diff --git a/src/allmydata/crypto/__init__.py b/src/allmydata/crypto/__init__.py
index 4ce95a24d..e69de29bb 100644
--- a/src/allmydata/crypto/__init__.py
+++ b/src/allmydata/crypto/__init__.py
@@ -1,21 +0,0 @@
-class BadSignature(Exception):
-    """
-    An alleged signature did not match
-    """
-
-
-class BadPrefixError(Exception):
-    """
-    A key did not start with the required prefix
-    """
-
-
-def remove_prefix(s_bytes, prefix):
-    """
-    Removes `prefix` from `s_bytes` safely
-    """
-    if not s_bytes.startswith(prefix):
-        raise BadPrefixError(
-            "did not see expected '{}' prefix".format(prefix)
-        )
-    return s_bytes[len(prefix):]
diff --git a/src/allmydata/crypto/util.py b/src/allmydata/crypto/util.py
new file mode 100644
index 000000000..571f55387
--- /dev/null
+++ b/src/allmydata/crypto/util.py
@@ -0,0 +1,34 @@
+"""
+Utilities used by allmydata.crypto modules
+"""
+
+
+class BadSignature(Exception):
+    """
+    An alleged signature did not match
+    """
+
+
+class BadPrefixError(Exception):
+    """
+    A key did not start with the required prefix
+    """
+
+
+def remove_prefix(s_bytes, prefix):
+    """
+    :param bytes s_bytes: a string of bytes whose prefix is removed
+
+    :param bytes prefix: the bytes to remove from the beginning of `s_bytes`
+
+    Removes `prefix` from `s_bytes` and returns the new bytes or
+    raises `BadPrefixError` if `s_bytes` did not start with the
+    `prefix` specified.
+
+    :returns: `s_bytes` with `prefix` removed from the front.
+    """
+    if not s_bytes.startswith(prefix):
+        raise BadPrefixError(
+            "did not see expected '{}' prefix".format(prefix)
+        )
+    return s_bytes[len(prefix):]