Int. J. Communications, Network and System Sciences, 2012, 5, 850-853
http://dx.doi.org/10.4236/ijcns.2012.512090 Published Online December 2012 (http://www.SciRP.org/journal/ijcns)
Improvement of the Round Key Generation of AES
Junshe Wang, Han Xu, Mingqiu Yao
Department of Communication and Information System, Hebei University of
Science and Technology, Shijiazhuang, China
Received August 13, 2012; revised September 27, 2012; accepted November 10, 2012
ABSTRACT
The key generation algorithm of AES was introduced, the weaknesses of the key generation design of AES were inves-
tigated. According to the key demand put forward a kind of new design idea, and this designing strategy was developed,
which can be used to improve the key generation algorithm of AES. An analysis shows that such improvement can en-
hance the safety of the original algorithm without reducing its efficiency.
Keywords: A ES; Data E nc r yption Standard (DE S); Key Generation; Rijndeal
1. Introduction
In modern society, computer network has already been
covered. In people’s daily lives, the information technol-
ogy industries have become ubiquitous. In the civil and
military, commercial’s security which is playing an im-
portant role is very prominent [1]. Therefore, the impor-
tance of information security has been paid more and
more attention. Encryption technology as an important
field of information security technology, which is widely
considered as the most effective means to ensure informa-
tion security. Because of advances in computer technol-
ogy and the needs of reality, the cryptology had the de-
velopment which progresses by leaps and bounds. In the
field of block ciphers, DES was already unable to satisfy
requirements of the security, the United States had col-
lected and selected the Rijndael algorithm as the new
Advanced Encryption Standard (Advanced Encryption
Standard = AES). Compared with 3DES, the security of
the AES algorithm is better, AES Algorithm is more sim-
ple and flexible.
First, algorithm of AES is based on group encryption
algorithm. Algorithm including massive shifting algo-
rithm, and the shift operation belongs to the time instruc-
tion. It cannot conduct simultaneously with other instruc-
tions, and reduces the efficiency of the algorithm. Second,
the rounds of encryption using a loop operation, cyclic
operation may cause the instruction block, so the instruc-
tion. Therefore, the paper made the improvement slightly
in the AES algorithm foundation, and reduced shift op-
eration and improved the round key, thereby reducing the
encryption and decryption without the premise of im-
proving the efficiency of its security.
2. Encryption Algorithm Round
Transformation
The AES encryption algorithm’s main body is the encryp-
ti on round the transfo rmation round transformation includes
mainly ByteSub, ShiftRow, MixColumn and AddRound-
Key.
A. ByteSub transformation
It is each byte in the state which is transformed by
ByteSub instead of s-boxes transformation. This trans-
formation made up of two steps:
1) Multiplicative inverses of each byte in the State.
2) The results which is obtained by (2) to (1) do trans-
formation y = f (x).
0
0
1
1
2 2
3 3
44
55
66
77
10001111 1
11000111 1
11100011 0
11110001 0
11111000 0
01111100 1
00111110 1
00011111 0
x
y
x
y
x y
x y
yx
yx
yx
yx




 

 

 

 

 


 

 

 

 

 
  

 

























B. ShiftRow transformation
ShiftRow transform the line of state which increasing the
offset of circulation moves left, first line unchanged, sec-
ond line loop le ft 1 by t e , t hi rd l i ne l oop l e ft 2 byt es, fourt h
line loop left 3 by t e s.
C. MixColumn transformation
MixColumn transformation makes confuse transforms
to columns in the state. In MixColumn transformation, the
data of state column as 32-bit, and then carries on the
C
opyright © 2012 SciRes. IJCNS

J. S. WANG ET AL. 851
matrix multiplication transformation to it:
 


Sjxc xSjx
 (1)

32
0.30.10.1 0.2cxx xx (2)
D. AddRoundKey transformati on
The AddRoundKey transformation let each byte in
state and the round sub-key corresponding to the byte
XOR, the value of the AddRoundKey transformation are
the result of state. The round key transformation in pseu-
do C code represent ed as fol l o ws:
Round(State,Roundkey[i]){
ByteSub(State);
ShiftRow(State);
MixColumn(St at e);
AddRoundKey (Stat e,R oundkey[i]);
}
3. Key Analysis of the AES Encryption
Algorithm
The AES algorithm expands directly the seed key to get
the keys. And each 32 bit word i and and k1i
k4i k

are related, in other words, if it obtained 4i and 1i
kk

can obtain i. Similarly, if it knew i and kk 1i
k

may
obtain 4i, knew i and 4i
kk k

may obtain 1i k

. Al-
though, each round word which produces in a round key
can be carried on by 4 integral multiples the complication,
the correlation of key generation cannot be changed by
this kind of the complication. Suppose, a round of the
AES key i, 1i, , 3 is known now. Then, it
may be through 1i, 2i obtain , through
kkk2k
i
ki k
1i
k1i k

,
2i obtain , through , 1i obtain 3i
k2i ki kk k

,
through 1i, i obtain 4i kk k

again, the preceding
round of the round keys of all sub-keys have been ob-
tained. It also through i, 3i obtain 4i, through
1i, obtain 5i
k, through 2i k, 5i obtain 6i k
kk k
k4i kk

,
through 3i, obtain k6i
k7i k

, and then get the last
round keys.
The above key generation process is analyzed to get the
following pr op ert i e s:
Nature 1: Direct expansion of the key enables itself to
have highly effective;
Nature 2: The preceding round key is only replied by
the generation of the new key, the generation of the new
key can participate encryption and decryption, immedi-
ately [2]. Therefore, the generation algorithm of the key
has timeliness;
Nature 3: If one of the round keys is obtained by the
aggressor then the complete seed key will be obtained by
the aggressor. Namely: The sub-key and the seed key had
the relevance.
Security of the key generation algorithm is reduced by
the nature 3. AES has the high efficiency of the nature 1,
basis of the direct expansion. The latter round key is ob-
tained by the preceding round key, through the first round
of the key as the seed key. Generally speaking, since the
latter round key can be promoted by preceding round key,
preceding round key is obtained by the latter round key,
this is the reason which the nature 3 appears [3].
the main reason is th e seed key which is condu cted on the
4. Algorithm Improvements
ing A. 44 keys are generated by dis patc h
The AES algorithm’s 128 seed key is produced by us-
ing 44 sub-keys. First, the seed key was divided into four
words: 0 k, 1 k, 2 k, 3 k, the remaining sub-keys are
produce ung a scheduling algori t hm in Figure 1. d bysi
And F is: 32 byte position rotate left one byte. Namely:
RotByte(a,b,c,d) = (b,c,d,a).
B. Cipher
The function cipher is the real McCoy, doing the actual
encryption of the 16 byte long input vector of plaintext
into the outpu t ciphertext vecto r ai illustrated in Figure 2 .
Further input parameters of the vipher, that have been
created by the initialization function aes_init are the sub-
stitution table s-box, the key schedule w, and theological
matrix poly_mat. [4]
The cipher rearranges the plaint ext vector into the state ma-
trix and iteratively loops the state through add_round _key,
sub_bytes, shif t_rows, and mix_coulum ns.
C. The new algorithm’s advantage—Round Keys Ex-
ch 0 round AES algorithm as the example, the ini-
tia
ange
Take 1
l 4 words seed key expands to 44 word sub-key.
Among them, Key dependent relationship for: i k de-
pends on 1i k

and 4i k



4,5, ,43i. First, it ner-
ates the oral key.en-keys generated
by the seed key and the gradual transformation as Figure
3. Taki
ge
igin Th, 40 words sub
ng 2nd round and the 3rd round key as a group,
Ex
kes 4 wo
10 w. Tn
change 7 k and 9 k;Taking 4th round and the 5th
round key a group, Exchange 15 k and 17 k; Taking
6th round and the 7th round key as a groupxchange
23
k and 25 k; Taking 8th round and the 9th round key as
oup, Exchange 31 k and 33 k; Taking 10th round and
the 11th round key a groupxchange 39 k and 41 k;
After exchanging 44 words sub-key, It still s
as new round of all 11 round. Take 2nd round and 3rd
round new sub-key as the example, it analyzes the corre-
lations of each round key after the exchange. Before the
exchange, it may derive the 3rd round key through the
2nd round key, it may also derive the 2nd round key ac-
cording to the 3rd round key [5]. 2nd round of keys be-
come 4 k, 5 k, 6 k, 9 k and 3rd round of keys becomes
8
k, 7 k10 kk af exchange the 8th word and the
thordhere-use the relationship between them, it
can only through the 2nd round key obtain the 3rd round
as
,
, E
a
a gras
ter
, Et rd
, 11

J. S. WANG ET AL. 852
k0k1k2k3
k7
k10
k5k4
k11
k6
k9k8
k43k42k41k40
F
F
F
F
Figure 1. Wheel key gene ration.
cipher
Sub_bytesAdd_round_key Shift_rows Mix_columns
plaintext ciphertext
S-box wPoly_mat
State,
round_
key
S-box Poly_mat
Figure 2. Encryaption funtion cipher.
k4
k8
k7k6k5
k11k10k9
k4
k8k7
k6k5
k10
k9
k11
The 2n
d
roun
d
The 3r
d
round
Figure 3. Column hybrid transformation.
key and through the 3rd round k ey can only obtain a
8
k
2nd rnd key 4 k. ou
It cannot ob tain
ance because of increasing just 5 times in exchange in the
original algorithm, and improving the security of the al-
gorithm. Such improvements cannot avoid the correlation
between each round key completely because of obtaining
one word of each round key, but it enhanced security of
the original algorithm actually [6]. It is the method that
the preceding round sub-key com-
pletely through the generation algorithm if the aggressor
obtains one of the new round sub-key. It maintains basi-
cally high speed of the original algorithm in the perform-

J. S. WANG ET AL. 853
the sub-key for the key rotation when produces two new
round sub-key every time, therefore, the nature 2 (real-
time) of the original AES algorithm have not been de-
stroyed by improving the algorithm, and it has not re-
duced the efficiency to strengthen security of the original
algorithm [7] .
D. New algorithm encryption experiment
The new improved algorithm is simulated by using
mhich obtains atlab. Cipher the Figure 4 completely, w
the encryption documents of Figure 4.
In the Figure 5, it is a result of the cipher of the im-
proved algorithm and the original algorithm. It can see
directly, the improved algorithm is more complex than the
original algorithm(the black spots are more crowded).
Response time of the two algorithms program as the fol-
lowing table (see Table 1).
Figure 4. The picture of original.
The origin gl al gorithm
50 100 150 200 250
50
100
150
200
250
The im proved algorit h m
50 100 150 200 250
50
100
150
200
250
Figure 5. The picture of cipher.
Table 1. Response time.
Time algorithmFirst (second)Second Third Fourth
Original 62.79114 62.39147 62.2351162.46756
Improved 62.34567 62.36778 62.2346862.87953
The table can be seen, response time of the original
algorithm is 62.23511 to 62.79114, response time of the
improved algorithm is 62.87953 to 62.67890 (seco nd).
5. Summary and Outlook
It improves sub-key of the original production algorithm
through research of the key production algorithm and
demand consideration, to strengthen the security of the
original algorithm without reducing efficiency. Algorithm
improvement extracts from many merits of improvement
algorithm when found one method to strengthen the secu-
rity of the origi nal al gori t hm without reducing effici ency .
It is unidirectional strategy that th e previous round key
has only been produced under the emphasis by the next
round key. Although this kind of strategy which research
in the AES algorithm is put forward, it also can be used
with other groups of the production key as a design
thought. This alg orithm will also be studied further in the
future.
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