"It seems very simple." "It is very simple. But if you don't know what the key is it's virtually indecipherable." Talking to Strange Men, Ruth Rendell

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Cryptography and

Network Security

Chapter 5

Fifth Edition

by William Stallings

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Chapter 5 –

Advanced Encryption

_{Advanced Encryption}

Standard

"It seems very simple."

"It is very simple. But if you don't know what

the key is it's virtually indecipherable."

—

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AES Origins

 clear a replacement for DES was needed_{clear a replacement for DES was needed}  have theoretical attacks that can break it_{have theoretical attacks that can break it}

 have demonstrated exhaustive key search attacks_{have demonstrated exhaustive key search attacks}

 can use Triple-DES – but slow, has small blocks_{can use Triple-DES – but slow, has small blocks}  US NIST issued call for ciphers in 1997_{US NIST issued call for ciphers in 1997}

 15 candidates accepted in Jun 98 _{15 candidates accepted in Jun 98}  5 were shortlisted in Aug-99 _{5 were shortlisted in Aug-99}

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The AES Cipher - Rijndael

 designed by Rijmen-Daemen in Belgium _{designed by Rijmen-Daemen in Belgium}  has 128/192/256 bit keys, 128 bit data _{has 128/192/256 bit keys, 128 bit data}  an _aniterativeiterative rather than rather than FeistelFeistel cipher cipher

 processes _processesdata as block of 4 columns of 4 bytesdata as block of 4 columns of 4 bytes  operates on entire data block in every round_{operates on entire data block in every round}

 designed to have:_{designed to have:}

 resistance against known attacks_{resistance against known attacks}

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AES

Encryption

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AES Structure

 _{data block of}_{data block of}_{4 columns of 4 bytes is state}_{4 columns of 4 bytes is state}  key is expanded to array of words_{key is expanded to array of words}

 has 9/11/13 rounds in which state undergoes: _{has 9/11/13 rounds in which state undergoes:}  byte substitution (1 S-box used on every byte) _{byte substitution (1 S-box used on every byte)}

 shift rows (permute bytes between groups/columns) _{shift rows (permute bytes between groups/columns)}  mix columns (subs using matrix multiply of groups) _{mix columns (subs using matrix multiply of groups)}  add round key (XOR state with key material)_{add round key (XOR state with key material)}

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Some Comments on AES

1.

1. an an iterativeiterative rather than rather than FeistelFeistel cipher cipher 2.

2. key expanded into array of 32-bit wordskey expanded into array of 32-bit words 1.

1. four words form round key in each roundfour words form round key in each round 3.

3. 4 different stages are used as shown4 different stages are used as shown 4.

4. has a simple structurehas a simple structure 5.

5. only only AddRoundKey uses keyAddRoundKey uses key 6.

6. AddRoundKey a form of Vernam cipherAddRoundKey a form of Vernam cipher 7.

7. each stage is easily reversibleeach stage is easily reversible 8.

8. decryption uses keys in reverse orderdecryption uses keys in reverse order 9.

9. decryption does recover plaintextdecryption does recover plaintext 10.

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Substitute Bytes

 a simple substitution of each byte_{a simple substitution of each byte}

 uses one table of 16x16 bytes containing a _{uses one table of 16x16 bytes containing a} permutation of all 256 8-bit values

permutation of all 256 8-bit values

 each byte of state is replaced by byte indexed by _{each byte of state is replaced by byte indexed by} row (left 4-bits) & column (right 4-bits)

row (left 4-bits) & column (right 4-bits)

 eg. byte {95} is replaced by byte in row 9 column 5_{eg. byte {95} is replaced by byte in row 9 column 5}  which has value {2A}_{which has value {2A}}

 S-box constructed using defined transformation _{S-box constructed using defined transformation} of values in GF(2

of values in GF(288)₎

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Shift Rows

 a circular byte shift in each each_{a circular byte shift in each each}  1₁stst row is unchanged_{row is unchanged}

 2₂ndnd row does 1 byte circular shift to left_{row does 1 byte circular shift to left}  3rd row does 2 byte circular shift to left_{3rd row does 2 byte circular shift to left}  4th row does 3 byte circular shift to left4th row does 3 byte circular shift to left  decrypt inverts using shifts to right_{decrypt inverts using shifts to right}

 since state is processed by columns, this step _{since state is processed by columns, this step} permutes bytes between the columns

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Mix Columns



_{each column is processed separately}



_{each byte is replaced by a value}

dependent on all 4 bytes in the column



_{effectively a matrix multiplication in GF(2}

88

)

₎

using prime poly m(x) =x

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AES Arithmetic



_{uses arithmetic in the finite field GF(2}

88

)

₎



_{with irreducible polynomial}

m(x) = x

m(x) = x88_{+ x}_{+ x}44_{+ x}_{+ x}33_{+ x + 1}_{+ x + 1}

which is

which is _(100011011)_(100011011)or or _{11b}_{11b}



_e.g.

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Mix Columns

 _{can express each col as 4 equations}_{can express each col as 4 equations}  to derive each new byte in col_{to derive each new byte in col}

 decryption requires use of inverse matrix_{decryption requires use of inverse matrix}  with larger coefficients, hence a little harder_{with larger coefficients, hence a little harder}  _{have an alternate characterisation}_{have an alternate characterisation}

 each column a 4-term polynomial_{each column a 4-term polynomial}  with coefficients in GF(2_{with coefficients in GF(2}88) ₎

 and polynomials multiplied modulo (x_{and polynomials multiplied modulo (x}44+1)₊₁₎

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Add Round Key



_{XOR state with 128-bits of the round key}



_{again processed by column (though}

effectively a series of byte operations)



_{inverse for decryption identical}

since XOR own inverse, with reversed keys_{since XOR own inverse, with reversed keys}



_{designed to be as simple as possible}

_{a form of Vernam cipher on expanded key}_{a form of Vernam cipher on expanded key}

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AES Key Expansion



_{takes 128-bit (16-byte) key and expands}

into array of 44/52/60 32-bit words



_{start by copying key into first 4 words}



_{then loop creating words that depend on}

values in previous & 4 places back

in 3 of 4 cases just XOR these together_{in 3 of 4 cases just XOR these together}

₁₁stst word in 4 has rotate + S-box + XOR round _{word in 4 has rotate + S-box + XOR round} constant on previous, before XOR 4

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Key Expansion Rationale



_{designed to resist known attacks}



design criteria included

_{design criteria included}

knowing part key insufficient to find many more_{knowing part key insufficient to find many more}

invertible transformation_{invertible transformation}

fast on wide range of CPU’s_{fast on wide range of CPU’s}

use round constants to break symmetry_{use round constants to break symmetry}

diffuse key bits into round keys_{diffuse key bits into round keys}

enough non-linearity to hinder analysis_{enough non-linearity to hinder analysis}

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AES

Example

Key

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AES

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AES

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AES Decryption



_{AES decryption is not identical to}

encryption since steps done in reverse



_{but can define an equivalent inverse}

cipher with steps as for encryption

but using inverses of each step_{but using inverses of each step}

with a different key schedule_{with a different key schedule}



works since result is unchanged when

_{works since result is unchanged when}

swap byte substitution & shift rows_{swap byte substitution & shift rows}

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Implementation Aspects



_{can efficiently implement on 8-bit CPU}

byte substitution works on bytes using a table _{byte substitution works on bytes using a table} of 256 entries

of 256 entries

shift rows is simple byte shift_{shift rows is simple byte shift}

_{add round key works on byte XOR’s}_{add round key works on byte XOR’s}

mix columns requires matrix multiply in _{mix columns requires matrix multiply in}GF(2GF(288) ₎ which works on byte values, can be simplified which works on byte values, can be simplified

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Implementation Aspects



_{can efficiently implement on 32-bit CPU}

redefine steps to use 32-bit words_{redefine steps to use 32-bit words}

can precompute 4 tables of 256-words_{can precompute 4 tables of 256-words}

then each column in each round can be _{then each column in each round can be} computed using 4 table lookups + 4 XORs computed using 4 table lookups + 4 XORs

at a cost of 4Kb to store tables_{at a cost of 4Kb to store tables}



_{designers believe this very efficient}

implementation was a key factor in its

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Summary



_{have considered:}

the AES selection process_{the AES selection process}

the details of Rijndael – the AES cipher_{the details of Rijndael – the AES cipher}

looked at the steps in each round_{looked at the steps in each round}

the key expansion_{the key expansion}