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M-105   AGAT
One-Time tape cipher machine - wanted item

The M-105 (codename: AGAT) was a Russian off-line cipher machine that was developed and built in the USSR in the late 1960s, as the successor to the nearly identical M-104 (AMETYST and AMETYST-2). The machines were used by all countries of the Warsaw Pact and contained a built-in key tape mixer that was fed with an 11-level punched paper tape with random data. In East-Germany (DDR), the M-104 was introduced in 1966 and the M-105 was first used in 1968 [1]. The machine is also known as Agat (Russian: АГАТ) or Achat and is a One-Time Tape cipher.
 
At first sight, the machine somewhat resembles the M-125 Fialka. It has nearly the same size, is painted in grey hammerite, is driven by an electric motor and has a latin/cyrillic keyboard at the front. But that is where the similarity ends.

The M-105 prints it output directly to a sheet of paper that runs through a carriage at the rear. A sophisticated matrix-type print head allows any of 30 characters (5 x 6) to be printed on the paper sheet. Depending on the language, some hardly used characters or numbers may have been omitted. The Polish variant is shown here.
  
M-105 AGAT with 11-level key tape and normal 5-level tape loaded

The M-105 has a built-in 5-level paper tape puncher at the right rear. It accepts standard 5-level punched paper tape, but does not use the standard Baudot encoding. Instead, it uses the same coding scheme as Fialka. Just like Fialka, the M-105 has a built-in 5-level paper tape reader at the front right. The paper can be guided through a ruler that runs along the full width of the machine, just above the keyboard. It is started and stopped with the two keys at the front right.

In East-Germany (DDR), the M-105 was introduced in 1968 and was used for communication with all partners of the Warsaw Pact. It was also used by the DDR Government for exchanging messages at the highest level. The M-105 was still in use with the East-German Intelligence Service MfS in 1982 for communication with their foreign radio monitoring stations [1].

The M-105 was succeeded by the fully electronic M-205 D in 1986 but many M-105 machines remained in use until the end of the DDR. For communication between the DDR Government and Hungary, the M-105 was replaced by the T-353 (Dudek) in 1987.
 
M-105 AGAT with 11-level key tape and normal 5-level tape loaded Print head seen from the front of the machine Feeding blank paper tape into the tape puncher at the right 5-level paper tape reader Starting the 5-level paper tape reader Selecting the mode of operation Close-up of the 11-level key tape reader The full keyboard

 
Keyboard
The image below shows the layout of the full keyboard. Each key cap has two letters: A cyrillic one in black at the left, and a latin one in red at the right. There are 30 normal keys, plus a black single-step key at the bottom left and a black CR/LF key at the bottom right. There is also a spacebar (at the bottom) which is shared with the Й2 key at the top left.

When entering text in cipher mode, the spacebar can be used and the Й2 key is blocked (i.e. covered by a metal plate). In decipher mode it is the other way around: the spacebar is blocked and the Й2 key is available. When the machine is used in plain-text mode (MODE selector set to 'O'), both the spacebar and the Й2 key can be used. The two black keys (single step and CR/LF) are not enciphered. They are only used in plain-text mode for formatting of the output.


Loosing the Russian letter Й is not much of a problem, as it can be replaced by the И without affecting readibility. Like with Fialka, only the 30 most frequently used Cyrillic letters are present, whilst 3 have been dropped. Although they are commonly used, this does not affect readibility. Judging from the layout [1] the keyboard shown here suggest this is probably the Polish variant of the machine. The following Latin characters are available in this version:

ABCDEFGHIJKLMNOPQRSTUVWXYZ 2578

As not all numbers are present, they had to be spelled out in full. When used in Cyrillic mode (Russian), numbers always had to be spelled out in full. There are no punctuation marks, except for the SPACE character, in which is swapped with the Й key at the top left. Switching between Latin and Cyrillic (Russian), involved swapping of the print head.
 
Key tape reader
Unlike Fialka, which uses 10 configurable electrical cipher wheels for creating a hard-to-predict pseudo-random key stream, the M-105 has a built-in mechanical key stream generator that is programmed with an 11-level key tape. The 11-level key tape is located on top of the machine.
 
The key tape itself has two rows of sprocket holes: one along the top of the tape and one just below the centre. Below the center sprocket hole are 5 data holes. the Remaining 6 data holes are located in between the two sprocket holes.

The key tape is fed into the reader from the right, like indicated in the image on the right. A knurled knob at the front of the reader allows the tape to be forwarded to the desired start position. To ensure that a key tape could only be used once for encryption, a Control Hole is punched when changing the mode of operation.
  
Close-up of the 11-level key tape reader

Once this Control Hole is present, the key tape can no longer be used for encryption. Decryption is still possible however, allowing a previously encrypted text to be tested [1]. Key tapes that were intended for decryption only, already had the Check Hole present when they were supplied, to prevent accidental use as encryption key.
 
Keyboard layout (note: the top left key is covered here) Single step character key Carriage Return (CR) and Line Feed (LF) with a single key 11-level key tape reader Closing the 110-level key tape reader Placing a key tape in the 11-level key tape reader Output selector: puncher, printer or both. At the right is the motor enable lever. Output selector: puncher, printer or both


Example of a key tape

 
Key material
The following key material was available for the M-105:
 
  • ТИ : Individual
    This key material was used for one-on-one communication. It consisted of two tapes: one for encryption and one for decryption. This material was supplied in a package that contained a table with 20 Indicator Groups (German: Kenngruppen) and 20 key tapes. The Indicator informed the operator at the other end about the key tape that was used.

  • ТЦ : Group
    For sending group messages (also known as NET), three or more identical key tapes were used. Although this was less secure (i.e. it didn't stricktly follow the rules of the One-Time Pad), it allowed net messages to be sent to a number of recipients. The key material was supplied in a Code Book that contained a table with Indicators and 25 key tapes, or a Cassette with an Indicator table and 75 key tapes.

  • Т-КПУ-М : Test
    This material was intended for using the Test and Security Feature of the M-105 (German: KSV, Kontroll- und Sicherungs-vorrichtung). It consisted of a Code Book with 25 test key tapes. As a security measure, the M-105 was able to test whether a key tape had previously been used for encryption. This was done by punching an extra hole in the key tape when switching from Cipher Mode (З) to Plain Text Mode (O). Once this hole was present, the tape could no longer be used for encryption. In decryption mode (Р), the tape could still be used for verification, as the M-105 does not test for the hole in that mode.

Each key tape consisted of 500 groups of 5 letters each and was identified by a tape number that was printed at the start of the tape. A standard code book consisted of a table with 20 Indicators and 20 key tapes. The following rules had to be followed:
 
  • Each key tape could only be used for encryption once.
  • For each key tape, the matching Indicator had to be used.
  • Each Indicator could only be used once and had to be ereased after use.
  • A key tape had to be used immediately after it was taken from the code book.
  • Unusued key tapes had to be accounted for.
5-level paper tape
The M-105 accepts a standard 5-level punched paper tape, just like many western telegraphic devices of the era, but does not use the standard Baudot encoding for this. Instead, the machine uses the same encoding as the M-125 Fialka. The 5-bit alphabet is illustrated in this diagram:


With 5 holes it is possible to create 25 = 32 different combinations. 30 of these are used for the standard 30 characters that are available on the keyboard. The two ramaining positions are taken by the SPACE character (10001) and the NULL character (00000), which is also known as STOP.
 
MODE selector
At the front left of the machine, just behind the keyboard is a black rotary knob that is used to select the required mode of operation. There are three possible settings: 'O' for plain-text (Открытый Текст), '3' for ciphering (ЗашифроватЬ) and 'P' for deciphering (РасшифровыватЬ).
 
In plain-text mode, the M-105 functions like a normal typewriter or teleprinter. It can be used to print text and create 5-level paper tape. In this mode, all keys can be used, including the space bar, the single-step and the CR/LF key.

Setting the MODE selector to either cipher mode (3) or decipher mode (P) results in a number of mechanical changes inside the machine. In cipher mode, the Й2 key is blocked and the spacebar is available. In decipher mode, this operation is reversed: the spacebar is blocked and the Й2 key can be used in the cipher text.
  
Mode selector (clear, encrypt, decrypt)

In many ways the operation of the MODE selector is similar to the MODE selector on other Russian cipher machines such as the M-125 Fialka or the M-130. The abbreviations used for the various modes are identical on all machines, as shown in the table below.
 
At the front of the 11-level key tape reader is a lever with three possible settings: П, ПК and К. It is used to select the destination of the encoded, decoded or plain-text output: to the puncher, to the printer or to both the puncher and printer.

The image on the right shows the front left of the machine. At the left is the key tape reader. The output selection lever is mounted to the front of the key tape reader, to the right of the kurdled knob. At the right is another lever which is used to release the motor (МОТОР / РУЧН.) in case of manual operation with the crank.
  
Output selector: puncher, printer or both. At the right is the motor enable lever.

 
Label Russian Phonetic English
О Открытый Текст Otkrytyj Tekst Plain text
З ЗашифроватЬ Zashifrovat Cipher
Р РасшифровыватЬ Rasshifrovyvat' Decipher
П Перфоратор Perforator Puncher
К Карта Karta Sheet
МОТОР МОТОР Motor Motor
РУЧН. РУЧНОЙ Ruchnoy Manual

 
Power Supply Unit
There were several different ways to power the M-105, resulting in different versions of the machine. The most common version has a built-in 24V DC motor that can be powered by an external 24V source. Especially for connection to a 110V DC network, a special version with a 110V DC motor was available. Existing 24V DC models could be modified for 110V DC.
 
In the field, the 24V model could be powered directly by the battery of, say, a truck. In most cases however, the unit was powered from the AC mains via the standard Power Supply Unit (PSU) that was supplied with each machine.

The image on the right shows the standard PSU that was used with the M-105. It is suitable for a wide variety of mains voltages (typically in the 110V or 220V range), so that it could be used virtually anywhere in the world. The same PSU was used with the M-125 Fialka cipher machine in most Warsaw Pact countries, such as the DDR.
  
Close-up of the front panel with the two main cables connected

The PSU came with several cables for connecting the M-105 to the AC mains or to an externa 24V DC source. These cables were usually stored inside a small storage compartment at the front of the PSU. The hinged lid of this compartment is held in place with two kurdled screws. More...
 
The M-105 is fitted with a fixed power cable that has a rather strange 3-pin plug at the end. It directly fits the 24V output socket of the PSU.

The M-105 could be converted for use on 110V DC networks, by swapping the existing 24V motor for a 110V one. Suitable conversion kits were available for this at the time. The 110V motor was connected to a spare socket (marked 110V) at the right right inside the machine.

 More about the PSU
  
Internal 24V connection

 
Internal 24V connection The standard PSU with the top lid closed The open cable compartment Standard PSU for M-125 (Fialka) and M-105 (Agat) Standard PSU with AC mains cable and 24V DC cable The five cables in front of the PSU Standard PSU The 3-pin male plug for the 24V output connection

 
Interior
The top cover of the machine can be removed by loosening a couple of screws in the outer case. This reveals the extremely complex but well-built interior of the M-105. The image below shows a compact mechanism with many cog-wheels at the center, driven by a motor at the right.
 
The printer mechanism and the paper carriage are barely visible at the top of the image. At the left is the 11-level key tape reader, whilst the 5-level tape reader is located at the bottom right.

The 30 keys from the keyboard are mechanically converted into 5-level code which is converted to electrical signals by means of 5 switches at the left rear. The 5-level paper tape reader operates in parallel with the keyboard; when a tape is played back, the keys automatically move up and down. The machine has a hole at the right through which a crank can be inserted.
  
Use the hand crank to operate the machine

The crank has several functions. It allows the machine to be operated manually in case of a power failure. As the encryption process is purely mechanical, the machine can read 5-level tapes and key tapes, and can print to paper without any electricity. The crank was also used to free the mechanism in case it got jammed (e.g. when a key was pressed whilst the motor wasn't running).

After encryption or decryption, the resulting 11 bits are (mechanically) converted back to 5-bit binary data, so that an ordinary telegraphy paper tape can be punched. The 5-bit data is also converted to electrical signals by means of 5 sensing switches at the left rear. These signals are available on the socket at the left of the machine (for which electricity is needed, or course).
 
The complex mechanical construction Internal 24V connection Electrical 5-bit encoder 11-level key tape reader Expansion connector at the left rear Printer mechanism Use the hand crank to operate the machine Right view of the interior

 
Operation
The M-105 internally uses a 5 x 6 matrix to address its 30 characters. This is done by translating each 5-bit character into a a row (6 bits) and column (5 bits). This result in 11 internal 'bits' that are used to drive the mechanism. For this reason, the key tape has 11 bits as well. If you closely examine the key tape, only one of the row/column bits are used in each step.


The block diagram above should explain the operation of the M-105 AGAT. At the left is the keyboard with its 30 keys that are mechanically converted to a 5-bit binary code. It operates in parallel with the key tape reader. The 5 data bits are then mechanically converted to an 11-bit row/column code (5 bits for the column and 6 bits for the row), which is fed into the cipher unit. In the example below, the letter 'V' generates the column/row bit pattern 01000 000010.

This is just an example that does not represent the actual layout

The 11-bit code from the key tape is then added to the 11-bit data stream and the result is converted back into 5-bit digital code. This 5-bit code is then used to drive the 5-level paper tape puncher. The 5-bit code is also electrically available from a socket at the left rear.

This is an example of how adding might have worked

At the moment it is not exactly known how the data from the key tape is added to the data stream, but it seems likely that the column/row codes from the key tape are used to shift the column/row codes from the data stream by a number of positions. When deciphering, the shift-direction could be reversed, or special deciphering key tapes had to be supplied. The above diagram shows how it might have worked, but it is not guaranteed that his is correct. It makes the M-105 effectively a One-Time Pad, or more precisely: a One-Time Tape machine (mixer).
 
References
  1. Jörg Drobick, Die M-105 N AGAT
    Website: Der SAS- und Chiffrierdienst (SCD). Retrieved May 2014.

  2. Anonymous source, M-105 AGAT cipher machine
    Owner of the M-105 featured here. Retrieved July 2013.

  3. Ministerium fur Staatssicherheit (MfS), GVS-XI/931/70
    Gebrauchsanweisung zum Verfahren AGAT. AGAT User Manual (German).
    BStU, MfS. Abt XI BV Neubrandenburg, Nr. 000192. BStU, MfS, BdL Nr. 000922. 1

  1. Via website Der SAS- und Chiffrierdienst (SCD) [1]

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