DES – Data Encryption Standard
Unit - 3
DES - Introduction
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It is a symmetric key block cipher. It uses Feistel Structure.
Block size of plain text: 64 bit
No of rounds: 16 rounds
Key size: 64 bit (converted to 56 bits)
Each round will have a separate independent key.
No of sub keys: 16 sub keys
Sub key size: 48 bit
Cipher text: 64 bit
Initial permutation will follow transposition order –
rearranging bit positions.
Round Function of DES
Round function
• After IP – Round1 will be performed 64 bit, divided into two
halves – 32 bits(L&R).
• Expansion Permutation will be applied and 16 more bits will
be added to the right 32 bits
Input – 32 bits output – 48 bits
Key generation
• Permuted choice 1 – we will eliminate the multiples of 8 (eg:
8,16,24…).
Input – 64 bits output – 56 bits
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56 bit is given as input to left circular shift.
56 bit is divided into two halves (28 bits (L&R)).
Left shift bits is decided depending upon round number.
After LCS apply permuted choice 2 (rearrange the bit positions
some 8 bits will be ignored/removed) – 48 bits.
• Output from permuted choice 2 is XORed with output from
expansion permutation.
• Output of the XORed function (48 bits) is given as an input to
S box and reduced to (32 bits).
Round
1
2
3
4
5
6
7
8
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10
Bits
shifted
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2
2
2
2
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2
Round
11
12
13
14
15
16
Bits
shifted
2
2
2
2
2
1
S box function
• Totally there are 8 S boxes.
Input – 48 bits output – 32 bits
< 48 bits>
6 Bits
6 Bits
6 Bits
6 Bits
6 Bits
6 Bits
6 Bits
6 Bits
S1
S2
S3
S4
S5
S6
S7
S8
4 Bits
4 Bits
4 Bits
4 Bits
4 Bits
4 Bits
4 Bits
4 Bits
< 32 bits (2 bits reduced for each S box)>
• Eg: consider 6 bits 100110
• First & last bit represent row number (2 bits).
• Between four bits – column number (4 bits).
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Max number of representation using 2 bits (00,01,10,11).
Max number of representation using 4 bits
(0000 – 1111)
From eg: F+L bit -> 10 (Row) which is equal to 2.
Remaining 4 bits 0011 (Column) which is equal to 3.
Result: 8 (1000) -> 4 bits.
In DES security lies in the S boxes.
AES Cipher
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Block size – 128 bits (PT).
Number of rounds – 10 rounds.
Key size – 128 bits (key is processed in terms of word means 32 bits).
1 word – 32 bits (128 bits – 4 words/ 16 bytes).
Number of sub keys – 44 sub keys.
Sub key size – 32 bits/1 word/4 bytes.
Each round – 4 sub keys (4X32 = 128bits/4 words/16bytes).
Pre round calculation – 4 sub keys (Before starting round functions).
Cipher size – 128 bits.
• Shift Rows – Circular right shift will be done.
• Mix columns – one word should be multiplied with the predefined matrix
(4x4).
• In round 10 small change will be there no need to have mix
columns.(eliminated)
• Keys - R2 [W8…W11] – K2 R3[W13…W16] – K3…. R10[w40…W43] –
K9.
State Array
Each column considered as one word – used to store intermediate results
S(0,0) – 0th byte of 0th word
Key
40 words are used in 10 rounds and 4 words are used in pre round.
128 bits/ 4 words
The key is expanded to 44 words
Substitution Bytes
Output gets stored in
state array
State Array
Input Array
In 8 bits- First Four – Row Number – 0…15.
Next Four – Column Number - 1…15.
O/P – 8 bits.
Size of S box – 16X16 but in DES it is 4X16.
Shift Rows
Depends upon row number we have to sift rows.
4X4
4X1 4X1
• Add Round Key (Performing XOR operation)
State Array
[O/P from Mix Columns]
XORed
Key (4 words)
Consider 1st column of state array (1st word) XORed with first word of
the key and store the result in the array.
Blowfish Encryption Algorithm
• Block cipher: 64-bit block
• Variable key length: 32 bits to 448 bits (default 128
bits)
• Number of rounds: 16
• Designed by Bruce Schneier
• Much faster than DES and IDEA
• Follows Fiestel Structure.
• There are two parts to this algorithm
– A part that handles the expansion of the key.
– A part that handles the encryption of the data.
The expansion of the key
• Break the original key into a set of sub keys. Ranges
from 32-448 bits.
• There is a P-Array and four 32 bit S-boxes.
• A P-Array contains 18 32bit subkeys, while each Sbox contains 256 entries.
The encryption of data
• 64 bit input is denoted with an x, while P-Array is
denoted by Pi.
Algorithm: Blowfish Encryption
1. Divide x into two 32-bit halves:xL, xR
2. For i = 1to 16:
3. xL = XL XOR Pi
4. xR = F(XL) XOR xR
5. Swap XL and xR
6. Swap XL and xR (Undo the last swap.)
7. xR = xR XOR P17
8. xL = xL XOR P18 Recombine xL and xR
Areas Of Applications
• Bulk encryption: The algorithm should be efficient in
encrypting data files or a continuous data stream.
• Random bit generation: The algorithm should be
efficient in producing single random bits.
• Packet encryption: The algorithm should be efficient
in encrypting packet-sized data.
• Hashing: The algorithm should be efficient in being
converted to a one-way hash function.
MD 5 Algorithm
• In cryptography, MD5 (Message-Digest algorithm 5)
is a widely used cryptographic hash function with a
128-bit hash value.
• MD5 has been employed in a wide variety of security
applications, and is also commonly used to check the
integrity of files.
Use of MD5 Algorithm
• To be considered cryptographically secure MD5
should meet two requirements:
– It is impossible to generate two inputs that
cannot produce the same hash function.
– It is impossible to generate a message having the
same hash value.
• How do the MD5 Algorithm works?
• MD5 produces an output of 128-bit hash value.
• This encryption of input of any size into hash values undergoes 5
steps
• Step1: Append Padding Bits
– Padding means adding extra bits to the original message.
– So in MD5 original message is padded such that its length in bits is
congruent to 448 modulo 512.
– Padding is done such that the total bits are 64 less being a multiple of
512 bits length.
– Padding is done even if the length of the original message is already
congruent to 448 modulo 512. In padding bits, the only first bit is 1 and
the rest of the bits are 0.
• Step 2: Append Length
– After padding, 64 bits are inserted at the end which is used to record the
length of the original input. The resulting message has a length multiple
of 512 bits.
• Step 3: Initialize MD buffer
– A four-word buffer (A, B, C, D) is used to compute the values
for the message digest. Here A, B, C, D are 32- bit registers.
• Step 4: Processing message in 16-word block
– MD5 uses the auxiliary functions which take the input as three
32-bit number and produces a 32-bit output. These functions use
logical operators like OR, XOR, AND.
• Step 5: Output (Message Digest in Buffers)
• Totally 64 constants
• CV – Chaining Variable.
Round 1
• a = b + ((a + F(b,c,d) + X[k] + T[i]) <<< s)
Round 2
• a = b + ((a + G(b,c,d) + X[k] + T[i]) <<< s)
Round 3
• a = b + ((a + H(b,c,d) + X[k] + T[i]) <<< s)
Round 4
• a = b + ((a + I(b,c,d) + X[k] + T[i]) <<< s)
• T[i] – Set of 64 Constants
• X[k] – Part of plain text
• F,G,H,I – Logical function
• <<< s – Circular Left Shift
Constants of MD5 Algorithm