Cannot push changes to git repo

[Dimitrios Eftaxiopoulos <eftaxi12@otenet.gr>]

Hello,
I am one of the  maintainers of the package mathgl. After the last changes on 
alioth I cannot push changes to the git repository. Below is what I get

$git push ssh://eftaxiop-guest@vasks.debian.org/git/debian-
science/packages/mathgl.git
The authenticity of host 'vasks.debian.org (217.196.43.140)' can't be 
established.
RSA key fingerprint is 8c:c0:b8:9f:0a:79:ee:1c:77:c4:b8:a1:70:55:b7:31.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'vasks.debian.org,217.196.43.140' (RSA) to the list 
of known hosts.
Permission denied (publickey).
fatal: The remote end hung up unexpectedly

Any help is appreciated
Dimitris