SSH, The Secure Shell: The Definitive Guide, 2nd Edition

A Cryptography Primer

We’ve covered the basic properties of SSH. Now we focus on cryptography, introducing important terms and ideas regarding the technology in general. There are many good references on cryptographic theory and practice, and we make no attempt here to be comprehensive. (For more detailed information, check out Bruce Schneier’s excellent book, Applied Cryptography, published by John Wiley & Sons.) We introduce encryption and decryption, plaintext and ciphertext, keys, secret-key and public-key cryptography, and hash functions, both in general and as they apply to SSH.

Encryption is the process of scrambling data so that it can’t be read by unauthorized parties. An encryption algorithm (or cipher) is a particular method of performing the scrambling; examples of currently popular encryption algorithms are RSA, AES, DSA, and Blowfish. The original, readable data is called the plaintext , or data “in the clear,” while the encrypted version is called the corresponding ciphertext.

The goal of an encryption algorithm is to convert plaintext to ciphertext. To do this, you pass two inputs to the encryption algorithm: the plaintext itself, and a key, a string that is typically a secret known only to you. From these inputs, the algorithm produces the ciphertext. An encryption algorithm is considered secure if it is infeasible for anyone to read (or decrypt) the encrypted ciphertext without the key. An attempt to decrypt data without its key is called cryptanalysis.

3.2.1 How ...