Notes

Message Authentication:-
MAC algorithm is a symmetric key cryptographic technique to provide message authentication. For establishing MAC process, the sender and receiver share a symmetric key K.Essentially, a MAC is an encrypted checksum generated on the underlying message that is sent along with a message to ensure message authentication.
The sender uses some publicly known MAC algorithm, inputs the message and the secret key K and produces a MAC value.
Similar to hash, MAC function also compresses an arbitrary long input into a fixed length output. The major difference between hash and MAC is that MAC uses secret key during the compression.
The sender forwards the message along with the MAC. Here, we assume that the message is sent in the clear, as we are concerned of providing message origin authentication, not confidentiality. If confidentiality is required then the message needs encryption.
On receipt of the message and the MAC, the receiver feeds the received message and the shared secret key K into the MAC algorithm and re-computes the MAC value.
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CBC-MAC:-
In cryptography, a cipher block chaining message authentication code (CBC-MAC) is a technique for constructing a message authentication code from a block cipher. The message is encrypted with some block cipher algorithm in CBC mode to create a chain of blocks such that each block depends on the proper encryption of the previous block. This interdependence ensures that a change to any of the plaintext bits will cause the final encrypted block to change in a way that cannot be predicted or counteracted without knowing the key to the block cipher.To calculate the CBC-MAC of message m one encrypts m in CBC mode with zero initialization vector.
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STREAM CIPHER:-
A stream cipher is a method of encryption where a pseudorandom cipher digit stream is combined with plain text digits. This pseudorandom cipher digit stream is applied to each binary digit, one bit at a time. This method of encryption uses an infinite number of pseudorandom cipher digits per key.
Stream cipher is also known as state cipher.
A stream cipher encrypts an arbitrary length of plain text, one bit at a time, with an algorithm that uses a key. For this form of encryption to remain secure, its psuedorandom cipher digits should be unpredictable and the key should never be used more than once. The pseudorandom cipher digits are generated through a number of random seed values that use digital shift registers. The encryption of each digit is dependent on the current state of the cipher, warranting the name state cipher for this. RC4 is a popular stream cipher that is widely used in software.
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Cryptography Hash functions
Hash functions are extremely useful and appear in almost all information security applications.
A hash function is a mathematical function that converts a numerical input value into another compressed numerical value. The input to the hash function is of arbitrary length but output is always of fixed length.Values returned by a hash function are called message digest or simply hash values.
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Features of Hash Functions
The typical features of hash functions are −
Fixed Length Output (Hash Value)
Hash function coverts data of arbitrary length to a fixed length. This process is often referred to as hashing the data.
In general, the hash is much smaller than the input data, hence hash functions are sometimes called compression functions.
Since a hash is a smaller representation of a larger data, it is also referred to as a digest.
Hash function with n bit output is referred to as an n-bit hash function. Popular hash functions generate values between 160 and 512 bits.
Efficiency of Operation
Generally for any hash function h with input x, computation of h(x) is a fast operation.
Computationally hash functions are much faster than a symmetric encryption.
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Advanced Encryption Standard
The more popular and widely adopted symmetric encryption algorithm likely to be encountered nowadays is the Advanced Encryption Standard (AES). It is found at least six time faster than triple DES.A replacement for DES was needed as its key size was too small. With increasing computing power, it was considered vulnerable against exhaustive key search attack. Triple DES was designed to overcome this drawback but it was found slow.
The features of AES are as follows −
Symmetric key symmetric block cipher
128-bit data, 128/192/256-bit keys
Stronger and faster than Triple-DES
Provide full specification and design details
Software implementable in C and Java
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Data Encryption Standard
The Data Encryption Standard (DES) is a symmetric-key block cipher published by the National Institute of Standards and Technology (NIST).DES is an implementation of a Feistel Cipher. It uses 16 round Feistel structure. The block size is 64-bit. Though, key length is 64-bit, DES has an effective key length of 56 bits, since 8 of the 64 bits of the key are not used by the encryption algorithm (function as check bits only).
DES Analysis
The DES satisfies both the desired properties of block cipher. These two properties make cipher very strong.
Avalanche effect − A small change in plaintext results in the very great change in the ciphertext.
Completeness − Each bit of ciphertext depends on many bits of plaintext.
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Difference between symmetric key encryption and public key encryption:-
Symmetric key encryption involves using a single key to encrypt and decrypt data. For example, suppose that you took a document and placed it in a file cabinet and then locked the cabinet with a key. For you or anyone else to access the document, you'd need the key to the file cabinet. Generally speaking, symmetric key encryption is fast and secure. On the other hand, symmetric key encryption works well locally, it doesn't work very well across networks. In order for the receiver of the encrypted packets to be able to decrypt the packets, they must use the key. Needless to say, this means that you must send them that key along with the message. The other problem is that the physical medium you're sending the packets across is insecure. If it were secure, there would be no reason to encrypt the message in the first place. Anyone who might be monitoring the network could steal the encrypted packets and the key necessary for decrypting them.
Public key encryption on the other hand uses a pair of keys: a public key that's sent along with the message and a private key which is always in the possession of the recipient. The private key is based on a derivative of the public key and only the two keys working together can decrypt the packets. Because the private key is never sent across the network, it remains secure. The down side of public key encryption is that it tends to be very slow and resource intensive. This makes it difficult to send large amounts of data using public key encryption.
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