A US Navy WWII Encryption Device

[Puster Bill]

Hopefully, this will be of use to you guys who play SHIV as a way to communicate in a more immersive way.

I decided to built a strip version of the US Navy CSP-488 "Wheel Cipher". The CSP-488 was the Navy designation of the US Army M-94 wheel cipher. Any wheel cipher can be turned into a strip cipher quite easily by "unrolling" the wheels, and doubling the mixed alphabets of each wheel on the individual strips.

The CSP-488 wheel cipher was used up until 1942 or 1943, and it was replaced by the more secure CSP-845 strip cipher, which had 100 strips to choose from and 30 positions for enciphering.

I chose to make a strip version of the CSP-488 though, because you only have to worry about setting up and cutting out 25 strips, not 100.

Anyway, here it is:



You can tell the device was first invented for use by the Army. If you look down the strip that is marked 'R17' at the top, you will notice that part of the alphabet reads "ARMYOFTHEUS". Wonder if that got on the nerves of Navy radiomen who had to use it to encipher!

How it works is that you set up the strips in a pre-arranged manner, according to a keylist. In this case, the 25 strips are ordered as follows:

YNXWSZJGFQRDVMTOHUPLEKICB

Once we've set the strips in the proper order, we slide the individual strips up and down until our desired plaintext is visible. Let's use 'SEND LAWYERS GUNS AND MONEY' as our plaintext:



I put the plaintext at the top of the bottom retention strip. Admittedly, it's an unlikely message for a US Navy sub to send, unless of course it's the USS Sea Tiger. I padded it out with two X's to make the message exactly 25 letters long.

You can pull the ciphertext from any line above. Lets say you pick the line 10 down from the bottom of the top retention strip. It reads like this:

DMWQIJIVUKIFABOQDXGKBMIFG

An operator would seperate that into groups, probably 5 letter groups, so it would look like this when sent:

DMWQI JIVUK IFABO QDXGK BMIFG

For a message longer than 25 characters, you would repeat the same procedure for each 25 character 'chunk' of plaintext, but you would generally chose a different line of ciphertext for each 25 character block. It doesn't matter which one you choose, as they will all result in the same plaintext, which 'jumps out' at you because it's the only intelligible result.

Construction notes:

The strips were individually typed into columns in an EXCEL spreadsheet based upon the wheel information found at the USS Pampanito website:
http://www.maritime.org/csp488.htm
They were then printed out on heavy cardstock and seperated using my wife's cutting board, my pocket knife, and a metal yardstick as a guide for the knife blade.

The strip holder is a piece of heavy cardstock with two sets of parallel cuts to hold the strips.

The US Navy CSP-845 strip cipher is vertically oriented instead of horizontally like my device, but I find that the horizontal orientation is more intuitive, and you can use the retaining strips as a guide.

[Puster Bill]

Forgot to mention: I have the EXCEL file all ready to be printed out if anyone wants it. That way you don't have to re-type everything.

[Rockin Robbins]

Whew! Not exactly Enigma, is it? I wonder how many of those "secret" messages were read.

[Puster Bill]

Originally Posted by Rockin Robbins

Whew! Not exactly Enigma, is it? I wonder how many of those "secret" messages were read.

For the M-94/CSP-488, probably quite a number of them. The system isn't really that secure, and it was only used for low level tactical communications. Although, quite frankly, Japanese signals intelligence left a *LOT* to be desired, so maybe not.

Certainly, today, the M-94/CSP-488 isn't secure.

Now, the M-138-A/CSP-845 is a different story. With 100 strips, only 25 or 30 of which would be used at once, it probably gives about as much security as a 4 rotor Naval Enigma. It's telling, though, that it was considered only "medium" security.

I settled on an M-94 variant because the wheel alphabets are published, and there are fewer strips to cut out, which is the most tedious part of making this.

I'm thinking about building a wheel version. I'm going to check around at the local hardware stores and see what I can use as the wheels. If I manage to build a decent one, I'll post pictures and a how-to.

[Puster Bill]

I went back and looked at it, and the key space (ie., the number of possible keys) is much larger for the M-138-A than it is for the Naval Enigma, implying that it would be more secure.

You have to be careful with that number, though, because systems can have weaknesses unrelated to the number of total keys. Simple monoalphabetic substitutions have a total key space of 403,291,461,126,605,635,584,000,000, more than the Naval Enigma at
409,183,233,484,479,582,535,680 possible keys, yet even kids can solve them in a short amount of time.

[razark]

Originally Posted by Rockin Robbins

Whew! Not exactly Enigma, is it? I wonder how many of those "secret" messages were read.

The question isn't how many were read, it's how many were read in time to be effectively used. A system like this could be useful for short, quick messages and carried by an individual on the field. A tabletop system like Enigma can't exactly be slipped into a pocket.

Enigma doesn't exactly have a reputation of being secure, either. The M-94 could just be read quicker.

Razark

[Puster Bill]

Originally Posted by razark

The question isn't how many were read, it's how many were read in time to be effectively used. A system like this could be useful for short, quick messages and carried by an individual on the field. A tabletop system like Enigma can't exactly be slipped into a pocket.

Enigma doesn't exactly have a reputation of being secure, either. The M-94 could just be read quicker.

Razark

That's a large part of it. A system just has to be secure *ENOUGH*, at least from a military standpoint.

Actually, you could use an Enigma today in a very secure manner by simply doubly enciphering like the 'OFFIZIER' messages, but without the tell-tale indicator and without transmitting the wheel settings (they'd have to be part of the pre-agreed upon key settings). That would pretty much stymie any potential attack that I am aware of.

[Puster Bill]

PREHN JXYCZ LHCGX DXKEQ NLYNV
GUNQP KYDDR UWVOC SETYO SJFJJ

For those who haven't built one yet, here is an electronic version:
http://members.aon.at/cipherclerk/VirtualM94.html
Key is the same as the previous message.