HMAC Encryption Experiment

# Chris M. Thomasson 9/19/2016

import math;
import random;
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;


def ct_hex_to_bytes(origin, offset):
    bytes = "";
    n = len(origin);
    t = "0123456789ABCDEF";

    i = offset + 1;
    while (i < n):

        nibh = 0;
        nibl = 0;

        try:
            nibh = t.index(origin[i - 1]);
            nibl = t.index(origin[i]);
        except ValueError:
            i = i + 1;
            continue;

        if (nibh > -1 and nibl > -1):
            c = nibh * 16 + nibl;
            bytes = bytes + chr(c);

        i = i + 2;

    return bytes;


# Generate n random bytes
def ct_rand_bytes(n):
    rb = "";
    for i in range(n):
        rb = rb + chr(random.randint(0, 255));
    return rb;

def ct_ptxt_display(ptxt):
    n = len(ptxt);
    print("\n\nPlaintext, size:%s" % (n));
    print("_________________________________________");
    print(ct_bytes_to_hex(ptxt, 0));
    print("_________________________________________");

def ct_ctxt_display(ctxt):
    n = len(ctxt[0]) + len(ctxt[1]);
    print("\n\nCiphertext, size:%s" % (n));
    print("_________________________________________");
    print("%s" % (ct_bytes_to_hex(ctxt[0], 0)));
    print("%s" % (ct_bytes_to_hex(ctxt[1], 0)));
    print("_________________________________________");



# HMAC Encrypt
#____________________________________________________________
def ct_hmac_encrypt(skey, ptxt):
    ctxt = "";
    n = len(ptxt);

    # Initialize HMAC for this session
    crand = ct_rand_bytes(32); # 32 bytes of random iv
    h = hmac.new((crand + skey).encode());
    h.update((skey + crand).encode());

    i = 0;
    while (i < n):
        # Grab HMAC buffer
        hbytes = h.digest();
        hn = len(hbytes);
        hi = 0;
        while (i < n and hi < hn):
            p = ord(ptxt[i]);
            c = p ^ hbytes[hi];
            h.update(ptxt[i].encode());
            h.update(chr(c).encode());
            ctxt = ctxt + chr(c);
            i = i + 1;
            hi = hi + 1;

    return (crand, ctxt);


# HMAC Decrypt
#____________________________________________________________
def ct_hmac_decrypt(skey, ctxt):
    ptxt = "";
    n = len(ctxt[1]);

    # Initialize HMAC for this session
    h = hmac.new((ctxt[0] + skey).encode());
    h.update((skey + ctxt[0]).encode());

    i = 0;
    while (i < n):
        hbytes = h.digest();
        hn = len(hbytes);
        hi = 0;
        while (i < n and hi < hn):
            c = ord(ctxt[1][i]);
            p = c ^ hbytes[hi];
            h.update(chr(p).encode());
            h.update(chr(c).encode());
            ptxt = ptxt + chr(p);
            i = i + 1;
            hi = hi + 1;

    return ptxt;



# Main Program
#____________________________________________________________
print("Experimental HMAC Encryption");
print("Chris M. Thomasson 9/6/2016");
print("==============================================================\n\n");


ptxt = "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA";

# Secret Key
skey = "This is the Secret HMAC Key! BTW, this should be longer...";
print("skey:%s" % (skey));


# Encrypt
print("\n\n\nEncrypted:");
print("\nptxt:%s" % (ptxt));
ct_ptxt_display(ptxt);
ctxt = ct_hmac_encrypt(skey, ptxt);
ct_ctxt_display(ctxt);

# Decrypt
dtxt = ct_hmac_decrypt(skey, ctxt);
print("\n\n\nDecrypted:");
ct_ptxt_display(dtxt);
print("\n\ndtxt:%s" % (dtxt));

if (dtxt != ptxt):
    print("Data Corrupted!");