"I cannot read this. I have no training in code breaking."
Major Yasha Ribakov explained the principles of ciphering. A cipher uses a system of fixed
rules (an "algorithm") to transform a legible message ("plaintext") into an apparently random
string of characters ("ciphertext"). For example, a cipher might be defined by the following rule:
"For every letter of plaintext, substitute a two-digit number specifying the plaintext letter's
position in the alphabet plus a constant between 1 and 73 that shall be agreed upon in advance."
If 46 is the agreed-upon constant, then the plaintext word ZAP enciphers to 724762 as follows:
*
*
*
Plaintext letter Z = ciphertext 72 (alphabet position 26 + 46).
Plaintext letter A = ciphertext 47 (alphabet position 1 + 46).
Plaintext letter P = ciphertext 62 (alphabet position 16 + 46).
Incorporation of a variable term into a fixed algorithm, as in this example, is typical of realworld ciphers. The variable component is termed a key. A real key would be longer and would
have a more complex relationship to the cipher algorithm than the key in this example, but its
basic role would be the same: a key fits into an algorithm so as to enable enciphering and
deciphering, just as a physical key fits into a lock to enable locking and unlocking. Without a
key, a cipher algorithm is missing an essential part. In fact, so important is the concept of the key
that in real-world ciphering it is not algorithms that are kept secret, but keys. Cipher designers
assume that their algorithms will always become known to their opponents, but design the
relationship between key and algorithm so that even knowing the algorithm it is almost
impossible to decipher a ciphertext without knowing the appropriate key. Before a cipher can
work, therefore, a key or set of keys must be in the possession of both the sender and the
receiver.
"You break them and you'll be pardoned and made a major in the Red Army."
"Are there any J Army plain language documents?"
"Yes," the colonel said producing a folder.
There were other Russians and Japanese in the project. Jongnay noticed that every Army
message began the same: To His Excellency Commanding Officer and looked for the pattern in
the coded messages. Indeed they were there. He found that the Germans had given the Japanese
the Enigma machine, which consisted of displacing an alphabetical character by another by
means of a series of complicated mathematical transformations. The Japanese could not use the
machine, because their writing system was syllabic, not alphabetical. However, they
1. Big clashes in Mongolia between Soviet and Japanese troops
2. Non-aggression treaty, March, 1941.
Jongnay was sent to the Gulag.
1. Gulag at a Coal Mine
1
The inmates were roused out of bed at 5, given a piece of stale bread, lined up, and
marched. The rickety over the frozen stream seemed on the point of giving way any time.
It was Mine #40. They had just blasted a hole in the side of the hill. It traveled about 50
feet horizontally, then began to drop vertically. A rail track was at the floor, down which fourwheeled cars were pushed. mouth of the coal mine. Daily descent
Jongnay was exhausted and couldn't get up, and fully expected to be starved and dumped
in the grave yard, a pit into which bodies were dumped and covered over with dirt.
The tools were pickaxes and shovels. The inmates dug chunks of coal and shoveled them
onto the cr
2. English
He was awakened from his stupor.
"The warden wants to speak to you," the jailer said, giving him a bowl of beef stew.
Resuscitated but still weak, Jongnay was escorted to the warden's office in the administration
building across the yard from the cell blocks.
"I note here in your record that you read English," Warden Sergey Murzin. "Can you read
this?"
He put in front of Jongnay an ID card, "Andrew Jackson, Captain, British Army, of the
17th Fusiliers."
"Good," Sergey said, asking him to read a sheaf of handwritten paper. It was the man's
diary, reporting daily chores and events, meal times, work quotas, interspersed with his
reflections on his family ad his past. Apparently, Jackson was a communist and became a Soviet
citizen, but mysteriously he had been arrested and sent to prison. Later pages were full of his
disillusionment with the Soviet system and downright hatred.
The warden had Jongnay keep accounts and make reports. He was spared the
backbreaking coal quotas.
The next year, perhaps because of the fudged accounts, NKVD came and arrested Sergey.
He wasn't put in the gulag he had administered but it was certain that he would be sent to a
gulag. Jongnay felt sorry about it, but he had to do what he had been told.
He expected to be sent back into the coal pits but the new warden retained him in the
position of clerk and secretary. This one occasionally had Jongnay translate his Korean poetry
into Russian and had him recite Pushkin.
Codes and Ciphers
Codes and ciphers are forms of cryptography, a term from the Greek kryptos, hidden, and
graphia, writing. Both transform legible messages into series of symbols that are intelligible only
to specific recipients. Codes do so by substituting arbitrary symbols for meanings listed in a
codebook; ciphers do so by performing rule-directed operations directly on original message
2
text. Because codes can only communicate concepts that are listed in their codebooks, they have
limited flexibility. Rather, modern cryptography relies almost entirely on ciphers implemented
by digital computers, and is widely employed in industry, diplomacy, espionage, warfare, and
personal communications.
Codes. A code is a set of symbolic strings ("code groups") that are listed, along with their
assigned meanings, in a code book.
Codes encrypt messages by substitution, that is, they substitute code groups for components of
the original message. "Kill the king at midnight" could thus be encoded, for example, as
"OAKEN 7890 SPINDRIFT." Without the code book, it would be difficult for a reader of the
encoded message to form an idea of its meaning.
Either a word or a number can be used as a code group. Code groups that are words are termed
code words and those that are numbers are termed code numbers. Note that a single code group
can encode a single word ("king") or an entire phrase ("deliver the films to agent number 3"). A
coded message may, therefore, be shorter than the original message. It can also be made as long
as or longer than the original message, if the codebook
The Morse Code, named for inventor Samuel Morse, was patented in this form in 1840. This
code is regarded as one of the great steps forward in international communication.
©BETTMANN/CORBIS.
provides lengthy code phrases for single concepts or nonsense code groups for padding purposes.
Such techniques can be used to make encoded messages harder for opponents to read.
Ciphers. A cipher uses a system of fixed rules (an "algorithm") to transform a legible message
("plaintext") into an apparently random string of characters ("ciphertext"). For example, a cipher
might be defined by the following rule: "For every letter of plaintext, substitute a two-digit
number specifying the plaintext letter's position in the alphabet plus a constant between 1 and 73
that shall be agreed upon in advance." If 46 is the agreed-upon constant, then the plaintext word
ZAP enciphers to 724762 as follows:
*
*
*
Plaintext letter Z = ciphertext 72 (alphabet position 26 + 46).
Plaintext letter A = ciphertext 47 (alphabet position 1 + 46).
Plaintext letter P = ciphertext 62 (alphabet position 16 + 46).
Incorporation of a variable term into a fixed algorithm, as in this example, is typical of realworld ciphers. The variable component is termed a key. A real key would be longer and would
have a more complex relationship to the cipher algorithm than the key in this example, but its
3
basic role would be the same: a key fits into an algorithm so as to enable enciphering and
deciphering, just as a physical key fits into a lock to enable locking and unlocking. Without a
key, a cipher algorithm is missing an essential part. In fact, so important is the concept of the key
that in real-world ciphering it is not algorithms that are kept secret, but keys. Cipher designers
assume that their algorithms will always become known to their opponents, but design the
relationship between key and algorithm so that even knowing the algorithm it is almost
impossible to decipher a cipher text without knowing the appropriate key. Before a cipher can
work, therefore, a key or set of keys must be in the possession of both the sender and the
receiver.
If the key were always the same, it would simply constitute a permanent part of the algorithm,
and keying would have no special advantage over trying to keep one's algorithm secret to begin
with. Keys must, therefore, be changed occasionally. A new key may be employed every day, for
every message, or on some other schedule.
Comparison of codes and ciphers. Codes have the advantage of simplicity. No calculations are
required to encode or decode messages, only lookups in a codebook. Further, because a code
uses no fixed system for associating code groups with their meanings (even the amount of
meaning assigned to a code word can vary, as seen above), a code may fail gracefully—that is,
an enemy may discern the meaning of a few code groups but still be unable to interpret others. In
contrast, a cipher produces ciphertext from plaintext (and vice versa) according to a fixed
algorithm. Thus, if an enemy determines the algorithm and steals or guesses a key, they can at
once interpret all messages sent using that key. Changing the key may restore cipher security,
unless the enemy has developed a system for guessing keys. One such system, always possible in
theory, is to try all possible keys until one is found that works.
Codes, however, have two great disadvantages. Users can only send messages that can be
expressed using the terms defined in the codebook, whereas ciphers can transmit all possible
messages. Additionally, all codes are vulnerable to codebook capture. If a codebook is captured,
there is no recourse but to distribute new codebooks to all users. In contrast, the key–algorithm
concept makes cipher secrecy dependent on small units of information (keys) that can be easily
altered.
Secure ciphers, however, entail complex calculations. This made the use of complex ciphers
impractical before
A 1968 miniature Kroger's codebook containing a series of numbers that was used by spies to
decode messages from Moscow, displayed beside an enlarged photocopy of the text.
©HULTON-DEUTSCH COLLECTION/CORBIS.
4
the invention of ciphering machines in the early twentieth century; codes and simple ciphers
were the only feasible methods of ciphering. Yet, a cipher that is simple to implement is
proportionately simple to crack, and a cracked cipher can be disastrous. It is better to have to
communicate "in the clear"—to send messages that can be easily read by the enemy—than to
suppose that one's communications are secret when they are not. Mary, Queen of Scots (1542–
1567) was executed for treason on the basis of deciphered letters that frankly discussed plans for
murdering Queen Elizabeth of England; likewise, simple ciphers used by the Confederacy during
the U.S. Civil War were easily cracked by Union cryptographers. What is more, even more
sophisticated ciphers, such as the Enigma cipher used by Nazi Germany during World War II or
implemented today on digital computers, are subject to attack. As soon as any new cipher is
invented, someone, somewhere starts attacking it. The result is that ciphers, like some antibiotics,
have limited lifespans, and must be regularly replaced.
Historical perspective. Throughout much of the ancient world, writing was either completely
unknown or was an arcane art accessible only to priests. There was little motive, therefore, to
develop coding or ciphering. Eventually, however, writing came to serve military, personal, and
commercial as well as sacred purposes, creating a need for secure communications. To meet this
need, ciphers based on scrambling the order of plaintext characters or on substituting other
characters for them were developed. The first recorded use of ciphering was by the Greek
general Lysander in the fifth century B.C. The Kamasutra, a Hindu text compiled in the A.D.
fourth century from manuscripts dating back as far as the fourth century B.C., recommends
monoalphabetic substitution ciphering—the replacement of each letter of a plaintext message
with a different letter of the alphabet—as one of the 64 arts to be mastered by an ideallyeducated woman. By the first century B.C., codes had also been developed.
Cryptography fell out of use during the early Middle Ages, but Arab scholars during the heyday
of medieval Muslim civilization, the Abbasid caliphate (A.D. 750–1258), revived it. Muslim
writers not only ciphered, but invented cryptanalysis, the systematic breaking of ciphers. Ninthcentury Arab philosopher Abu Yusuf al-Kindi wrote the earliest known description of the
cryptanalytic technique known as frequency analysis, which breaks substitution ciphers by
matching ciphertext letters with plaintext letters according to their frequency of use in the
language. In English, for example, the most frequently used letter is E; in an English-language
ciphertext produced using a monoalphabetic substitution cipher, therefore, the most frequently
used character probably stands for E.
During the late Middle Ages and the Renaissance, a literate ruling class arose throughout Europe,
and ciphering regained importance in that part of the world for purposes of intrigue, espionage,
and war. English monk and scientist Roger Bacon (1220–1292) wrote a book describing several
cryptographic methods; Italian artist Leon Battista Alberti (1404–1472) wrote the first European
text on cryptanalysis in 1466. Under pressure from cryptanalysis, codes and cipher systems
gradually became more complex.
Beginning in the mid-nineteenth century, the importance of coding and ciphering was rapidly
amplified by the invention of electronic information technologies: the telegraph (1837), the
telephone (1876), radio (1895), and electronic computers (1940s). Non-secret commercial codes
were developed in conjunction with telegraphy to make messages more compact (therefore
5
cheaper); ciphers were widely used (and cracked) during the U.S. Civil War and the first and
second world wars. The cracking of German and Japanese ciphers by Allied cryptographers
during World War II was of particular importance, enabling the British and Americans to avoid
submarines, intercept ships and aircraft, and otherwise frustrate enemy plans. Ciphering has
since become basic to military and government communications. Since the 1960s, commercial
and personal communications have become increasingly dependent on digital computers, making
sophisticated ciphering a practical option for those sectors as well. In the late 1970s, the U.S.
government defined a cipher algorithm for standard use by all government departments,
available also to the public; this now-elderly algorithm, the Digital Encryption Standard, is today
in the process of being replaced by a new algorithm, the Advanced Encryption Standard.
Types of codes. Codes can be generally divided into one-part and two-part codes. In a one-part
code, the same codebook is used for encipherment and decipherment. The problem with this
system is that some systematic ordering of the code groups and their assigned meanings must be
made, or it will be difficult to locate code groups when enciphering or their meanings when
deciphering. (A randomly ordered list of words or numbers thousands of terms long is difficult to
search except by computer.) Thus, code groups tend to be arranged in alphabetic or numerical
order in a one-part code, an undesirable property, since an opponent seeking to crack the code
can exploit the fact that code groups that are numerically or alphabetically close probably encode
words or phrases that are alphabetically close. To avoid this weakness, a two-part code employs
one codebook for encipherment and another for decipherment. In the encipherment codebook,
alphabetically ordered meanings (e.g., A, ABDICATE, ABLE) are assigned randomly ordered
code groups (e.g., 6897, 1304, 0045). In the decipherment codebook, the code groups are
arranged in order (e.g., 0045, 1304, 6897), for easy location.
Code security can be improved by combining ciphering with coding. In this technique, messages
are first encoded and then enciphered; at the receiving end, they are first deciphered and then
decoded. A standard method for combining coding and ciphering is the "code plus additive"
technique, which employs numbers as code groups and adds a pseudorandom number to each
code group to produce a disguised code group. The pseudorandom numbers used for this purpose
are generated by modulo-arithmetic techniques closely related to those used in stream ciphering.
Block ciphers. Ciphers that encrypt whole blocks of characters at once—such as 10 letters at a
time, or 128 bits—are termed block ciphers. Block ciphers have the advantage that each
character in each ciphertext block can be made to depend complexly on all characters of the
corresponding message block, thus scrambling or smearing out the message content over many
characters of ciphertext. The widely used Digital Encryption Standard (DES) is a block cipher
that employs a 56-bit key to encrypt 56-bit blocks. In DES, the key and each message block are
used as inputs to a complex algorithm that produces a 56-bit block of ciphertext. The same key is
used to decode the block of ciphertext at the receiving end.
Stream ciphers. Stream ciphers operate upon series of binary digits ("bits," usually symbolized
as 1s and 0s), enciphering them one by one rather than in blocks of fixed length. In stream
encipherment, a series of bits termed the key-stream is made available by some means to both
the sender and receiver. This stream is as long as the message to be sent. At the sending end, the
key-stream is combined with the message-stream in a bit-by-bit fashion using the exclusive or
6
operation of Boolean algebra, producing the ciphertext. At the receiving end, the same keystream is combined again with the ciphertext to recover the message stream. This system of
ciphering is unbreakable in both theory and practice if the key-stream remains secret. Ongoing
breakthroughs in quantum cryptography may soon make perfectly secret key-streams available
by exploiting certain properties of photons. If these techniques can be made technologically
practical, truly unbreakable cipher systems will have become available for the first time in
history.
Public-key ciphers. All ciphers require the use of a secret key. Public-key ciphers, first
developed in the late 1970s, are no exception. However, public-key ciphers have the important
advantage that the secret key possessed by the sender need not be the same secret key possessed
by the receiver; thus, no secure transfer of keys between the sender and receiver is ever
necessary.
Public-key ciphers exploit the computational difficulty of discovering the prime factors of large
numbers. (The prime factors of a number are the primes that, when multiplied together, produce
the number: e.g., the prime factors of 15 are 5 and 3.) To create a public key, two large (50-digit
or longer) primes are chosen and their product calculated. This number (r) is made public.
Further mathematical operations by the user produce two numbers based on r; one of these is the
user's public key kp, and the other is retained as the user's private key ks. Anyone that knows r
and a given user's public key kp can send encrypted messages to that particular user; the recipient
decrypts the message using their private key ks.
Public-key cryptography has seen wide use since the 1970s. Its security is limited by the ability
of opponents to determine the prime factors of r, and the difficulty of this task is a function both
of the size of r and of the speed of available digital computers. (Large r also makes encryption
and decryption more computation-intensive, so it is not practical to defeat opponents by simply
making r extremely large.)
Software for a powerful public-key cipher algorithm known as Pretty Good Privacy (PGP) is
downloadable for free from many sites on the Internet.
Attacking codes and ciphers. Codes and ciphers can be attacked by two basic means. The first
is theft of codebooks or keys—espionage. The second is cryptanalysis, which is any attempt to
crack a code or cipher without direct access to keys or codebooks. Cryptanalysis may proceed
either by trial and error or by systematic analysis of plaintext and ciphertext. The analytic
approach may involve both looking for patterns in ciphertext and solving mathematical equations
representing the encryption algorithm.
Cryptanalysis by trial and error usually means guessing cipher keys. A cipher key can be guessed
by trying all possible keys using a computer. However, designers of encryption systems are
aware of this threat, and are constantly employing larger and larger keys to keep ahead of
growing computer speed. Systematic cryptanalysis may seek patterns in ciphertext, either by
itself or in conjunction with a known plaintext (the so-called "known-plaintext attack").
Mathematical modeling of cipher algorithms may assist trial-and-error methods by reducing the
number of guesses required to within (or near) practical limits. For example, in 2002,
7
cryptographers announced that the recently-standardized Advanced Encryption Standard of the
U.S. government might be vulnerable to a mathematical attack that would reduce the number of
computations needed for a successful trial-and-error attack from order 2256 to order 2100. The
latter number is still not computationally practical, but may be soon.
Quantum cryptography holds out the promise of truly attack-proof ciphering. In a quantumcryptographic system, not only would messages be undecipherable if intercepted, but also the act
of interception would always be detectable by the intended receiver. Such systems may become
available to military and government users around 2010.
8