Symmetric cryptography

symmetric key cryptography, also called secret key cryptography key cryptography) or cryptography of a key (in English single-key cryptography), is a cryptographic method in which the same key is used to encrypt and decrypt messages at the sender and the receptor. The two communicating parties must agree beforehand on the key to use. Once both parties have access to this key, the sender encrypts a message using the key, sends it to the recipient, and the recipient decrypts it with the same key.

The algorithms used in symmetric cryptography are mainly boolean and transpose operations.

Security

A good encryption system puts all the security in the key and none in the algorithm. In other words, it should be of no help to an attacker to know the algorithm being used. Only if the attacker obtained the key would it help him to know the algorithm. Widely used encryption algorithms have these properties.. (for example: AES).

Since all security is in the key, it's important to make it very difficult to guess the key type. This means that the range of possible keys, that is, the space of key possibilities, must be wide. Richard Feynman was famous in Los Alamos for his ability to open safe deposit boxes; to fuel the legend around him, he carried a set of tools that included a stethoscope. In reality, he used a variety of tricks to reduce the number of combinations he had to try to a small number, and from there he simply tried until he guessed the right combination. In other words, he was reducing the size of the key possibilities.

Computers today can crack keys extremely quickly, which is why key size is important in modern cryptosystems. The DES encryption algorithm uses a 56-bit key, which means there are 2⁵⁶ possible keys (72,057,594,037,927,936 keys). This represents a very high number of keys, but a generic computer can check the possible set of keys in a matter of days. A specialized machine can do it in hours. Newer design encryption algorithms like 3DES, Blowfish, and IDEA use 128-bit keys, which means there are 2¹²⁸ possible keys. This equates to many, many more keys, and even if a large number of machines were cooperating, it would take a long time to find the key.

Symmetric Key Ciphers in Computing

Flow ciphers: they encrypt the message with bit-to-bit correspondences on the flow (stream). Some stream ciphers are RC4 or RC6.

Block Ciphers: they encrypt the message by dividing the stream into blocks of k bits. Each block corresponds to a different one. For example, a block with k=3 "010" could correspond to "110". An example of a block cipher is the AES algorithm.

Examples

As an example of a symmetrical system is Enigma. This was a system used by Germany during World War II, in which the keys were distributed daily in the form of code books. Each day, a radio operator, receiver or transmitter, consulted his copy of the code book to find the day's key. All traffic sent by radio waves during that day was encrypted and decrypted using the keys of the day.

England used machines to decipher the keys during that war and although the aforementioned German system, Enigma, was provided with a wide range of keys, the English designed specialized computing machines, the Bombes, to check the keys mechanically until that the key of the day was found. This meant that they sometimes found the day's key within hours of it being put to use, but also that on other days they couldn't find the correct key. The Bombes were not general computing machines, but rather the precursors of today's computers (computers).

Some examples of symmetric algorithms are DES, 3DES, RC5, AES, Blowfish, and IDEA.

Disadvantages

The main problem with symmetric encryption systems is not linked to their security, but to the exchange/distribution of keys. Once the sender and recipient have exchanged keys they can use them to communicate securely, but what secure communication channel did they use to transmit the keys to each other? It would be much easier for an attacker to try to intercept a key than to try the possible combinations of the key space.

Another issue is the number of keys needed. If we have n number of people who need to communicate with each other, a total of n(n-1)/2 keys are needed for all couples of people who have to communicate privately. This may work with a small group of people, but it would be impossible to do with larger groups.

To solve these problems we could have symmetric key distribution centers. This could work for example for military organizations. Although there would always be a risk of possible information leaks about which keys are used in certain communications. However, its use in the private sector would lead to inevitable leaks, bureaucratic logjams and a constant threat of leaks.

Alternatives

To solve the key distribution problem and those derived from it, there are asymmetric cryptography and hybrid cryptography.