Add support for KMS Encryption Context

[flatiron-nick]

Credstash added support for KMS Encryption Contexts in this pull request.

Would it be possible to add support for Encryption Contexts in unicreds too?

[wolfeidau]

Great suggestion,

I am just read over the blog post detailing how it works.

http://blogs.aws.amazon.com/security/post/Tx2LZ6WBJJANTNW/How-to-Protect-the-Integrity-of-Your-Encrypted-Data-by-Using-AWS-Key-Management

I am most certainly interested in supporting this feature!

Thanks for mentioning it.

[flatiron-nick]

Awesome, thanks so much! 👍

[wolfeidau]

Added some examples https://github.com/Versent/unicreds#examples I used the existing flags parser.