The Caesar cipher | Journey into cryptography | Computer Science | Khan Academy

Brit explains the Caesar cipher, the first popular substitution cipher, and shows how it was broken with "frequency analysis"

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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.