Begin to describe the methods
diff --git a/docs/primitives/symmetric-encryption.rst b/docs/primitives/symmetric-encryption.rst
index acb5fc1..cf92437 100644
--- a/docs/primitives/symmetric-encryption.rst
+++ b/docs/primitives/symmetric-encryption.rst
@@ -20,3 +20,11 @@
# The ciphertext
[...]
+Here ``key`` is the encryption key (which must be kept secret), and ``iv`` is
+the initialization vector (which should be random). Exactly what form these
+values should take is described for each of the ciphers and modes.
+
+``encrypt()`` should be called repeatedly with additional plaintext, and it
+will return the encrypted bytes, if there isn't enough data, it will buffer it
+internally. ``finalize()`` should be called at the end, and will return
+whatever data is left.