Hello and welcome to a new video on the CrypTool 2 YouTube channel. In this video I want
to give you an introduction to the basics
of cryptology. I structured this video into
several sections. In the first section we want to talk about what is cryptology and in the
second section we want to talk about the basic terms of cryptography. And as you can see
here the third part is written in gray color
and this is because the third part will
be covered in the next video. So
what is cryptology? Cryptology consists of
two parts. The first part is cryptography, which is the art of making ciphers and then we
have cryptanalysis which is part of breaking
ciphers. And here on the right side you see
a tree, where I tried to further divide parts
of cryptology. On the left side you see the
cryptography. On the right side you see the cryptanalysis. And the cryptography can be
split into classical and modern cryptography. And same can be done with the cryptanalysis
which also can be separated into classical cryptanalysis, which was mostly done by hand
or maybe with some early machines. And of course we have the modem cryptanalysis which is done
with modern computing techniques. Then on
the left side the classical cryptography can
be split into a substitution and transposition ciphers. And the modern cryptography or at
least the ciphers that we have with modern cryptography can be split into symmetric
and asymmetric ciphers. In this video we want to have to look at cryptology, cryptography,
classical cryptography and the substitution and transposition. Here are the first basic terms
that you will find in every book about cryptology or cryptography when we speak about cryptography.
The first thing is a cipher and the cipher
is an encryption method or algorithm. In many
books and papers you also find cipher for ciphertext but we define cipher only as the encryption
method or algorithm. Then we have of course the plaintext. The plaintext is a non-encrypted
text that we want to encrypt. Then we have the ciphertext, which is the result of applying
a cipher on a plaintext and the ciphertext
of course is encrypt. Then we have the key.
And the key is some kind of secret information that is used for encryption and of course
which is needed for decryption. And the key is the only thing that we keep secret. We don't
keep the cipher secret and we don't keep how the cipher works secret. So a crypto algorithm
or method has to be secure by an attacker knowing the complete cipher itself but not
having the key. Then we have the alphabet. And the alphabet can be split into a plaintext
alphabet and ciphertext alphabet. And an alphabet is an order set of letters
or symbols which are used for encryption
or decryption. Here, I tried to show how all
these words are connected. So, as you can see here on the left,  the plaintext goes into the
cipher. Also the key goes into the cipher.
The cipher then performs a methods or an algorithm
on the plaintext with the help of the key.
And the result of the cipher then is a ciphertext.
Here is a short example for the a world
cipher. In this case the Caesar cipher. On the
left side you see here the plaintext alphabet that is used with Caesar. That's a Latin
alphabet from A to Z. Then we have a
plaintext message here, which is "Hello world".
In the middle we have the Caesar cipher that we use for encryption, which also needs a
key. In the case of the Caesar cipher it's
a shift key and its value is 13. If you're
further interest in the Caesar cipher I've already made a video about this so you may have a
look at this. And here on the right side we now see the ciphertext alphabet which is
shifted by 13 so it doesn't start at A it
starts at N and it doesn't end with Z,
it ends with an M. So the A is encrypted
to N. The B is encrypted to O and so on and
so on. And the Z is encrypted to M. And the result of course of the Casear cipher is
the ciphertext here which is U R
Y Y B J B E Y Q. There are three types of
classical ciphers. Two main types and an additional one. The first main type is the substitution
ciphers type and substitution ciphers replace letters by other letters or for example symbols.
An example for a substitution cipher is a
Caesar cipher, or the simple monoalphabetic substitution
cipher, or the Vigenère cipher. We already had videos on all these three so you may have to look
at the classical ciphers series on this channel. Then we have the second type of ciphers, which
also transposition ciphers. These change the order of the plaintext letters but the plaintext
alphabet is the same as the ciphertext
alphabet then. Only the position is changed.
Examples for this are the Scythale cipher or the columnar transposition cipher. We also had
videos on these. And the third type of ciphers are the composed ciphers. And the composed
ciphers are ciphers that are a combination of, for instance, different substitution ciphers
or different transposition ciphers, or  also a combination of substitution and transposition
ciphers. Examples for this are the ADFGVX cipher. We already had a video about this. And the granite
cipher, which was used in the Cold War in Germany. Then, there are other important words that
we have with cryptography. For instance the keyspace. The keyspace is a set of all
possible keys of a cipher. For instance with the Caesar cipher i have an example here. The keyspace
are all numbers from 0 to 25. These are all possible shift values for the Caesar cipher
and the identity, that is the shift key equal to zero, is also included in the keyspace
The keyspace size of course then is
the size of the set of all possible keys of
a cipher and usually cryptanalysts or cryptographs give these as a rounded up power off
two. In the case of the Caesar cipher the keyspace size of course is 26, because we have
26 numbers here. And this is about two to the power of five. With substitution ciphers we
also have some additional basic terms. First of all we have monoalphabetic substitution
ciphers. The monoalphabetic substitution cipher only has one ciphertext alphabet. An example
for this is the Casear cipher or the simple
monoalphabetic substitution cipher. Then,
we have the polyalphabetic substitution ciphers. And with these ciphers, the alphabet is changed during
the encryption. An example for this is the
Vigenère cipher or the Enigma machine.
With the Vigenère cipher and the Enigma machine we also had videos on this channel. So you
may have to look at these. Then we have the homophonic substitution ciphers and with
a homophonic substitution a letter is encrypted by
more than one letter or symbol. So A is for instance
encrypted with 01 and 05. And B is encrypted with 09 and 07, and so on. Examples for this
are the Zodiac killer ciphers (we also had a video on this) or historic ciphers that were
used in the Vatican. Then, we have polyphonic substitution ciphers and with this ciphers, different
plaintext letters are encrypted by the same ciphertext letter. And this makes this kind
of cipher problematic, because they are
non-deterministic and the decryption is ambigious.
And the problem is when you encrypt an A
with an X and a B you also encrypt
with an X. Then when you have the X in the ciphertext and want to decrypt you don't know
was it an A or was it a B. So you need the context of the letter, for instance you know
three letters before and three letters after this and then you know, okay, this can only be an A.
And examples for this are historic ciphers
that were also used in the Vatican. So we have
examples for polyphonic substitution ciphers and they're really hard and difficult for cryptanalysis.
Then, we have other basic terms for cryptography. For instance we have monographic ciphers.
And with a monographic cipher one letter is encrypted at the same time. An example
for this is the Casear cipher, where you first encrypt the first letter, then you encrypt the
second letter and so on. Then we have bigraphic ciphers. And with biraphic ciphers
letter pairs are encrypted at the same time. So you encrypt the first two letters, than the
second two letters, then the third two letters, and so on. An example for this is a Plaifair cipher
where we also had a video on this channel. Then we have monopartite ciphers. And with a
monopartite cipher the replacement of a
letter or maybe more letters is a single
letter. Also the Caesar cipher is an example for this. We replace a single letter
with another single letter. And then we have the bipartite cipher. And with the bipartite cipher we replace
a letter for instance with two letters. An
example for this is the ADFGVX, where we
first or in the first step where we encrypt
a letter for instance an A with DD.
Or a B with AX. And so on. So we replaced
one letter by two letters. So the "graphic" here
also refers to the plaintext. So what we do
with the plaintext. And the "partite" here always
refers to what we do in the ciphertext.
An example is for instance the simple monoalphabetic
substitution cipher or short simple MASC,
which is a monoalphabetic monographic
monopartite substitution cipher. So this
was everything that I wanted to discuss with
terminology. So now we will have to look in CrypTool 2 at substitution ciphers. Then,
we will have to look at transposition ciphers. And finally we will have a short look at composed
ciphers. I am here now CrypTool 2 and I want to first show you some examples for substitution
ciphers or classical substitution ciphers.
And you find the substitution ciphers by
going to "cryptography" and "classical". And here we have substitution and transposition ciphers.
And a first example for simple substitution cipher is the Caesar cipher. Just double
click it then it opens a work space. Make some more space. And in CrypTool 2 we have examples
for all kinds of ciphers and here's a Caesar cipher and to execute it in CrypTool 2 you just
click the "play" button. And what do we see here. We have a shift of 10. So, as I already
said the Caesar cipher shifts the alphabet - the plaintext alphabet. And you can specify
here the alphabet that that should be used. And for instance the T is substituted by D.
The H by R, and so on. And as always in CrypTool 2 you can type and it will live encrypt
what you are doing. "Hello, this is a test of the Caesar cipher". And the Caesar cipher is now set to ignore. No its not set to
ignore it should contain the source case. So if you write lowercase and uppercase it will encrypt
also in lowercase and uppercase. So let's
have a look at another substitution cipher.
For instance, we could have a look at the Vigenère cipher here. So we just
double click the Vigenère cipher. And we will write a text. "Hello, this is a test of the Vigenere cipher". We already had videos about
this, so I don't describe in detail how these ciphers work. So if you're interested in
this, you should have you should have a look at the other videos. And of course we have
to click play. And then, we see here the Vigenère cipher -  encryption and the decryption. Now let's
have a look at an example transposition cipher. For this I go to the columnar transposition.
So, we have here the transposition cipher. Double-click! And then, we write "Hello this is a test of the transposition cipher".
And you also click on play. You can specify the key here. The plaintext here. Here you see
how the transposition cipher actually works. We also made a video about this so you could
have a look at this video. And here we have the decryption - also transposition cipher. Here we have the ciphertext
and here we have the decrypted ciphertext. So let's have a final look at a composed
cipher. For that we have the ADFGVX. And we just open this here. And we have our plaintext
here: "This is a test of the ADFGVX cipher". We also had a video about the ADFGVX. So for details please have a look
at this video. And the ADFGVX cipher
is, as I said, a composed cipher. So it
needs a substitution password (=key). This is "substitution". And a transposition password or key
not password: "transposition". And we copy this also to the second component - ADFGVX component -
because this decrypts. We have here encryption and decryption. And let's test it. So it first
uses a substitution to substitute the text,
then it transposes the text. And then we have here the ciphertext, which only consists of A D F G V X.
And of course the decryption here: "This is a test of the ADFGVX cipher". So, in this video
I showed you the basic terminology of cryptography and the cryptanalysis part will be in the
next video. There, I will show you different attacks on the ciphers and the terminology of cryptanalysts
used for these attacks. And, yeah, thank you very much for watching! I hope you like this
video! If you do please give a thumbs up and if you're not yet subscribed to our channel
I would be really happy if you subscribe to
our channel. And see you in the next video!
