Support kex algorithm - curve25519-sha256

[Izorkin]

Error connection with curve25519-sha256 kex algorithm.
SSHD configuration:

...
KexAlgorithms curve25519-sha256
Ciphers chacha20-poly1305@openssh.com
MACs hmac-sha2-512-etm@openssh.com
...

SSHD log:

авг 05 20:16:02 test systemd[1]: Starting SSH Daemon (192.168.248.143:57579)...
авг 05 20:16:02 test systemd[1]: Started SSH Daemon (192.168.248.143:57579).
авг 05 20:16:02 test sshd[93798]: debug1: inetd sockets after dupping: 3, 4
авг 05 20:16:02 test sshd[93798]: Connection from 192.168.248.143 port 57579 on 192.168.248.100 port 22 rdomain ""
авг 05 20:16:02 test sshd[93798]: debug1: Local version string SSH-2.0-OpenSSH_8.3
авг 05 20:16:02 test sshd[93798]: debug1: Remote protocol version 2.0, remote software version PuTTY_KiTTY
авг 05 20:16:02 test sshd[93798]: debug1: no match: PuTTY_KiTTY
авг 05 20:16:02 test sshd[93798]: debug2: fd 3 setting O_NONBLOCK
авг 05 20:16:02 test sshd[93798]: debug2: Network child is on pid 93799
авг 05 20:16:02 test sshd[93798]: debug1: permanently_set_uid: 498/65534 [preauth]
авг 05 20:16:02 test sshd[93798]: debug1: list_hostkey_types: ssh-ed25519 [preauth]
авг 05 20:16:02 test sshd[93798]: debug1: SSH2_MSG_KEXINIT sent [preauth]
авг 05 20:16:02 test sshd[93798]: debug1: SSH2_MSG_KEXINIT received [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: local server KEXINIT proposal [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: KEX algorithms: curve25519-sha256 [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: host key algorithms: ssh-ed25519 [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: ciphers ctos: chacha20-poly1305@openssh.com [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: ciphers stoc: chacha20-poly1305@openssh.com [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: MACs ctos: hmac-sha2-512-etm@openssh.com [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: MACs stoc: hmac-sha2-512-etm@openssh.com [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: compression ctos: none [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: compression stoc: none [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: languages ctos:  [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: languages stoc:  [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: first_kex_follows 0  [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: reserved 0  [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: peer client KEXINIT proposal [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: KEX algorithms: curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1,rsa2048-sha256,rsa1024-sha1,diffie-hellman-group1-sha1 [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: host key algorithms: ssh-ed25519,ssh-rsa,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-dss [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: ciphers ctos: chacha20-poly1305@openssh.com,aes256-ctr,aes256-cbc,rijndael-cbc@lysator.liu.se,aes192-ctr,aes192-cbc,aes128-ctr,aes128-cbc,blowfish-ctr,blowfish-cbc,3des-ctr,3des-cbc,arcfour256,arcfour128 [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: ciphers stoc: chacha20-poly1305@openssh.com,aes256-ctr,aes256-cbc,rijndael-cbc@lysator.liu.se,aes192-ctr,aes192-cbc,aes128-ctr,aes128-cbc,blowfish-ctr,blowfish-cbc,3des-ctr,3des-cbc,arcfour256,arcfour128 [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: MACs ctos: hmac-sha2-256,hmac-sha1,hmac-sha1-96,hmac-md5,hmac-sha2-256-etm@openssh.com,hmac-sha1-etm@openssh.com,hmac-sha1-96-etm@openssh.com,hmac-md5-etm@openssh.com [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: MACs stoc: hmac-sha2-256,hmac-sha1,hmac-sha1-96,hmac-md5,hmac-sha2-256-etm@openssh.com,hmac-sha1-etm@openssh.com,hmac-sha1-96-etm@openssh.com,hmac-md5-etm@openssh.com [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: compression ctos: none,zlib,zlib@openssh.com [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: compression stoc: none,zlib,zlib@openssh.com [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: languages ctos:  [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: languages stoc:  [preauth]
авг 05 20:16:02 test sshd[93798]: debug2: first_kex_follows 0  [preauth]
авг 05 20:16:02 test systemd[1]: sshd@21-192.168.248.100:22-192.168.248.143:57579.service: Succeeded.
авг 05 20:16:02 test sshd[93798]: debug2: reserved 0  [preauth]
авг 05 20:16:02 test sshd[93798]: debug1: kex: algorithm: (no match) [preauth]
авг 05 20:16:02 test sshd[93798]: Unable to negotiate with 192.168.248.143 port 57579: no matching key exchange method found. Their offer: curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1,rsa2048-sha256,rsa1024-sha1,diffie-hellman-group1-sha1 [preauth]
авг 05 20:16:02 test sshd[93798]: debug1: do_cleanup [preauth]
авг 05 20:16:02 test sshd[93798]: debug1: monitor_read_log: child log fd closed
авг 05 20:16:02 test sshd[93798]: debug1: do_cleanup
авг 05 20:16:02 test sshd[93798]: debug1: Killing privsep child 93799

With KexAlgorithms curve25519-sha256@libssh.org worked.

[BoberMod]

ed25519 support was added to putty in 0.71 release (March 2019).
kitty based on putty 0.71 but doesn't support ed25519 🤔

UPD: ed25519 works, but PPK V3 (default in puttygen) is not supported

[cyd01]

Issue closed due to inactivity