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 user@sftp.xxxxxx.com 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 user@sftp.xxxxxx.com again and I am still prompted for a password, but now the password doesn't work...

Connecting to user@sftp.xxxxxx.com...
user@sftp.xxxxxx.com's password: 
Permission denied, please try again.
user@sftp.xxxxxx.com's password: 
Permission denied, please try again.
user@sftp.xxxxxx.com'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 user@sftp.xxxxxx.com

....
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?

Answer 1

Running in verbose mode should help you:

sftp -vvv user@sftp.xxxxxx.com

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

Answer 2

First link that came up on how to do what you did: http://ask-leo.com/how_can_i_automate_an_sftp_transfer_between_two_servers.html

Seems to describe the same steps you took, but without the resulting trouble. Did you make sure your private keys aren't using a password? The site implies the keys are matched automatically by SFTP; I'm guessing it's using the key fingerprints.

Also, have a look at the man page for SFTP and try specifying the desired options to prefer key-based authentication over password authentication.

Update:
Perhaps the server-side isn't properly configured? For example, the authorized_keys file has an incorrect entry specifying the user@host?