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Text Elements
Input
128bits
(16 bytes)
010101111010110101010011101101011011101
Key
128bit (16bytes) 192bit (24bytes) 256bit (32bytes)
010101111010110101010011101101011011101
Output
Same length as key
Encryption Algorithm
Input:
16 bytes.
A 4x4 matrix is made from these bytes
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Add round key
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Sub bytes
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Shift Rows
Repeated ‘n’ rounds.
if key = 128 bits, n= 10 if key = 192 bits, n= 12 if key = 256 bits, n= 14
Confusion
A single change does not have a predictable change in the output
101101
→ 22222211
→ 22222222
101100
101100 → 0920319203
Diffusion
Certain words in the english language appear more often.
To make the cipher secure, information in the plaintext, should be spread out evenly.
the rascal fox jumped out
re traf jth eutc slomuhs
As a convention, for every single bit changed in the plaintext, there is a change to about half of the bits in the ciphertext
1011101010101
0
1011
plain:
cipher
1010101110
1011101010
Input is turned into a matrix
Then, it goes through a cycle of ‘n’ rounds
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XOR
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Input
Key ‘n’
- Add round key
Single key turned to many
XOR performed with the corresponding round key
- Sub bytes
The # of keys corresponds to how many rounds are required
-
Mix Columns
-
Add round key
Skipped last round
Then there is a key expansion.
A B C D E F G H …
F I B O Q J M Z
Symmetric Cipher
Plaintext
Key
Any encryption algorithm where the same key is used to encrypt and decrypt
and
A special substitution cipher where each byte is replaced with another byte.
They use the S-Box to determine what to replace with
- Mix Columns
Goal is to diffuse information.
For each column, there is an expression for how to combine its 4bytes to get a new value for each byte
A is evaluated first, then B, then C, then D. subsequent bytes are dependent on the previous byte’s value
- Shift rows
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No shift for top row
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1 left shift for row 2
2 left shift for row 3
3 left shift for row 4
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