Types of Cryptography

Following Types of Encryption
Types of Cryptography Based on the key used for encryption and decryption, cryptography can be divided into two, namely:  
1. Symmetric key cryptography
2. Asymmetric key cryptography 

1. Symmetry Key Cryptography In symmetric key cryptography system, the same key for encryption key for decryption, so-called symmetric key cryptography or symmetric cryptography. Another term for symmetric cryptography is a cryptographic private key (private key cryptographi), secret key cryptography (secret key cryptographi), or conventional cryptography (conventional cryptographi). Symmetric cryptographic systems assume the sender and the recipient have to share the same key before exchanging messages. Symmetric cryptographic security lies in the secrecy of the key. Symmetric cryptography is the only type of cryptography known in historical records until 1976. All classical cryptographic algorithms included in the cryptographic system symmetry. On the other hand, there are dozens of modern cryptographic algorithms which belong to the symmetric cryptographic system, such as DES (Data Encryption Standard), Blowfish, Twifish, Triple-DES, IDEA, Serpent and the newest is the AES (Advanced Encryption Standard).
In general, cipher which belong to the symmetric cryptographic operation in the bits and can be grouped into two categories, namely:  
1. Cipher block (block cipher)
Cryptographic algorithms operate on the plaintext / ciphertext block in the form of bits, which in this case a series of bit-blik divided into blocks of bits whose length is predetermined. For example, the block length is 64 bits, then that means the encryption algorithm to treat 8 characters each time encryption (1 character equals 8 bits in ASCII encoding.) Example that uses cryptographic methods are DES, TripleDES, IDEA, AES, and others. 
2. Cipher stream (stream cipher)
Cryptographic algorithms operate on the plaintext / ciphertext in the form of a single bit, which in this case a series of bits encrypted / didekripsikan bit by bit or byte by byte. Examples of cryptography that uses the RC4 stream cipher method (Ron's Code 4), A5 and others. Symmetry of the main applications of cryptography is to protect the confidentiality of data sent over telecommunications media and protect the confidentiality of data stored on storage media. The weakness of this system is both the sender and recipient must have the same key, so that the sender of the message should be seeking a safe way to inform the recipient's key messages. 

2. Asymmetric key cryptography 
If the key for encryption is not the same key for decryption, then kriptografinya called asymmetric key cryptography or asymmetric cryptography. Another name is public key cryptography (public key cryptographi), because no secret key for encryption and can be known by anyone (made public), while the key for decryption is known only by the recipient of the message. In this type of cryptography, each person who communicates has the key pair, namely the private key and public key. The sender encrypts the message using the recipient's public key. Only the recipient can decrypt the message because only he who knows the corresponding private key itself. Advantages of public key cryptography is twofold. First there is no need to distribute the private key cryptography as the symmetry. The public key can be sent to the receiver via the same channels with the channel used to send messages. Note that the channel to send messages is generally not safe. Second, the number of keys can be pressed. To communicate in secret with a lot of people, do not need a secret key sebayak number of people, simply create two keys, namely the public key for encryption of messages and correspondent for the private key for the recipient of a message to describe the message. Unlike symmetric key cryptography where the number of keys that made as many as the number of parties are invited to correspond Examples of public key cryptography algorithms such as RSA, ElGamal, DSA, and so many else.

Comparison of Key Cryptography Key Cryptography Symmetry and Asymmetry
Both symmetric key cryptography or asymmetric key, both have advantages and disadvantages of each.
Excess key cryptography symmetry: 1. Symmetric cryptographic algorithms are designed so that the process of encryption / decryption takes a short time. 2. Symmetric key size is relatively short. Symmetric cryptographic algorithms can be used to generate random numbers. 3. Symmetric key algorithm can be arranged for resulting in a stronger cipher. 4. Sender authentication messages immediately known from the received ciphertext, because the key only known by the sender and the recipient only.
Symmetric key cryptography Weaknesses: 1. Symmetric key must be sent via a secure channel. Both entities must maintain the confidentiality of communication is key. 2. Keys must be changed often, perhaps every communication session.
The advantages of asymmetric key cryptography: 1. Only the private key that needs to be maintained by any entity of anonymizing communication (but public key authentication must be guaranteed). There is no need to send the private key as symmetric key cryptography. 2. Pair of public key / private key does not need to be changed, even in waktun a long period. 3. Can be used to secure the delivery of key symmetry 4. Some public key algorithms can be used to provide digital signatures on messages.
The weakness of asymmetric key cryptography: 1. Encryption and decryption of data is generally slower than symmetric cryptographic system, because the encryption and decryption using bilangna large and involve great powers of operation. 2. Ciphertext size larger than the plaintext. 3. Relatively larger key sizes than symmetric key sizes. 4. Because the public key is widely known and can be used every person the ciphertext does not give information about the sender authentication.
Because of the advantages and weaknesses in each of cryptography, it is not true that replacing public-key cryptography key cryptography symmetry. Because public key cryptography has a disadvantage in terms of computation time and the size of ciphertexts compared to symmetric key cryptography, then this has implications in the practice and usage. Most security systems use a combination of symmetric key cryptography and public key cryptography (hybrid cryptosystem). In this hybrid system encryption / decryption messages using symmetric key cryptography, while the symmetric key encryption / decryption using the public key. Symmetric key (which is also called a session key) generated by one party and encrypts the message with the key. Furthermore, the session key encrypted with the recipient's public key and sent along with the message that is encrypted. Recipient first decrypt the session key with the corresponding private key, then decrypt with key pasan session.

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