OpenSSL is over four times faster than Go's built-in GCM implementation.
$ cd internal/stupidgcm
$ go test -bench .
PASS
Benchmark4kEncStupidGCM-2 50000 25860 ns/op 158.39 MB/s
Benchmark4kEncGoGCM-2 10000 116050 ns/op 35.29 MB/s
ok github.com/rfjakob/gocryptfs/internal/stupidgcm 3.667s