This probably looks like gibberish to you,
and it should because it's a cryptogram,
a message in code.
But, if I told you that all I did was
shift every letter in the sentence
to the next one in the alphabet,
then you'd know that it translates to this.
To encrypt a message, you need two main parts -
the cypher and the key
The cipher is the set of rules
that are using to encode the information
for example shifting the alphabet by
certain number of letters
The key tells you how to arrange those rules
otherwise they'd be the same every time
and it would be easy to decode the message in
this case the key would be one because
we shifted the alphabet by one letter to
decrypt the information you need to know
what kind of cipher was used and also
have the key
or you can just crack the code either by
trying all possible combinations you can
think of or by analyzing the code and
working backward from it, known as deciphering.
But is it possible to come
up with a combination of a cipher and
key that could never be determined
Is there such a thing as an unbreakable code?
Well people keep coming up with new and
better ciphers,
but it's hard to make them unbreakable
because, no matter what,
you're using a set of rules to encrypt
the information and with enough time and
enough data
someone can usually uncover those rules.
That puzzle i just gave you is one of
the oldest and simplest ways to encrypt
a message.
It's usually called a caesar cipher
and in this case the key was just
a number representing how many letters
of the alphabet I shifted it but it's
also very easy to crack.
Even if you didn't know the key it would
take you at most 25 tries to decode the
message because you know the whole
alphabet has to be shifted by a certain amount
Since there are 26 letters in the
alphabet, there are only 25 other options
A caesar cipher is one simple type of 
monoalphabetic ciphers
a class of
ciphers for the whole code is based on
one letter of the alphabet standing in
for another letter consistently
throughout the whole message basically
just scramble the alphabet in that case
the key would just be a list of which
letters correspond to which
like this one
There are over 400 septillion possible
ways to encrypt this kind of message
so you'd think it would be hard to crack
and it is but only a little bit because
there are lots of ways to decode
messages just trying all of the possible
keys to a code is probably the most
obvious and least subtle and it has an
equally unsubtle name - brute force
But you can try a more
sophisticated technique something called
frequency analysis,  which is based on the
idea that every language has its own specific patterns
In English for example
the letter E shows up a lot
I used it seven times in just the last
sentence
And some words like "the", are so common
that it's hard to even use a
sentence without them
Cryptographers call these words cribs
So frequency analysis looks for common
words and also common letters or sets of
letters like "ed" or "ing" at the end of words.
If you find that the letter X is
showing up a lot in a message and so is
the three letter word IRX you might
guess that in the key X corresponds to
the letter E and IRX spells THE and
once you've figured out those letters
you can figure out the rest by
recognizing other words and using the
process of elimination
And since longer cryptograms contain
more clues
they're easier to crack. So
monoalphabetic ciphers are fun but
they're not hard to break. If you want to
get a little fancier with your
encryption you can use polyalphabetic
ciphers instead they're much more
effective. In a polyalphabetic cipher the
way you scramble the alphabet actually
changes throughout the message. In the
first word s might translate to w
but in the last word s might translate
to h. It all depends on the particular
encryption method you're using and on
your key.
One of the earliest polyalphabetic
ciphers was the VIGENERE cipher
developed in the 16th century was pretty
simple because the key was just a word.
So let's say you want to encrypt "SciShow is the greatest" using a VIGENERE cipher
well the first thing you need to do is
write out a VIGENERE square.
The alphabet goes across the top and along
the left side and each row contains the
letters A to Z shifted over by one
So the first line starts with A and ends
with Z but the second starts with B goes
all through the letters until Z and then
ends with A and so on. You end up with 26
differently scrambled alphabets and now
you're ready to encode the message
You just have to pick a key
Let's just say your key is Michael, you
write out your key multiple times until
it fills the same number of letters as
your message so "SciShow is the greatest"
would correspond to this. Then to encrypt
it, you take each letter of the message
and move along its row in the
VIGENERE square until you get to the
column of the corresponding letter in
Michael. So "scishow is the greatest" turns
into this
That's much tougher to decode unless you
have the key because those letter
frequencies are all different now. Since
the keyword Michael is seven letters long
each letter of your message is encrypted
using seven different scrambled
alphabets. But if your text is long
enough it's still crackable using a type
of frequency analysis developed in the
19th century by cryptographer
Charles Babbage Babbage realized
that in a long enough message some
patterns and the coded message will
still show up like if your key only has
seven letters that means that there are
only seven ways to encrypt the word the
but if your message uses the word the
eight times there are definitely going
to be repeat. So he just counted how many
letters separated those repeated
patterns. If they were separated by 7 14
or 21 letters, he knew that the key was
probably seven letters long
and from there he would just use
frequency analysis to figure out the
seven scrambled alphabet
Babbages method
is just one example of why it's so hard
to create an unbreakable cipher.
Your key
creates a pattern within the encrypted message
And with enough work a spy can
uncover that pattern
It turns out that the only way to have a really unbreakable cipher is to use what's
known as a one-time pad encryption, which uses a key that is as long as the message itself.
That way there aren't any patterns in the
encrypted text. There's nothing to
analyze, so there's no way to work
backwards.
The sender and the recipient both have the same pad
and each sheet contains a long set of random letters
which is used as the key once she is
used to decode a message you destroy it
Then you use the next 
sheet for the next message so you never repeat a key.
As long as you keep the pad safe the
message can't be decrypted by anyone else.
But you can't always use one time
pad encryption.
Let's say you needed to get a message to
someone halfway across the world whom
you'd never met, you wouldn't have a
chance to give them a matching pad.
In warfare that sort of situation comes up
a lot which is why in the early 20th
century there was suddenly plenty of
incentive to come up with better ciphers
Remote communication using technology
like the telegraph was incredibly
valuable during wartime but it was
essential that only your allies
understood what you were saying.
The Germans experimented with a new more
complicated mono-alphabetic cipher
during World War I
but eventually the French managed to
crack it.
Then during World War II the Germans again came up with the new cipher
And this time its security seemed perfect, 
maybe you've heard of it
The Enigma machine.
The machine used a
poly-alphabetic cipher that scrambled the
alphabet in a different way each time
you typed a new letter.
As far as the Germans knew the only way to decipher the message, was to have your own Enigma machine
and set it up using a secret key
that changed every day.
The machine was meant to work like a
one-time pad in the sense that the
alphabet was re-scrambled for every letter of the message.
But instead of having to distribute a set of sheets to everyone
you could just use a key that told the users how to set up their Enigma machines
And you could change that key is
often as you wanted, but it had a few flaws.
For example no letter could be encoded as itself. That might not sound like a
big deal but it ended up being a fatal
weakness.
British mathematician Alan Turing along
with the rest of his team designed a
machine of their own that could crack the Enigma code as long as they knew
around 20 of the characters contained in the message - which they often did because
some phrases tended to show up a lot in Nazi communicates, especially nice things about Hitler
So part of the strategy of Turnings team
was to look for cribs - those common words
and phrases - and see where they might fit.
For instance if they knew a message
contained the word Führer, they could look
for places in the text that didn't have
the letter F, since they knew that the F
in Führer couldn't be encoded as itself
Those Clues helped them figure out how the Enigma rotors were set up to encrypt the message.
Cracking the Enigma code was a
huge advantage for the Allies.
Many historians attribute some of the most
important victories during the war to
information the Allies got from the
Enigma encrypted messages.
These days encryption is mostly important in digital computing, and that isn't perfect either
When websites announced that hackers
now know everything about you
that's because their encryption methods were breakable
Companies that store your data have to take into account a whole new set of considerations
Like how when you can complete billions of operations per second
brute force suddenly becomes a lot more practical. So the same principles that
Vigenere and Turing used are the same ones that allow you to pay your bills online
and keep North Korea out of your email, most of the time.
But HOW, is a story for another episode
Thank you for watching this episode of scishow,
which was brought to you by our patrons on patreon.
If you want to help us keep making
videos like this check out patreon.com/scishow
And don't forget to go to
youtube.com/scishow and subscribe.
