The Code Book Part 12

Stage 3: Monoalphabetic Cipher with h.o.m.ophones [image]

Stage 4: Vigenere Cipher [image]

Stage 5 [image]

Stage 6 [image]

Stage 7 [image]

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Stage 8 [image]

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Stage 9 [image]

Stage 10 Shorter message [image]

Longer message [image]

Appendices

Appendix A The Opening Paragraph of A Void A Void by Georges Perec, translated by Gilbert Adair by Georges Perec, translated by Gilbert Adair Today, by radio, and also on giant h.o.a.rdings, a rabbi, an admiral notorious for his links to masonry, a trio of cardinals, a trio, too, of insignificant politicians (bought and paid for by a rich and corrupt Anglo-Canadian banking corporation), inform us all of how our country now risks dying of starvation. A rumor, that's my initial thought as I switch off my radio, a rumor or possibly a hoax. Propaganda, I murmur anxiously-as though, just by saying so, I might allay my doubts-typical politicians' propaganda. But public opinion gradually absorbs it as a fact. Individuals start strutting around with stout clubs. "Food, glorious food!" is a common cry (occasionally sung to Bart's music), with ordinary hardworking folk hara.s.sing officials, both local and national, and cursing capitalists and captains of industry. Cops shrink from going out on night s.h.i.+ft. In Macon a mob storms a munic.i.p.al building. In Rocadamour ruffians rob a hangar full of foodstuffs, pillaging tons of tuna fish, milk and cocoa, as also a vast quant.i.ty of corn-all of it, alas, totally unfit for human consumption. Without fuss or ado, and naturally without any sort of trial, an indignant crowd hangs 26 solicitors on a hastily built scaffold in front of Nancy's law courts (this Nancy is a town, not a woman) and ransacks a local journal, a disgusting right-wing rag that is siding against it. Up and down this land of ours looting has brought docks, shops and farms to a virtual standstill.

First published in France as La Disparition La Disparition by Editions Denoel in 1969, and in Great Britain by Harvill in 1994. Copyright by Editions Denoel 1969; in the English translation Harvill 1994. Reproduced by permission of the Harvill Press. by Editions Denoel in 1969, and in Great Britain by Harvill in 1994. Copyright by Editions Denoel 1969; in the English translation Harvill 1994. Reproduced by permission of the Harvill Press.

Appendix B Some Elementary Tips for Frequency a.n.a.lysis (1) Begin by counting up the frequencies of all the letters in the ciphertext. About five of the letters should have a frequency of less than 1 per cent, and these probably represent j, k, q, x and z. One of the letters should have a frequency greater than 10 per cent, and it probably represents e. If the ciphertext does not obey this distribution of frequencies, then consider the possibility that the original message was not written in English. You can identify the language by a.n.a.lyzing the distribution of frequencies in the ciphertext. For example, typically in Italian there are three letters with a frequency greater than 10 per cent, and nine letters have frequencies less than 1 per cent. In German, the letter e has the extraordinarily high frequency of 19 per cent, so any ciphertext containing one letter with such a high frequency is quite possibly German. Once you have identified the language you should use the appropriate table of frequencies for that language for your frequency a.n.a.lysis. It is often possible to unscramble ciphertexts in an unfamiliar language, as long as you have the appropriate frequency table.

(2) If the correlation is sympathetic with English, but the plaintext does not reveal itself immediately, which is often the case, then focus on pairs of repeated letters. In English the most common repeated letters are ss, ee, tt, ff, ll, mm and oo. If the ciphertext contains any repeated characters, you can a.s.sume that they represent one of these.

(3) If the ciphertext contains s.p.a.ces between words, then try to identify words containing just one, two or three letters. The only one-letter words in English are a and I. The commonest two-letter words are of, to, in, it, is, be, as, at, so, we, he, by, or, on, do, if, me, my, up, an, go, no, us, am. The most common three-letter words are the and and.

(4) If possible, tailor the table of frequencies to the message you are trying to decipher. For example, military messages tend to omit p.r.o.nouns and articles, and the loss of words such as I, he, a and the will reduce the frequency of some of the commonest letters. If you know you are tackling a military message, you should use a frequency table generated from other military messages.

(5) One of the most useful skills for a crypta.n.a.lyst is the ability to identify words, or even entire phrases, based on experience or sheer guesswork. Al-Khall, an early Arabian crypta.n.a.lyst, demonstrated this talent when he cracked a Greek ciphertext. He guessed that the ciphertext began with the greeting "In the name of G.o.d." Having established that these letters corresponded to a specific section of ciphertext, he could use them as a crowbar to prize open the rest of the ciphertext. This is known as a crib.

(6) On some occasions the commonest letter in the ciphertext might be E, the next commonest could be T, and so on. In other words, the frequency of letters in the ciphertext already matches those in the frequency table. The E in the ciphertext appears to be a genuine e, and the same seems to be true for all the other letters, yet the ciphertext looks like gibberish. In this case you are faced not with a subst.i.tution cipher, but with a transposition cipher. All the letters do represent themselves, but they are in the wrong positions.

Crypta.n.a.lysis by Helen Fouche Gaines (Dover) is a good introductory text. As well as giving tips, it also contains tables of letter frequencies in different languages, and provides lists of the most common words in English. by Helen Fouche Gaines (Dover) is a good introductory text. As well as giving tips, it also contains tables of letter frequencies in different languages, and provides lists of the most common words in English.

Appendix C The So-called Bible Code In 1997 The Bible Code The Bible Code by Michael Drosnin caused headlines around the world. Drosnin claimed that the Bible contains hidden messages which could be discovered by searching for equidistant letter sequences (EDLSs). An EDLS is found by taking any text, picking a particular starting letter, then jumping forward a set number of letters at a time. So, for example, with this paragraph we could start with the "M" in Michael and jump, say, five s.p.a.ces at a time. If we noted every fifth letter, we would generate the EDLS mesahirt.... by Michael Drosnin caused headlines around the world. Drosnin claimed that the Bible contains hidden messages which could be discovered by searching for equidistant letter sequences (EDLSs). An EDLS is found by taking any text, picking a particular starting letter, then jumping forward a set number of letters at a time. So, for example, with this paragraph we could start with the "M" in Michael and jump, say, five s.p.a.ces at a time. If we noted every fifth letter, we would generate the EDLS mesahirt....

Although this particular EDLS does not contain any sensible words, Drosnin described the discovery of an astonis.h.i.+ng number of Biblical EDLSs that not only form sensible words, but result in complete sentences. According to Drosnin, these sentences are biblical predictions. For example, he claims to have found references to the a.s.sa.s.sinations of John F. Kennedy, Robert Kennedy and Anwar Sadat. In one EDLS the name of Newton is mentioned next to gravity, and in another Edison is linked with the lightbulb. Although Drosnin's book is based on a paper published by Doron Witzum, Eliyahu Rips and Yoav Rosenberg, it is far more ambitious in its claims, and has attracted a great deal of criticism. The main cause of concern is that the text being studied is enormous: in a large enough text, it is hardly surprising that by varying both the starting place and the size of the jump, sensible phrases can be made to appear.

Brendan McKay at the Australian National University tried to demonstrate the weakness of Drosnin's approach by searching for EDLSs in Moby d.i.c.k Moby d.i.c.k, and discovered thirteen statements pertaining to a.s.sa.s.sinations of famous people, including Trotsky, Gandhi and Robert Kennedy. Furthermore, Hebrew texts are bound to be particularly rich in EDLSs, because they are largely devoid of vowels. This means that interpreters can insert vowels as they see fit, which makes it easier to extract predictions.

Appendix D The Pigpen Cipher The monoalphabetic subst.i.tution cipher persisted through the centuries in various forms. For example, the pigpen cipher was used by Freemasons in the 1700s to keep their records private, and is still used today by schoolchildren. The cipher does not subst.i.tute one letter for another, rather it subst.i.tutes each letter for a symbol according to the following pattern.

[image]

To encrypt a particular letter, find its position in one of the four grids, then sketch that portion of the grid to represent that letter. Hence: [image]

If you know the key, then the pigpen cipher is easy to decipher. If not, then it is easily broken by: [image]

Appendix E The Playfair Cipher The Playfair cipher was popularized by Lyon Playfair, first Baron Playfair of St. Andrews, but it was invented by Sir Charles Wheatstone, one of the pioneers of the electric telegraph. The two men lived close to each other, either side of Hammersmith Bridge, and they often met to discuss their ideas on cryptography.

The cipher replaces each pair of letters in the plaintext with another pair of letters. In order to encrypt and transmit a message, the sender and receiver must first agree on a keyword. For example, we can use Wheatstone's own name, CHARLES, as a keyword. Next, before encryption, the letters of the alphabet are written in a 5 5 square, beginning with the keyword, and combining the letters I and J into a single element: [image]

Next, the message is broken up into pairs of letters, or digraphs. The two letters in any digraph should be different, achieved in the following example by inserting an extra x between the double m in hammersmith, and an extra x is added at the end to make a digraph from the single final letter: [image]

Encryption can now begin. All the digraphs fall into one of three categories-both letters are in the same row, or the same column, or neither. If both letters are in the same row, then they are replaced by the letter to the immediate right of each one; thus mi becomes NK. If one of the letters is at the end of the row, it is replaced by the letter at the beginning; thus ni becomes GK. If both letters are in the same column, they are replaced by the letter immediately beneath each one; thus ge becomes OG. If one of the letters is at the bottom of the column, then it is replaced by the letter at the top; thus ve becomes CG. thus ge becomes OG. If one of the letters is at the bottom of the column, then it is replaced by the letter at the top; thus ve becomes CG.

If the letters of the digraph are neither in the same row nor the same column, the encipherer follows a different rule. To encipher the first letter, look along its row until you reach the column containing the second letter; the letter at this intersection then replaces the first letter. To encipher the second letter, look along its row until you reach the column containing the first letter; the letter at this intersection replaces the second letter. Hence, me becomes GD, and et becomes DO. The complete encryption is: [image]

The recipient, who also knows the keyword, can easily decipher the ciphertext by simply reversing the process: for example, enciphered letters in the same row are deciphered by replacing them by the letters to their left.

As well as being a scientist, Playfair was also a notable public figure (Deputy Speaker of the House of Commons, postmaster general, and a commissioner on public health who helped to develop the modern basis of sanitation) and he was determined to promote Wheatstone's idea among the most senior politicians. He first mentioned it at a dinner in 1854 in front of Prince Albert and the future Prime Minister, Lord Palmerston, and later he introduced Wheatstone to the Under Secretary of the Foreign Office. Unfortunately, the Under Secretary complained that the system was too complicated for use in battle conditions, whereupon Wheatstone stated that he could teach the method to boys from the nearest elementary school in 15 minutes. "That is very possible," replied the Under Secretary, "but you could never teach it to attaches."

Playfair persisted, and eventually the British War Office secretly adopted the technique, probably using it first in the Boer War. Although it proved effective for a while, the Playfair cipher was far from impregnable. It can be attacked by looking for the most frequently occurring digraphs in the ciphertext, and a.s.suming that they represent the commonest digraphs in English: th, he, an, in, er, re, es.

Appendix F The ADFGVX Cipher The ADFGVX cipher features both subst.i.tution and transposition. Encryption begins by drawing up a 6 6 grid, and filling the 36 squares with a random arrangement of the 26 letters and the 10 digits. Each row and column of the grid is identified by one of the six letters A, D, F, G, V or X. The arrangement of the elements in the grid acts as part of the key, so the receiver needs to know the details of the grid in order to decipher messages.

[image]

The first stage of encryption is to take each letter of the message, locate its position in the grid and subst.i.tute it with the letters that label its row and column. For example, 8 would be subst.i.tuted by AA, and p p would be replaced by AD. Here is a short message encrypted according to this system: would be replaced by AD. Here is a short message encrypted according to this system: [image]

So far this is a simple monoalphabetic subst.i.tution cipher, and frequency a.n.a.lysis would be enough to crack it. However, the second stage of the ADFGVX is a transposition, which makes crypta.n.a.lysis much harder. The transposition depends on a keyword, which in this case happens to be the word MARK, and which must be shared with the receiver. Transposition is carried out according to the following recipe. First, the letters of the keyword are written in the top row of a fresh grid. Next, the stage 1 ciphertext is written underneath it in a series of rows, as shown below. The columns of the grid are then rearranged so that the letters of the keyword are in alphabetical order. The final ciphertext is achieved by going down each column and then writing out the letters in this new order. the following recipe. First, the letters of the keyword are written in the top row of a fresh grid. Next, the stage 1 ciphertext is written underneath it in a series of rows, as shown below. The columns of the grid are then rearranged so that the letters of the keyword are in alphabetical order. The final ciphertext is achieved by going down each column and then writing out the letters in this new order.

[image]

The final ciphertext would then be transmitted in Morse code, and the receiver would reverse the encryption process in order to retrieve the original text. The entire ciphertext is made up of just six letters (i.e. A, D, F, G, V, X), because these are the labels of the rows and columns of the initial 6 6 grid. People often wonder why these letters were chosen as labels, as opposed to, say, A, B, C, D, E and F. The answer is that A, D, F, G, V and X are highly dissimilar from one another when translated into Morse dots and dashes, so this choice of letters minimizes the risk of errors during transmission.

Appendix G The Weaknesses of Recycling a Onetime Pad For the reasons explained in Chapter 3 Chapter 3, ciphertexts encrypted according to a onetime pad cipher are unbreakable. However, this relies on each onetime pad being used once and only once. If we were to intercept two distinct ciphertexts which have been encrypted with the same onetime pad, we could decipher them in the following way.

We would probably be correct in a.s.suming that the first ciphertext contains the word the somewhere, and so crypta.n.a.lysis begins by a.s.suming that the entire message consists of a series of the's. Next, we work out the onetime pad that would be required to turn a whole series of the's into the first ciphertext. This becomes our first guess at the onetime pad. How do we know which parts of this onetime pad are correct?

We can apply our first guess at the onetime pad to the second ciphertext, and see if the resulting plaintext makes any sense. If we are lucky, we will be able to discern a few fragments of words in the second plaintext, indicating that the corresponding parts of the onetime pad are correct. This in turn shows us which parts of the first message should be the.

By expanding the fragments we have found in the second plaintext, we can work out more of the onetime pad, and then deduce new fragments in the first plaintext. By expanding these fragments in the first plaintext, we can work out more about the onetime pad, and then deduce new fragments in the second plaintext. We can continue this process until we have deciphered both plaintexts.

This process is very similar to the decipherment of a message enciphered with a Vigenere cipher using a key that consists of a series of words, such as the example in Chapter 3 Chapter 3, in which the key was CANADABRAZILEGYPTCUBA.

Appendix H The Daily Telegraph Daily Telegraph Crossword Solution Crossword Solution ACROSS.

DOWN DOWN.

1. Troupe 1. Tipstaff 1. Tipstaff 4. Short Cut 2. Olive oil 2. Olive oil 9. Privet 3. Pseudonym 3. Pseudonym 10. Aromatic 5. Horde 5. Horde 12. Trend 6. Remit 6. Remit 13. Great deal 7. Cutter 7. Cutter 15. Owe 8. Tackle 8. Tackle 16. Feign 11. Agenda 11. Agenda 17. Newark 14. Ada 14. Ada 22. Impale 18. Wreath 18. Wreath 24. Guise 19. Right nail 19. Right nail 27. Ash 20. Tinkling 20. Tinkling 28. Centre bit 21. Sennight 21. Sennight 31. Token 23. Pie 23. Pie 32. Lame dogs 25. Scales 25. Scales 33. Racing 26. Enamel 26. Enamel 34. Silencer 29. Rodin 29. Rodin 35. Alight 30. Bogie 30. Bogie

Appendix I Exercises for the Interested Reader Some of the greatest decipherments in history have been achieved by amateurs. For example, Georg Grotefend, who made the first breakthrough in interpreting cuneiform, was a schoolteacher. For those readers who feel the urge to follow in his footsteps, there are several scripts that remain a mystery. Linear A, a Minoan script, has defied all attempts at decipherment, partly due to a paucity of material. Etruscan does not suffer from this problem, with over 10,000 inscriptions available for study, but it has also baffled the world's greatest scholars. Iberian, another pre-Roman script, is equally unfathomable.

The most intriguing ancient European script appears on the unique Phaistos Disk, discovered in southern Crete in 1908. It is a circular tablet dating from around 1700 B.C B.C. bearing writing in the form of two spirals, one on each side. The signs are not handmade impressions, but were made using a variety of stamps, making this the world's oldest example of typewriting. Remarkably, no other similar doc.u.ment has ever been found, so decipherment relies on very limited information-there are 242 characters divided into 61 groups. However, a typewritten doc.u.ment implies ma.s.s production, so the hope is that archaeologists will eventually discover a h.o.a.rd of similar disks, and shed light on this intractable script.

One of the great challenges outside Europe is the decipherment of the Bronze Age script of the Indus civilization, which can be found on thousands of seals dating from the third millennium B.C B.C. Each seal depicts an animal accompanied by a short inscription, but the meaning of these inscriptions has so far evaded all the experts. In one exceptional example the script has been found on a large wooden board with giant letters 37 cm in height. This could be the world's oldest billboard. It implies that literacy was not restricted to the elite, and raises the question as to what was being advertised. The most likely answer is that it was part of a promotional campaign for the king, and if the ident.i.ty of the king can be established, then the billboard could provide a way into the rest of the script.

Appendix J The Mathematics of RSA What follows is a straightforward mathematical description of the mechanics of RSA encryption and decryption.

(1) Alice picks two giant prime numbers, p p and and q q. The primes should be enormous, but for simplicity we a.s.sume that Alice chooses p p = 17, = 17, q q = 11. She must keep these numbers secret. = 11. She must keep these numbers secret.

(2) Alice multiplies them together to get another number, N N. In this case N N = 187. She now picks another number e, and in this case she chooses = 187. She now picks another number e, and in this case she chooses e e = 7. = 7.

(e and (p 1) (q 1) should be relatively prime, but this is a technicality.) (3) Alice can now publish e e and and N N in something akin to a telephone directory. Since these two numbers are necessary for encryption, they must be available to anybody who might want to encrypt a message to Alice. Together these numbers are called the public key. (As well as being part of Alice's public key, in something akin to a telephone directory. Since these two numbers are necessary for encryption, they must be available to anybody who might want to encrypt a message to Alice. Together these numbers are called the public key. (As well as being part of Alice's public key, e e could also be part of everybody else's public key. However, everybody must have a different value of N, which depends on their choice of could also be part of everybody else's public key. However, everybody must have a different value of N, which depends on their choice of p p and q.) and q.) (4) To encrypt a message, the message must first be converted into a number, M M. For example, a word is changed into ASCII binary digits, and the binary digits can be considered as a decimal number. M M is then encrypted to give the ciphertext, is then encrypted to give the ciphertext, C C, according to the formula C = Me (mod (mod N N) (5) Imagine that Bob wants to send Alice a simple kiss: just the letter X X. In ASCII this is represented by 1011000, which is equivalent to 88 in decimal. So, M M = 88. = 88.

(6) To encrypt this message, Bob begins by looking up Alice's public key, and discovers that N N = 187 and = 187 and e e = 7. This provides him with the encryption formula required to encrypt messages to Alice. With = 7. This provides him with the encryption formula required to encrypt messages to Alice. With M M = 88, the formula gives = 88, the formula gives C = 88 = 887 (mod 187) (mod 187) (7) Working this out directly on a calculator is not straightforward, because the display cannot cope with such large numbers. However, there is a neat trick for calculating exponentials in modular arithmetic. We know that, since 7 = 4 + 2 + 1, 887 (mod 187) = [88 (mod 187) = [884 (mod 187) 88 (mod 187) 882 (mod 187) 88 (mod 187) 881 (mod 187)] (mod 187) (mod 187)] (mod 187) 881 = 88 = 88 (mod 187) = 88 = 88 (mod 187) 882 = 7,744 = 77 (mod 187) = 7,744 = 77 (mod 187) 884 = 59,969,536 = 132 (mod 187) = 59,969,536 = 132 (mod 187) 887 = 88 = 881 88 882 88 884 = 88 77 132 = 894,432 = 11 (mod 187) = 88 77 132 = 894,432 = 11 (mod 187) Bob now sends the ciphertext, C C = 11, to Alice. = 11, to Alice.

(8) We know that exponentials in modular arithmetic are one-way functions, so it is very difficult to work backward from C C = 11 and recover the original message, = 11 and recover the original message, M M. Hence, Eve cannot decipher the message.

(9) However, Alice can decipher the message because she has some special information: she knows the values of p p and and q q. She calculates a special number, d d, the decryption key, otherwise known as her private key. The number d d is calculated according to the following formula is calculated according to the following formula e d d = 1 (mod ( = 1 (mod (p-1) (q-1)) 7 d d = 1 (mod 16 10) = 1 (mod 16 10) 7 d d = 1 (mod 160) = 1 (mod 160) d = 23 = 23 (Deducing the value of d d is not straightforward, but a technique known as Euclid's algorithm allows Alice to find is not straightforward, but a technique known as Euclid's algorithm allows Alice to find d d quickly and easily.) quickly and easily.) (10) To decrypt the message, Alice simply uses the following formula, M = = C Cd (mod 187) (mod 187) M = 11 = 1123 (mod 187) (mod 187) M = [11 = [111 (mod 187) 11 (mod 187) 112 (mod 187) 11 (mod 187) 114 (mod 187) 11 (mod 187) 1116 (mod 187)] (mod 187) (mod 187)] (mod 187) M = 11 121 55 154 (mod 187) = 11 121 55 154 (mod 187) M = 88 = = 88 = X X in ASCII. in ASCII.

Rivest, Shamir and Adleman had created a special one-way function, one that could be reversed only by somebody with access to privileged information, namely the values of p p and q. Each function can be personalized by choosing and q. Each function can be personalized by choosing p p and and q q, which multiply together to give N N. The function allows everybody to encrypt messages to a particular person by using that person's choice of N N, but only the intended recipient can decrypt the message because the recipient is the only person who knows p p and and q q, and hence the only person who knows the decryption key, d d.

Glossary ASCII American Standard Code for Information Interchange, a standard for turning alphabetic and other characters into numbers. American Standard Code for Information Interchange, a standard for turning alphabetic and other characters into numbers.

asymmetric key cryptography A form of cryptography in which the key required for encrypting is not the same as the key required for decrypting. Describes public key cryptography systems, such as RSA. A form of cryptography in which the key required for encrypting is not the same as the key required for decrypting. Describes public key cryptography systems, such as RSA.

Caesar-s.h.i.+ft subst.i.tution cipher Originally a cipher in which each letter in the message is replaced with the letter three places further on in the alphabet. More generally, it is a cipher in which each letter in the message is replaced with the letter Originally a cipher in which each letter in the message is replaced with the letter three places further on in the alphabet. More generally, it is a cipher in which each letter in the message is replaced with the letter x x places further on in the alphabet, where places further on in the alphabet, where x x is a number between 1 and 25. is a number between 1 and 25.

cipher Any general system for hiding the meaning of a message by replacing each letter in the original message with another letter. The system should have some built-in flexibility, known as the key. Any general system for hiding the meaning of a message by replacing each letter in the original message with another letter. The system should have some built-in flexibility, known as the key.

cipher alphabet The rearrangement of the ordinary (or plain) alphabet, which then determines how each letter in the original message is enciphered. The cipher alphabet can also consist of numbers or any other characters, but in all cases it dictates the replacements for letters in the original message. The rearrangement of the ordinary (or plain) alphabet, which then determines how each letter in the original message is enciphered. The cipher alphabet can also consist of numbers or any other characters, but in all cases it dictates the replacements for letters in the original message.

ciphertext The message (or plaintext) after encipherment. The message (or plaintext) after encipherment.

code A system for hiding the meaning of a message by replacing each word or phrase in the original message with another character or set of characters. The list of replacements is contained in a codebook. (An alternative definition of a code is any form of encryption which has no built-in flexibility, i.e., there is only one key, namely the codebook.) A system for hiding the meaning of a message by replacing each word or phrase in the original message with another character or set of characters. The list of replacements is contained in a codebook. (An alternative definition of a code is any form of encryption which has no built-in flexibility, i.e., there is only one key, namely the codebook.) codebook A list of replacements for words or phrases in the original message. A list of replacements for words or phrases in the original message.

crypta.n.a.lysis The science of deducing the plaintext from a ciphertext, without knowledge of the key. The science of deducing the plaintext from a ciphertext, without knowledge of the key.

cryptography The science of encrypting a message, or the science of concealing the meaning of a message. Sometimes the term is used more generally to mean the science of anything connected with ciphers, and is an alternative to the term cryptology. The science of encrypting a message, or the science of concealing the meaning of a message. Sometimes the term is used more generally to mean the science of anything connected with ciphers, and is an alternative to the term cryptology.

cryptology The science of secret writing in all its forms, covering both cryptography and crypta.n.a.lysis. The science of secret writing in all its forms, covering both cryptography and crypta.n.a.lysis.

decipher To turn an enciphered message back into the original message. Formally, the term refers only to the intended receiver who knows the key required to obtain the plaintext, but informally it also refers to the process of crypta.n.a.lysis, in which the decipherment is performed by an enemy interceptor. To turn an enciphered message back into the original message. Formally, the term refers only to the intended receiver who knows the key required to obtain the plaintext, but informally it also refers to the process of crypta.n.a.lysis, in which the decipherment is performed by an enemy interceptor.

decode To turn an encoded message back into the original message. To turn an encoded message back into the original message.

decrypt To decipher or to decode. To decipher or to decode.

DES Data Encryption Standard, developed by IBM and adopted in 1976. Data Encryption Standard, developed by IBM and adopted in 1976.

Diffie-h.e.l.lman-Merkle key exchange A process by which a sender and receiver can establish a secret key via public discussion. Once the key has been agreed, the sender can use a cipher such as DES to encrypt a message. A process by which a sender and receiver can establish a secret key via public discussion. Once the key has been agreed, the sender can use a cipher such as DES to encrypt a message.

digital signature A method for proving the authors.h.i.+p of an electronic doc.u.ment. Often this is generated by the author encrypting the doc.u.ment with his or her private key. A method for proving the authors.h.i.+p of an electronic doc.u.ment. Often this is generated by the author encrypting the doc.u.ment with his or her private key.

encipher To turn the original message into the enciphered message. To turn the original message into the enciphered message.

encode To turn the original message into the encoded message. To turn the original message into the encoded message.