Describe that hashes get real broken over time
diff --git a/docs/hazmat/primitives/cryptographic-hashes.rst b/docs/hazmat/primitives/cryptographic-hashes.rst
index 20fa23c..e7b4f2d 100644
--- a/docs/hazmat/primitives/cryptographic-hashes.rst
+++ b/docs/hazmat/primitives/cryptographic-hashes.rst
@@ -27,6 +27,12 @@
>>> digest.finalize()
'l\xa1=R\xcap\xc8\x83\xe0\xf0\xbb\x10\x1eBZ\x89\xe8bM\xe5\x1d\xb2\xd29%\x93\xafj\x84\x11\x80\x90'
+ Keep in mind that attacks against cryptographic hashes only get stronger
+ with time, and that often algorithms that were once thought to be strong,
+ become broken. Because of this it's important to include a plan for
+ upgrading the hash algorithm you use over time. For more information, see
+ `Lifetimes of cryptographic hash functions`_.
+
.. method:: update(data)
:param bytes data: The bytes you wish to hash.
@@ -109,3 +115,6 @@
MD5 is a deprecated cryptographic hash function. It has a 128-bit message
digest and has practical known collision attacks.
+
+
+.. _`Lifetimes of cryptographic hash functions`: http://valerieaurora.org/hash.html