Support signing commits with ssh keys

My SSH-key requires a passphrase. I get an error when i try to rebase a nonsigned branch on top of a signed one. Can you help me out?

T:\git\software\artos> git.exe -c credential.helper= -c "credential.helper=C:/Program\ Files/SmartGit/bin/smartgitc.exe --credential=4349099657397060193" -c credential.useHttpPath=true -c advice.mergeConflict=false rebase 4a4211196a004720731bc6907f5e30d3a9ad48bb feature/ARTOS-1251_savem.x_add_$root_before_tabs.cc
skipped previously applied commit 907528277
hint: use --reapply-cherry-picks to include skipped commits
hint: Disable this message with "git config set advice.skippedCherryPicks false"
Rebasing (1/1)
Enter passphrase for "c:\Users\al15591\.ssh\id_ed25519": Load key "c:\\Users\\al15591\\.ssh\\id_ed25519": incorrect passphrase supplied to decrypt private key?
failed to write commit object
hint: Could not execute the todo command
hint:
hint:     pick cb87aed0cc8a48c1bc73f41afe662aa1ecb033c2 # savem.x now loads artos/tabs.css including $root (ARTOS-1251)
hint:
hint: It has been rescheduled; To edit the command before continuing, please
hint: edit the todo list first:
hint:
hint:     git rebase --edit-todo
hint:     git rebase --continue


skipped previously applied commit 907528277
hint: use --reapply-cherry-picks to include skipped commits
hint: Disable this message with "git config set advice.skippedCherryPicks false"
Rebasing (1/1)
hint: Could not execute the todo command
hint:
hint:     pick cb87aed0cc8a48c1bc73f41afe662aa1ecb033c2 # savem.x now loads artos/tabs.css including $root (ARTOS-1251)
hint:
hint: It has been rescheduled; To edit the command before continuing, please
hint: edit the todo list first:
hint:
hint:     git rebase --edit-todo
hint:     git rebase --continue

It seems smartgit does not ask for the password. If I do this with git, it works.

On Windows I can use this via Powershell: 

$askpass = "C:\Program Files\Git\mingw64\bin\git-askpass.exe"

[Environment]::SetEnvironmentVariable("SSH_ASKPASS", $askpass, "User")

[Environment]::SetEnvironmentVariable("SSH_ASKPASS_REQUIRE", "force", "User")

[Environment]::SetEnvironmentVariable("DISPLAY", "dummy", "User")