Symmetric-key encryption: the same key is used for both encryption and decryption
Symmetric-key algorithms are algorithms for cryptography that use the same cryptographic keys for both the encryption of plaintext and the decryption of ciphertext. The keys may be identical, or there may be a simple transformation to go between the two keys. The keys, in practice, represent a shared secret between two or more parties that can be used to maintain a private information link. The requirement that both parties have access to the secret key is one of the main drawbacks of symmetric-key encryption, in comparison to asymmetric-key encryption (also known as public-key encryption). However, symmetric-key encryption algorithms are usually better for bulk encryption. With the exception of the one-time pad they have a smaller key size, which means less storage space and faster transmission. Due to this, asymmetric-key encryption is often used to exchange the secret key for symmetric-key encryption.
When used with asymmetric ciphers for key transfer, pseudorandom key generators are nearly always used to generate the symmetric cipher session keys. However, lack of randomness in those generators or in their initialization vectors is disastrous and has led to cryptanalytic breaks in the past. Therefore, it is essential that an implementation use a source of high entropy for its initialization.
Reciprocal cipher
editA reciprocal cipher is a cipher where, just as one enters the plaintext into the cryptography system to get the ciphertext, one could enter the ciphertext into the same place in the system to get the plaintext. A reciprocal cipher is also sometimes referred as self-reciprocal cipher.
Practically all mechanical cipher machines implement a reciprocal cipher, a mathematical involution on each typed-in letter. Instead of designing two kinds of machines, one for encrypting and one for decrypting, all the machines can be identical and can be set up (keyed) the same way.
Examples of reciprocal ciphers include:
- Atbash
- Beaufort cipher
- Enigma machine
- the self-reciprocal cipher which Marie Antoinette and Axel von Fersen communicated with.
- the Porta polyalphabetic cipher which is self-reciprocal.
- Purple cipher
- RC4
- ROT13
- XOR cipher
- Vatsyayana cipher
The majority of all modern ciphers can be classified as either a stream cipher, most of which use a reciprocal XOR cipher combiner, or a block cipher, most of which use a Feistel cipher or Lai–Massey scheme with a reciprocal transformation in each round.Delfs, Hans; Knebl, Helmut (2007). "Symmetric-key encryption". Introduction to cryptography: principles and applications. Springer. ISBN 9783540492436.