title: ssh works on one machine, but not another
body: <p>I'm running a simple command on two separate machines:</p>
  
  <pre><code>ssh -vvvT git@github.com
  </code></pre>
  
  <p>On one machine, this works. On the other machine, ssh fails with these messages:</p>
  
  <pre><code>debug1: Server accepts key: pkalg ssh-rsa blen 535
  debug2: input_userauth_pk_ok: fp bd:5a:d9:2b:d0:36:1c:f1:dc:f8:83:05:17:b9:04:e0
  debug3: sign_and_send_pubkey: RSA bd:5a:d9:2b:d0:36:1c:f1:dc:f8:83:05:17:b9:04:e0
  debug1: key_parse_private2: missing begin marker
  debug1: key_parse_private_pem: PEM_read_PrivateKey failed
  debug1: read PEM private key done: type &lt;unknown&gt;
  debug1: read_passphrase: can't open /dev/tty: No such device or address
  debug2: no passphrase given, try next key
  debug2: we did not send a packet, disable method
  debug1: No more authentication methods to try.
  Permission denied (publickey).
  </code></pre>
  
  <p>I am using the same private key on both machines. The private key has the begin marker (i.e. -----BEGIN RSA PRIVATE KEY-----). The private key was generated without a passphrase. The permissions for the .ssh directory are the same on both machines (700). The permissions for the private key file are the same on both machines (600). Here's the contents of my .ssh/config:</p>
  
  <pre><code>Host github.com
      StrictHostKeyChecking no
      IdentityFile ~/keyfile
  </code></pre>
  
  <p>I'm completely confused on why the output says "missing begin marker", why it's attempting to read a passphrase, and of course why it's failing at all. Is there anything I should try to help diagnose this issue?</p>