In the previous post, we learned how to decrypt RSA by getting the factors of the big number N and computing for the inverse of e (the encoding number) modulo N. There is also another way to decrypt an RSA encrypted message. This is when you are able to get the period of the ciphertext. If c is the ciphertext, the **period** r is the smallest integer that satisfies:

Once we get the period, we compute for , the inverse of e modulo r:

The inverse can then be used to decrypt the ciphertext:

In our previous example, we encrypted the message

THIS IS A SECRET MESSAGE

using public key p=53, q=59, N=pq=3127 and e=7 and private key d=431. The “plain text” is

1907 0818 2608 1826 0026 1804 0217 0419 2612 0418 1800 0604

and the ciphertext is:

0794 1832 1403 2474 1231 1453 0268 2223 0678 0540 0773 1095

Let’s compute the period of the first block of our ciphertext:

Using the python script below, we can compute the period

for r in range(1,100):
p=pow(794,r,N)
if p == 1:
print "%d %d" % (r,p)

The result of running the above program gives r=58. We can then compute using the following equation:

The above equation is satisfied when m=3 and . Using this value of , we can compute for

which gives us the original message!

However, unlike using the private key, you need to compute the period r and for every block of the ciphertext (unless the ciphertext is composed of only one block). However, that should not stop a cracker from deciphering all the blocks.

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