"Cryptography - Study of Secure Communication"
Cryptography comes from the Greek words
"kryptos" meaning 'hidden or secret' and
"graphein" meaning 'writing'
Another related concept is Steganography.
Steganography is the combination of two greek words,
"steganos" meaning 'covered concealed or
protected' and "graphein" meaning 'writing'.
Both are the techniques used for secure
communication.
So, what are the differences between these two?
In cryptography it is assumed that the
encrypted message is accessible by all,
but only the intended recipient should
be able to decrypt the message. On the
other hand, steganography focuses on
hiding the messages in such a way that
only the recipient knows that the
message even exists. These hidden
messages can either be encrypted or not
depending on the requirement.
To sum it up, steganography adds another level of
security by hiding the encrypted message.
Moving ahead with cryptography, it
consists of two processes, encryption or
ciphering and decryption or deciphering.
Ordinary information called 'plaintext' is
encrypted into unintelligible
information called 'ciphertext'.
This conversion from 'plaintext' to 'ciphertext'
and back from 'ciphertext' to 'plaintext' is
done using a 'specific set of information'
called KEY.
Just like traditional locks have a physical key which locks and unlocks it. In cryptography, set of
information known as key is used to
encrypt and decrypt the messages.
Cryptography can be implemented in two
ways, depending on the type of key used.
Symmetric key or Private key Cryptography and, Asymmetric key or Public key Cryptography
Symmetric key or Private key cryptography uses the same key to encrypt and decrypt the messages.
A simple example is the key which shifts
the letters three places. When Alice
wants to send a message to Bob, she will leftshift
the letters three places. 'E' will be
sent as 'B'. She will also have to send the
key to Bob. On the receiving side, Bob
will use the key sent by Alice to recover the message.
Letters will be right shifted three places and he will read,
'B' as 'E'. As the same key is required for
encryption and decryption therefore a
secure channel is required to transmit
the symmetric key. Anyone who gets hold
of this key can read all their messages
If Alice wants to send a message to Bob
using asymmetric public key cryptography
then she will have to once use a key
generation program to generate a
public/private key pair. As the name
suggests, public key is meant to be
published and transmitted freely over
unsecured channel, while private key is
kept secret. The strength of public key
cryptography lies in the fact that it is
not computationally feasible to deduce
private key from its corresponding known public key.
Here, Alice will use her private key
to encrypt the message and share her
public key with the recipient Bob. And,
Bob using Alice's public key will be
able to retrieve the message.
There are pros and cons of both
symmetric and asymmetric key cryptography.
Symmetric key or private key cryptography uses less complex algorithms and are faster.
Hence it is better suited for larger volumes of data,
while in public key cryptography the
private key is never transmitted and
hence it does not require a secure channel.
Most practical systems use a combination of both types of cryptography to get the best out from each method.
These are called 'Hybrid Cryptographic Systems'.
Before continuing, pause the video here and click on the card or link in the description to watch a short
video about 'Applications of Public Key Cryptography'.
This will be useful for better understanding of the next part.
Alice and Bob want to have a secure
communication on an unsecured channel.
Both, Alice and Bob will generate their
private-public key pair.
They will exchange their public keys while keeping
their private key secret.
This exchange of public keys is done only once.
To send a message, Alice will use a
'symmetric key' to encrypt the message into 'ciphertext'.
In this case, symmetric
key is called 'session key'.
Next, she will use Bob's public
key to encrypt the session key and send
it to Bob once at the beginning of every session.
At the receiving side, Bob will use his private key to decrypt the session key.
Note that, only Bob can decrypt the session key as it was encrypted using his public key.
Once he retrieves the session key, Bob can decipher all the messages sent by Alice in that session.
This is used in web
browsers. Every time when you log in to a
website, a new session is created and the
session expires after logout or certain time of inactivity.
Cryptography is tricky, watch the video repeatedly to understand it better.
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