pyUmbral/docs/examples/umbral_simple_api.py

"""
Copyright (C) 2018 NuCypher

This file is part of pyUmbral.

pyUmbral is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

pyUmbral is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with pyUmbral. If not, see <https://www.gnu.org/licenses/>.
"""

#1
# Sets a default curve (secp256k1)
import random
from umbral import pre, keys, config, signing

config.set_default_curve()

#2
# Generate an Umbral key pair
# ---------------------------
# First, Let's generate two asymmetric key pairs for Alice:
# A delegating key pair and a Signing key pair.

alices_private_key = keys.UmbralPrivateKey.gen_key()
alices_public_key = alices_private_key.get_pubkey()

alices_signing_key = keys.UmbralPrivateKey.gen_key()
alices_verifying_key = alices_signing_key.get_pubkey()
alices_signer = signing.Signer(private_key=alices_signing_key)

#3
# Encrypt some data for Alice
# ---------------------------
# Now let's encrypt data with Alice's public key.
# Invocation of `pre.encrypt` returns both the `ciphertext`,
# and a `capsule`. Anyone with Alice's public key can perform
# this operation.

plaintext = b'Proxy Re-encryption is cool!'
ciphertext, capsule = pre.encrypt(alices_public_key, plaintext)
print(ciphertext)

#4
# Decrypt data for Alice
# ----------------------
# Since data was encrypted with Alice's public key,
# Alice can open the capsule and decrypt the ciphertext with her private key.

cleartext = pre.decrypt(ciphertext=ciphertext, 
						capsule=capsule, 
						decrypting_key=alices_private_key)
print(cleartext)


#5
# Bob Exists
# -----------

bobs_private_key = keys.UmbralPrivateKey.gen_key()
bobs_public_key = bobs_private_key.get_pubkey()

#6
# Bob receives a capsule through a side channel (s3, ipfs, Google cloud, etc)
bob_capsule = capsule

#7
# Attempt Bob's decryption (fail)
try:
    fail_decrypted_data = pre.decrypt(ciphertext=ciphertext, 
    								  capsule=bob_capsule, 
    								  decrypting_key=bobs_private_key)
except:
    print("Decryption failed! Bob doesn't has access granted yet.")

#8
# Alice grants access to Bob by generating kfrags 
# -----------------------------------------------
# When Alice wants to grant Bob access to open her encrypted messages, 
# she creates *threshold split re-encryption keys*, or *"kfrags"*, 
# which are next sent to N proxies or *Ursulas*. 
# She uses her private key, and Bob's public key, and she sets a minimum 
# threshold of 10, for 20 total shares

kfrags = pre.split_rekey(delegating_privkey=alices_private_key,
                         signer=alices_signer,
                         receiving_pubkey=bobs_public_key,
                         threshold=10,
                         N=20)


#9
# Ursulas perform re-encryption
# ------------------------------
# Bob asks several Ursulas to re-encrypt the capsule so he can open it. 
# Each Ursula performs re-encryption on the capsule using the `kfrag` 
# provided by Alice, obtaining this way a "capsule fragment", or `cfrag`.
# Let's mock a network or transport layer by sampling `threshold` random `kfrags`,
# one for each required Ursula.

import random
kfrags = random.sample(kfrags,  # All kfrags from above
                       10)      # M - Threshold

# Bob collects the resulting `cfrags` from several Ursulas. 
# Bob must gather at least `threshold` `cfrags` in order to activate the capsule.

cfrags = list()                 # Bob's cfrag collection
for kfrag in kfrags:
	cfrag = pre.reencrypt(kfrag=kfrag, capsule=bob_capsule)
	cfrags.append(cfrag)        # Bob collects a cfrag

assert len(cfrags) == 10


#10
# Bob attaches cfrags to the capsule
# ----------------------------------
# Bob attaches at least `threshold` `cfrags` to the capsule;
# then it can become *activated*.

bob_capsule.set_correctness_keys(delegating=alices_public_key,
                                 receiving=bobs_public_key,
                                 verifying=alices_verifying_key)

for cfrag in cfrags:
	bob_capsule.attach_cfrag(cfrag)

#11
# Bob activates and opens the capsule
# ------------------------------------
# Finally, Bob activates and opens the capsule,
# then decrypts the re-encrypted ciphertext.


bob_cleartext = pre.decrypt(ciphertext=ciphertext, capsule=bob_capsule, decrypting_key=bobs_private_key)
print(bob_cleartext)
assert bob_cleartext == plaintext