This change does two things:
1. Fix a length checking bug in the Decrypt function.
2. Use binary.LittleEndian for byte conversions.
Fixesgolang/go#19881
Change-Id: I9d33b92f2bd7e6ca8f69308388f1e8a5c6df2be8
Reviewed-on: https://go-review.googlesource.com/39954
Reviewed-by: Adam Langley <agl@golang.org>