# The Work of Symmetric Encryption and Decryption

Subject: Tech & Engineering 3 779 3 min

Encryption is the method by which data can be coded and securely transferred from one location to another. The unauthorized access of the data, as well as protection of data, is the main objective in encryption and decryption of data. The encrypted information is referred to as ciphertext. When it comes to encryption of data, two forms of encryption are possible, the symmetric encryption of the data and the asymmetric encryption of data. Symmetric encryption is also known as single-key encryption.

Symmetric encryption is the oldest and best-known technique. A secret key, which can be a number, a word, or just a string of random letters, is applied to the text of a message to change the content in a particular way. This might be as simple as shifting each letter by a number of places in the alphabet. As long as both sender and recipient know the secret key, they can encrypt and decrypt all messages that use this key.

In the case of symmetric encryption, the encryption key is shared between two parties who use the key on the sending end to encrypt the data, and on the receiving end to decrypt the data. In this case, the key is known to the sender and the receiver, who use the key to encode the message at the sender end and decode the message at the receiver end. As per the symmetric encryption approach, the individual bits and blocks in the message are encrypted one by one, one after the next, e.g., in a text of ‘xyz’ first ‘x’ is encrypted, then ‘y’ and then ‘z’. ROT13 is a symmetric algorithm which “works by shifting all letters in the alphabet thirteen places to the right – A becomes N, B becomes O, Z becomes M, etc. Because there are twenty-six characters in the English alphabet, performing ROT13 a second-time result in the original text again – N becomes A, O becomes B, and M becomes Z.

There are two main types of symmetric key algorithms for encryption. One is the stream cipher, while the other is the block cipher. The stream cipher has the amiability to encrypt small portions and bits of the message at a time while block ciphers on the other hand have the capability to take more than one bit and encrypt them as a block or a gathered collective unit. The bock unit which is mostly used in block cipher encryption is 64 bits.

Informally, for symmetric encryption scheme SE to be secure against chosen plaintext attack, the IND-CPA advantage of an adversary must be small, no matter what strategy the adversary tries. However, we have to be realistic in our expectations, understanding that the advantage may grow as the adversary invests more effort in its attack. Security is a measure of how large the advantage of the adversary might when compared against the adversary’s resources.

The advantage that symmetric algorithms for encryption have over asymmetric algorithms is that processing and encryption in symmetric key algorithms are very fast as compared to asymmetric encryption. The symmetric algorithms are less intensive in terms of their computation as a result they can be processed faster. As a result, the symmetric encryption method is used for the encryption of sessions. The high speed of symmetric encryption is suitable for bulk processing of encryption. As mentioned earlier, there are many forms of symmetric algorithms of which DES and DEA are the most popular and widely used.

DES uses 64-bit blocks with a 64-bit key (although only 56 bits are significant; the other 8 are parity bits). The bits within a block are shuffled and XOR’ed with the key in a sequence of 16 substitutions called “rounds”, to create the ciphertext. Applying the same process (with the same key) to the ciphertext restores the original plaintext, so the process is symmetric. It has been very widely used, by the US government and commercial organizations around the world, including many financial institutions. It is easy to code (and there are good public domain implementations) and as it only involves bit-shifting operations combined with a few small look-up tables it doesn’t impose too much of a computational load.

DES is a form of symmetric algorithm which uses simple bit shifting. With supercomputers, it is now possible to crack the DES encryption, and with the way technology is progressing, in the future, even desktops can be used to crack DES making the symmetric encryption form a not very safe and secure option for encryption.