cURL sftp public key authentication fails "Callback Error"

Question

I have some php code which works well uploading files using cURL to hosts which are simply using user & password ftp, now I have to upload to a server which only allows public key auth and am getting the error: "* SSH public key authentication failed: Callback returned error"

I had a problem with the keys as they were not in the right format, but have since put them in the correct single line format and this stopped the "not base64 encoded" errors. I can't find much help on this callback error online.

my code is as follows.

$ch = curl_init();
curl_setopt($ch, CURLOPT_VERBOSE, TRUE); 
curl_setopt($ch, CURLOPT_URL, 'sftp://user:@12.12.12.12:22/testfile.gz');
curl_setopt($ch, CURLOPT_PROTOCOLS, CURLPROTO_SFTP);
curl_setopt($ch, CURLOPT_SSH_PUBLIC_KEYFILE,'C:\keys\public.pub');
curl_setopt($ch, CURLOPT_SSH_PRIVATE_KEYFILE,'C:\keys\private.ppk');
curl_setopt($ch, CURLOPT_SSH_HOST_PUBLIC_KEY_MD5,'2acfe24108c37a276a93ac3398a5oe8f');
curl_setopt($ch, CURLOPT_SSH_AUTH_TYPES,CURLSSH_AUTH_PUBLICKEY);
curl_setopt($ch, CURLOPT_UPLOAD, 1);
$fp = fopen($localfile, 'r');
curl_setopt($ch, CURLOPT_INFILE, $fp);
curl_setopt($ch, CURLOPT_INFILESIZE, filesize($localfile));
$sR = curl_exec ($ch);

here is the output from running the test

* About to connect() to 12.12.12.12 port 22 (#0)
*   Trying 12.12.12.12...
* connected
* Connected to 12.12.12.12 (12.12.12.12) port 22 (#0)
* SSH MD5 fingerprint: ebbc61b886c798b25073c912833ffers
* SSH authentication methods available: publickey
* Using ssh public key file C:\keys\public.pub
* Using ssh private key file C:\keys\private.ppk
* SSH public key authentication failed: Callback returned error
* Authentication failure
* Closing connection #0

any help appreciated

Answer 1

There are cases (debian-based distros) when your libssh2 is built with libgcrypt. In those, use PEM-encoded private key file:

$ openssl rsa -in ~/.ssh/id_rsa -outform pem > id_rsa.pem