How can I calculate an AWS API signature (v4) in python?

Question

I'm attempting to generate a signature for an Amazon Glacier upload request, using the example requests and example functions provided by the AWS documentation, but I can't make it work. At this point, I'm certain I'm missing something incredibly obvious:

#!/bin/env python

import hmac
import hashlib

# This string to sign taken from: http://docs.amazonwebservices.com/amazonglacier/latest/dev/amazon-glacier-signing-requests.html#example-signature-calculation
sts = """AWS4-HMAC-SHA256
20120525T002453Z
20120525/us-east-1/glacier/aws4_request
5f1da1a2d0feb614dd03d71e87928b8e449ac87614479332aced3a701f916743"""

# These two functions taken from: http://docs.amazonwebservices.com/general/latest/gr/signature-v4-examples.html#signature-v4-examples-python
def sign(key, msg):
    return hmac.new(key, msg.encode('utf-8'), hashlib.sha256).hexdigest()

# The fake secret key is provided by the referenced docs
def getSignatureKey():
    kDate = sign(("AWS4" + "wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY").encode('utf-8'), "20120525")
    kRegion = sign(kDate, "us-east-1")
    kService = sign(kRegion, "glacier")
    kSigning = sign(kService, "aws4_request")
    return kSigning

signature = sign(getSignatureKey(), sts)
print signature

If I run my program, I get the following hash:

$ python test.py
3431315da57da4df28f92895c75364d94b36c745896ad3e580c0a6ae403b1e05

Yet the docs clearly state:

If the secret access key, wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY, is used, then the calculated signature is:

3ce5b2f2fffac9262b4da9256f8d086b4aaf42eba5f111c21681a65a127b7c2a

What am I missing?

Answer 1

Your function differs from theirs in one respect. You're doing

def sign(key, msg):
  return hmac.new(key, msg.encode('utf-8'), hashlib.sha256).hexdigest()

but they're doing

def sign(key, msg):
  return hmac.new(key, msg.encode('utf-8'), hashlib.sha256).digest()

So your derived key is wrong. You only want to be using hexdigest at the last step of the process, not when calculating the signing key.