The Data Encryption Techniques and Standard
•The DES (Data Encryption Standard) algorithm is a symmetric-key block cipher
•The algorithm takes the plain text in 64-bit blocks and converts them into ciphertext using 48-bit keys.
•Since it’s a symmetric-key algorithm, it employs the same key in both encrypting and decrypting the data.
•DES is based on the Feistel block cipher, called LUCIFER.
• DES uses 16 rounds of the Feistel structure, using a different key for each round.
•DES became the approved federal encryption standard in November 1976 .For the longest time, DES
was the data encryption standard in information security.
•DES’s dominance came to an end in 2002, when the Advanced Encryption Standard (AES) replaced the
DES encryption algorithm as the accepted standard.
•Triple DES is a symmetric key-block cipher which applies the DES cipher in triplicate. It encrypts with the
first key (k1), decrypts using the second key (k2), then encrypts with the third key (k3).
The DES algorithm is a combination of two fundamental building blocks of encryption: substitution
The algorithm derives its strength from repeated application of these two techniques, for a total of 16
The algorithm begins by encrypting the plaintext as blocks of 64 bits.
The key is 64 bits long - it can be any 56-bit number.
The 8 bits are often used as check digits
Block size = 64 bits
– Key size = 56 bits (in reality, 64 bits, but 8 are used as
parity-check bits for error control)
– Number of rounds = 16
– 16 intermediary keys, each 48 bits
The Feistel (F) function
The F-function operates on half a block (32 bits) at a time and consists of four stages:
Four Stages of DES:
1. Expansion — 32-bit half-block is expanded to 48 bits using the expansion
Permutation ( duplicating half of the bits)
The output consists of eight 6-bit (8 * 6 = 48 bits) pieces.
2. Key mixing — Result is combined with a subkey using an XOR operation.
16 48- bit subkeys — one for each round — are derived from the main key using the key
3. Substitution — The block is divided into eight 6-bit pieces before processing by the S-boxes, or substitution
Each of the eight S-boxes replaces its six input bits with four output bits according to a non-linear
The S-boxes provide the core of the security of DES
4. Permutation — 32 outputs from the S-boxes are rearranged according to a fixed permutation, the P-box.
This is designed so that, after permutation, each S-box's output bits are spread across 4 different S boxes in the next
The alternation of substitution from the S-boxes, and permutation of bits from the P-box and E-expansion provides
- "confusion and diffusion" respectively.
Advanced Encryption Standard ( AES )
Commercial-grade symmetric algorithm
AES is based on a design principle known as a substitution-permutation network
Combination of both substitution and permutation
It is fast in both software and hardware
AES does not use a Feistel network.
AES has a fixed block size of 128 bits, and a key size of 128, 192, or 256 bits.
AES operates on a 4×4 column-major order matrix of bytes
Most AES calculations are done in a special finite field.
The key size used for an AES cipher specifies the number of repetitions of transformation
rounds that convert the plaintext, into the ciphertext.
The number of cycles of repetition are as follows:
10 cycles of repetition for 128-bit keys.
12 cycles of repetition for 192-bit keys.
14 cycles of repetition for 256-bit keys.
High-level description of the algorithm
● KeyExpansions—round keys are derived from the cipher key using Rijndael's key schedule.
AES requires a separate 128-bit round key block for each round plus one more.
○ AddRoundKey—each byte of the state is combined with a block of the round key using bitwise xor.
○ SubBytes—a non-linear substitution step where each byte is replaced with another according to a lookup
○ ShiftRows—a transposition step where the last three rows of the state are shifted cyclically a certain number
○ MixColumns—a mixing operation which operates on the columns of the state, combining the four bytes in
● Final Round (no MixColumns)
The SubBytes steps
In the ShiftRows step, bytes in each row of the state are shifted
cyclically to the left. The number of places each byte is shifted differs
for each row.
The ShiftRows step
In the SubBytes step, each byte in the state is
replaced with its entry in a fixed 8-bit lookup
table, S; bij =S(aij).
The MixColumns step
In the MixColumns step, each column of the state is multiplied
with a fixed polynomial c(x).
The AddRoundKey step
In the AddRoundKey step, each byte of the state is
combined with a byte of the round subkey using the
● A class of cryptographic schemes to provide security in scenarios where a group of
participants can communicate securely over some computer network in such a way that
the exchanged messages would be unintelligible for outsiders and non-pertaining
1. Broadcast encryption: To prevent unauthorized users from extracting data, the broadcaster
encrypts the message and only the authorized users have the decryption keys to recover the data.
1. Traitor tracing: these traitor-tracing schemes use a secret-key encryption scheme to encrypt data.
A public-key traitor tracing allows everyone to perform encryption, and thus anyone can broadcast
messages to authorized users securely.
3. Threshold cryptosystems: Threshold cryptosystems allow one to send encrypted messages
to a group, while only a group achieving a “threshold" has the ability to reconstruct the plaintext.
Steganography and its types
● Steganography is a technique of hiding
communication by concealing the secret message
into a fake message.
Types of Steganography
● The image Steganography is used to hide a secret message inside an image.
The most widely used technique to hide secret bit inside the LSB of the cover
● The most widely used technique to hide secret bit inside the LSB of the cover
● Audio stenography can conceal the secret message in the audio file with the
help of its digital representation.
● Video Steganography brings more possibilities of disguising a large amount of
data because it is a combination of image and sound.
● Text Steganography uses data compression. Data compression encodes
information in one representation into another representation. The new
representation of data is smaller in size.
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