chk_dec.py

#BSD 3-Clause License
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# Copyright (c) 2018, Joseph deBlaquiere <jadeblaquiere@yahoo.com>
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from FSPKE import CHKPKE, Element
from argparse import ArgumentParser
import base64
import sys
from hashlib import sha256
import asn1
import pysodium


def ensure_tag(decoder, expected):
    tag = decoder.peek()
    if tag.nr != expected:
        raise ValueError("Error in DER format, expected tag %d, got %d" %
                         (expected, tag.nr))


desc = ('chk-dec decrypts a message encrypted by chk-enc using the CHK '
        'forward secure encryption scheme.')

parser = ArgumentParser(description=desc)
parser.add_argument('-k', '--key', default=None, help='file path for file containing private key')
parser.add_argument('-f', '--file', default=None, help='read ciphertext from file instead of stdin')
clargs = parser.parse_args()

if clargs.key is None:
    sys.exit('Error: -k / --key option for public key file is mandatory.')

with open(clargs.key, 'r') as keyfile:
    PEMkey=keyfile.read()
DERkey = base64.b64decode(PEMkey.split('-----')[2].encode())
try:
    privkey = CHKPKE(privkey=DERkey)
except ValueError:
    sys.exit('Error: Unable to import private key, aborting.')

if privkey is None:
    sys.exit('Error: Unable to import private key, aborting.')

if clargs.file is None:
    PEMtxt = sys.stdin.read()
else:
    with open(clargs.file, 'r') as msgfile:
        PEMtxt=msgfile.read()

DERtxt = base64.b64decode(PEMtxt.split('-----')[2].encode())

decoder = asn1.Decoder()
decoder.start(DERtxt)
ensure_tag(decoder, asn1.Numbers.Sequence)
decoder.enter()
ensure_tag(decoder, asn1.Numbers.Sequence)
decoder.enter()
ensure_tag(decoder, asn1.Numbers.OctetString)
tag, enckey = decoder.read()
ensure_tag(decoder, asn1.Numbers.OctetString)
tag, nonce = decoder.read()
ensure_tag(decoder, asn1.Numbers.Integer)
tag, interval = decoder.read()
decoder.leave()
ensure_tag(decoder, asn1.Numbers.OctetString)
tag, ctext = decoder.read()

# re-write additional data into a single DER structure
encoder = asn1.Encoder()
encoder.start()
encoder.enter(asn1.Numbers.Sequence)
encoder.write(enckey, asn1.Numbers.OctetString)
encoder.write(nonce, asn1.Numbers.OctetString)
encoder.write(interval, asn1.Numbers.Integer)
encoder.leave()
AD = encoder.output()

try:
    e = privkey.decrypt(enckey, interval)
except ValueError:
    print("<Error>: Unable to derive key for interval")
    sys.exit(1)
chachakey = sha256(e.to_bytes()).digest()

message = pysodium.crypto_aead_chacha20poly1305_ietf_decrypt(ciphertext=ctext,
    ad=AD, nonce=nonce, key=chachakey)

print(message.decode(), end='')