after reading the book Digital Gold by Nathaniel Popper I learned this:
I quote:

ADDRESSES AND SECRET KEYS Anyone joining the Bitcoin network can generate his or her own Bitcoin address (generally a string of thirty- four letters and numbers), and a corresponding private key (generally a string of sixty- four characters).
As an example, one actual Bitcoin address is:
16R5PtokaUnXXXjQe4Hg5jZrfW69fNpAtF
The private key for this particular address is: 5JJ5rLKjyMmSxhauoa334cdZNCoVEw6oLfMpfL8H1w9pyDoPMf3
Unquote.
I assume Mr Popper is right and up to date because his book is a recent one.

I have two questions and I hope someone can help me.
1.) After I registred at a bitcoin exchange, I bought bitcoins (june 2015) I received a Bitcoin address and this contained the 34 characters as mentioned in the book.
I did not generate or receive a private key of 64 characters.
However I did get, a so called Identifyer. The Identifyer is 36 characters instead of the 64 mentioned by Mr Popper.

2.) If the identifyer is the equivalent to the private key, should I worry that this key was send to me via email by/via a third party.

Thanks for any input