Public-key cryptography, or asymmetric cryptography, is a cryptographic system that uses pairs of keys: public keys (which may be known to others), and private keys (which may never be known by any except the owner). The generation of such key pairs depends on cryptographic algorithms which are based on mathematical problems termed one-way functions. Effective security requires keeping the private key private; the public key can be openly distributed without compromising security.
In such a system, any person can encrypt a message using the intended receiver's public key, but that encrypted message can only be decrypted with the receiver's private key. This allows, for instance, a server program to generate a cryptographic key intended for a suitable symmetric-key cryptography, then to use a client's openly-shared public key to encrypt that newly generated symmetric key. The server can then send this encrypted symmetric key over an insecure channel to the client; only the client can decrypt it using the client's private key (which pairs with the public key used by the server to encrypt the message). With the client and server both having the same symmetric key, they can safely use symmetric key encryption
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— DES (Data Encryption Standard) is the most popular computer encryption algorithm. DES is a U.S. and international standard. It is a symmetric algorithm; the same key is used for encryption and decryption.
— RSA (named for its creators—Rivest, Shamir, and Adleman) is the most popular public-key algorithm. It can be used for both encryption and digital signatures.
— DSA (Digital Signature Algorithm, used as part of the Digital Signature Standard) is another public-key algorithm. It cannot be used for encryption, but only for digital signatures.
In a transposition cipher the plaintext remains the same, but the order of characters is shuffled around. In a simple columnar transposition cipher, the plaintext is written horizontally onto a piece of graph paper of fixed width and the ciphertext is read off vertically. Decryption is a matter of writing the ciphertext vertically onto a piece of graph paper of identical width and then reading the plaintext off horizontally. Since the letters of the ciphertext are the same as those of the plaintext, a frequency analysis on the ciphertext would reveal that each letter has approximately the same likelihood as in English. This gives a very good clue to a cryptanalyst, who can then use a variety of techniques to determine the right ordering of the letters to obtain the plaintext. Putting the ciphertext through a second transposition cipher greatly enhances security.
There are even more complicated transposition ciphers, but computers can break almost all of them. The German ADFGVX cipher, used during World War I, is a transposition cipher combined with a simple substitution. It was a very complex algorithm for its day but was broken by Georges Painvin, a French cryptanalyst . Although many modern algorithms use transposition, it is troublesome because it requires a lot of memory and sometimes requires messages to be only certain lengths. Substitution is far more common.
A substitution cipher is one in which each character in the plaintext isbstituted for another character in the ciphertext. The receiver inverts thesubstitution on the ciphertext to recover the plaintext.In classical cryptography, there are four types of substitution ciphers:
— A simple substitution cipher, or monoalphabetic cipher, is one inwhich each character of the plaintext is replaced with a correspondingcharacter of ciphertext. The cryptograms in newspapers are simplesubstitution ciphers.
— A homophonic substitution cipher is like a simple substitutioncryptosystem, except a single character of plaintext can map to one ofseveral characters of ciphertext. For example, “A” could correspond toeither 5, 13, 25, or 56, “B” could correspond to either 7, 19, 31, or 42,and so on.
— A polygram substitution cipher is one in which blocks ofcharacters are encrypted in groups. For example, “ABA” couldcorrespond to “RTQ,” “ABB” could correspond to “SLL,” and so on.
— A polyalphabetic substitution cipher is made up of multiple simple substitution cipher.
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