# Introduction to Cryptography

in #blockchain3 years ago

Cryptography comes from the ancient greek meaning: "crypto" = secret, "graphy" = writing.

Let's say that Messi wants to send Pelé a message, but he doesn't want anyone else to see the content of the message. Messi must send the message to Pelé via a non secure way, like the Internet or via Radio. However, he is not sure that the message won't be seen by any other person. What if there is a middleman trying to read the secret message?
The solution for Messi is simple: use cryptography!
If he encrypts the message he won't have to worry if someone will get it. Pelé then needs to decrypt the message to read it.

### Some terminology

Cryptography: method to send secret messages using a code
Crytanalysis: trying to break the code and read those messages
Plaintext: message in its original form. very easy to understand its contents
Ciphertext: message in coded form
Key: mathematical operations we can perform on a value
Encryption: transforming PT to CT
Decryption: transforming CT to PT
Cipher: the method used for encryption
Steganography: the practice of hiding information

### How do we know if a cryptographic technique is "secure"?

We let lots of smart people try to break it (cryptanalysis).
If they can't, we assume it is secure.

### Frequency analysis

Cryptanalysis technique described in the 9th century by Al-Kindi in Iraq.
Not all letters in a language occur with the same frequency.
Some languages have certain characteristics. For example, here are some patterns of the English language:

• E is most common letter
• vowels tend to be separated by consonants
• Q tends to be followed by U
• etc.

### Caesar algorithm

Substitute letters by N letters away (e.g. N = 3. A is D, B is E, C is F...)
How do we break it? try all possible values for N.
Example:

If we try here all the 26 letters we will easily get to the key 3.

### Vigenére

It's a poly-alphabetic cipher where one plaintext letter can become different ciphertext letters. It uses a text based key and a modulo arithmetic to perform the encryption. Using frequency analysis is possible, but much more difficult.
Example:

Let's chose a key: MONKEY
We now add the numbers of the letters and get the result letter:

Vigénere cipher with a randomly chosen key as long as the message. The key needs to be shared between parties beeforehand and can NEVER be re-used. Provable unbreakable without the key.
This is the only perfect cryptography.
Example:

All of the previous techniques have two basic components:

1. algorithm (what you do to the message)
2. key (the secret that you need in order to encrypt/decrypt properly)

When using these algorithms, the key is the secret. The algorithm is public.

More details:
Cryptography (Wikipedia)

Check out my Blockchain website: Operation Blockchain

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