Amazon AWS Ubuntu Server Need to login with new users

The AWS Instance is in Ubuntu Server. I can able to login through Ubuntu Server through Ubuntu login (ubuntu@Public DNS value).

I gave the below Host Name in Putty.

ubuntu@ec2-54-67-100-126.us-west-1.compute.amazonaws.com

And also created PPK file from PEM file.(The PPK file name is devsignapi.ppk)

I can able to login to the amazon server with these credentials. And created a user in the Ubuntu Server in the below commands.

   sudo adduser dev1
and set up the password also.

To login with dev1@ec2-54-67-100-126.us-west-1.compute.amazonaws.com I Created the Public SSH key file from PPK file generated from Amazon Private PPK file.The PPK file name is:

Using username "ubuntu".
Authenticating with public key "imported-openssh-key"
Last login: Wed Feb  4 05:40:27 2015 from 106.208.43.99
ubuntu@ip-172-31-23-230:~$ mkdir -p ~/.ssh/
ubuntu@ip-172-31-23-230:~$ ls
docsignapipublic.ppk
ubuntu@ip-172-31-23-230:~$ cat devsignapipublic.ppk >> ~/.ssh/authorized_keys

ubuntu@ip-172-31-23-230:~$ cd ~/.ssh
ubuntu@ip-172-31-23-230:~/.ssh$ cat authorized_keys
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC7xbVAxh7d7JXLqQOtnOOh7qdZK4+ZCPLUl6hqbInjQbtuhfa2ixfVIVjxNdiOHAEh7cRF7zRBOq7jiuop9OyXdOfwhgGCYynJzvLvQB3aJat/15aFPEyNyoVldOgmNYkGZC5Q0vQK1b1TYffgnJqBMfTPtSAB0MZBwaOWdZ1vqam9o/cooo7huXVee7zqwG6hRouCU69cRTg6ZDRVTLitw41UyJPLhdwgHnDCgHoAih8O4Sx4vCjnqsYeyRqcgIvio6BOXCBjoVeTqmBCV8PazS/EsMmjvTqjrxjq/Wh1YRNFu0dLhOx6tmO3Q8bPTNlXZZYx+4FfUO2UeLOTTnFF devsignapi
---- BEGIN SSH2 PUBLIC KEY ----
Comment: "imported-openssh-key"
AAAAB3NzaC1yc2EAAAADAQABAAABAQC7xbVAxh7d7JXLqQOtnOOh7qdZK4+ZCPLU
l6hqbInjQbtuhfa2ixfVIVjxNdiOHAEh7cRF7zRBOq7jiuop9OyXdOfwhgGCYynJ
zvLvQB3aJat/15aFPEyNyoVldOgmNYkGZC5Q0vQK1b1TYffgnJqBMfTPtSAB0MZB
waOWdZ1vqam9o/cooo7huXVee7zqwG6hRouCU69cRTg6ZDRVTLitw41UyJPLhdwg
HnDCgHoAih8O4Sx4vCjnqsYeyRqcgIvio6BOXCBjoVeTqmBCV8PazS/EsMmjvTqj
rxjq/Wh1YRNFu0dLhOx6tmO3Q8bPTNlXZZYx+4FfUO2UeLOTTnFF
---- END SSH2 PUBLIC KEY ----
ubuntu@ip-172-31-23-230:~/.ssh$ authorized_keys

Now i am try to login using the created User dev1. So in Putty, i gave the below Host Name

dev1@ec2-54-67-100-126.us-west-1.compute.amazonaws.com

And also decsignapipublic.ppk file. But i am getting the below error.

Unable to use key file "C:\Users\Murugan\Desktop\desktop\db structure\Hubino\Amazon Web Services\devsignapipublic.ppk" (not a private key) Using username "dev1".