added some more references and ideas for future code break parties
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misc/Proj RSA2_ Cracking a Short RSA Key (15 pts.).html
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<!-- saved from url=(0041)https://samsclass.info/141/proj/pRSA2.htm -->
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<html><head><meta http-equiv="Content-Type" content="text/html; charset=windows-1252">
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<title>Proj RSA2: Cracking a Short RSA Key (15 pts.)</title>
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</head>
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<body bgcolor="#cccccc">
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<h1>Proj RSA2: Cracking a Short RSA Key (15 pts.)</h1>
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What you need:
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<ul>
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<li>A Mac or Linux computer with Python.
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</li></ul>
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<h2>Purpose</h2>
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To break into RSA encryption without prior knowledge of the
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private key. This is only possible for small RSA keys,
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which is why RSA keys should be long for security.
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<p>
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</p><h2>Summary</h2>
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Here's a diagram from the textbook showing the
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RSA calculations.
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<blockquote><img src="./Proj RSA2_ Cracking a Short RSA Key (15 pts.)_files/pRSA1-1.png"></blockquote>
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<h2>Problem Statement</h2>
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Meghan's public key is (10142789312725007, 5).
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Find her private key.
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<h2>1. Factoring n</h2>
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<h3>Finding the Square Root of n</h3>
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n = 10142789312725007. This is the product of
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two prime numbers, p and q.
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<p>
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How large are p and q? Well, they can't both be
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larger than the square root of n, or they'd be larger
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than n when multiplied together.
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</p><p>
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Start Python in interactive mode. On a Mac
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or Linux box, you can do that by
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typing this command into a Terminal window:
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</p><blockquote><b><big><code>
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python
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</code></big></b></blockquote>
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Execute these commands:
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<blockquote><b><big><code><pre>import math
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n = 10142789312725007
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print math.sqrt(n)
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</pre></code></big></b></blockquote>
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The square root prints out,
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as shown below.
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<blockquote><img src="./Proj RSA2_ Cracking a Short RSA Key (15 pts.)_files/pRSA2-1.png"></blockquote>
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<h3>Displaying More Decimal Places</h3>
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It's not clear from that output whether the result
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is an integer, or just rounded off to one decimal
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place. To see more decimal places, we'll use
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the repr() function.
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<p>
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Execute this command:
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</p><blockquote><b><big><code><pre>print repr(math.sqrt(n))
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</pre></code></big></b></blockquote>
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Now more decimal places appear,
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as shown below.
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<blockquote><img src="./Proj RSA2_ Cracking a Short RSA Key (15 pts.)_files/pRSA2-2.png"></blockquote>
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<h3>Testing 20 Candidates</h3>
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So one of the prime factors must be a prime number
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less than 100711415. All we have to do is try dividing
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n by odd numbers starting at 100711413 and going down until
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we get an integer result. (We don't need to test 100711415 because
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it's divisible by 5 and therefore not a prime number.)
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<p>
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A good way to do this is to calculate n mod c, where c is a candidate. If c is a factor of n, the result will be zero.
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</p><p>
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We can test the first 20 candidates with a for loop.
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</p><p>
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Execute these commands:
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</p><blockquote><b><big><code><pre>c = 100711413
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for i in range(c, c-40, -2):
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print i, n%i
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</pre></code></big></b></blockquote>
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Press Enter twice after the last command to terminate the loop.
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<p>
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The third candidate is the winner, with a remainder of zero,
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as shown below.
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</p><blockquote><img src="./Proj RSA2_ Cracking a Short RSA Key (15 pts.)_files/pRSA2-3.png"></blockquote>
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<h3>Calculating q</h3>
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We now know p and we can calculate q.
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<p>
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Execute these commands:
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</p><blockquote><b><big><code><pre>p = 100711409
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q = n / p
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print p, q, n, p*q, n - p*q
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</pre></code></big></b></blockquote>
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The calculation worked, so the last value is zero,
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as shown below.
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<blockquote><img src="./Proj RSA2_ Cracking a Short RSA Key (15 pts.)_files/pRSA2-4.png"></blockquote>
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<h2>2. Compute phin = (p-1) * (q-1)</h2>
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Execute these commands:
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<blockquote><b><big><code><pre>phin = (p-1) * (q-1)
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print p, q, n, phin
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</pre></code></big></b></blockquote>
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The parameters print out, as shown below.
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<blockquote><img src="./Proj RSA2_ Cracking a Short RSA Key (15 pts.)_files/pRSA2-6.png"></blockquote>
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<h2>3. Compute Private Key d</h2>
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We need to find a <b>d</b> with this
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property:
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<blockquote>
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<b> (d * e) mod phin = 1 </b>
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</blockquote>
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We know that e = 5 from the Problem Statement.
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<p>
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It's not obvious how to find d, but
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<a href="https://stackoverflow.com/questions/4798654/modular-multiplicative-inverse-function-in-python">there's
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a simple way to do it in Python</a>, using the
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"gmpy> library.
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</p><p>
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Open a <b>new Terminal window</b>, not the one
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running Python, and execute this command to
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download and install a few dependencies and gmpy:
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</p><blockquote><b><big><code><pre>brew install gmp mpfr mpc
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pip install gmpy
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</pre></code></big></b></blockquote>
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<p>
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In the Terminal window running python,
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execute these commands.
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</p><blockquote><b><big><code><pre>e = 5
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import gmpy
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d = gmpy.invert(e, phin)
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print d, e, d*e %phin
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</pre></code></big></b></blockquote>
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We get the value of d, and, to verify it,
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we see that d*e %phin is indeed 1,
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as shown below.
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<blockquote><img src="./Proj RSA2_ Cracking a Short RSA Key (15 pts.)_files/pRSA2-7.png"></blockquote>
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<h2>4. Encrypting a Message</h2>
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<h3>Encoding the Message as a Number</h3>
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Cueball wants to send Meghan this message:
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<blockquote><b><big><code><pre>Hi!
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</pre></code></big></b></blockquote>
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We can only send numbers.
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Let's convert that message to three bytes of ASCII
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and then interpret it as a 24-bit binary value.
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<p>
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In the Terminal window running python,
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execute these commands.
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</p><blockquote><b><big><code><pre>x1 = ord('H')
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x2 = ord('i')
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x3 = ord('!')
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x = x1*256*256 + x2*256 + x3
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print x
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</pre></code></big></b></blockquote>
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We get the value of x,
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as shown below.
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<blockquote><img src="./Proj RSA2_ Cracking a Short RSA Key (15 pts.)_files/pRSA2-8.png"></blockquote>
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<h3>Encrypting the Message with the Public Key</h3>
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A public key contains two numbers: <b>n</b>
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and <b>e</b>. To encrypt a message <i>x</i>,
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use this formula:
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<blockquote><img src="./Proj RSA2_ Cracking a Short RSA Key (15 pts.)_files/pRSA1-11.png"></blockquote>
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<p>
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Execute these commands:
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</p><blockquote><b><big><code><pre>y = x ** e % n
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print y
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</pre></code></big></b></blockquote>
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The encrypted message appears,
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as shown below.
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<blockquote><img src="./Proj RSA2_ Cracking a Short RSA Key (15 pts.)_files/pRSA2-9.png"></blockquote>
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<h2>5. Decrypting a Message</h2>
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To decrypt a message, use this formula:
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<blockquote><img src="./Proj RSA2_ Cracking a Short RSA Key (15 pts.)_files/pRSA1-13.png"></blockquote>
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Execute these commands:
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<blockquote><b><big><code><pre>xx = y ** d % n
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print xx
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</pre></code></big></b></blockquote>
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Python crashes,
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as shown below. It cannot handle such large numbers.
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<blockquote><img src="./Proj RSA2_ Cracking a Short RSA Key (15 pts.)_files/pRSA2-10.png"></blockquote>
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To compute such a number, we must use the pow() function.
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<p>
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Execute these commands to restart python and restore
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all the values we calculated previously:
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</p><blockquote><b><big><code><pre>python
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n = 10142789312725007
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p = 100711409
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q = 100711423
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phin = (p-1) * (q-1)
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e = 5
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import gmpy
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d = gmpy.invert(e, phin)
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x1 = ord('H')
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x2 = ord('i')
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x3 = ord('!')
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x = x1*256*256 + x2*256 + x3
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y = x ** e % n
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</pre></code></big></b></blockquote>
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Your screen should look like the image
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below.
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<blockquote><img src="./Proj RSA2_ Cracking a Short RSA Key (15 pts.)_files/pRSA2-11.png"></blockquote>
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Let's try that decryption again with the pow()
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function. Execute these commands:
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<blockquote><b><big><code><pre>xx = pow(y, d, n)
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print xx
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</pre></code></big></b></blockquote>
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Now it works, showing our original message in
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numerical form.
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<blockquote><img src="./Proj RSA2_ Cracking a Short RSA Key (15 pts.)_files/pRSA2-12.png"></blockquote>
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<h3>Converting the Message to Readable Text</h3>
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To convert that number back to letters,
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execute these commands:
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<blockquote><b><big><code><pre>xx1 = xx / (256*256)
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xx2 = (xx - 256*256*xx1) / 256
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xx3 = xx - 256*256*xx1 - 256*xx2
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msg = chr(xx1) + chr(xx2) + chr(xx3)
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print xx1, xx2, xx3, msg
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</pre></code></big></b></blockquote>
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Now it works, showing the original message in
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readable form, as shown below.
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<blockquote><img src="./Proj RSA2_ Cracking a Short RSA Key (15 pts.)_files/pRSA2-13.png"></blockquote>
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<hr>
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<h2>Challenge 2a: Encrypt "OBEY!"</h2>
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Using the same system and keys, encrypt this message:
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<blockquote><b><big><pre>OBEY!
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</pre></big></b></blockquote>
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<i>Hint 1: The message, converted to a number, is 12 digits long and ends in 41.</i>
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<p>
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<i>Hint 2: The encrypted message is 16 digits long and ends in 81.</i>
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</p><p>
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Use the form
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below to put your name on the
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<a href="http://ad.samsclass.info/python/RSAchal2a-winners.htm">
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<b>WINNERS PAGE</b></a>.
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</p><blockquote>
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<form action="http://ad.samsclass.info/python/RSAchal2a.php" method="post">
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<table cellpadding="5" border="10"><tbody><tr><td>
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<table cellpadding="5">
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<tbody><tr><td><big><b>Your Name (without spaces):</b></big></td>
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<td><textarea name="u" rows="1" cols="25"></textarea></td></tr>
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<tr><td><big><b>Encrypted message:</b></big></td>
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<td><textarea name="p" rows="1" cols="30"></textarea></td></tr>
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<tr><td colspan="2" align="center"><big><b>
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<input type="submit" value="SUBMIT"></b></big></td></tr>
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</tbody></table>
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</td></tr></tbody></table>
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</form>
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</blockquote>
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<hr>
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<h2>Challenge 2b: Message to Cueball</h2>
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Cueball's public key is:
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<blockquote><b><big><pre>(111036975342601848755221, 13)
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</pre></big></b></blockquote>
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Meghan sends this message to Cueball. Decrypt it.
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<blockquote><b><big><pre>80564890594461648564443
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</pre></big></b></blockquote>
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Use the form
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below to put your name on the
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<a href="http://ad.samsclass.info/python/RSAchal2b-winners.htm">
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<b>WINNERS PAGE</b></a>.
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<blockquote>
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<form action="http://ad.samsclass.info/python/RSAchal2b.php" method="post">
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<table cellpadding="5" border="10"><tbody><tr><td>
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<table cellpadding="5">
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<tbody><tr><td><big><b>Your Name (without spaces):</b></big></td>
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<td><textarea name="u" rows="1" cols="25"></textarea></td></tr>
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<tr><td><big><b>Cleartext Message:</b></big></td>
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<td><textarea name="p" rows="1" cols="25"></textarea></td></tr>
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<tr><td colspan="2" align="center"><big><b>
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<input type="submit" value="SUBMIT"></b></big></td></tr>
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</tbody></table>
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</td></tr></tbody></table>
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</form>
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</blockquote>
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<hr>
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<h2>Challenge 3: Message to Rob</h2>
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Rob public key is:
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<blockquote><b><big><pre>(1234592592962967901296297037045679133590224789902207663928489902170626521926687, 5557)
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</pre></big></b></blockquote>
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Meghan sends this message to Rob. Decrypt it.
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<blockquote><b><big><pre>272495530567010327943798078794037733865151017104532777645776936985235709526002
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</pre></big></b></blockquote>
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<i>Hint:
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<a href="https://stackoverflow.com/questions/10725522/arbitrary-precision-of-square-roots">make square root calculations more precise</a>.</i>
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<p>
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Use the form
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below to put your name on the
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<a href="http://ad.samsclass.info/python/RSAchal2c-winners.htm">
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<b>WINNERS PAGE</b></a>.
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</p><blockquote>
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<form action="http://ad.samsclass.info/python/RSAchal2c.php" method="post">
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<table cellpadding="5" border="10"><tbody><tr><td>
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<table cellpadding="5">
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<tbody><tr><td><big><b>Your Name (without spaces):</b></big></td>
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<td><textarea name="u" rows="1" cols="25"></textarea></td></tr>
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<tr><td><big><b>Cleartext Message:</b></big></td>
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<td><textarea name="p" rows="1" cols="25"></textarea></td></tr>
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<tr><td colspan="2" align="center"><big><b>
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<input type="submit" value="SUBMIT"></b></big></td></tr>
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</tbody></table>
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</td></tr></tbody></table>
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</form>
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</blockquote>
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<hr>
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<h2>Sources</h2>
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<a href="https://stackoverflow.com/questions/4078902/cracking-short-rsa-keys">
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Cracking short RSA keys</a>
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<p>
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<a href="http://www.numberempire.com/primenumbers.php">
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Prime Numbers Generator and Checker</a>
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</p><p>
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<a href="https://stackoverflow.com/questions/10725522/arbitrary-precision-of-square-roots">Arbitrary precision of square roots</a>
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</p><hr>
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Posted 3-31-16 by Sam Bowne <br>
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Winners pages added 8-6-16 <br>
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Attack server name updated 4-4-17 <br>
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https://puzzling.stackexchange.com/questions/49099/another-alien-message-but-from-where?noredirect=1&lq=1
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https://puzzling.stackexchange.com/questions/4705/solve-an-alien-message
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