sftp and public keys

Question

I am trying to sftp into an a server hosted by someone else.

To make sure this worked I did the standard sftp [email protected] i was promted with the password and that worked fine.

I am setting up a cron script to send a file once a week so have given them our public key which they claim to have added to their authorized_keys file.

I now try sftp [email protected] again and I am still prompted for a password, but now the password doesn't work...

Connecting to [email protected]...
[email protected]'s password: 
Permission denied, please try again.
[email protected]'s password: 
Permission denied, please try again.
[email protected]'s password: 
Permission denied (publickey,password).
Couldn't read packet: Connection reset by peer

I did notice however that if I simply pressed enter (no password) it logged me in fine...

So here are my questions:

1. Is there a way to check what privatekey/pulbickey pair my sftp connection is using?
2. Is it possible to specify what key pair to use?
3. If all is setup correctly (using correct key pair and added to authorized files) why am I being asked to enter a blank password?

Thanks for your help in advance!

UPDATE

I have just run sftp -vvv [email protected]

....
debug1: Authentications that can continue: publickey,password
debug3: start over, passed a different list publickey,password
debug3: preferred gssapi-with-mic,publickey,keyboard-interactive,password
debug3: authmethod_lookup publickey
debug3: remaining preferred: keyboard-interactive,password
debug3: authmethod_is_enabled publickey
debug1: Next authentication method: publickey
debug1: Offering public key: /root/.ssh/id_rsa
debug3: send_pubkey_test
debug2: we sent a publickey packet, wait for reply
debug1: Server accepts key: pkalg ssh-rsa blen 277
debug2: input_userauth_pk_ok: SHA1 fp 45:1b:e7:b6:33:41:1c:bb:0f:e3:c1:0f:1b:b0:d5:e4:28:a3:3f:0e
debug3: sign_and_send_pubkey
debug1: read PEM private key done: type RSA
debug1: Authentications that can continue: publickey,password
debug1: Trying private key: /root/.ssh/id_dsa
debug3: no such identity: /root/.ssh/id_dsa
debug2: we did not send a packet, disable method
debug3: authmethod_lookup password
debug3: remaining preferred: ,password
debug3: authmethod_is_enabled password
debug1: Next authentication method: password

It seems to suggest that it tries to use the public key... What am I missing?

Solution

Running in verbose mode should help you:

sftp -vvv [email protected]

Which will show you what keys it is using and hopefully why its asking for a password.

update: You can tell ssh which private key to use: http://www.cyberciti.biz/faq/force-ssh-client-to-use-given-private-key-identity-file/ but Im unsure how you would do the same server side

Comments

[Fred Foo]

Are the keys installed in /root/.ssh or your own homedir?

[Anita]

/root/.ssh (note currently logged in as root)

[bahamat]

It looks to me like the guy who runs that server as wrecked his PAM configuration. It's nearly impossible to say why without being able to see it.

[Fran]

The line debug3: no such identity: /root/.ssh/id_dsa in the above output seems to be the point where publickey authentication fails. Does that file exist on your machine? If it exists, is it readable by anyone other than you (it shouldn't be: it should have permissions 400 or 600). This applies to it's containing directory too (which chould have permissions 700).