[e-mail],hmac-sha2-256,hmac-sha1,umac-128(a)openssh.com,hmac-sha2-512
debug2: compression ctos: none,zlib(a)openssh.com,zlib
debug2: compression stoc: none,zlib(a)openssh.com,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,[e-mail],ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1,kexguess2(a)matt.ucc.asn.au
debug2: host key algorithms: rsa-sha2-256,ssh-rsa,ssh-dss
debug2: ciphers ctos: chacha20-poly1305(a)openssh.com,aes128-ctr,aes256-ctr
debug2: ciphers stoc: chacha20-poly1305(a)openssh.com,aes128-ctr,aes256-ctr
debug2: MACs ctos: hmac-sha1,hmac-sha2-256
debug2: MACs stoc: hmac-sha1,hmac-sha2-256
debug2: compression ctos: zlib(a)openssh.com,none
debug2: compression stoc: zlib(a)openssh.com,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: rsa-sha2-256
debug1: kex: server->client cipher: chacha20-poly1305(a)openssh.com MAC: <implicit>
compression: none
debug1: kex: client->server cipher: chacha20-poly1305(a)openssh.com MAC: <implicit>
compression: none
debug1: kex: curve25519-sha256 need=64 dh_need=64
debug1: kex: curve25519-sha256 need=64 dh_need=64
debug3: send packet: type 30
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug3: receive packet: type 31
debug1: SSH2_MSG_KEX_ECDH_REPLY received
Bad server host key: Invalid key length

When trying with another edge distro like opensuse tumbleweed, I get 

OpenSSH_8.9p1, OpenSSL 1.1.1q  5 Jul 2022
debug1: Reading configuration data /usr/etc/ssh/ssh_config
debug1: /usr/etc/ssh/ssh_config line 24: include /etc/ssh/ssh_config.d/*.conf matched no
files
debug1: /usr/etc/ssh/ssh_config line 25: include /usr/etc/ssh/ssh_config.d/*.conf matched
no files
debug1: /usr/etc/ssh/ssh_config line 27: Applying options for *
debug2: resolve_canonicalize: hostname 192.168.225.130 is address
debug3: expanded UserKnownHostsFile '~/.ssh/known_hosts' ->
'/home/carlos/.ssh/known_hosts'
debug3: expanded UserKnownHostsFile '~/.ssh/known_hosts2' ->
'/home/carlos/.ssh/known_hosts2'
debug3: ssh_connect_direct: entering
debug1: Connecting to 192.168.225.130 [192.168.225.130] port 22.
debug3: set_sock_tos: set socket 3 IP_TOS 0x10
debug1: Connection established.
debug1: identity file /home/carlos/.ssh/id_rsa type -1
debug1: identity file /home/carlos/.ssh/id_rsa-cert type -1
debug1: identity file /home/carlos/.ssh/id_ecdsa type -1
debug1: identity file /home/carlos/.ssh/id_ecdsa-cert type -1
debug1: identity file /home/carlos/.ssh/id_ecdsa_sk type -1
debug1: identity file /home/carlos/.ssh/id_ecdsa_sk-cert type -1
debug1: identity file /home/carlos/.ssh/id_ed25519 type -1
debug1: identity file /home/carlos/.ssh/id_ed25519-cert type -1
debug1: identity file /home/carlos/.ssh/id_ed25519_sk type -1
debug1: identity file /home/carlos/.ssh/id_ed25519_sk-cert type -1
debug1: identity file /home/carlos/.ssh/id_xmss type -1
debug1: identity file /home/carlos/.ssh/id_xmss-cert type -1
debug1: identity file /home/carlos/.ssh/id_dsa type -1
debug1: identity file /home/carlos/.ssh/id_dsa-cert type -1
debug1: Local version string SSH-2.0-OpenSSH_8.9
debug1: Remote protocol version 2.0, remote software version dropbear_2022.82
debug1: compat_banner: no match: dropbear_2022.82
debug2: fd 3 setting O_NONBLOCK
debug1: Authenticating to 192.168.225.130:22 as 'sysadmin'
debug3: record_hostkey: found key type RSA in file /home/carlos/.ssh/known_hosts:1
debug3: load_hostkeys_file: loaded 1 keys from 192.168.225.130
debug1: load_hostkeys: fopen /home/carlos/.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
debug3: order_hostkeyalgs: prefer hostkeyalgs:
[e-mail],rsa-sha2-256-cert-v01(a)openssh.com,rsa-sha2-512,rsa-sha2-256
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,[e-mail],ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,[e-mail],diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1,ext-info-c
debug2: host key algorithms:
[e-mail],[e-mail],rsa-sha2-512,rsa-sha2-256,[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],ssh-ed25519,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,[e-mail],sk-ecdsa-sha2-nistp256(a)openssh.com
debug2: ciphers ctos:
[e-mail],aes128-ctr,aes192-ctr,aes256-ctr,[e-mail],aes256-gcm(a)openssh.com
debug2: ciphers stoc:
[e-mail],aes128-ctr,aes192-ctr,aes256-ctr,[e-mail],aes256-gcm(a)openssh.com
debug2: MACs ctos:
[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],umac-128(a)openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-sha1
debug2: MACs stoc:
[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],umac-128(a)openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-sha1
debug2: compression ctos: none,zlib(a)openssh.com,zlib
debug2: compression stoc: none,zlib(a)openssh.com,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,[e-mail],ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1,kexguess2(a)matt.ucc.asn.au
debug2: host key algorithms: rsa-sha2-256,ssh-rsa,ssh-dss
debug2: ciphers ctos: chacha20-poly1305(a)openssh.com,aes128-ctr,aes256-ctr
debug2: ciphers stoc: chacha20-poly1305(a)openssh.com,aes128-ctr,aes256-ctr
debug2: MACs ctos: hmac-sha1,hmac-sha2-256
debug2: MACs stoc: hmac-sha1,hmac-sha2-256
debug2: compression ctos: zlib(a)openssh.com,none
debug2: compression stoc: zlib(a)openssh.com,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: rsa-sha2-256
debug1: kex: server->client cipher: chacha20-poly1305(a)openssh.com MAC: <implicit>
compression: none
debug1: kex: client->server cipher: chacha20-poly1305(a)openssh.com MAC: <implicit>
compression: none
debug1: kex: curve25519-sha256 need=64 dh_need=64
debug1: kex: curve25519-sha256 need=64 dh_need=64
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:oH0saNfFWms66HfgQF/Bn/FazwRdg6AGCFtV6xuL6z8
debug3: record_hostkey: found key type RSA in file /home/carlos/.ssh/known_hosts:1
debug3: load_hostkeys_file: loaded 1 keys from 192.168.225.130
debug1: load_hostkeys: fopen /home/carlos/.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 '192.168.225.130' is known and matches the RSA host key.
debug1: Found key in /home/carlos/.ssh/known_hosts:1
debug3: send packet: type 21
debug2: ssh_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: ssh_set_newkeys: mode 0
debug1: rekey in after 134217728 blocks
debug1: get_agent_identities: bound agent to hostkey
debug1: get_agent_identities: ssh_fetch_identitylist: agent contains no identities
debug1: Will attempt key: /home/carlos/.ssh/id_rsa 
debug1: Will attempt key: /home/carlos/.ssh/id_ecdsa 
debug1: Will attempt key: /home/carlos/.ssh/id_ecdsa_sk 
debug1: Will attempt key: /home/carlos/.ssh/id_ed25519 
debug1: Will attempt key: /home/carlos/.ssh/id_ed25519_sk 
debug1: Will attempt key: /home/carlos/.ssh/id_xmss 
debug1: Will attempt key: /home/carlos/.ssh/id_dsa 
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=&lt;ssh-ed25519,[e-mail],ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,sk-ecdsa-sha2-nistp256(a)openssh.com,rsa-sha2-256,ssh-rsa,ssh-dss&gt;
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,password
debug3: start over, passed a different list publickey,password
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: Trying private key: /home/carlos/.ssh/id_rsa
debug3: no such identity: /home/carlos/.ssh/id_rsa: No such file or directory
debug1: Trying private key: /home/carlos/.ssh/id_ecdsa
debug3: no such identity: /home/carlos/.ssh/id_ecdsa: No such file or directory
debug1: Trying private key: /home/carlos/.ssh/id_ecdsa_sk
debug3: no such identity: /home/carlos/.ssh/id_ecdsa_sk: No such file or directory
debug1: Trying private key: /home/carlos/.ssh/id_ed25519
debug3: no such identity: /home/carlos/.ssh/id_ed25519: No such file or directory
debug1: Trying private key: /home/carlos/.ssh/id_ed25519_sk
debug3: no such identity: /home/carlos/.ssh/id_ed25519_sk: No such file or directory
debug1: Trying private key: /home/carlos/.ssh/id_xmss
debug3: no such identity: /home/carlos/.ssh/id_xmss: No such file or directory
debug1: Trying private key: /home/carlos/.ssh/id_dsa
debug3: no such identity: /home/carlos/.ssh/id_dsa: No such file or directory
debug2: we did not send a packet, disable method
debug3: authmethod_lookup password
debug3: remaining preferred: ,password
debug3: authmethod_is_enabled password
debug1: Next authentication method: password
sysadmin(a)192.168.225.130&#39;s password: 
debug3: send packet: type 50
debug2: we sent a password packet, wait for reply
debug3: receive packet: type 52
Authenticated to 192.168.225.130 ([192.168.225.130]:22) using "password".
debug1: channel 0: new [client-session]
debug3: ssh_session2_open: channel_new: 0
debug2: channel 0: send open
debug3: send packet: type 90
debug1: Entering interactive session.
debug1: pledge: filesystem
debug3: receive packet: type 91
debug2: channel_input_open_confirmation: channel 0: callback start
debug2: fd 3 setting TCP_NODELAY
debug3: set_sock_tos: set socket 3 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 SESSION_MANAGER
debug3: Ignored env COLORTERM
debug3: Ignored env XDG_CONFIG_DIRS
debug3: Ignored env LESS
debug3: Ignored env XDG_MENU_PREFIX
debug3: Ignored env MACHTYPE
debug3: Ignored env G_BROKEN_FILENAMES
debug3: Ignored env HISTSIZE
debug3: Ignored env HOSTNAME
debug3: Ignored env FROM_HEADER
debug3: Ignored env MINICOM
debug3: Ignored env JAVA_ROOT
debug3: Ignored env JAVA_HOME
debug3: Ignored env AUDIODRIVER
debug3: Ignored env JRE_HOME
debug3: Ignored env SSH_AUTH_SOCK
debug3: Ignored env CPU
debug3: Ignored env JAVA_BINDIR
debug3: Ignored env DESKTOP_SESSION
debug3: Ignored env GPG_TTY
debug3: Ignored env PWD
debug3: Ignored env QEMU_AUDIO_DRV
debug3: Ignored env XDG_SESSION_DESKTOP
debug3: Ignored env LOGNAME
debug3: Ignored env XDG_SESSION_TYPE
debug3: Ignored env MANPATH
debug3: Ignored env SYSTEMD_EXEC_PID
debug3: Ignored env XAUTHORITY
debug3: Ignored env LS_OPTIONS
debug3: Ignored env XKEYSYMDB
debug3: Ignored env XNLSPATH
debug3: Ignored env HOME
debug3: Ignored env USERNAME
debug1: channel 0: setting env LANG = "en_US.UTF-8"
debug2: channel 0: request env confirm 0
debug3: send packet: type 98
debug3: Ignored env LS_COLORS
debug3: Ignored env XDG_CURRENT_DESKTOP
debug3: Ignored env PYTHONSTARTUP
debug3: Ignored env VTE_VERSION
debug3: Ignored env WAYLAND_DISPLAY
debug3: Ignored env OSTYPE
debug3: Ignored env GNOME_TERMINAL_SCREEN
debug3: Ignored env LESS_ADVANCED_PREPROCESSOR
debug3: Ignored env GNOME_SETUP_DISPLAY
debug3: Ignored env LESSCLOSE
debug3: Ignored env XDG_SESSION_CLASS
debug3: Ignored env TERM
debug3: Ignored env G_FILENAME_ENCODING
debug3: Ignored env HOST
debug3: Ignored env XAUTHLOCALHOSTNAME
debug3: Ignored env LESSOPEN
debug3: Ignored env USER
debug3: Ignored env GNOME_TERMINAL_SERVICE
debug3: Ignored env MORE
debug3: Ignored env CSHEDIT
debug3: Ignored env DISPLAY
debug3: Ignored env SHLVL
debug3: Ignored env WINDOWMANAGER
debug3: Ignored env PAGER
debug3: Ignored env XDG_RUNTIME_DIR
debug3: Ignored env MANPATHISSET
debug3: Ignored env XDG_DATA_DIRS
debug3: Ignored env VENDOR
debug3: Ignored env PATH
debug3: Ignored env GDMSESSION
debug3: Ignored env DBUS_SESSION_BUS_ADDRESS
debug3: Ignored env PROFILEREAD
debug3: Ignored env MAIL
debug3: Ignored env HOSTTYPE
debug3: Ignored env LESSKEY
debug3: Ignored env _
debug3: Ignored env OLDPWD
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 24576 rmax 32759
debug3: receive packet: type 99
debug2: channel_input_status_confirm: type 99 id 0
debug2: PTY allocation request accepted on channel 0
debug3: receive packet: type 99
debug2: channel_input_status_confirm: type 99 id 0
debug2: shell request accepted on channel 0

[stan]

On Wed, 28 Sep 2022 16:36:52 -0000
"Carlos Martinez" <jcmartinez2129(a)gmail.com&gt; wrote:

>  After updating to Fedora 37 beta, I was not able to login to a
> device
>  that uses rsa to authenticate anymore.
>  
>  The message shown is: "Bad server host key: Invalid key length" 
[snip]

debug1: kex: algorithm: curve25519-sha256
debug1: kex: host key algorithm: rsa-sha2-256
debug1: kex: server->client cipher: chacha20-poly1305(a)openssh.com MAC: <implicit>
compression: none
debug1: kex: client->server cipher: chacha20-poly1305(a)openssh.com MAC: <implicit>
compression: none
debug1: kex: curve25519-sha256 need=64 dh_need=64
debug1: kex: curve25519-sha256 need=64 dh_need=64
debug3: send packet: type 30
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug3: receive packet: type 31
debug1: SSH2_MSG_KEX_ECDH_REPLY received
Bad server host key: Invalid key length

debug1: kex: algorithm: curve25519-sha256
debug1: kex: host key algorithm: rsa-sha2-256
debug1: kex: server->client cipher: chacha20-poly1305(a)openssh.com MAC: <implicit>
compression: none
debug1: kex: client->server cipher: chacha20-poly1305(a)openssh.com MAC: <implicit>
compression: none
debug1: kex: curve25519-sha256 need=64 dh_need=64
debug1: kex: curve25519-sha256 need=64 dh_need=64
debug3: send packet: type 30
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug3: receive packet: type 31
debug1: SSH2_MSG_KEX_ECDH_REPLY received

This is the critical bit.  It looks like a difference in enforcement of
rsa key lengths.  Here is an old link that had the same error, though
the size is probably out of date.

https://stackoverflow.com/questions/48055006/ssh-add-invalid-key-length

I'm guessing that Fedora increased the minimum size of RSA keys in F37.
Is it possible to generate a longer key pair?  I don't think there is
an alternative other than recompiling openssh as the third
response to the answer suggests (with a corresponding loss of security).

[František Šumšal]

On 9/28/22 18:36, Carlos Martinez wrote:
>  After updating to Fedora 37 beta, I was not able to login to a device
> that uses rsa to authenticate anymore.
>  
>  The message shown is: "Bad server host key: Invalid key length" 
My wild guess would be this commit [0] fom the crypto-policies package requiring at least
2048-bit RSA key:

# grep RSAMinSize /etc/crypto-policies/back-ends/*
/etc/crypto-policies/back-ends/openssh.config:RSAMinSize 2048
/etc/crypto-policies/back-ends/opensshserver.config:RSAMinSize 2048

# rpm -q crypto-policies
crypto-policies-20220815-1.gite4ed860.fc37.noarch

Given that the remote side appears to use dropbear (so it might be some smaller embedded
system), I suspect
the key might be smaller than 2048-bits.

Check if you can connect to the remote side with the LEGACY policy:

# update-crypto-policies --set LEGACY
# ssh ...

[0]
https://gitlab.com/redhat-crypto/fedora-crypto-policies/-/commit/e4ed8604...

-- 
Frantisek Sumsal
GPG key ID: 0xFB738CE27B634E4B