Friday, December 25, 2009

Rotor Machines

Rotor machines electro-mechanical implementation of polyalphabetic substitution ciphers in which the substituted letter is chosen electrically from huge number of possible combinations. Since one or more of the disks rotated mechanically with each plaintext letter enciphered, with 5 disk rotor machine for the English language, the number of combinations is large as 265. Rotor machines are easily implement in hardware, requires a little amount of memory and fast. Although rotor machines have these advantages, there are some security issues in them. In the following section I discuss some of the major advantages of this kind of rotor machine for the language English and some weaknesses of the machine when considering the design of it.


  • Most of the ciphers which are built using pen and paper alone can be easily broken using the cipher text only cryptanalysis. But this kind of rotor machine gives large number of possible mappings which make it difficult for cipher text only attacks.
  • Frequency analysis of the characters and the brute force type (which tries all possible keys) of analysis is much more difficult since there are large number of mappings with 5 disk rotor machine for English.
  • Because of the constant alternation of the electrical paths, there it produces a very long period before the key sequence or substitution alphabet repeats and it makes it difficult to do cryptanalysis since it is difficult to detect the repetition.


  • It is possible to do cipher text only cryptanalysis by exploiting insecure protocols which explains the message settings. One such incident happened with earlier rotor machine type called enigma and cryptanalysis was done by Polish cryptographers. This kind of attack can use virtual bank of rotor machines, each testing one possible rotor order. This is the step which has most dominating cost in the attack. Then attacker can find the best ring settings for this message key and the assumed rotor order. Finally has to recover the plugboard settings assuming the correctness of the recovered rotor order, ring settings, and the message key settings.
  • In other encryption mechanisms the key is the most important and should be protected from the enemy. But in rotor machines there is an internal wiring structure which is also important aspect when breaking them by cryptographers by deducing the logical structure.
  • In rotor machines such as Enigma, reflector is a fundamental feature which helps to the cryptanalysts. Reflector ensures that no letter could be encrypted as itself. Cribs are any known plaintext or suspected plaintext at some point of the encrypted message. Cryptanalysts can use the reflector factor along with the knowledge of cribs to perform known plain text kind of attacks. With cribs and the fact that no letter could be encrypted as itself, a corresponding cipher text fragment can be tested by trying every possible alignment of the crib against the cipher text. This procedure is known as the crib-dragging.
  • The plugboard connections of the rotor machines are reciprocal. That means if A is plugged in to H, then H is plugged in to A. That means the encryption is performed identically to the decryption. It makes the task of cryptanalysts easy by considerably reducing the number of scrambler settings that need to consider.
  • Operating short comings or the way that the machine is used can be a major factor other than the design characteristics of the machine. Mistakes of the operators are common and this can lead to related key attacks. Operators sometimes set rotors incorrectly. If the operator then corrected the rotor positions and retransmitted the same plaintext, the cryptanalysts would have a single plain text encrypted in two related keys.

No comments:

Post a Comment