HMAC Stream Cipher with Feedback v:0.0.0...

# Chris M. Thomasson Copyright 2018 (c)
# Experimental HMAC Stream Cipher
#____________________________________________________________


# Our external libs
#____________________________________________________________
import random;
import hashlib;
import hmac;


# Some Utilities
#____________________________________________________________
def ct_bytes_to_hex(origin, offset):
    hex = "";
    n = len(origin);
    t = "0123456789ABCDEF";
    for i in range(offset, n):
        c = ord(origin[i]);
        nibl = c & 0x0F;
        nibh = (c & 0xF0) >> 4;
        hex = hex + t[nibh];
        hex = hex + t[nibl];
        hex = hex + " ";
        if (not ((i + 1) % 16) and i != n - 1):
            hex = hex + "\r\n";
    return hex;


# Generate n random bytes
# These need should ideally be from a truly random, non-repeatable
# source. TRNG!
def ct_rand_bytes(n):
    rb = "";
    for i in range(n):
        rb = rb + chr(random.randint(0, 255));
        #rb = rb + chr(i);
    return rb;


# Secret Key
SK_hmac_key = "Password";
SK_hmac_algo = hashlib.sha384;
SK_rand_n = 13;


# Alice ENCRYPTS!!!
P = "XAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAX";

print("P:%s\n" % ct_bytes_to_hex(P, 0));

# Generate IV
IV = ct_rand_bytes(SK_rand_n);

H = hmac.new(SK_hmac_key.encode(), None, SK_hmac_algo);
H.update(IV.encode());
D = H.digest();

cidx = 0;

C = "";

for i in range(0, len(P)):
    CI = ord(P[i]) ^ D[cidx];
    C = C + chr(CI);
    H.update(chr(CI).encode());
    H.update(chr(cidx).encode());
    cidx = cidx + 1;
    if (cidx == len(D)):
        D = H.digest();
        cidx = 0;

print("IV:%s\n" % ct_bytes_to_hex(IV, 0));
print("ciphertext:%s\n" % ct_bytes_to_hex(C, 0));




# Bob DECRYPTS!!!
H = hmac.new(SK_hmac_key.encode(), None, SK_hmac_algo);
H.update(IV.encode());
D = H.digest();

cidx = 0;

BOB = "";

for i in range(0, len(C)):
    CI = ord(C[i]) ^ D[cidx];
    BOB = BOB + chr(CI);
    H.update(C[i].encode());
    H.update(chr(cidx).encode());
    cidx = cidx + 1;
    if (cidx == len(D)):
        D = H.digest();
        cidx = 0;

print("BOB:%s\n" % ct_bytes_to_hex(BOB, 0));