I found the instructions in this link. In profile of putty, in the username field i must not add @hostname (ex user@srv1).
We create the keys with puttygen and we add the public key in the file ~/.ssh/authorized_keys.
if you cannot connect and you receive this mesasge:
Authentication refused: bad ownership or modes for directory /home/user
change the permissions of the file authorized_keys:
chmod go-w ~/ chmod 700 ~/.ssh chmod 600 ~/.ssh/authorized_keys
Convert key for SSH android app
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