ssh on cygwin (win10): read_passphrase: can't open /dev/tty:

Documenting my solution in case anyone else has the problem: It appears that a Windows version of OpenSSH was introduced in the 2018 spring update, and it is being called instead of Cygwin's ssh. (See e.g. https://www.zdnet.com/article/openssh-arrives-in-windows-10-spring-update/)

% which ssh
/cygdrive/c/WINDOWS/System32/OpenSSH/ssh

The reason for the sudden problem with ssh is that the cygwin PATH has /cygdrive/c/WINDOWS/System32 ahead of /usr/bin/. Or at least, mine did.

So the issue is solved by calling /usr/bin/ssh explicitly. (Reordering the PATH is also a possible solution, but not for my environment.)

If A fresh install of cygwin and having the sames issues, make sure to select openssh packages as part of the installation

The ssh client come with windows 10 only can be used via cmd.exe not cygwin terminal