#!/usr/bin/env python
import base64
import binascii
import boto3
import datetime as dt
import hashlib
import hmac

# http://pythonhosted.org/srp/
# https://github.com/cocagne/pysrp
import srp

bytes_to_hex = lambda x: "".join("{:02x}".format(ord(c)) for c in x)

cognito = boto3.client('cognito-idp', region_name="us-east-1")

username = "foobar@foobar.com"
password = "123456"

user_pool_id = u"us-east-1_XXXXXXXXX"
client_id = u"XXXXXXXXXXXXXXXXXXXXXXXXXX"

# Step 1:
# Use SRP lib to construct a SRP_A value.

srp_user = srp.User(username, password)
_, srp_a_bytes = srp_user.start_authentication()

srp_a_hex = bytes_to_hex(srp_a_bytes)

# Step 2:
# Submit USERNAME & SRP_A to Cognito, get challenge.

response = cognito.initiate_auth(
    AuthFlow='USER_SRP_AUTH',
    AuthParameters={ 'USERNAME': username, 'SRP_A': srp_a_hex },
    ClientId=client_id,
    ClientMetadata={ 'UserPoolId': user_pool_id })

# Step 3:
# Use challenge parameters from Cognito to construct
# challenge response.

salt_hex         = response['ChallengeParameters']['SALT']
srp_b_hex        = response['ChallengeParameters']['SRP_B']
secret_block_b64 = response['ChallengeParameters']['SECRET_BLOCK']

secret_block_bytes = base64.standard_b64decode(secret_block_b64)
secret_block_hex = bytes_to_hex(secret_block_bytes)

salt_bytes = binascii.unhexlify(salt_hex)
srp_b_bytes = binascii.unhexlify(srp_b_hex)

process_challenge_bytes = srp_user.process_challenge(salt_bytes,
                                                     srp_b_bytes)

timestamp = unicode(dt.datetime.utcnow().strftime("%a %b %d %H:%m:%S +0000 %Y"))

hmac_obj = hmac.new(process_challenge_bytes, digestmod=hashlib.sha256)
hmac_obj.update(user_pool_id.split('_')[1].encode('utf-8'))
hmac_obj.update(username.encode('utf-8'))
hmac_obj.update(secret_block_bytes)
hmac_obj.update(timestamp.encode('utf-8'))

challenge_responses = {
    "TIMESTAMP": timestamp.encode('utf-8'),
    "USERNAME": username.encode('utf-8'),
    "PASSWORD_CLAIM_SECRET_BLOCK": secret_block_hex,
    "PASSWORD_CLAIM_SIGNATURE": hmac_obj.hexdigest()
}

# Step 4:
# Submit challenge response to Cognito.

response = cognito.respond_to_auth_challenge(
    ClientId=client_id,
    ChallengeName='PASSWORD_VERIFIER',
    ChallengeResponses=challenge_responses)