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aws_srp.py
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aws_srp.py
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import base64
import binascii
import datetime
import hashlib
import hmac
import re
import boto3
import os
import six
from .exceptions import ForceChangePasswordException
# https://github.com/aws/amazon-cognito-identity-js/blob/master/src/AuthenticationHelper.js#L22
n_hex = 'FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD1' + '29024E088A67CC74020BBEA63B139B22514A08798E3404DD' + \
'EF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245' + 'E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7ED' + \
'EE386BFB5A899FA5AE9F24117C4B1FE649286651ECE45B3D' + 'C2007CB8A163BF0598DA48361C55D39A69163FA8FD24CF5F' + \
'83655D23DCA3AD961C62F356208552BB9ED529077096966D' + '670C354E4ABC9804F1746C08CA18217C32905E462E36CE3B' + \
'E39E772C180E86039B2783A2EC07A28FB5C55DF06F4C52C9' + 'DE2BCBF6955817183995497CEA956AE515D2261898FA0510' + \
'15728E5A8AAAC42DAD33170D04507A33A85521ABDF1CBA64' + 'ECFB850458DBEF0A8AEA71575D060C7DB3970F85A6E1E4C7' + \
'ABF5AE8CDB0933D71E8C94E04A25619DCEE3D2261AD2EE6B' + 'F12FFA06D98A0864D87602733EC86A64521F2B18177B200C' + \
'BBE117577A615D6C770988C0BAD946E208E24FA074E5AB31' + '43DB5BFCE0FD108E4B82D120A93AD2CAFFFFFFFFFFFFFFFF'
# https://github.com/aws/amazon-cognito-identity-js/blob/master/src/AuthenticationHelper.js#L49
g_hex = '2'
info_bits = bytearray('Caldera Derived Key', 'utf-8')
def hash_sha256(buf):
"""AuthenticationHelper.hash"""
a = hashlib.sha256(buf).hexdigest()
return (64 - len(a)) * '0' + a
def hex_hash(hex_string):
return hash_sha256(bytearray.fromhex(hex_string))
def hex_to_long(hex_string):
return int(hex_string, 16)
def long_to_hex(long_num):
return '%x' % long_num
def get_random(nbytes):
random_hex = binascii.hexlify(os.urandom(nbytes))
return hex_to_long(random_hex)
def pad_hex(long_int):
"""
Converts a Long integer (or hex string) to hex format padded with zeroes for hashing
:param {Long integer|String} long_int Number or string to pad.
:return {String} Padded hex string.
"""
if not isinstance(long_int, six.string_types):
hash_str = long_to_hex(long_int)
else:
hash_str = long_int
if len(hash_str) % 2 == 1:
hash_str = '0%s' % hash_str
elif hash_str[0] in '89ABCDEFabcdef':
hash_str = '00%s' % hash_str
return hash_str
def compute_hkdf(ikm, salt):
"""
Standard hkdf algorithm
:param {Buffer} ikm Input key material.
:param {Buffer} salt Salt value.
:return {Buffer} Strong key material.
@private
"""
prk = hmac.new(salt, ikm, hashlib.sha256).digest()
info_bits_update = info_bits + bytearray(chr(1), 'utf-8')
hmac_hash = hmac.new(prk, info_bits_update, hashlib.sha256).digest()
return hmac_hash[:16]
def calculate_u(big_a, big_b):
"""
Calculate the client's value U which is the hash of A and B
:param {Long integer} big_a Large A value.
:param {Long integer} big_b Server B value.
:return {Long integer} Computed U value.
"""
u_hex_hash = hex_hash(pad_hex(big_a) + pad_hex(big_b))
return hex_to_long(u_hex_hash)
class AWSSRP(object):
NEW_PASSWORD_REQUIRED_CHALLENGE = 'NEW_PASSWORD_REQUIRED'
PASSWORD_VERIFIER_CHALLENGE = 'PASSWORD_VERIFIER'
def __init__(self, username, password, pool_id, client_id, pool_region=None,
client=None, client_secret=None):
if pool_region is not None and client is not None:
raise ValueError("pool_region and client should not both be specified "
"(region should be passed to the boto3 client instead)")
self.username = username
self.password = password
self.pool_id = pool_id
self.client_id = client_id
self.client_secret = client_secret
self.client = client if client else boto3.client('cognito-idp', region_name=pool_region)
self.big_n = hex_to_long(n_hex)
self.g = hex_to_long(g_hex)
self.k = hex_to_long(hex_hash('00' + n_hex + '0' + g_hex))
self.small_a_value = self.generate_random_small_a()
self.large_a_value = self.calculate_a()
def generate_random_small_a(self):
"""
helper function to generate a random big integer
:return {Long integer} a random value.
"""
random_long_int = get_random(128)
return random_long_int % self.big_n
def calculate_a(self):
"""
Calculate the client's public value A = g^a%N
with the generated random number a
:param {Long integer} a Randomly generated small A.
:return {Long integer} Computed large A.
"""
big_a = pow(self.g, self.small_a_value, self.big_n)
# safety check
if (big_a % self.big_n) == 0:
raise ValueError('Safety check for A failed')
return big_a
def get_password_authentication_key(self, username, password, server_b_value, salt):
"""
Calculates the final hkdf based on computed S value, and computed U value and the key
:param {String} username Username.
:param {String} password Password.
:param {Long integer} server_b_value Server B value.
:param {Long integer} salt Generated salt.
:return {Buffer} Computed HKDF value.
"""
u_value = calculate_u(self.large_a_value, server_b_value)
if u_value == 0:
raise ValueError('U cannot be zero.')
username_password = '%s%s:%s' % (self.pool_id.split('_')[1], username, password)
username_password_hash = hash_sha256(username_password.encode('utf-8'))
x_value = hex_to_long(hex_hash(pad_hex(salt) + username_password_hash))
g_mod_pow_xn = pow(self.g, x_value, self.big_n)
int_value2 = server_b_value - self.k * g_mod_pow_xn
s_value = pow(int_value2, self.small_a_value + u_value * x_value, self.big_n)
hkdf = compute_hkdf(bytearray.fromhex(pad_hex(s_value)),
bytearray.fromhex(pad_hex(long_to_hex(u_value))))
return hkdf
def get_auth_params(self):
auth_params = {'USERNAME': self.username,
'SRP_A': long_to_hex(self.large_a_value)}
if self.client_secret is not None:
auth_params.update({
"SECRET_HASH":
self.get_secret_hash(self.username,self.client_id, self.client_secret)})
return auth_params
@staticmethod
def get_secret_hash(username, client_id, client_secret):
message = bytearray(username + client_id, 'utf-8')
hmac_obj = hmac.new(bytearray(client_secret, 'utf-8'), message, hashlib.sha256)
return base64.standard_b64encode(hmac_obj.digest()).decode('utf-8')
def process_challenge(self, challenge_parameters):
user_id_for_srp = challenge_parameters['USER_ID_FOR_SRP']
salt_hex = challenge_parameters['SALT']
srp_b_hex = challenge_parameters['SRP_B']
secret_block_b64 = challenge_parameters['SECRET_BLOCK']
# re strips leading zero from a day number (required by AWS Cognito)
timestamp = re.sub(r" 0(\d) ", r" \1 ",
datetime.datetime.utcnow().strftime("%a %b %d %H:%M:%S UTC %Y"))
hkdf = self.get_password_authentication_key(user_id_for_srp,
self.password, hex_to_long(srp_b_hex), salt_hex)
secret_block_bytes = base64.standard_b64decode(secret_block_b64)
msg = bytearray(self.pool_id.split('_')[1], 'utf-8') + bytearray(user_id_for_srp, 'utf-8') + \
bytearray(secret_block_bytes) + bytearray(timestamp, 'utf-8')
hmac_obj = hmac.new(hkdf, msg, digestmod=hashlib.sha256)
signature_string = base64.standard_b64encode(hmac_obj.digest())
response = {'TIMESTAMP': timestamp,
'USERNAME': user_id_for_srp,
'PASSWORD_CLAIM_SECRET_BLOCK': secret_block_b64,
'PASSWORD_CLAIM_SIGNATURE': signature_string.decode('utf-8')}
if self.client_secret is not None:
response.update({
"SECRET_HASH":
self.get_secret_hash(self.username, self.client_id, self.client_secret)})
return response
def authenticate_user(self, client=None):
boto_client = self.client or client
auth_params = self.get_auth_params()
response = boto_client.initiate_auth(
AuthFlow='USER_SRP_AUTH',
AuthParameters=auth_params,
ClientId=self.client_id
)
if response['ChallengeName'] == self.PASSWORD_VERIFIER_CHALLENGE:
challenge_response = self.process_challenge(response['ChallengeParameters'])
tokens = boto_client.respond_to_auth_challenge(
ClientId=self.client_id,
ChallengeName=self.PASSWORD_VERIFIER_CHALLENGE,
ChallengeResponses=challenge_response)
if tokens.get('ChallengeName') == self.NEW_PASSWORD_REQUIRED_CHALLENGE:
raise ForceChangePasswordException('Change password before authenticating')
return tokens
else:
raise NotImplementedError('The %s challenge is not supported' % response['ChallengeName'])
def set_new_password_challenge(self, new_password, client=None):
boto_client = self.client or client
auth_params = self.get_auth_params()
response = boto_client.initiate_auth(
AuthFlow='USER_SRP_AUTH',
AuthParameters=auth_params,
ClientId=self.client_id
)
if response['ChallengeName'] == self.PASSWORD_VERIFIER_CHALLENGE:
challenge_response = self.process_challenge(response['ChallengeParameters'])
tokens = boto_client.respond_to_auth_challenge(
ClientId=self.client_id,
ChallengeName=self.PASSWORD_VERIFIER_CHALLENGE,
ChallengeResponses=challenge_response)
if tokens['ChallengeName'] == self.NEW_PASSWORD_REQUIRED_CHALLENGE:
challenge_response = {
'USERNAME': auth_params['USERNAME'],
'NEW_PASSWORD': new_password
}
new_password_response = boto_client.respond_to_auth_challenge(
ClientId=self.client_id,
ChallengeName=self.NEW_PASSWORD_REQUIRED_CHALLENGE,
Session=tokens['Session'],
ChallengeResponses=challenge_response)
return new_password_response
return tokens
else:
raise NotImplementedError('The %s challenge is not supported' % response['ChallengeName'])