diff --git a/tests/test_check.py b/tests/test_check.py
new file mode 100644
index 0000000..85b2d8b
--- /dev/null
+++ b/tests/test_check.py
@@ -0,0 +1,66 @@
+from basetest import *
+from zfs_autobackup.util import *
+
+
+class TestZfsEncryption(unittest2.TestCase):
+
+    def setUp(self):
+        pass
+
+    def test_blockhash(self):
+        # sha1 sums of files, (bs=4096)
+        # da39a3ee5e6b4b0d3255bfef95601890afd80709  empty
+        # 642027d63bb0afd7e0ba197f2c66ad03e3d70de1  partial
+        # 3c0bf91170d873b8e327d3bafb6bc074580d11b7  whole
+        # 2e863f1fcccd6642e4e28453eba10d2d3f74d798  whole2
+        # 959e6b58078f0cfd2fb3d37e978fda51820473ff  whole_whole2
+        # 309ffffba2e1977d12f3b7469971f30d28b94bd8  whole_whole2_partial
+
+
+        self.assertEqual(
+            list(block_hash("tests/data/empty", count=1)),
+            []
+        )
+
+        self.assertEqual(
+            list(block_hash("tests/data/partial", count=1)),
+            [(0, "642027d63bb0afd7e0ba197f2c66ad03e3d70de1")]
+        )
+
+        self.assertEqual(
+            list(block_hash("tests/data/whole", count=1)),
+            [(0, "3c0bf91170d873b8e327d3bafb6bc074580d11b7")]
+        )
+
+        self.assertEqual(
+            list(block_hash("tests/data/whole_whole2", count=1)),
+            [
+                (0, "3c0bf91170d873b8e327d3bafb6bc074580d11b7"),
+                (1, "2e863f1fcccd6642e4e28453eba10d2d3f74d798")
+            ]
+        )
+
+        self.assertEqual(
+            list(block_hash("tests/data/whole_whole2_partial", count=1)),
+            [
+                (0, "3c0bf91170d873b8e327d3bafb6bc074580d11b7"), #whole
+                (1, "2e863f1fcccd6642e4e28453eba10d2d3f74d798"), #whole2
+                (2, "642027d63bb0afd7e0ba197f2c66ad03e3d70de1")  #partial
+            ]
+        )
+
+        self.assertEqual(
+            list(block_hash("tests/data/whole_whole2_partial", count=2)),
+            [
+                (0, "959e6b58078f0cfd2fb3d37e978fda51820473ff"), #whole_whole2
+                (1, "642027d63bb0afd7e0ba197f2c66ad03e3d70de1")  #partial
+            ]
+        )
+
+        self.assertEqual(
+            list(block_hash("tests/data/whole_whole2_partial", count=10)),
+            [
+                (0, "309ffffba2e1977d12f3b7469971f30d28b94bd8"), #whole_whole2_partial
+            ]
+        )
+
diff --git a/zfs_autobackup/util.py b/zfs_autobackup/util.py
new file mode 100644
index 0000000..acd8e99
--- /dev/null
+++ b/zfs_autobackup/util.py
@@ -0,0 +1,39 @@
+import hashlib
+
+# root@psyt14s:/home/psy/zfs_autobackup# ls -lh /home/psy/Downloads/carimage.zip
+# -rw-rw-r-- 1 psy psy 990M Nov 26  2020 /home/psy/Downloads/carimage.zip
+# root@psyt14s:/home/psy/zfs_autobackup# time sha1sum /home/psy/Downloads/carimage.zip
+# a682e1a36e16fe0d0c2f011104f4a99004f19105  /home/psy/Downloads/carimage.zip
+#
+# real	0m2.558s
+# user	0m2.105s
+# sys	0m0.448s
+# root@psyt14s:/home/psy/zfs_autobackup# time python3 -m zfs_autobackup.ZfsCheck
+#
+# real	0m1.459s
+# user	0m0.993s
+# sys	0m0.462s
+
+# NOTE: surprisingly sha1 in via python3 is faster than the native sha1sum utility, even in the way we use below!
+def block_hash(fname, count=10000, bs=4006):
+    """yields sha1 hash per count blocks.
+    yields(chunk_nr, hexdigest)
+
+    yields nothing for empty files.
+    """
+
+    with open(fname, "rb") as f:
+        hash = hashlib.sha1()
+        block_nr = 0
+        chunk_nr = 0
+        for block in iter(lambda: f.read(4096), b""):
+            hash.update(block)
+            block_nr = block_nr + 1
+            if block_nr % count == 0:
+                yield (chunk_nr, hash.hexdigest())
+                chunk_nr = chunk_nr + 1
+                hash = hashlib.sha1()
+
+        # yield last (incomplete) block
+        if block_nr % count != 0:
+            yield (chunk_nr, hash.hexdigest())