pbkdf2: add benchmarks

Add benchmarks for PBKDF2-HMAC-SHA1 and PBKDF2-HMAC-SHA256.
This is to help measure the crypto/hmac changes in CL 27458.

Change-Id: I17ef12f3a4641ba44f7bb917a9d87a3ed7c97c67
Reviewed-on: https://go-review.googlesource.com/84380
Reviewed-by: Filippo Valsorda <hi@filippo.io>

Author: magical

Diff for: pbkdf2/pbkdf2_test.go

@@ -155,3 +155,22 @@ func TestWithHMACSHA1(t *testing.T) {
 func TestWithHMACSHA256(t *testing.T) {
 	testHash(t, sha256.New, "SHA256", sha256TestVectors)
 }
+
+var sink uint8
+
+func benchmark(b *testing.B, h func() hash.Hash) {
+	password := make([]byte, h().Size())
+	salt := make([]byte, 8)
+	for i := 0; i < b.N; i++ {
+		password = Key(password, salt, 4096, len(password), h)
+	}
+	sink += password[0]
+}
+
+func BenchmarkHMACSHA1(b *testing.B) {
+	benchmark(b, sha1.New)
+}
+
+func BenchmarkHMACSHA256(b *testing.B) {
+	benchmark(b, sha256.New)
+}