title: PHP mcrypt_get_iv_size () for Salt Size Generation
body: <p>I am writing a PHP script to authenticate users. I want to use SHA512 for the hash and use a salt to prepend to the password. To generate the salt, I want to use mcrypt_create_iv. But first, I must figure out the initialization Vector size. For this, I see php has: mcrypt_get_iv_size. But I have a question, please:</p>
  
  <p>For mcrypt_get_iv_size() what do I use for the cipher string and the mode string? Please keep in mind I am using SHA512, so the salt needs to be at LEAST as long as the sha512 hash. For experimenting, I tried " mcrypt_get_iv_size(CRYPT_SHA512, MCRYPT_MODE_CFB) " but php complained.</p>