•Decryption by applying same transformations in reverse on cipher
text.
•This method called symmetric key encryption as encryption and
decryption performed using same key.
•Normally the encryption/decryption algorithm is publicised. Only
key is secret.
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Symmetric Encryption
• Problem is to ensure secrecy of key when it is sent to partner.
• If the key is to be sent to many partners need for separate key for
each partner.Directory of who was sent which key is to be kept and
used for each transaction.Directory should be secure.
• If large number of partners are there key distribution very difficult.
• Advantage of symmetric key is easy and fast to transform plain
text to cipher text.
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Digital Encryption Standard
DES - Proposed by IBM in 1975
Standardised by US Govt in 1977
Reasonably secure
It is a combination of permutation and substitution on blocks of
64 bits. A message is broken up into 64 bit blocks and each
block is separately encrypted.
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Digital Encryption Standard
#General
idea used in DES
M = PLAINTEXT
01101100
11011000
11011010
K = KEY
10101111
00101100
01011011
E =
M + K
11000011
11110100
10000001
encryption
M=
E + K
01101100
11011000
11011010
decryption
See simplicity of Transformation using Exclusive OR
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Digital Encryption Standard Algorithm
Before applying DES the text is split up into the 64 bit blocks.
DES applied on each 64 bit block.
Encryption method
Step 1: Apply an initial permutation on a block.Result is B=IP(P)
where P is the 64 bit block IP Initial Permutation function and
B the result.
Step 2: Split B into 32 bit blocks
Li = leftmost 32 bits
Ri = rightmost 32 bits.
Step 3: Pick a 56 bit key. Permute it
Step 4: Left circular shift it by 1 bit giving K
1
.
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Digital Encryption Standard Algorithm
Step 5: Perform a complex sequence of operations and obtain
X
1
= F(R
1
,K
1
) (The complex set of operations include table look up
and dropping bits).
Step 6: Find R
2
= L
1
+ X
1
Step 7: Set L
2
= R
1
Repeat steps 2 to 7 16 times to get B
16
= L
16
,R
16
Step 8: Apply inverse of initial permutation on B
16
The result is the encrypted block
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Digital Encryption Standard Algorithm
• In summary the DES encryption applies the following
transformation 16 times.The i
th
round transformation are
L
i+1
= R
i
R
i+1
= L
i
+ F(R
i
,K
i
)
• Each round has a different key K
i
• For Decryption the process of encryption is reversed.The
encrypted block is permuted using IP
-1.
On this
transformations are applied starting with K
16
and going to
K
1
last.The keys and F are same as those used in encryption
process.
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13.3.15

Digital Encryption Standard Algorithm
• The encryption process uses simple binary operations.
They can thus be realised in hardware as an integrated
circuit chip.