Introduction to Text Encryption

Encryption

Encryption:

Encryption = the process of encoding messages (usually text) in such a way that eavesdroppers or hackers cannot read it, but that authorized parties can

Process of encryption:

The input text (or plaintext) is processed by an encryption algorithm:
The encryption algorithm uses an additional input that changes its behavior.
The encrypted text is called cyphertext
The cyphertext can only be correctly decrypted by the decryption algorithm using the correct decryption key:

Note:

The encryption and the decription algorithms are assumed to be public
(Meaning: everyone know exactly how the encryption and the decription algorithms operates)
The decription key must be kept secret
(or else, everyone can decrypt the cyphertext !!!)

Types of encryption schemes

There are two types of encryption schemes:

Symmetric-key schemes:

In a symmetric-key scheme, the encryption and decryption keys are the same:

Note:

Because the decryption key must be kept secret, and the encryption key is equal to the decryption key...
Therefore:

Asymmetric-key schemes:

In a Asymmetric-key scheme, the encryption and decryption keys are the different:

Note:

Because the decryption key must be kept secret, and since the encryption key is not equal to the decryption key...
Therefore:

A simple symmetric encryption scheme

Rot13:

ROT13 ("rotate by 13 places") is a simple letter substitution cipher that:

Example:

Try it out yourself: click here

Note:

The Caesar Cypher

Caesar Cypher:

each letter in the plaintext is replaced by a letter at some fixed number of positions down the alphabet.

Example:

Suppose we use shift right 3 positions to encode:

Encode:

ABCDEFHIJKLMNOPQRSTUVWXYZ XYZABCDEFHIJKLMNOPQRSTUVW --> shift right 3 positions Plaintext: HELLO Cyphertext: DBIIL

Decode: shift left 3 positions

ABCDEFHIJKLMNOPQRSTUVWXYZ DEFHIJKLMNOPQRSTUVWXYZABC <-- shift left 3 positions Cyphertext: DBIIL Plaintext: HELLO

Note:

The encryption key used in Caesar cypher is the amount of shift (direction can be encoded as positive or negative)
If you find out that the cyphertext is encrypted with the Caesar cypher method, you cannot it !!
unless you know:

Note:

Application of encryption
- Secure communication:

Communication paradigm using Symmetric keys

Secure communication using symmetric key must following the following procedure:

Before any secure communication can occur, the communicating parties must first obtain the encryption/decription key:
Secure communication is achieved by keeping the key secret:
The adversary cannot decode the cyphertext because he/she does not have the key.

Where are symmetic-key schemes used:

Military
Coorporation
Etc.

In general:

Symmetric-key schemes can be used in situations where:

Strength and weakness of the symmetric-key scheme

Advantage:

Disadvantage:

The communicating parties must meet in private (at least once) to establish a common key before the symmetric-key scheme canbe used
Consequently:

Communication paradigm using asymmetric-key encryption

Secure communication using asymmetric key must following the following procedure:

Before any secure communication can occur, each communicating party must first publish his/her encryption key:
Each communicating party must keep his/her decription key secret !!!

Secure communication is achieved as follows:

The sender uses the receipient's encoding key to encode the message:
The cyphertext sent (and may be intercepted by the adversary)
The cyphertext can only be decoded using the associated decription key:

In assymetric key encoding, the cyphertext that is encoded with a encryption key can only be decode using the associated decryption key !!!

The decryption key must be kept secret !!!

Usages of asymmmetric key encryption:

Protect emails that you want to send to someone that you have never met before
(This kind of communication is popular in underground organizations whose members don't have the opportunity to meet in person)

Public key encryption

Fact:

The asymmetric key encoding scheme cannot use the same key for encryption and decryption
Because:

Historical development:

The idea of assymmetric key encoding was published back in 1874 !!!
In 1997, it was publicly disclosed that asymmetric key algorithms were secretly developed by James H. Ellis, Clifford Cocks, and Malcolm Williamson at the Government Communications Headquarters (GCHQ) in the UK in 1973.

An asymmetric-key cryptosystem was published in 1976 by Whitfield Diffie and Martin Hellman who, influenced by Ralph Merkle's work on public-key distribution, disclosed a method of public-key agreement.

This method of key exchange, which uses exponentiation in a finite field, came to be known as DiffieHellman key exchange.

This was the first published practical method for establishing a shared secret-key over an authenticated (but not private) communications channel without using a prior shared secret.
Merkle's "public-key-agreement technique" became known as Merkle's Puzzles, and was invented in 1974 and published in 1978.

A generalization of Cocks's scheme was independently invented in 1977 by Ron Rivest, Adi Shamir and Leonard Adleman, all then at MIT.