Written by @EdOverflow & @YouPunk.
Table of contents
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1. Foundations
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2. Classical Cryptography
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3. WWI
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4. WWII
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5. Cold War
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6. Modern-day Cryptography
1. Foundations
The origin of cryptography
Cryptography began thousands of years ago as a way of hiding, encrypting and protecting secrets.
Early encryption methods are known as classical cryptography. They were not very sophisticated
and were done with pen and paper or really basic mechanical aids. The first signs of cryptography
were discovered in Egypt, where non-standard hieroglyphs carved into monuments have been
found. The ancient Greeks are also known to have used very simple ciphers.
Encryption and decryption
Encryption is a process converts data into cipher text so that it can be safely stored or sent and
then decrypted. Decryption turns the hidden and protected cipher text into readable plaintext. For
instance, you can encrypt a message and then send it to a friend, who can decrypt the message and
then read the contents. If an eavesdropper were to get hold of the message they should not be
able to read the contents.
Modern cryptography
Encryption has evolved drastically over the years. Not so long ago we were able to decrypt some
ciphers with a pen and paper. Today we use very complicated algorithms and keys (see chapter
3.24) to encrypt information and our brain power is not sufficient enough to decrypt these new
encryption methods. They can only be decrypted with the help of computers. Around 1990, the use
of the Internet was getting bigger and things such as online transactions had to be encrypted.
How & where is cryptography used today?
Every single time someone makes a phone call, sends a message, purchases something online or
even at the store with your credit card, cryptography is used, in fact all of these everyday appliances
would be very insecure without encryption.
Terminology
What is confidentiality?
Confidentiality is the protection of personal information. If you talk with your friend about a
sensitive matter, you want your conversation to be kept in confidence. Since 1990 and the use of
the Internet, confidentiality has become more and more important.
What is integrity?
In order for the client to know whether they are using the desired (expected) information (e.g.,
website), there must be some mechanism in place to assure integrity.
What is authentication?
Authentication provides proof of the identity of the provided data. When contacting somebody you
want to ensure they are who you believe they are.
2. Classical Cryptography
Egypt
According to David Kahn's "The Codebreakers"[1], the earliest use of cryptography is found in an
Egyptian tomb belonging to Khnumhotep II. Researchers found unusual hieroglyphs carved into the
walls that date back to 2000 BC. This method is known as symbol substitution and is a very
important element of cryptography that is still used today. Interestingly though, Khnumotep II's
intentions were not to hide the text, but rather to impress the reader.
Ancient India
We have well documented evidence, that cryptography was prevalent in ancient India too,
especially among traders, thieves and robbers. Number 44 of the 64 arts that Vatsyayana listed in
his book "Vatsyayana's Kama Sutra" is Mlecchita Vikalpa: "The art of understanding writing in cipher,
and the writing of words in a peculiar way."[2] Ancient Indian ciphers such as the Muladeviya, a
simplified form of Kautiliya, consisted mainly of letter substitutions, which were based on phonetic
relations.
Plain: | a | kh | gh | c | t | ñ | n | r | l | y |
Cipher: | k | g | ṅ | ṭ | p | ṇ | m | ṣ | s | ś |
Rome
One of the earliest known and simplest ciphers, the Caesar cipher, is a shift cipher, which means it
encrypts by shifting all the letters in a piece of text by a certain number of places. It was named
after Julius Caesar, who used it to encrypt messages sent to his generals. Caesar ciphers are very
simple to create, but are also very easy to crack. The key for this cipher is a letter which represents
the number of shifts in the alphabet. So for example, a key of E means "shift 4 places". The key Z
can either mean "shift 25 places" or "shift one place backwards".
:mortar_board: Example
The word "HELP" with the key J becomes "ROVZ".
ABCDEFGHIJKLMNOPQRSTUVWXYZ
KLMNOPQRSTUVWXYZABCDEFGHIJ
Caesar's method is a weak encryption method, since there are only 26 possibilities. The Enigma
machine used in World War II by the German military had over 158 quintillion combinations! To
crack the Caesar cipher we can also use a method called frequency analysis. This is where the
number of times each letter occurs is analysed. The frequency of the most commonly occurring
letters in the cipher text correspond to the most common in the plaintext (etaoin shrdlu).
:mortar_board: Example
The following cipher text is given:
RYYVBGWHFGNGRPU
After analysing the text we get:
Letter Percentage Number of occurrences
N
6.67%
1
P
6.67%
1
U
6.67%
1
F
6.67%
1
W
6.67%
1
V
6.67%
1
B
6.67%
1
H
6.67%
1
R
13.33%
2
Y
13.33%
2
G
20.0%
3
Now we compare the most common letters from the cipher text with the most common English
letters: etaoin shrdlu
If cipher text `G` is plaintext `E`, then the shift is `2`.
Result: pwwtzeufdelepns
Else if cipher text `G` is plaintext `T`, then the shift is `13`.
Result: elliotjustatech
:hourglass: Challenge
It is believed that Julius Caesar used a key of E when encrypting messages. Try to decrypt this
message Caesar sent to one of his generals.
M leh vexliv fi jmvwx mr e zmppeki xler wigsrh ex Vsqi.
Solution: I had rather be first in a village than second at Rome.
The Hebrews
The Hebrews used a simple monoalphabetic substitution cipher known as "atbash". The alphabet is
simply reversed as follows:
Plain: ABCDEFGHIJKLMNOPQRSTUVWXYZ
Cipher: ZYXWVUTSRQPONMLKJIHGFEDCBA
Ancient Greece & Sparta
The Ancient Greeks and the Spartans used a scytale to transport their messages safely. A scytale is
a rod with a strip wrapped around it. The message is written on the strip and in order for the
receiver to decipher the secret message, they must wrap the strip around a rod of same diameter
as the one used to encrypt the message.
:mortar_board: Example
Hermes comes running up to you and hands you his belt. You realise that there are letters
engraved into the belt. You look back up and Hermes has already vanished. When placing the belt
flat on the table, the following can be read: BSRITOOEO:NILT) You have not got a stick lying
around, so you start by drawing tables. There are 15 letters in total, which means, in order to fill all
fields, we must draw a 3x5 table.
| B | S | R | I | T |
--------------------| O | O | E | O | : |
--------------------| N | I | L | T | ) |
Now when you read the table from top to bottom you get: Bonsoir Eliott :)
3. WWI
The Importance of Cryptography during World War I
Encryption was vital during WWI, because nations did not want their enemies to be able to read
their military secrets. Plans and military operations had to be kept under strict protection.
Messages that were intercepted and decrypted by enemies changed the war drastically. The most
notable case was the Zimmerman telegram, decrypted by the British Naval intelligence that helped
bring the United States into the war.
WWI cryptography was not very sophisticated, cryptographers would mostly use substitution
ciphers or mathematical encryption for important messages. The ciphers they used had to be
explained in codebooks that were distributed to military personnel. This was a major weakness,
since the books could be stolen by spies or enemy forces.
The Zimmermann Telegram
The Zimmermann Telegram was a note from Germany requesting Mexican allegiance. It was
wirelessly intercepted and then decrypted by the British. The contents of the letter gave the U.S.A
another reason to join World War I.
On 17 January 1917, the team of British codebreakers from Room 40 at Admiralty received the
intercepted letter. They soon realized the number 13042 was a variation of 13040, the title number
of a German codebook. The codebreakers managed to recreate the codebook with all sorts of
intercepted messages they had managed to get hold of. Using the book the signature 97556
decoded to Zimmermann, revealing that Arthur Zimmermann had sent the note to the German
ambassador in Mexico.
4. WWII
The Importance of Cryptography during World War II
Cryptography played a much bigger role in World War II as opposed to World War I. In World War II
cryptography was more sophisticated and the ciphers and machines they used were far more
complex. The practice of cryptanalysis was also much more advanced and every country tried to
improve their ability to encrypt/decrypt messages.
The most important event was the decryption of the German Enigma machine by the
cryptographers at Bletchley Park. With the help of Ultra, the decrypted Enigma messages, the allies
were able to read Germany's military intelligence throughout the war.
The Enigma Machine
Enigma machines were a series of rotor cipher machines, originally used by companies, banks and
later by the German military during WWII to encrypt messages. The main difference to classical
cryptographic methods, was the fact that the cipher was polyalphabetic. The key changed for each
individual letter you enciphered. So for example, you might press a J and end up with an E the first
time and a B the second time.
The early Enigma machine consisted of a keyboard, a lampboard and three rotors with 26 letters
each. When a key was pressed a letter flashed on the lampboard and the right rotor turned one
step. Once the right rotor had made one full revolution, it kicked the middle rotor by one step and
so forth, just like an odometer. So if you set the rotors to A-B-C, the following would happen:
A-B-C -> A-B-D ... A-B-Z -> A-C-A
When the German military saw the potential of the machine, they used various methods to insure
better security. First they increased the number of rotors to four 4 * 3 * 2 = 24 permutations.
They then added a plugboard, which paired letters together with 10 wires 26! / (6! * 10! * 2^10)
= 150'738'274'937'250 permutations.
Total combinations: 1054560 * 150,738,274,937,250 = 158,962,555,217,826,360,000 ≈
1.59 * 10^20
So now that we know the different parts of an Enigma machine, lets have a look at how to set up
the machine to encrypt and decrypt a message:
The rotor order: Numbered from one to three in Roman numerals (I-III), each rotor was wired up
differently and when placed on the rack in different order, they gave a different output.
The ring setting: The numbers displayed through little holes next to the rotors, indicated the ring
setting. Each rotor was assigned a ring setting.
The ground setting: The key.
Plugging: Two letters could be connected together with a wire.
The reflector wheel: The circuit passed through the rotors and was then reflected back through the
three rotors.
:mortar_board: Example
You have got hold of an Enigma machine and a stolen list for this month. The list says the following:
Datum (date DD/MM/YY):
Walzenlage (rotor order):
Ringstellung (ring setting):
Steckerverbindungen (plug setting):
Kenngruppen (identification group):
04/09/1939
| I | II | III |
| 12 | 25 | 01 |
AF DU LQ VZ PI WX HM OF GD SC
LOL
1. This means you must set rotor I to 12, rotor II to position 25 and rotor III to 1.
2. Next you place rotor I in the left slot, rotor II in the middle slot and rotor III in the right slot.
Rotor III will spin the fastest.
3. Now that all of that is done, you must connect all the pairs listed above together. A with F, D with
U and so on.
The following steps were taken to ensure that both ends could communicate:
1. Both sender and recipient had to have the same machine, or at least both machines had to be
configured so as to be in the same state. This means the same reflector wheel, rotor order, ring
setting and ground setting were used to encrypt and decrypt the message.
2. Everyday these settings changed and so German operators were handed out keylists of all the
different settings for the month. If the enemy got hold of a list, which they did sometimes, the list
became useless once the month was over. Obviously, if there was any suspicion of a missing
keylist, the Germans would have generated a new one. Keylists were either distributed in printed
form or over radio as Morse code. The German Navy used ink that disappeared in water, so that
when a ship sank the keylist was destroyed.
Now if both ends have followed all these steps accordingly, the sender could encrypt HELLO with his
machine, which might output something like UZCFS. The receiver can then type UZCFS into his
machine and it should output HELLO.
The Polish Bomba
The Polish mathematician and cryptologist Marian Rejewski was the first one to design a machine
that was able to decrypt messages encrypted with an Enigma machine. Because the Germans used
to write the combination key at the start of every transmission, Marian Rejewski was able to
recreate the internal wirings of the Enigma machine. In order to speed up the process of decrypting
the key, he created the "Bomba Kryptologiczna" in October 1938. Six Bombas were built in Warsaw
before September 1939. It took a Bomba about two hours to find out the daily keys. The Polish
cryptographers were already reading Enigma encrypted messages by 1939 but they did not tell
their British or French allies. Then by the end of 1938 the Germans added two more rotors, so that
an operator had to choose three out of five rotors to put in the Enigma machine. The Polish
cryptologists were not able to decrypt the messages anymore. In 1939 with the invasion of Poland,
the Polish cryptographers decided to share their data with the French and the British.
The Bombe
The Bombe was a device used by the British cryptologists at Bletchley Park to decode German
Enigma messages during WWII. The Bombe was designed in 1939 at the UK Government Code and
Cypher School (GC&CS), a peace-time codebreaking agency, by Alan Turing and Gordon Welchman.
In order to decrypt Enigma messages the Bombe had to find out the relative starting positions of
the Enigma rotors, the cross-pluggings of the Enigma plugboard and the order of the rotors. The
Bombe could not solve everything on its own, therefore some work had to be done by the
codebreakers manually. For instance likely pieces of plain-text known as Cribs had to be found by
the codebreakers and then inserted into the Bombe machine.
:radio: The Machine
The Bombe had to do one thing, it had to replicate the action of several Enigma machines. A
standard British Bombe replicated the actions of 36 Enigma machines wired together, each with 3
drums connected to them to produce the same effect as the Enigma rotors. To recreate the Enigma
rotors actions, each rotor drum of the Bombe had two sets of contact and 104 wire brushes that
were arranged in four circles, 2 input and 2 output circles. The circles were arranged, so that the 2
output circles were connected with the current passing through the rotors, and the 2 input circles
were connected with the current passing in the other direction.
Lorenz Cipher
Another cipher machine, which the German military used towards the end of WWII is the Lorenz
machine. The Lorenz machine was a rotor stream cipher machine often used by Nazi High
Command and famously known to have been Adolf Hitler's cipher of choice. Lorenz machines were
attached to teleprinters, in order to encrypt important messages. Each machine model starts with
an SZ abbreviation (SZ-40, SZ-42, etc.), which stands for "Schlüssel-Zusatz", the German for "cipher
attachment".
:radio: The Machine
The Lorenz cipher machine, although not as famous as Enigma, was far more complex than Enigma.
Lorenz machines had 12 rotor wheels and each wheel had a different number of pins, which could
be set to on or off. The rotor wheels were divided into 3 groups, χ ("chi"), ψ ("psi") and μ ("mu"). χ
and ψ wheels are key wheels and by adding both together they output a key.
Key = χ-Key + ψ-Key
χ wheels rotate after every letter that is typed, while ψ wheels do not change regularly. The two
middle μ wheels are responsible for the rotation of the χ and ψ wheels, and are therefore called
motor wheels.
Rotor number: 1 2 3 4 5 6 7 8 9 10 11 12
Type:
ψ ψ ψ ψ ψ μ μ χ χ χ χ χ
Number of pins: 43 47 51 53 59 37 61 41 31 29 26 23
Lorenz machines used an enciphering method proposed by the American scientist, Gilbert Vernam,
to encrypt teleprinter messages. Letters were represented as dots ("•") and crosses ("x") and in
order to encrypt the message, the typed letter was XOR'd with the key letter. The XOR of (•,•) is •;
(•,x) is x; (x,•) is x; and (x,x) is •.
Letter A: x x • • •
Key C: • x x x •
----------------------- (XOR)
Cipher F: x • x x •
The Codebreakers
The codebreakers at Bletchley Park who managed to crack Lorenz cipher messages referred to
Lorenz machine traffic as Tunny, the old English word for tuna. When the first messages were
intercepted, there were clear signs that this could not come from an Enigma machine.
Bletchley Park
Bletchley Park was the home of the British Codebreakers during World War II, and was run by the
Government Code and Cypher School (GC&CS). The park is located in Buckinghamshire, England.
The advantage of the parks location was its centrality. It was just in the middle of the Oxford and
Cambridge universities who supplied many code-breakers. Bletchley Park is famous for cracking
naval Enigma and Lorenz codes. Although many incredible minds worked together to decode
messages at Bletchley, the most famous codebreaker remains Alan Turing. In 1945 Bletchley Park
had about 10‘000 code-breakers at their service, 75% of them were women, due to the lack of men,
who had been sent to war. The extreme secrecy around Bletchley Park meant that many of the
achievements were not revealed until decades later. All staff members had to sign the Official
Secrets Act to ensure their loyalty. They were told not to talk about Britain's "Ultra secret" or any of
the intelligence acquired by decrypting enemy messages and signals. Members were not even
allowed to share this information with their relatives or discuss it at Bletchley Park. This discretion
was partially respected, since Germany had no idea about Britain's success, even though we know
today, that Bletchley Park had been infiltrated by a Soviet spy that leaked information about the
"Ultra secret". Today, Bletchley Park is a famous tourist attraction.
:hourglass: Challenge
Why is it important that codebreaking agencies are hidden and operated in secret?
Solution: If enemies were to find out about a codebreaking agency, they could send spies, improve
their encryption methods and possibly send fake messages to put off the enemy.
Purple
In the early 1930s, the Japanese Navy purchased a German Enigma machine and improved the
security of the machine. This new machine was codenamed "Red" by the U.S. government and was
one of the most secure cryptographic machines in the world. The American Army Signal Intelligence
Service (SIS) led by William Friedman managed to crack the machine in 1936, thus being able to
read Japan's top secret communications. However in 1939, Japan's Foreign Ministry presented a
newly improved cipher machine called "Purple". This machine was so secure, and so much better
than its predecessor "Red", that even the U.S. thought it was unbreakable. Purple was used in
World War II to encrypt classified Japanese information. Purple, unlike Enigma, was only used in
Japanese embassies, and not on the battlefield, which means that enemies and even allies of Japan
had no idea such a machine existed. This is also why the U.S. had a lot of trouble cracking the
machine, since they had no idea how it looked like, they could not even start to try and break it.
The message, that broke off negotiations and then led to a war between America and Japan, was
sent through Purple.
In 1929, the U.S. Army hired William F. Friedman, to lead the SIS and to crack the Purple machine.
The best way to crack a cipher machine, was to create an exact replica of it. In 1940, the U.S. with
the help of spies managed to create eight replica Purple machines, and they were finally able to
decode many Purple encrypted messages. Decoded messages of the Purple cipher were
codenamed "Magic" by the U.S. government, which is similar to "Ultra", the British Enigma
decryptions. The U.S. Army was aware of Japan's most secret communications. Thanks to Magic, the
U.S. Army was able to read Japanese plans and obtained vital information regarding the war in
Europe. Spies around the world tried to tell the Japanese government, that Purple had been broken,
but the Japanese were so confident in the security of their machines that they simply ignored all
warnings.
:radio: The Machine
A Purple machine consisted of two electronic typewriters connected to a plugboard and a box
containing the cryptographic elements within a network of wiring. The plugboard had a plug for
each letter in the Roman alphabet. JU.S.t like the German Enigma machine, Purple would not
encipher any letter to itself, meaning that an A entered on the electronic typewriter would never be
outputted as an A in the cipher text. When plaintext was typed on the first electronic typewriter, the
input keyboard, it passed through a plugboard which separated the letters. The separated letters
were then sent to two different groups. The first group consisted of 6 vowels (AEIOUY) and the
second group of 20 consonants (BCDFGHJKLMNPQRSTVWXZ). This feature of Purple was called the "6-20
split." The first group was called the "sixes" and the second group was called the "twenties". After
passing through the input plugboard the letter is denoted by S in the image below, if the letter is in
the "sixes". The letter is then sent to the output plugboard. On the other hand, if the letter is in the
"twenties" the letter passes through three permutations, L, M, and R in the image below, and is then
sent to the output plugboard. As you can see below the "sixes" will be permuted to "sixes" and the
"twenties" will be permuted to "twenties" which is the weakness of the Purple machine and was
exploited by the United States.
The Japanese also used modified versions of Purple (Coral and Jade), these cipher machines did not
use the "6-20 split." Unlike the German Enigma machine, which used rotors to permute letters, the
Purple machine uses switches, each step of a switch permutes the letter. That is why Purple is
referred as a "stepping switch machine". Unlike the Enigma machine, the Purple permutations were
not made to be changed, that is another major weakness of the cipher machine. [3]
5. Cold War
6. Modern-day Cryptography
The Key
In cryptography, a key is used to encrypt or decrypt plaintext into cipher text and is usually a small
piece of information. The security of an encrypted system relies on the secrecy of the key and
keeping keys secret is the most difficult part in cryptography. If a third party is able to intercept the
key, it could read the original message. Some encryption algorithms use the same key to encrypt
and decrypt data, they are called symmetric key algorithms. More secure encryption algorithms
use different keys to encrypt and decrypt data, a public key and a private key. These encryption
algorithms are called asymmetric key algorithms. The length of such a key is always different.
Some algorithms, for example cipher algorithms, use keys that are much shorter than the plaintext.
Other systems, such as the One Time Pad (see later) use keys that are exactly as long as the
plaintext.
Exclusive or (XOR)
XOR or "Exclusive OR" is a logical operator that outputs True when the inputs differ and False when
two inputs are the same. The XOR operation is denoted as ⊕ or ⊻.
When "adding" two binary strings together with XOR, for every bit that differs the output will be 1
and for every bit that corresponds the output will be 0.
X Y X XOR Y
000
011
101
110
If you have a binary string of 00110 and you XOR it with the string 10101 the result will be 11001.
00110 ⊕ 10101 = 11001
Let us look at an example where your message is the string 01010111 01101001 and the key is
11110011 11110011:
01010111 01101001 // Message
⊕ 11110011 11110011 // Key
---------------------= 10100100 10011010 // Encrypted Message
Now if you want to decrypt your encrypted string you would have to XOR it with the same key that
you used to encrypt your message.
10100100 10011010 // Encrypted Message
⊕ 11110011 11110011 // Key
---------------------= 01010111 01101001 // Message
:flashlight: Psst...
If you XOR the same string of bits the result is always 0.
1100110011 ⊕ 1100110011 = 0
Random & Pseudo-random
The problem with generating random numbers with a computer, is that there is nothing random in
a computer. The only thing computers really do is solve arithmetic operations. Functions in a
computer take inputs and give outputs. This means that outputs are always somehow related to
inputs, so if you want a random output, your input must be random. Computers are made to be
deterministic, they will always do the same thing when an instruction is given, and their action is
predictable, which is the complete opposite of randomness.
We can create pseudo-random numbers, which are the closest we can get to creating a random
number with a computer. There are several ways to create pseudo-random numbers, all in all the
only procedure is to add so many "layers", until the result resembles a random number. You could
have a function that takes the current date, adds the current time, divides by 6, adds 27 and divides
by 4. This is a pseudo-random number generator (PRNG), even though this one is very basic and
insecure. More advanced generators, take an input value that is outside the computer. For example,
measuring noise or electric current will give you a number that is seemingly random.
One of the first pseudo-random number generators was invented by John von Neumann and is
called the "Middle-square method". He created an algorithm that simulated the effect of
randomness. He first took a truly random number a "seed", the seed could have come from the
measurement of noise or the current time. After having calculated a seed, it was used as an input
to a simple calculation. First multiply the seed by itself and output the middle of the result, this
output was then used as the next seed. This process was repeated as many times as needed.
234 * 234 = 54756
Output: 475
Next seed: 475
The only difference between these pseudo-random generated numbers and truly random numbers,
is that at some point, unlike random numbers, the process will be repeated and a predictable
pattern will be generated with enough calculations.
Prime Numbers
Prime numbers are numbers divisible only by themselves and one. Prime numbers are very useful
for modern-day cryptography, here is why:
Start with two really big prime numbers, let us call them P1 and P2 and multiply them together to
get a natural number called C.
P1 * P2 = C
61 * 89 = 5429
It is very easy for a computer to multiply two numbers together, but it is very troublesome if you
start with C and try to work back to get P1 and P2.
C
= P1 * P2
5429 = ? * ?
This number C is used to generate something called a public key, a code everyone can use to
encrypt messages. Your bank, for example, would send you its public key to encrypt your data. P1
and P2 together are used to decrypt this code, so obviously they have to be kept secret and they
are what are called a private key. Only your bank knows what P1 and P2 are, so they are the only
ones that can access your credit card. If a thief does intercept the public key it would take them
thousands of years to factor the numbers and find out P1 and P2 and eventually decrypt the code.
:flashlight: Psst...
The largest known prime number 2^74207281-1 was discovered in January 2016, and consists of
22‘338‘618 decimal digits.
One-Time Pads (OTP)
The ultimate question in cryptography is and always has been: "How can I design an unbreakable
cipher?"
The answer is complete randomness.
Imagine you want to encrypt the word "HELP". In this case you can roll a 26 sided die 4 times to
generate a list of shifts (e.g., 4, 23, 15, 9), because the word "HELP" has 4 letters. The recipient must
be able to decrypt the message later so they must have the same list of shifts.
To encrypt the message you now use the list of shifts.
HELP --> LBAY
You then send this encrypted message to the recipient (LBAY) and they will be able to decrypt it.
Any third party intercepting the message will have no chance to decrypt it, because of two very
powerful properties. The shifts do not have any repetitive pattern, since the numbers are chosen
randomly and the message has a uniform frequency distribution meaning that there is no
frequency differential, every letter appears only once. Therefore it is impossible to break the
encryption without the list of shifts. This is the strongest possible method of encryption, but it is
also very time consuming to perform. Remember that each One-time Pad can only be used once —
hence the name — and the receiver has to have the exact same One-time Pad as you, which is very
difficult, because it is very hard to transmit the One-time Pad to the receiver, before encrypting a
message, considering, that it has to be long enough to fulfil present and future communications
with the receiver. Therefore, we could not use this method in everyday applications.
One-Time Pads and XOR
You can create a One-Time Pad using XOR. If you were to XOR your plaintext with a random key of
the same length, an attacker would have no chance to decrypt your message.
Here is a graphical representation of a one-time pad using XOR.
Your message:
Your key:
If you XOR them together you get the following encrypted message:
:flashlight: Psst...
If you encrypt another message with the same exact key, you would get another encrypted
message, but when XOR'd with the first encrypted message the result would be both original
messages. This is why it is very dangerous to reuse the same key.
Cryptographic Protocols
A protocol is a set of instructions that describes how different algorithms should be used. In
cryptography a protocol is designed for secure communication and can be divided into the
following categories:
●
Authentication protocols
●
Key transport protocols
●
Key-agreement protocols