Secret ID and Source are the same

[baruchiro]

BTW, I used the Source to give some extra information (and it's still unique), for example: git show 797f277509117a61b81eb97e1c6f509b5addf0c2:testdata/example.txt.

I think we should change the Secret.Description to this information, and move the description: Private Key to be under a new field called SecretType.

Originally posted by @baruchiro in #109 (comment)