In previous article, we have explained the common basic phrases associated with encryption. In principle, a distinction is made between different classical and modern symmetric encryption methods and the asymmetric encryption methods, which have only been known since the 1970s. Classical encryption methods can be classified according to the alphabet used.
Symmetric encryption
Symmetric encryption methods use the same key for encryption and decryption. In the case of historical methods, two classes of encryption can be distinguished. In modern symmetric methods, a distinction is made between stream encryption and methods based on block encryption. In stream encryption, the characters of the plaintext are encrypted individually and one after the other. Block encryption, on the other hand, splits the plaintext into blocks of a certain size in advance. How the blocks are then encrypted is determined by the operating mode of the encryption method.
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Interestingly, even modern block ciphers, such as the DES (Data Encryption Standard), which became the standard over several decades towards the end of the 20th century, are based on the two classic methods of substitution and transposition. They use these two basic principles in combination and derive their strength from the repeated use of such combinations, often in dozens of “rounds”. In this way, comparable to the repeated kneading of dough, the plaintext is increasingly encrypted. The strength of the encryption usually increases with the number of rounds used.
Asymmetric encryption
In asymmetric encryption, there are two different keys, the public key for encryption and the private key for decryption. For centuries, it was believed that there was no alternative to symmetric encryption and the associated key distribution problem.
It was not until the 1970s that asymmetric encryption (public-key cryptography) was developed. The characteristic of asymmetric encryption is that a completely different key is used for encryption than for decryption. A distinction is made here between the “public key”, which is used to encrypt, and the “private key”, which is used to decrypt the ciphertext. The private key is never passed on or even published, while the public key is handed over or published to the communication partner. It can then be used by anyone to encrypt messages. However, in order to be able to decrypt them, you need the appropriate private key. This is the only way to decrypt the encrypted message. This means that not even the encryptor himself is able to decrypt his own message, which he has encrypted with the public key.
Incidentally, the method can also be used “the other way around”, in which a person uses his private key to encrypt a piece of information. Now, anyone who has access to the public key is able to use it to decrypt the message. This is usually not about keeping a message secret, but about authenticating a person or the digital signature of a message, for example. Anyone can easily verify and recognize that the encrypted information can only come from this one person, because only this person has the necessary private key. For signing alone, it is sufficient to leave the message text unencrypted as plain text, and, for example, to append only a checksum of it in encrypted form. If the author’s public key exposes a correct checksum when decrypted, both the author and the authenticity of the message are confirmed.
Since asymmetric methods are algorithmically more complex than symmetric methods and therefore slower to execute, combinations of both, so-called hybrid methods, are usually used in practice. For example, a randomly generated individual session key is first exchanged using an asymmetric method, and then it is used together as a key for a symmetric encryption method, which encrypts the information to be communicated.
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