When Julius Caesar sent
information to his generals, he never believed his messengers. So he interchanges
every A with a D, every B with an E, and so on through the alphabet in his
message. Only authenticated person who knew the key could decipher his
messages. By doing this only authenticated person could understand his message.
This is somewhat called cryptography.

**Cryptography**: - Cryptography is a science and study of secret writing. It is a method in which data is stored and transmitted in a specific form so that only those for whom it is proposed can read and practice it. Following figure shows it in simple way.

Fig- Cryptography process

**Plaintext**- Original Data to be transmitted.

**Ciphertext**- Modified or encrypted data.

**Cryptography**

**key: -**By using key, one can decrypt the ciphertext and get original data. Two types of key are there, public key and private key.

An overview of some cryptographic
techniques is given below:-

**1. RSA algorithm**

RSA uses a

*public key*and a private key*.*The public key is designed for everyone to encrypt a original data. The original data which is to be transmitted is encrypted using public key and by using private key it can be decoded in a reasonable amount of time. If a message M is transmitted than ciphertext would be
C=M

^{E }mod n.
Where E is
public key and n is product of two prime numbers.

**2. RC4 algorithm**

It is a stream cipher with byte
oriented operations whose key size can be varied. Random permutations are used
in this algorithm for encryption and decryption process. Length of the key can
be in between 1 and 256 bytes to
initialize a 256-byte state table. This state table is used to generate a
sequence of pseudo random bytes and a stream. By xoring this stream with the
plaintext, we get a ciphertext. Each
and every element in the state table must be swapped at least once. Limitation
of RC4 is that in every 256 keys there is one weak key.

**3. DES algorithm**

Data Encryption
Standard (DES) is a symmetric key algorithm that encrypt 64-bit blocks of
plaintext into 64-bit of ciphertext by using 56-bit long keys. The encryption process
completes in 16 rounds. DES
encryption/decryption times are faster than those of RC4 encryption/decryption
times. DES is used to secure
confidential information of smart cards and ciphering of images and so on.

**4. 3DES algorithm**

It is also Symmetric Key Algorithms. It
is called Triple DES(3DES) because it encrypts original data
three times as compared to DES, first encrypted it with 56-bit key K by
applying DES encryption and then applying DES decryption with 56-bit key K2 final applying DES 56-bit
key K1. Same operation takes place for Decryption but in reverse order.

**5. IDEA algorithm**

IDEA (international
data encryption algorithm) operates on 64-bit blocks and uses a key
of length 128-bit, and consists of a series of eight similar rounds or stages
and a half output stage. Thus a total of 8 and half rounds are needed to
encrypt message. IDEA is a much secure method of encryption because of its different
operations such as- modular addition and
multiplication, and bitwise exclusive OR (XOR) — which are algebraically incompatible in some manner. It
used in steganography and other applications.

**6. AES algorithm**

AES is Symmetric Key Algorithms. Three
different keys of different sizes can be used in this algorithm such as: 128, 192, or 256 bits
long keys. If we use 128 bit key than 10 rounds are needed for encryption, if
192-bit key is used than 12 rounds needed, and for 256-bit keys, 14 rounds
needed. AES is more protected than DES and 3DES,
as this algorithm uses longer length key. AES provides faster encryption than
DES and 3DES, and this feature of AES makes it ideal for software applications and
those of hardware which require low-latency or high throughput.

**7**.

**Blowfish algorithm**

Blowfish is a
symmetric block cipher just like DES or IDEA. It takes a variable-length key. It’s
key length could be in between 32 to 448 bits. It is designed such as it is a
speedy alternative to that of existing cryptographic algorithms.

**Comparison in terms of encryption time(TE), decryption time(TD) and total time(TT) as per literature survey**

**Comparison of encryption algorithms on the bases of hardware implementation as per the literature survey**

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