* adds PrivateKey to handle different algorithms
* fixes input check problem of decrypt_credential_source
* addresses comments
* version number not encrypted
* version number test
* adds a credential size test
* removes the algorithm from credential encoding
They assume blocks are encrypted/decrypted one at a time. To avoid one syscall
per block, it is preferable to encrypt/decrypt the data at once.
Change-Id: I795c8f5b7901a1b55fa3b06fa45fe57ab19d06ea
