Code and ciphers: Julius Caesar, the Enigma and the internet

values are repeating at the appropriate interval; something like 150 key
values would be sufficient. In addition, the full size Hagelin has features
which make the task of solving it significantly harder, as has already been
indicated, and as we now see.
Additional features of the Hagelin machine
The Hagelin machine that has been described and analysed so far is the
most basic type and we have ignored the fact that two of the possible key
values, 0 and 1, are ambiguous and might really be 26 and 27 respectively.
This means that if the stretch of recovered key contains any values which are
0 or 1, the cryptanalyst will have to consider 26 and 27 as alternatives when
differencing. This could involve examining many alternative versions of
the key stream; a failure of the differencing attack would indicate that one
or more of the ambiguous values has been wrongly identified. In compensation
the cryptanalyst does get some reward for correctly identifying a key of
0 or a key of 27 since the former implies that all six wheels are inactive at
that point and the latter implies that they are all active; furthermore, a
genuine key value of 1 implies that there is a wheel with a kick of 1, as does a
key of 26 (we are assuming that all 27 bars are used, with no ‘overlapping’).
On balance, though, the task is made harder by these ambiguities.
On top of this complication, which applies to all models of the
Hagelin, there are two additional features on the majority of models that
add significantly to its security:
(1) the ‘slide’;
(2) ‘overlapping’.
The slide
On Hagelin machines possessing this feature there is a small wheel on the
outside which has the alphabet around its rim. This can be turned to any
of the 26 positions and remains in that position whilst the message is
being enciphered, or deciphered. The effect of the slide is to increase the
key by a constant, so if the slide is E the key values are all increased by 4. In
general the encipherment rule changes from
to
cipher letter�key�(plain letter) (mod 26)
cipher letter�(key�slide)�(plain letter) (mod 26),
The Hagelin cipher machine 147