One-time pad |
In is combined with a random pad the same length as the plaintext. The pad part of the name comes from early implementations of the key material as a pad of gummed paper (for easy concealment, the pad was often physically very small, e.g. [http://www.ranum.com/security/computer_security/papers/otp-faq]).
The one-time pad is also sometimes known as the Vernam cipher, after Gilbert Vernam. Vernam s system was a cipher that combined a message with a key read from paper tape. In its original form, Vernam s system was not theoretically unbreakable — this came only later when Joseph Mauborgne recognized that the key tape needs to be completely random. Because of this, the term Vernam cipher is also sometimes used to describe any scheme where the plaintext is combined with the key symbol by symbol; that is, an additive stream cipher, even if it is not theoretically unbreakable.
probability of a plaintext message M given the corresponding ciphertext.
Despite the strong proof of security, the one-time pad has drawbacks in practice: it requires perfectly random one-time pads; secure generation and exchange of the one-time pad material, which must be at least as long as the message; and careful treatment to make sure that it is disposed of correctly and never reused — hence one time . These implementation difficulties have led to examples of one-time pad systems being broken (for example, VENONA), and are so serious that they have prevented the one-time pad from being adopted as a widespread tool in information security. In addition, the one-time pad gives no guarantees about the integrity of a message.
= Principle =
Each character in the message is combined with one from the (random, secret, and used only once) pad in the manner of a Vernam cipher. So the pad must be at least the length of the message. Theoretically there is no way to decipher the message without knowing the contents of the pad. For this reason it is very important that the pad be protected (ie, secret), random (ie, unpredictable by anyone), and used only once, lest the cipher be easily compromised.
=History=
==Technical development==
The history of the one time pad is marked by four separate but closely related discoveries.
The first one time pad system was electrical. In 1917, Gilbert Vernam (of AT&T) invented and later patented () a cipher based on teletype machine technology. Each character in a message was combined with a character on a paper tape key. Captain Joseph Mauborgne (then a Captain in the United States Army and later chief of the Signal Corps) recognized that the character on the key tape could be completely random. Together they invented the first one time tape system.
The second development was the paper pad system. Diplomats had long used codes for confidentiality and to minimize telegraph costs. Words and phrases were converted to groups of numbers (typically 4 or 5 digits) using a dictionary-like codebook. For added security, secret numbers could be added to each code group before transmission, with the secret numbers being changed periodically. In the early 1920s, three German cryptographers, Werner Kunze, Rudolf Schauffler and Erich Langlotz, who were involved in breaking such systems, realized that they could never be broken if a separate additive number was used for every code group. They had duplicate paper pads printed up with lines of random number groups. Each page had a serial number and eight lines. Each line had six 5-digit numbers. A page would be used as a work sheet to encode a message and then destroyed. The serial number of the page would be sent with the encoded message. The recipient would reverse the procedure and then destroy his copy of the page. The German foreign office put this system into operation by 1923.
A separate notion was the use of a one time pad of letters to encode plaintext directly as in the example below. Leo Marks describes inventing such a system for the British Special Operations Executive during World War II, though he suspects it was already known in the highly compartmentalized world of cryptography.
The final discovery was by Claude Shannon in the 1940s who recognized and proved the theoretical significance of the one time pad system.
==Application==
One time pads were employed by Soviet Union espionage agencies for covert communications with agents and agent controllers. Counterespionage analysis has shown that these pads were generated by typists using typewriters. This method is of course not truly random, as it makes certain convenient key sequences more likely than others, yet it proved to be effective. As this method was effectively unbreakable it became necessary to compromise it by obtaining an image of the pad that was used, this by covert entry to a suspected communicator s residence or office followed by extremely careful search and subsequent photography. The search would be quite difficult since each item moved would have to be replaced and it would be simple for the target of the search to set up intrusion indicators. Should the search be revealed to the person targeted the results would become useless as the entire pad could then be discarded. It was only the unknowing use of a compromised pad that would be useful to the counterespionage agency.
= Example =
Suppose . The numerical values of corresponding message and key letters are added together, modulo 26. If key material begins with,
X M C K L
and the message is HELLO , then the coding is done as follows:
23 (X) 12 (M) 2 (C) 10 (K) 11 (L) key + 7 (H) 4 (E) 11 (L) 11 (L) 14 (O) message = 30 16 13 21 25 key + message = 4 (E) 16 (Q) 13 (N) 21 (V) 25 (Z) key + message (mod 26)
Note that if a number is larger than 25, then in modular arithmetic fashion, 26 is subtracted from the number to make it less than 26.
The ciphertext to be sent to Bob is thus EQNVZ. Bob uses the matching key page and the same process, but in reverse, to obtain the plaintext. Here, the key is subtracted from the ciphertext, again using modular arithmetic:
4 (E) 16 (Q) 13 (N) 21 (V) 25 (Z) ciphertext - 23 (X) 12 (M) 2 (C) 10 (K) 11 (L) key = -19 4 11 11 14 ciphertext - key = 7 (H) 4 (E) 11 (L) 11 (L) 14 (O) ciphertext - key (mod 26)
Similar to above, if a number is negative, 26 is added to make the number positive.
Thus, Bob produces Alice s plaintext, the vital message, HELLO . Both Alice and Bob destroy the key sheet immediately after use, thus preventing reuse and an essentially trivial attack against the cipher. The KGB often issued its espionage one-time pads printed on tiny sheets of flash paper —paper chemically converted to nitrocellulose, which burns almost instantly and leaves no ash.
The classical one-time pad of espionage (which often required actual pads of minuscule paper, a sharp pencil and the use of some mental arithmetic) can be translated into a software program using data files as input (plaintext) and output (ciphertext) and key material (the required random sequence). XOR can be used to combine the plaintext and the key. Ensuring that the key material is used only once and completely destroyed after use is not elementary. The auxiliary parts of a software one-time pad implementation present real challenges: secure handling/transmission of plaintext, truly random keys, and one-time-only use of the key.
= Security =
One-time pads are information theoretic security in that the encrypted message (ie, the ciphertext) provides no information about the original message to a cryptanalyst. This is a very strong notion of security, and it was first proven, mathematically, by Claude Shannon during World War II. His result was published in the Bell Labs Technical Journal in 1949. Properly used one-time pads are secure in this sense even against adversaries with infinite computational power. To continue the example from above, Eve intercepts Alice s ciphertext: EQNVZ , if Eve had infinite computing power, she would quickly find that the key: XMCKL would produce the plaintext HELLO . However, she would also try the key material sequence TQURI giving the plaintext LATER , an equally plausible message: 4 (E) 16 (Q) 13 (N) 21 (V) 25 (Z) ciphertext - 19 (T) 16 (Q) 20 (U) 17 (R) 8 (I) possible key = -15 0 -7 4 17 ciphertext-key = 11 (L) 0 (A) 19 (T) 4 (E) 17 (R) ciphertext-key (mod 26) In fact, it s possible to decrypt any message whatsoever with the same number of characters out of the ciphertext simply by using a different key.
The one-time pad would not be made less secure by a proof that Complexity classes P and NP, one of the central outstanding unsolved problems of computer science; many other encryption algorithms are likely to have their security brought into question if P=NP (it is widely believed that P≠NP and many doubt this question has any practical relevance to cryptography).
= Universal unbreakability =
Claude Shannon s work showed that any perfect encryption system requires that the number of possible keys exceeds the number of possible plaintext messages. If the number of possible keys and plaintexts are measured in, say, bits, this is equivalent to saying that the key must be at least as long as the plaintext.
At the same time, when implementing a one-time pad system, there are a number of problems and limitations which have the potential to greatly reduce security in practice.
= Controversy about one time pads in practice =
Some argue that one-time pads are not practical for use in real-world systems:
But many criticisms of the one time pad apply in some degree to other cryptosystems as well:
The one time pad does have a couple of real advantages:
At the end of this controversy one may say: I want to use a theoretically secure cipher, but my high throughput OTP RNG could get an undetected failure. Why can t I use both kinds of cipher (i.e. use a conventional cipher and then add an OTP layer).
= Historical uses =
In some diplomatic or espionage situations, the one-time pad is useful because it can be computed by hand with only pencil and paper. Indeed, nearly all other high quality ciphers are entirely impractical without computers. Spies can receive their pads in person from their handlers. Embassies can receive theirs by diplomatic pouch.
One-time pads have been used in special circumstances since the early 1900s. The Weimar Republic Diplomatic Service began using the method in about 1920. The breaking of poor Union of Soviet Socialist Republics cryptography by the United Kingdom, with messages made public for political reasons in two instances in the 1920s, appear to have induced the USSR to adopt one-time pads for some purposes by around 1930. KGB spies are also known to have used pencil and paper one-time pads more recently. Examples include Colonel Rudolf Abel, who was arrested and convicted in New York City in the 1950s, and the Krogers (ie, Morris and Lona Cohen), who were arrested and convicted of espionage in the United Kingdom in the early 1960s. Both were found with physical one-time pads in their possession.
The U.S. and Britain used one time pad systems for their most sensitive traffic in World War II and through the 1950s. The NSA describes one time tape systems like SIGTOT and 5-UCO as being used for intelligence traffic until the introduction of the electronic cipher based KW-26. Leo Marks reports that the British Special Operations Executive used one time pads to encode traffic between its offices. One time pads for use with its overseas agents were introduced late in the war.
The World War II voice scrambler SIGSALY was a one-time pad system. It added (analog) noise to the signal at one end and removed it at the other end. The noise was distributed to the channel ends in the form of large shellac records of which only two were made. There were both starting synchronization and longer term phase drift problems which arose and were solved before the system could be used.
Beginning in the late 1940s, U.S. and U.K. intelligence agencies were able to break some of the Soviet one-time pad traffic to Moscow during WWII as a result of errors made in generating and distributing the key material. One suggestion is that Moscow Centre personnel were somewhat rushed by the presence of German troops just outside Moscow in late 1941 and early 1942, and they produced more than one copy of same key material during that period. This decades-long effort was finally codenamed VENONA (BRIDE had been an earlier name); it produced a considerable amount of information, including more than a little about some of the Soviet atom spies. Even so, only a small percentage of the intercepted messages were either fully or partially decrypted (a few thousand out of several hundred thousand).
In 1945 the U.S. discovered that Canberra-Moscow messages were being encrypted first using a code-book and then using a one-time pad. However the one-time pad used was the same one used by Moscow for Washington, DC-Moscow messages. Combined with the fact that some of the Canberra-Moscow messages included known British government documents, this allowed some of the encrypted messages to be broken.
The s, military aircraft to carry the couriers, and so on.
During the 1983 Invasion of Grenada, U.S. forces found a supply of pairs of one time pad books in a Cuban warehouse. The continued presence of number stations on shortwave radio suggests one time pads are still used by spies.
A related notion is the code (cryptography)#One time code—a signal, used only once, of A for mission completed and B for mission failed cannot be decrypted in any reasonable sense of the word. Understanding the message will require additional information, often depth of repetition, or some traffic analysis. However, such strategies (though often used by real operatives, and baseball coaches) are not a cryptographic one-time pad in any significant sense.
= True randomness requirements =
In discussing the one time pad, two notions of security have to be kept distinct. The first is the theoretical security of the one-time pad system as proved by Shannon (Shannon security). The second is the security offered by state-of-the-art ciphers (e.g. AES) designed with principles learned in the long history of code breaking and subjected to extensive testing in a standardization process, either in public or by a top notch security service (empirical security). The former is mathematically proven, subject to the practical availability of random numbers. The later is unproven but relied upon by most governments to protect their most vital secrets (insofar as publicly known thus far).
==Methods that offer empirical security but not Shannon security==
If the key material is generated by a deterministic program then it is not random and the encryption system cannot claim the theoretical security of the one-time pad system. Instead it is called a stream cipher. These employ a short key that is used to seed a long pseudorandom stream, which is then combined with the message using some such mechanism as those used in one-time pads. Stream ciphers can be secure in practice, but they cannot be absolutely secure in the same provable sense as the one-time pad. The FISH (cryptography) used by the German military in WWII turned out to be insecure stream ciphers, not practical automated one-time pads as their designers had intended. Bletchley Park broke one of them, the Lorenz cipher machine, regularly.
However, if a modern so-called cryptographically secure pseudo-random number generators is used, it can form the basis for an empirically secure stream cipher. There are many well-vetted designs in the public domain, ranging from the simplicity of RC4 to using a top-secret rated block cipher like AES in counter mode. There is little reason to invent new stream ciphers.
==Methods that offer neither empirical security nor Shannon security==
The similarity between stream ciphers and one-time pads often leads the cryptographically unwary to invent insecure stream ciphers under the mistaken impression that they have developed a practical version of the one-time pad. An especially insecure approach is to use any of the Pseudorandom number generator that are distributed in many (perhaps most) computer programming language runtime support packages or as operating system system calls. These typically produce sequences that pass some (or even many) statistics tests, but are nonetheless breakable by cryptoanalytic techniques. For some time the ANSI C standard restricted the C language random number routine output to a single precision integer, for most implementations that would be 16-bits, giving at most 32768 different values before repeating. This is entirely insecure and is easily breakable by brute force attack. Standard computer random number generators are not suitable for cryptographic purposes, specifically including the one-time pad. In particular, the relatively newly developed and widely admired Mersenne twister algorithm, while sufficiently random for most research or simulation uses, better than most any other such generator, and quite fast as well, should not be used to generate one-time pad key material. The algorithm is deterministic and was not designed for cryptographic security.
As well, publicly known values such as the terminal digits of Marathon (sport) race times, closing stock prices from any sourse however obscure, daily temperatures or atmospheric pressures, etc, though seemingly random, are predictable -- after the fact. Indeed, even truly random sequences which have been published cannot be used as they are now predictable if identified. An example is the Rand Corp 1950s publication of a million random number table; it has passed every statistical test for randomness thus far and is thought to be actually random. But, having been published, it is fully predictable. So are the digits of pi, e, Golden_ratio, and other irrational, or transcendental, numbers; the sequences may be random (an open question, actually), but are fully predictable nonetheless.
==Achieving Shannon security==
To achieve Shannon security, a source of perfectly unpredictable random data is needed. One theoretical basis for the physical existence of unpredictability is over time.
For use in a one time pad, data should exhibit perfect randomness. Most practical sources exhibit some imperfection or bias. The quality of randomness is measured by Information entropy. A perfectly random bit has an entropy of one. An idea due to Von Neumann is to use an algorithm to combine multiple, imperfectly random bits, each with entropy less than one, to create a single bit with entropy equal to one. This process is called entropy distillation or Von Neumann whitening and can allow the practical generation of random data suitable for use in one time pads. Von Neumann whitening (as described in [http://www.cryptography.com/resources/whitepapers/VIA_rng.pdf E VIA C3 NRNG (pdf)]) is as follows:
In Linux (and some other Unix-like systems) the kernel s random number generator, /dev/random, uses environmental noise to generate random data and is better than many such system call designs. It attempts to estimate the amount of entropy it collects and blocks if the entropy pool is exhausted. It is intended to be, and is widely thought to actually be, better than most such generators, and if so is rather closer to satisfactorily random. But this process will be slow on systems which have few usable noise sources. It can, however, be fed additional entropy by reading from an attached noise generating device.
Linux also provides /dev/urandom which uses a deterministic algorithm to generate the data whenever environmental noise is unavailable. Improved designs, such as the Yarrow algorithm are available. One-time pad key material generated in this way (ie, from deterministic random number generators) lacks the information-theoretic security of a one-time pad. Yarrow offers at least as much strength as a block cipher based on 3DES.
To minimize the risk of computer virus, a computer used to generate one time pad key material should never be connected to any computer network and preferably should not be used for any other purpose. For the same reason, key material should be collected on new, blank media (e.g. floppy disks or CD-Rs). If paper pads are to be produced, the printer should be dedicated as well. The computer and any peripherals (the fewer the better) should be stored in a safe when not generating pads. OTP generation would be a good use for an older laptop, purged and rebuilt with a fresh, traceable copy of an open source operating system such as Linux or BSD.
=Making one time pads by hand=
One time pads were originally made without the use of a computer and this is still possible today. The process can be tedious, but if done correctly and the pad used only once, the result is unbreakable.
There are two components needed to make a one time pad: a way to generate letters at random and a way to record two copies of the result. The traditional way to do the latter was to use a typewriter and carbon paper. Typewriters are scarce these days and add a requirement to destroy the carbon paper and typewriter ribbon, from which the pad data can often be recovered. A more modern approach is to hand write the letters neatly in groups of five on two part carbonless copy paper sheets, which can be purchased at office supply stores. Each sheet should be given a serial number or some other unique marking.
The simplest way to generate random letters is to obtain 26 identical objects with each letter of the alphabet marked on one object. Tiles from the game Scrabble can be used as long as only one of each letter is selected. Kits for making name charm bracelets are another possibility. One can also write the letters on 26 penny (U.S. coin) with a marking pen. The objects are placed in a box or cup and shaken vigorously, then one object is withdrawn and its letter is recorded. The object is returned to the box and the process is repeated.
= See also =
= External links =
*Marcus Ranum s [http://www.ranum.com/security/computer_security/papers/otp-faq/ One-Time Pad FAQ] *[http://www.vadiumtech.com/resources/ A FAQ by a company selling one time pad software] *[http://www.schneier.com/crypto-gram-0210.html#7 Arguments against one-time pad systems] — comments by Bruce Schneier *The FreeS/WAN [http://www.freeswan.org/freeswan_trees/freeswan-2.06/doc/glossary.html#OTP glossary entry] with a discussion of OTP weaknesses *[http://patft.uspto.gov/netacgi/nph-Parserpatentnumber=1310719 US patent 1310719] — Note that some browsers may not be able to display these TIFF images. *[http://www.fgerlach.com/clock_otp.html A clock-jitter based One-Time Pad generator]|
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