Introduction to Cryptography

Cryptography involves two processes: Encryption and Decryption. Encryption is the process of transforming a plaintext message into a relatively meaningless message (the ciphertext) through means of a function (encryption algorithm) and a key. Decryption is the process of transforming a ciphertext back into the original plaintext through means of a function (decryption algorithm) and a key. Decryption is the reverse process of encryption. There are two basic key-based cryptographic algorithms, symmetric (secret key) and asymmetric (public key) algorithms.

Secret Key Cryptosystem

In symmetric algorithms, the same key is used for encryption and decryption. Security of the symmetric algorithms relies heavily on the key. For secure communication between two parties, the key should be kept secret since the algorithm is usually available in the public domain. Symmetric algorithms are much faster than asymmetric algorithms.

The current industry standard for symmetric algorithm is Advanced Encryption Standard (AES), using the Rijndael algorithm. AES utilizes block cipher, that is, by encrypting a single block of information at a time. AES uses 128-bit (with 10 rounds of encryption), 192-bit (with 12 rounds of encryption), or 256-bit (with 14 rounds of encryption) keys to encrypt 128-bit blocks of data.

Public Key Cryptosystem

Public key cryptosystem requires the use of two different keys — one for the encryption process and another for the decryption process. The strength of the public key system lies in the fact that it is not computationally feasible to derive one key from the other. The encryption key is usually made public and hence known as the public key. The decryption key is kept secret and is known as the private key. This means that anyone can encrypt a message using the public key, but only the intended receiver can decrypt the ciphertext using his private key. Thus, there is no need for the users to agree on a same key prior to the cryptographic process.

Rivest–Shamir–Adleman (RSA) is the current industry standard for public key cryptosystem. RSA has a variable key size between 1024-bit to 2048-bit.

Conclusion

There are a lot of applied technologies that are actually based on cryptography, such as HTTPS (SSL/TLS), Digital Signatures, Block chain, Cryptocurrencies, etc. Most of these utilize a hybrid type of cryptosystem, using public key cryptography for key transfer, while utilizing secret key algorithm for the encryption/decryption of data. Having a basic understanding on cryptography is essential to fully understand these applied technologies.