Cannot unlock password protected SSH key in OS X Mavericks
Apparently, the private key I was using on my machine was an old one. My key did not contain the encryption information added to the private key file if I generate a new one:

-----BEGIN RSA PRIVATE KEY-----
Proc-Type: 4,ENCRYPTED
DEK-Info: AES-...


To solve the problem, I decrypted the key with openssl:

$ cd ~/.ssh
$ cp id_rsa id_rsa.bck
$ openssl rsa -in id_rsa -out id_rsa


...and then re-encrypted it:

$ openssl rsa -in id_rsa -aes256 -out id_rsa
$ chmod 0600 id_rsa


And just in case, regenerate the public key as well:

$ ssh-keygen -y -f id_rsa &gt; id_rsa.pub