The Enigma Encryption The Enigma Encryption SystemSystem

Andrew Regan

CSC 290

Key Components for Encryption/Decryption

• Plugboard

• 3 Rotating Removable Rotors

• Reflector

Sample Encryption/Decryption

Enigma

Enigma Properties

• 3 rotors 26x26x26 = 17,576 initial settings• Rotor order 3x2x1 = 6 possible arrangements• Swapping 6 letters 100,391,791,500• Approximately 1016 = 10,000,000,000,000,000

keys• Later versions of the Enigma had even more

keys.– Swapped more than 6 letters– Selected 3 rotors from set of 5

Ciphertext-Only AttackDeveloped by James Gillogly.

• Find initial rotor setting and rotor order.

• Uses Index of Coincidence.

• Iterate through all possible initial rotor settings and rotor orders with no plugboard.

• Largest IOC is the most likely rotor arrangement.

( 1)

( 1)

z

i ii a

f fIC

N N

• Uses rotor settings from step 1.

• Finds one new plugboard setting for each iteration.

• Decrypt the text with every possible character pair. – 25x26 = 650 pairs.

• Trigram analysis on the 650 decrypted texts. Add the setting to the plugboard with the best decrypted text.

Step 1: Step 2:

Ciphertext-Only Attack Step 1 Results

0

20000

40000

60000

80000

100000

120000

50 100 150 200 250 300 350 400 450 500

Number of Characters

Index of IOC

t01t02t03t04t05t06t07t08t09t10t11t12t13t14t15t16t17t18t19t20t21t22t23t24t25t26t27t28t29t30t31t32t33t34t35t36t37t38t39t40

Ciphertext-Only Attack Step 1 Results Cont.

# Characters Avg. IOC Index out of 105,456

50 19405.025

100 10059.05

150 2878.45

200 453.725

250 91.75

300 10.6

350 1.15

400 .2

450 .05

500 0

# Characters Successful Decryptions

50 0.0

100 0.0

150 .1

200 .3

250 .475

300 .675

350 .875

400 .925

450 .975

500 1

Current Trigram Results

• Attempted analyzing text based on most probable trigrams in English plaintext.– +1 for “good” trigram, -1 for “bad.”– Highest score wins.– To many swaps initially to be accurate.

• Currently working on developing trigram statistics for partially decrypted Enigma texts to use in the initial rounds of finding the plugboard settings.