CMSI 662: Secure Software Development: Homework #4 Answers

A code is a straight-up table that maps words to words or phrases to phrases. A cipher is an algorithmic process for producing a ciphertext from a message.

Vigenere autokey, using the ciphertext as the rest of the key:

message    : TAKEACOPYOFYOURPOLICYTONORMAWILCOXONTHETHIRDFLOOR
key        : QUARKJUKVKLIZTYQGNNPFUYVRDNMIFUZMENFBABBSUHFUZCYI
ciphertext : JUKVKLIZTYQGNNPFUYVRDNMIFUZMENFBABBSUHFUZCYIZKQMZ

Here’s how I generated it:

from string import ascii_uppercase as a

message = "TAKEACOPYOFYOURPOLICYTONORMAWILCOXONTHETHIRDFLOOR"
key = "QUARK"
ciphertext = ""
for i in range(0, len(message)):
    c = a[(a.index(message[i]) + a.index(key[i])) % 26]
    ciphertext += c
    key += c
print(ciphertext)

You had to write the program for this one. Here’s mine:

cryptoexample.mjs

import { createCipheriv, createDecipheriv } from 'crypto'

function encrypt(data, key, iv) {
  const cipher = createCipheriv('aes-256-cbc', key, iv);
  return cipher.update(data, 'utf-8', 'hex') + cipher.final('hex');
}

function decrypt(data, key, iv) {
  const cipher = createDecipheriv('aes-256-cbc', key, iv);
  return cipher.update(data, 'hex', 'utf-8') + cipher.final('utf-8');
}

if (process.argv.length !== 6) {
  console.error(`Syntax: node cryptoexample.mjs (-e|-d) message-or-hexstring key iv
    where:
      -e means decrypt
      -d means encrypt
    If -e, the next argument must be text, and the program will respond with a hex string
    If -d, the next argument must be a hex string, and the program will respond with text`);
} else if (!['-d', '-e'].includes(process.argv[2])) {
  console.error('First argument must be -d or -e');
} else if (process.argv[2] === '-e') {
  console.log(encrypt(process.argv[3], process.argv[4], process.argv[5]));
} else {
  console.log(decrypt(process.argv[3], process.argv[4], process.argv[5]));
}

Here’s the script I used to figure this out:

p = 100392089237316158323570985008687907853269981005640569039457584007913129640081
q = 90392089237316158323570985008687907853269981005640569039457584007913129640041
n = p * q
e = 65537
d = pow(e, -1, (p-1)*(q-1))
m = "Scaramouche, Scaramouche, will you do the Fandango? 💃🏽"
m = int(''.join(f'{x:02x}' for x in bytearray(m, "utf8")), 16)
c = pow(m, e, n)
print(f"N  = {n}")
print(f"d  = {d}")
print(f"m  = {m:x}")
print(f"c  = {c:x}")
print(f"m' = {pow(c, d, n):x}")

Result:

N  = 9074650689060089248199307400991055468098862103321403389047443270291029585149437412899788230307477461518354291801534660904940424431810965948411931416083321
d  = 3440604854078842449902442746842634638010902628184540510109569714515334896803156693630578151012041316815494692897384029619755303913249828307540510260406273
m  = 53636172616d6f756368652c2053636172616d6f756368652c2077696c6c20796f7520646f207468652046616e64616e676f3f20f09f9283f09f8fbd
c  = 1510726ec4756e595c4b5ce1f3a1974798a34369eb8f43f7462d4093f30973994849a5b63d6b28e33c2200bfea7f7005bd7642e74302832b739be60d966a926b
m' = 53636172616d6f756368652c2053636172616d6f756368652c2077696c6c20796f7520646f207468652046616e64616e676f3f20f09f9283f09f8fbd

Quickest way to get this answer is to use Python

$ import hashlib
$ hashlib.sha384(bytes("Російський військовий корабель, іди нахуй", "utf-8")).hexdigest()
'c358ff602ada470dfb85fad41bd1fe277d587ace98d09c7eb70f48ef1048b76a2ec1103f67d54871cd18046cbd6fe816'