Classical classical cryptographic algorithms implement Replace Cipher Convert Cipher 15

Classical encryption algorithms are divided into substitution algorithms and displacement shift methods.

1. Alternative algorithms.

The substitution algorithm replaces the letters of the plaintext with other letters or numbers or symbols. The most well-known alternative algorithm is the Caesar cipher. The principle of the Caesar cipher is very simple, it is actually a single letter substitution.

Example: Plaintext: abcdefghijklmnopq

Ciphertext: defghijklmnopqrst

2. Displacement and displacement method.

One of the most well-known types of ciphers that use the permutation-shift method is called the Virginia cipher. It replaces the cipher with a cyclical replacement based on a displacement shift.

In the Virginia cipher, the encryption key is an arbitrarily specified string. The encryption key characters are applied to the plaintext information characters one by one. The length of the plaintext information is often greater than the length of the key string, and each character of the plaintext needs to have a corresponding key character, so the key needs to be looped until each character in the plaintext corresponds to a key character.

Other common encryption algorithms.

1. The DES algorithm is a symmetric cryptosystem in the cryptosystem, which changes the 64-bit plaintext input block into a 64-bit ciphertext output block, and the key it uses is also 64-bit.

DES is a symmetric algorithm based on DES, which encrypts a piece of data three times with three different keys, which is stronger.

3. RC2 and RC4 are symmetric algorithms, which encrypt a large amount of data with a variable-length key, which is faster than DES.

4. The IDEA algorithm is developed on the basis of the DES algorithm, which is implemented as an iterative block cipher, using a 128-bit key and 8 loops.

5. RSA is invented by RSA company, which is a public key algorithm that supports variable-length keys, and the length of the file block that needs to be encrypted is also variable, asymmetric.

6. DSA, or digital signature algorithm, is a standard DSS (digital signature standard), which is not strictly speaking, an encryption algorithm.

7. AES is an advanced encryption standard symmetric algorithm, which is the next generation of encryption algorithm standard, with fast speed and high security level, and an implementation of AES standard in the 21st century is Rijndael algorithm.

Additive passwords.

A and B are alphabets with n letters.

Define a mapping from A to B: f:a B

f(ai )= bi=aj

j=i+k mod n

The addition cipher is replaced by the k-th letter of the alphabet in plaintext.

k=3 is the famous Caesar cipher.

The Caesar cipher – the first cryptographic technology in history.

The "Caesar Code" was a cryptographic system used by Julius Caesar in ancient Rome to protect important military information during the rescue of Cicero (Gallic Wars).

Multiplication cipher. A and B are alphabets with n letters. Define a mapping from A to B: F:

a b f(ai) = bi= aj j=ik mod n where (n,k) = 1. Note: Only (n,k)=1 can be decrypted correctly.

A key phrase replaces a password.

Randomly select a word, remove the repeating letters, write it to the first line of the matrix, remove the letters from the plaintext alphabet, and write the rest of the alphabet to the matrix in order. Then take out the letters by column to form the ciphertext alphabet Single table instead of password is not very secure, one reason is that a plaintext letter is replaced by only one ciphertext letter. Frequency analysis can be used to decipher.

Therefore, a more secure multi-table substitution cipher is generated, that is, multiple ciphertext alphabets are constructed, and a series of substitution tables are used to replace the letter sequence of the plaintext message in turn under the control of the key. Well-known multi-table substitution passwords include vigene passwords, etc.

vernam password.

Plaintext, ciphertext, and keys are all represented as binary bits:

m=m1,m2,… mn k =k1,k2,… kn c =c1,c2,… cn

playfair password.

A matrix is generated with key control and then substituted every two letters.

Hill password (product password).

Built on matrix multiplication, but not resistant to known plaintext attacks.