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## Computer science

### Course: Computer science > Unit 2

Lesson 1: Ancient cryptography- What is cryptography?
- The Caesar cipher
- Caesar Cipher Exploration
- Frequency Fingerprint Exploration
- Polyalphabetic cipher
- Polyalphabetic Exploration
- The one-time pad
- Perfect Secrecy Exploration
- Frequency stability property short film
- How uniform are you?
- The Enigma encryption machine
- Perfect secrecy
- Pseudorandom number generators
- Random Walk Exploration

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# The Caesar cipher

The Caesar Cipher, used by Julius Caesar around 58 BC, is a substitution cipher that shifts letters in a message to make it unreadable if intercepted. To decrypt, the receiver reverses the shift. Arab mathematician Al-Kindi broke the Caesar Cipher using frequency analysis, which exploits patterns in letter frequencies. Created by Brit Cruise.

## Want to join the conversation?

- why would Caesar use ciphers?(27 votes)
- Caesar used ciphers so that important information, such as the location of a attack or the date it would be carried out, would be unknown to enemies but know to the rest of his troop. If his messages were ever intercepted, the enemy would't immediately understand what the cipher meant.(20 votes)

- At2:18, what if one uses a 'Random' shift like A is D, D is K.....rather using shift 3 or 4 or anything? Then Frequency fingerprint won't work right?(16 votes)
- It doesnt really matter as long as all letters receive the same treatment meaning ALL As turn to D and ALL Ds to Ks. The key thing is hear is universality for each letter.(3 votes)

- Decoding the Caesar Cipher based on the "fingerprint" requires a large sample space . I mean lets say if the message contains few words or only single word even that frequency distribution wont help in that case ..(25 votes)
- To the original question, yes, shorter messages make it harder to detect the frequency distribution, but you'd be surprised how quickly it shows up.

To Skylear's comment: A Caesar Cipher does have a sample space. The random variable is the number used for the shift. In your example, you encoded JASON IS BLUE using a shift of 2, but 2 could have been 1 or 23 or 14. In fact, it could have been any number from 1 to 26. So the sample space has 26 possibilities (there are 26 different ways to apply a caesar's cipher to the message).(12 votes)

- So if the shift is 3, wouldnt a Z be a C?(7 votes)
- Yes it would, since there are no more letters after Z so you'll have to start the beginning which A then B and finally C.(8 votes)

- A cipher method I found quite interesting is the "Pig Pen Cipher" which is, according to wikipedia; "a geometric simple substitution cipher which exchanges letters for symbols which are fragments of a grid". I found the basic substitution quite simple but what about Pig Pen Cipher's that contain code words that disrupt the order of letters in the grid?(9 votes)
- I do not quite understand what you are saying, but if you are saying something about cracking the Pig Pen cipher I can give you some advice. If you want to now how the Pig Pen cipher works, skip the next paragraph.

The Pig Pen cipher is one of the many symbol ciphers, where a symbol is designated to each letter in the alphabet. Now, the Pig Pen cipher is very common code, so many people might know it, but say a random code is made up. You would collect the sample, and analyze it the same way. You would make a chart of the symbol frequency though, and compare each symbols height to a letter.

If you wanted to know how a Pig Pen cipher works, read below.

A Pig Pen Cipher is a symbolic cipher, with a different symbol representing each letter of the alphabet. It is a set code, and never changes. The link below shows a chart with each letter in its part of the grid. (Obviously you do not draw the letter in the secret note.)

https://www.google.com/search?q=pig+pen+cipher&safe=active&espv=210&es_sm=91&source=lnms&tbm=isch&sa=X&ei=jLkLU4rnIcTuyQGe04DoDQ&ved=0CAkQ_AUoAQ&biw=1598&bih=815&dpr=0.9#facrc=_&imgrc=Nu1Ck4zZZcOcZM%253A%3B9RAZ76S0Tq0H5M%3Bhttp%253A%252F%252Fupload.wikimedia.org%252Fwikipedia%252Fen%252F7%252F7e%252FAc2_cipher_key.png%3Bhttp%253A%252F%252Fen.wikipedia.org%252Fwiki%252FPigpen_cipher%3B600%3B600(4 votes)

- How would you find the frequency of an encrypted message?(3 votes)
- Hello Josiah,

You could do it manually by printing the note on paper. Then use a scissors to cut the individual letters. Next sort the letters into 26 bins ( a through z). Finally count the number of times each letter occurred.

Most would argue this is an inefficient way to do the operation but that is what the statistical analysis tells us to do...

In reality you would uses a computer program to count the number of times a letter occurred. The computer could do the calculation for the entire dictionary in minutes.

Ref: https://en.wikipedia.org/wiki/Letter_frequency

Regards,

APD(8 votes)

- At0:37MEET is encrypted as PHHN. M is shifted "ahead" 3 letters to P, E is shifted "ahead" 3 letters to H (twice over), but then T is encrypted by shifting to the letter that PRECEDES it (by three)... why?(5 votes)
- Because it's wrong; the 't' in "at" or "elephant" should be the same letter but it's W. The sequence should be 'p', 'h', 'h', 'w', 'p', 'h', 'd', 'w', 'h', 'o', 'h', 's', 'k', 'd', 'q', 'w', 'o', 'd', 'n', 'h'(2 votes)

- When did Caesar start using this cipher?(3 votes)
- He used it whenever he needed to communicate to his officers, but I'm not sure when he started to use it.(5 votes)

- I am not able to understand how shifts are determined using frequency?(3 votes)
- Well for example E is the most common letter in the English alphabet. So if you look at some cryptotext and see that C is the most common symbol, you might infer that C actually represents the letter E. Shifting by -2 would give that result (E --> D --> C), so you might try shifting by +2 to decrypt.(4 votes)

- Is Caesar's code still used in today's world? If so, what sort of things use it?(3 votes)
- what does obvious mean because i`m in 1st grade.(1 vote)

## Video transcript

SPEAKER 1: The first well known
cipher, a substitution cipher, was used by Julius
Caesar around 58 BC. It is now referred to
as the Caesar Cipher. Caesar shifted each letter
in his military commands in order to make them
appear meaningless should the enemy intercept it. Imagine Alice and Bob decided
to communicate using the Caesar Cipher First, they would need
to agree in advance on a shift to use-- say, three. So to encrypt her
message, Alice would need to apply a shift
of three to each letter in her original message. So A becomes D, B becomes
E, C becomes F, and so on. This unreadable, or
encrypted message, is then sent to Bob openly. Then Bob simply subtracts
the shift of three from each letter in order to
read the original message. Incredibly, this
basic cipher was used by military leaders for
hundreds of years after Caesar. JULIUS CAESAR: I
have fought and won. But I haven't conquered
over man's spirit, which is indomitable. SPEAKER 1: However,
a lock is only as strong as its weakest point. A lock breaker may look
for mechanical flaws. Or failing that,
extract information in order to narrow down
the correct combination. The process of lock breaking and
code breaking are very similar. The weakness of
the Caesar Cipher was published 800 years later
by an Arab mathematician named Al-Kindi. He broke the Caesar Cipher
by using a clue based on an important
property of the language a message is written in. If you scan text from
any book and count the frequency of
each letter, you will find a fairly
consistent pattern. For example, these are the
letter frequencies of English. This can be thought of as
a fingerprint of English. We leave this
fingerprint when we communicate without
realizing it. This clue is one of
the most valuable tools for a codebreaker. To break this
cipher, they count up the frequencies of each
letter in the encrypted text and check how far the
fingerprint has shifted. For example, if H is
the most popular letter in the encrypted
message instead of E, then the shift was likely three. So they reverse
the shift in order to reveal the original message. This is called
frequency analysis, and it was a blow to the
security of the Caesar cipher.