Support signing a pre-calculated hash (#87)

* Split the hashing out of the sign method

This code change adds support to split the hashing of a message
and the actual signing of the message.

* Updating unit test and documentation

This commit updates the unit test and usage docs. In addition,
This change removes a redundant error check inside rsa.sign().

* Refactore unit tests and code comments

Removed the print statements from the unit test and refactored a
few code comments to improve readability.

* Rename hash function

The new hash function had the same name as a function in the
standard library. This commit changes the name to avoid conflicts.

* Rename hash function to compute_hash()

This commit renames the hash function to compute_hash().

1 files changed, 9 insertions, 0 deletions

diff --git a/doc/usage.rst b/doc/usage.rst
index b4f8426..6ac9e82 100644
--- a/doc/usage.rst
+++ b/doc/usage.rst
@@ -203,6 +203,15 @@ This hashes the message using SHA-1. Other hash methods are also
 possible, check the :py:func:`rsa.sign` function documentation for
 details. The hash is then signed with the private key.
 
+It is possible to calculate the hash and signature in separate operations
+(i.e for generating the hash on a client machine and then sign with a
+private key on remote server). To hash a message use the :py:func:`rsa.compute_hash`
+function and then use the :py:func:`rsa.sign_hash` function to sign the hash:
+
+    >>> message = 'Go left at the blue tree'
+    >>> hash = rsa.compute_hash(message, 'SHA-1')
+    >>> signature = rsa.sign_hash(hash, privkey, 'SHA-1')
+
 In order to verify the signature, use the :py:func:`rsa.verify`
 function. This function returns True if the verification is successful: