Code N13 of the Railway Troops of the NKVD

NKVD insignia The NKVD, or Народный Комиссариат Внутренных Дел (People's Commissariat of Internal Affairs) was the forerunner of the infamous KGB from 1934 through the years of the Second World War.

This is a 5-figure code, used with a complex superencryption system by the NKVD Railway Troops in the last years of World War II. The code book has been almost completely reconstructed, as well as most of the keys used for superencrypting it.

Keys

Below is a scan of a reconstructed key list. The legend, in Swedish reads:
Railway NKVD Code N13.
Signal group [indicator] 95216
Date / Call sign / Frequency / Key
Scan of NKVD Key List
From the above key list, we learn that on November 21 the net used the frequency 3250kHz, their callsigns were OTD and 5MN.

We can also see what superencryption keys to use. There are three different types of sub-keys, named AB, C, and DE, each having an identification number.

The AB-key for November 21 had identification number 5546, and it looks like this in reconstructed format:
Scan of AB-key 5546

The C-key for the above date was number 53 and it looks like this, together with keys 51 through 58:
Scan of C-keys 51 through 58

And, finally the reconstructed DE-key known as number 1, looks like this:
Scan of DE-key 1

Decoding

Let's say we have received the code group 92736. This is divided into AB-group 92, C-group 7, and DE-group 36 (the letters simply indicate where in a group the figures stand, A being the first figure, B the second, and so on).

We use the AB-key to decipher the first two figures. On row 9, column 2 in AB-key 5546, we find the plain AB-group 02.

In the C-key, we look in row 53 for the middle figure of the encrypted code group, i.e. 7. We find 7 together with 1 and 3 (when encrypting, the clerk could choose between 1, 3, or 7; they all have the same meaning when row 53 is used as key), in the column headed by 3, so 3 is the plain C-group.

Finally, we consult DE-key 1 to decipher the last two figures of the encrypted code group. On row 3, column 6, we find the plain DE-group 09 (row 8, column 2 also means 09. There are only 50 possible plain code values for position DE, and 100 possible superencrypted values, and all plain code values are given two different superencrypted representations in the keys I've examined.).

Putting it all together, we now have the plain code group 02309. The first two figures give the main column in the code book, the third gives the sub-column, and the last two give the row. Let's look the plain text up:

(Part of Code Book N13 showing the first four main columns)
Scan of part of Code Book

The word found in main column 02, sub-column 3, row 09 is безопасно, which means secure-ly/ity, safe-ly/ty (the exact meaning depending on inflexion).

Encoding a message

Let's say we want to encrypt Полковник Антонов находится в Москве (Colonel Antonov is in Moscow). We find the first part Полковник in main column 19, sub-column 3, row 10 (not shown in the above scan), which gives the plain code group 19310. The name Antonov will have to be spelled out, and we can do this in a number of ways. One way is to locate the trigram Ант in the code, and it is found in main column 01, sub-column 3, row 22 (see scan), or plain code 01322. The bigram он becomes the plain code group 17123, and ов is found to be 16431.

Continuing in the same way, and using the key sheets shown above, we can encrypt this message in the following way:

Plain text:ПолковникАнт--он--овнаход--итсявМоскв(а)-е
Plain code:19-3-1001-3-2217-1-2316-4-3115-1-2311-4-4703-3-0614-3-0309-3-14
Superencrypted code:20-3-1977-7-8607-2-6231-8-8125-4-6263-0-5559-3-07 14-7-8054-1-43
Final cryptogram:

20319 77786 07262 31881 25462 63055 59307 14780 54143

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© Torbjörn Andersson.Torbjцrn Andersson Fecit