 # Symmetric encryption algorithms

##### The most important and interesting about encryption.
###### A series of articles understandable even to non-specialists.

One of the well-known and widely used algorithms of symmetric encryption was DES (Data Encryption Standard). This algorithm was developed by IBM Corporation (USA) in 1977. As of today, the DES is obsolete because it has ceased to be crypto-resistant due to advancements in computing power. DES is a block encryption algorithm that uses 16 rounds of encryption, a 56-bit key and a 64 bits block size.
The DES algorithm encryption scheme looks like this:

#### Features of the DES Algorithm Encryption

The possible number of keys for the DES algorithm is given by 2 raised to the power of 56. This gives 72 057 594 037 927 940 variants of possible keys. In 1998, 20 years ago, aided by a super-powerful computer called DES-Cracker at the time, DES was successfully cracked in 3 days.
Nowadays, the most relevant symmetric encryption algorithm is AES (Advanced Encryption Standard). It was declared the encryption standard in 2002.
AES is a block algorithm that has a block size of 128 bits, and its keys could vary among 128, 192 and 256 bits long. Its number of rounds depends on the length of the key and could be 10, 12 or 14 rounds, respectively. When the AES algorithm was tested, it was found to be resistant to attacks after the 8th round. Thus, the minimum number of encryption rounds is 10.
The AES algorithm applies operations such as shift, mixing, substitution using tables, and various algebraic calculations. For example, one of the operations looks like this:

As for the resistance of the AES algorithm, cryptographers have repeatedly conducted studies on its cryptographic robustness. As a result, theoretical models of attacks on this algorithm were developed, and specialists in the field of cryptography, after comparison, resulted that supposing the DES algorithm could be hacked in one second using a mythical supercomputer, it would take the same computer 149 trillion years to hack the AES algorithm.

Symmetric encryption algorithms are good because they have a high speed of processing information. These algorithms are easy to modify due to the relative simplicity of operations. In addition, symmetric algorithms can encrypt large amounts of data with a relatively small key size.

But symmetric algorithms have one major drawback.
Imagine that Alice left Bob a letter in the mailbox, and locked the box. Now, this key must be somehow passed to Bob so he could get access to the letter and its information from that box. Should Alice send the key by pigeon post or by courier, there is a great risk that Eve will intercept the pigeon or courier, take possession of the key and read the letter that was intended for Bob. It is still a question of whether Bob will receive the letter, and if he does, Eve would have already known the secrets of Bob and Alice.
It turns out that the need to pass the key to decrypt the message is one of the main problems of symmetric encryption algorithms.

Accordingly, Alice needs to somehow pass the key to Bob in a safe way. And this can be done with asymmetric encryption algorithms.

### What is Encryption: basic concepts

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