Welcome to demystifying cryptography. Thank you
for joining this course. So let's get started
with Section 1 introduction.
In this first video we look at
what that's cryptography mean.
We'll see how this definition evolved between
ancient time and modern time.
We also Define the Fundamental problems that today's
cryptography. Aims to solve.
So cryptography is an ancient science.
Probably it started some centuries
or even some thousands of
years ago during long time cryptography has
been the science of secret codes that
ensure the confidentiality of information through an
insecure Channel.
So let's stop and try to grasp all
the terms in this definition.
So basically we have some parties more than
I mean two or more parties and
there is some insecure Channel between these parties and
the parties would like to share some information through
this channel. They would like to ensure the
confidentiality of the information.
And to achieve that they are using secret
codes.
What is a code the code is just system of
symbols that represent that information?
And the purpose the confidentiality is
this insurance that given information
is not accessed by an authorized parties.
So only the parties that
agree to communicate through that channel should be
able to retrieve the information any unauthorized
party. That is somehow managed
to catch the message passing
or is listening to their Channel.
It should not be able to retrieve the information.
Let's an example of the very famous
secret code probably the
most famous ancient code is scissor
code for scissor code. We have
a transformation of the alphabet. So the
transformation can be represented by aligning
to alphabets. We see the plane
alphabet which is written in
small case. And then the encrypted alphabet is
the rotation of the plane original
Alphabet by some number
of positions to the left or to
the right.
Here for instance. We have scissor code
using a left rotation of three places.
As you see the a in the original alphabet
the small a became a
big D in the encrypted alphabet the
B became e Etc when
which the end for example with X. There
is no letter beyond the Z. So
we start again. That's why the encryption of
X is a
so remember we set in definition of cryptography that
it is a secret code.
So the secret here is the number of positions.
If the code is not secret for some unauthorized party.
Then that party can of course apply the reverse process and
retrieve the information. So here
for example, we see this original text
meet you at 6 and that's the the
corresponding encrypted sentence or
information. So, of course this text the
encrypted text will go through the
insecure Channel and as long as
the attack already unauthorized party does not know
the secret code which is here the
rotation by three positions to the left
then that party which would be hard for that
party to retrieve the original information meteor at
6.
Scissor code or also called scissor Cipher
is named after a Julius Caesar
the Roman general who used
it with the shift of 3, like in our example to
protect messages of military significance.
Okay in the previous slide, we basically defined cryptography
as a secrecy science.
This definition is not accurate anymore in
our modern time.
Better definition for our modern cryptography is that
it is the science of information protection against
an authorized Parties by preventing from unauthorized alteration
of use.
So we extend the guarantee.
From the retrieval of information to the alteration
of use which is more General guarantee
in the previous or in
ancient time cryptography was mainly used
for military purposes today. We
use it in almost all our
devices or applications cryptography
is what provides security of
bank cards of internet communication
https protocol relies on
cryptography mobile telephony protocols.
Also mobile communication protocols such
as Bluetooth Wi-Fi Etc.
The identity documents like biometric passports.
They also use pieces of cryptography inside
obviously systems of
access control as well and electronic voting
for instance. Aims to replace
The physical mechanisms to verify identity of
the people and to protect the
secrecy of the ballot Etc.
So all these can be achieved as well using
cryptography and Elite for electronic voting
for for instance. We can even have stronger guarantees likely
very viability with electronic voting.
You can check that your ballot was
counted in during when
they were counting the ballots which is
not possible with physical protein for instance.
And we come to the last application of
cryptography probably today is
it's the most famous application of cryptography which
is the blockchain or the
cryptocurrencies. They rely of course on underlying
cryptography and that's what makes them
secure and it allows the
appearance of the stick Roach.
Let's clarify a bit more the meaning of
alteration of use that we have seen in
the previous slide. So we set in
cryptography. We have some insecure Channel between two
parties. So here we
represent those parties by this lady that
will call Alice and the man that
will call a bob and through this
channel. Alice will send some information represented
by X to a
bob.
And then there is some unauthorized party on
on this channel that is represented by this
kind of mean character that will
call the adversary or sometimes. If
so throughout the course will have
such illustration and these characters will come
again.
Okay. So what are the guarantees we have
seen, of course the confidentiality Insurance of Confidential
Care guarantee. So the information should
not click to any an expert party. So
basically only the legitimate receiver which is
here above can get the X the second guarantee that
cryptography provides is integrity.
So the information is protected against any
malicious modification.
Which means that they received message is necessarily
equal to X.
if this adversary
Does any modification to the message then
Bob should have mechanisms or to detect
this modification?
the third guarantee of cryptography is the authentication so
the authentication is basically
the guarantee that
the information makes clear who the
author of it is.
so only the legitimate sender can insert
X if this adversary try
to take the role of Alice and
sends the message to Bob
then
Bob should be able to know that
it comes from the adversary from adversary and
not from Alice. So these
are the main principles or
core principles of cryptography
of modern cryptography and throughout
our course will focus on these
guarantees and how we can achieve
them.
In the next sections, we'll study in detail
a rich variety of modern cryptographic
methods that we let
us ensure confidentiality integrity and
Authentication.
We first start with the confidentiality and we'll
study the encryption systems more specifically
the Symmetry key cryptographic encryption
systems. So see you
in the next video.