J1 (cipher)

J1 (Nisan)

A generic contracting unbalanced Feistel network, similar to J1.
General
Designers	Unit 28, Air Force Intelligence Directorate
First published	1974 (issued) 1998 (declassified)
Cipher detail
Key sizes	242 bits
Block sizes	36 bits
Structure	Unbalanced Feistel network
Rounds	127
Best public cryptanalysis
Correlation attack using 245 ciphertext characters against several weak long-term keys. As of 2018, no feasible attack has been found against long-term keys used operationally.

The J1 cipher, also known by its code name of Nisan (Arabic: نيسان, English: April), is a symmetric key block cipher with a key length of 242 bits and a block size of 36 bits. It was developed by Unit 28 (Cryptography) of the Riysian Air Force Intelligence Directorate (Mukhabarat) as a cipher for the encryption of military, government, and diplomatic teletype and later data networks. The algorithm was issued in 1974, but due to its sensitive nature, it was only declassified in 1998, after the hardware using it had largely been retired. It is notable for being the first Riysian block cipher designed for digital encryption machines, and for using a structure similar to an unbalanced Feistel network, which was considered highly complex in its time.

Though it has been superseded by more modern algorithms, the J1 continues to make a substantial impact on Riysian cryptography. Lessons learned from its development and cryptanalysis continue to be utilized in the development of cipher algorithms, as well as some of its underlying concepts. In particular, the concept of the unbalanced Feistel cipher remains very popular, with even the recent J3 algorithm using a modified Feistel network.

Description

A J1 long-term key.

J1 uses a block size of 36 bits, with a key size of 242 bits and an initialization vector (IV) of 61 bits randomly generated from an LSFR. The internal structure is similar to an unbalanced Feistel network, with four branches and 127 rounds. Its key schedule is very simple, forming two subkeys each 121 bits in length by splitting the main key in half, with a periodic repetition every 121 rounds. Like most ciphers of its age, it is implemented in specific hardware.

An unusual feature of the J1 cipher is that it accepts two keys, a short-term key and a plug-in long-term key. The short-term key is the nominal secret key for encryption, while the long-term key is a printed circuit board with unique wiring per series. This long-term key performs a function similar to both the S-boxes and P-box of modern block ciphers, accepting 9 individual inputs - each similar to an S-box - of 4 bits each, and then performing a permutation as in a P-box. The implementation of the long-term key is similar to the concept of a tweak. The long term key has an effective size of anywhere from 83 to 94 bits, depending on the key's wiring.

Though designed as a block cipher, J1 was almost always used in a stream cipher mode. When used as a stream cipher, the block cipher is run through its full 127 cycles, with a function on the long-term key circuit board extracting a single bit. To encrypt a single 5-bit Baudot character, 13 bits are required, for a total of 1,651 cycles; 3 bits of these 13 are randomly chosen to be dropped, with the remaining 10 bits used to doubly-encrypt the character in a 5+5 scheme. As the IV is needed in the decryption process, the 61-bit IV would be transmitted in the clear as part of a 125-bit/25-character "synchronization sequence".

Cryptanalysis

Despite its age, J1 remains a substantially strong algorithm, especially when compared to some of its contemporaries such as DES. This is largely thanks both to the large number of rounds used and the extremely low number of bits generated during encryption, as well as the use of a long-term key. The long key length of 242 bits also helps to increases security. Analysis of some of the publicly available long-term keys indicate that the bits were specifically chosen with the goal of defending against basic differential and linear cryptanalysis, nearly 20 years before the techniques were publicly discovered, but around the same time that major intelligence agencies around the world became aware of it.

However, the security of the J1 cipher is heavily dependent on the long-term key; overall weak long-term keys can open up the cipher to a variety of attacks. Furthermore, the simple, periodic key schedule allows for some strong correlation attacks against the cipher despite the presence of a randomly generated IV. To protect against these potential weaknesses, a J1 long-term key must historically meet 40 different design requirements. Most - but not all - keys delivered to military and diplomatic systems met these requirements, but the stringent requirements significantly increased the difficulty and cost of manufacturing them.

Currently, the best attack against J1 is a correlation attack published in January 2018, which uses 2⁴⁵ ciphertext characters to break a number of weak long-term keys. Long-term keys that were known to have been used operationally were found to be strongly resistant to this attack. Due to this difference, J1 is not yet considered broken.

Technology/Hardware

An AT-262A with an AK-120 teletype/printer.

The J1 was created specifically for the AT-262 family of encryption machines, the first Riysian digital electronic - as opposed to analog or electromechanical - cipher machines. These were highly successful, being used throughout the Armed Forces and government ministries, as well as for diplomatic communication with embassies.

AT-262A

The AT-262A is the first and main member of the AT-262 family. It is an cipher machine used for both online and offline point-to-point communications by means of teletype, designed to replace old electromechanical cipher machines in that role. Combined with its control panel it weighs 45 kilograms, is capable of both asynchronous and synchronous transmission, with transmission speeds of either 50 or 100 baud respectively, and can operate in duplex or half-duplex mode. The short-term key takes the form of a punched card, which is inserted into a card reader in the machine and is changed regularly, while the long-term key is inserted into the machine as a plug-in board and is only changed when necessary. It was capable of operating in an "quiet" mode, where it would automatically go online upon receipt of an encrypted telex, and decipher and print it out.

It was generally used with the AK-120 semi-electronic teletype/teleprinter machine, which featured an electronic keyboard and encoding, but an electromechanical matrix-type printer and opto-electronic tape reader. It could be operated with all components - keyboard, tape reader, and printer - together, as a standalone printer, or with only the keyboard or tape reader. Weight with all components together is 40 kilograms, and speed rate is adjustable to 50, 75, or 100 baud. Both the AK-120 and AT-262A were "caged" by metal casings in the field, and rated as not having enough EM emissions to warrant operating restrictions.

This was by far the most popular model, with just under 4,000 being produced in total from 1974 to 1982 at a peak rate of 50 machines a month. AT-262As were used for internal government communication, for communication with foreign embassies and consulates, Ministry of Interior communications, and on the operational level upwards in the Armed Forces. These machines remained in service until the early 1990s, being replaced by modern systems capable of handling higher data rates, but some remained in diplomatic and administrative service until the mid 2000s.

AT-262B

The AT-262B was the first attempt to improve on the AT-262 design, and is the second member of the AT-262 family. Faced with ever increasing data usage, this version was designed to be able to encode/decode point-to-point data signals in ITA5/ASCII 8-bit code, in addition to the 5-bit Baudot code teleprinter messages from the original AT-262A. In order to keep costs down, it was designed to have minimal changes compared to the AT-262A, which would also allow for the older model to be upgraded rather than merely replaced. As the AT-262 could only recognize 5-bit code, ITA5 signals had to be converted to ITA2 Baudot before coding, encrypted as 5-bit Baudot, and then re-converted back to 8-bit code, which constrained the maximum data rate of the AT-262B to 100 baud as with teletype signals.

Only 70 machines were built in 1983, and a further 80 were upgraded from 1983-1984. This model didn't achieve the popularity as its predecessor, as dedicated data encryption units with better performance than the AT-262B were already entering widespread military use by 1984. Most machines went to the Navy and Air Force, with all being used in the military.

AT-262T

The final attempt