document HashAlgorithm
diff --git a/docs/hazmat/primitives/interfaces.rst b/docs/hazmat/primitives/interfaces.rst
index 2adad91..f31e9f4 100644
--- a/docs/hazmat/primitives/interfaces.rst
+++ b/docs/hazmat/primitives/interfaces.rst
@@ -204,6 +204,34 @@
The public exponent. Alias for :attr:`public_exponent`.
+Hash Algorithms
+~~~~~~~~~~~~~~~
+
+.. class:: HashAlgorithm
+
+ .. versionadded:: 0.2
+
+ .. attribute:: name
+
+ :type: str
+
+ The standard name for the hash algorithm, for example: ``sha1`` or
+ ``sha256``.
+
+ .. attribute:: digest_size
+
+ :type: int
+
+ The size of the resulting digest in bytes.
+
+ .. attribute:: block_size
+
+ :type: int
+
+ The internal block size of the hash algorithm in bytes.
+
+
+
Key Derivation Functions
~~~~~~~~~~~~~~~~~~~~~~~~