This would be it for theory.
Let's now see it in practice.
The everyday use of the solutions is as follows.
Symmetric ciphers have their shortcomings.
The biggest limitation they have is they require managing a huge amount of keys and don't offer the
option to exchange them
repudiation and authenticity cannot be assured either asymmetric ciphers on the other hand are not useful
for encrypting long messages.
There are slow and inefficient basaltic key exchange problem and provide for the ability to prove ascenders
identity and establish the authenticity of a message by combining the best of the two solutions you
can arrive at a hybrid cryptographic system.
All systems today in fact use a hybrid scheme SSL FS and H TTP has all these hybrid cryptography.
The idea behind it is that while messages are encrypted using symmetric cryptography The key is used
for encryption or encrypted using asymmetric cryptography.
The keys are put in a digital envelope and sent along with the ciphertext.
The word send is used in a broad sense.
It doesn't require for example sending data over a network between users A and B.
The data can also be put on a disk.
The message circulates in time until someone finds it and decrypts it.
So let's take a look at the encryption process in a hybrid system.
You have a message that must be encrypted using symmetric cryptography.
For example using the advancing Christian standard to do this you need a key.
Let's call it a symmetric session key.
It will be pseudo random and secure.
You have successfully generated.
Next you should asymmetrically encrypt the generated symmetric key using for example RSA or El-Gamal
the output is saved with the ciphertext.
This will allow you to include copies of the key that are encrypted using other system user's keys.
If the ciphertext is intended for 6 recipients you should repeat the operation six times.
The recipient's public keys will be used for encrypting the session key the number of session key copies
matches the number of recipients each copy is encrypted using a different public key.
How do you decrypt this message.
You get a cipher text to start to decrypt anything you need to have a key an encrypted session key has
to be taken from the envelope decrypt it using a private key.
There's a private key that corresponds to the public key which means that only an authorized user can
perform this operation.
The user is the intended recipient of the message.
If you have a session key the remaining process is straightforward decrypt the message using symmetric
cryptography the output is the original text of the message.
By the way of ending this module let me say a few concluding remarks.
Remember that developing a cryptographic system is always difficult.
And creating a cryptographic system that is secure and not vulnerable is perplexing and nearly unfeasable
don't use algorithms you designed yourself in a production environment.
While developing or modifying cryptographic algorithms can be a great intellectual diversion if you're
not a professional cryptographer if your work hasn't been entered into a competition and tested by experts
in the field don't even think of using it and their computer system the same reservations can be applied
to using classified ciphers following Kirchhoff's principle.
And even if you decide to use a well known untested algorithm that has an appropriate strength don't
experiment with implementations of the algorithm modes of operation don't fall into the trap the electronic
codebook fell into
the Modu uses the Advanced Encryption Standard long plaintexts inputs that is split into blocks on your
own and then apply encryption.
Don't do this.
Even the relatively weak algorithms like R-S.C. for that are vulnerable to adaptive known plaintext
attacks if implemented correctly are resistant to the threats.
Even a slight modification for example adding an additional round or an additional permutation don't
modify things on your own.
Apart from this keep in mind at all times the crucial rule your system is only as secure as the weakest
element.
System vendors often try to obscure this by claiming for example that their solution guarantees total
security because connection to some banks service requires using a four kilobyte RSA key.
What they hide is that a formula by key is then used to encrypt a 56 bit key and that the following
transmission which is symmetric since a hybrid scheme is used proceeds with the weak and vulnerable
DSL.
Rhythm of potential attacker will not try to crack for kilobyte RSA key there will directly target the
D.S. encryption.
Thank you for your attention.