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CHAPTER 1
INTRODUCTION
As the culmination of a four-year competitive process, NIST (the National Institute of
Standards and Technology) has selected the AES (Advanced Encryption Standard), the successor
to the venerable DES (Data Encryption Standard). The competition was an open one, with
public participation and comment solicited at each step of the process. The AES, formerly
known as Rijndael, was chosen from a field of five finalists.
AES is suitable for any application that requires strong encryption technology. This new
encryption standard may replace the previously used triple-DES where the superior efficiency of
Rijndael algorithm can be used to gain much increased data throughput for less logic real–estate.
Typical applications might include secure communications, program content protection for
digital media applications, storage area, networks, VPN, secure VoIP, wireless LAN, electronic
banking etc..
AES is a 128-bit symmetric cryptographic algorithm. It is symmetric since same key is
used for encryption and decryption.The general Rijndael algorithm is a block cipher with
multiple options for its block and key size. The NIST approved AES is a subset of these options
with a fixed block size of 128-bits, but the key may be 128, 192 or 256-bits in length. This
means, that a basic AES engine is capable of encrypting plain text data in blocks of 128-bits
using any of the specified key sizes. Higher levels of security can be achieved by using bigger
key sizes.
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1.1 OUTLINE OF REPORT
This standard specifies the Rijndael algorithm, a symmetric block cipher that can process
data blocks of 128 bits, using cipher keys with lengths of 128, 192, and 256 bits. Rijndael was
designed to handle additional block sizes and key lengths; however they are not adopted in this
standard. Throughout the remainder of this standard, the algorithm specified herein will be
referred to as “The AES Algorithm.”
This specification includes the following sections:
Chapter 2. Evolution of Cryptography which briefs emergence of thoughts about
cryptography.
Chapter 3. Basics of Cryptography describing its principles.
Chapter 4. Security, Cryptography and Privacy depicts security and privacy aspects in
cryptography.
Chapter 5. Symmetric Key Cryptography describes the key standard used in AES.
Chapter 6. Implementation Tool – MatLab 7.6 briefs the details of project platform.
Chapter 7. AES with CBC algorithm which specifies idea of algorithm and steps involved in
it.
Chapter 8. High-level description of algorithm which explains the steps: SubBytes,
ShiftRows, MixColumns and AddRoundKey in detail.
Chapter 9. Flow diagram which describes flow of algorithm in both encryption and decryption.
Chapter 10. Block diagrams which describes Standard core encryption, decryption and
encryption/decryption standards in detail.
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Chapter 11. Inputs & Outputs given to and taken from the function.
Chapter 12. Key Expansion Algorithm & Flowchart which describes the key expansion
scheme used in AES
Chapter 13. Substiution Boxes which involves construction details of SBox, ISBox and
MixColumn matrices
Chapter 14. AES Encryption Procedure illustrates full encryption algorithm in a 4X4 matrix
Chapter 15. AES Decryption Procedure illustrates full decryption algorithm in a 4X4 matrix
Chapter 16. Algorithms illustrate full AES program in simple steps
Chapter 17. Block Cipher: Modes of Operation which describes and compares different block
modes
Chapter 18. Comparison with previous standard involves comparison between DES and AES.
Chapter 19. Attacks and Security which point outs the possible attacks on AES and the
security in AES against it
Chapter 20. Advantages of AES over other encryption techniques.
Chapter 21. Applications of AES Algorithm
Chapter 22. Limitations of the Algorithm
Chapter 23. Conclusion of the project
Chapter 24. References comprising of paper and web references.
Chapter 25. Appendix used for the implementation of project.
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CHAPTER 2
EVOLUTION OF CRYPTOGRAPHY
The secure transport of messages was the concern of many early civilizations.
Since then, different methods have been developed to assure that only the sender and the
addressee would be able to read a message, while it would be illegible or without significant
meaning to a third party. Today, this practice continues with more fervor. Wireless, wired, and
optical communication networks are able to transport unimaginable amounts of data and thus
privacy of information and security of the network are of the utmost concern because a good part
of the transported information may be very sensitive and/or confidential. Confidentiality of
information has been particularly popularized with the explosive growth of the Internet, which
has touched most people’s lives. However, from the outset, the Internet was based on an open
network architecture with computer-based nodes and without network security, and thus was
vulnerable to attackers and hackers. The development of unbreakable cipher keys, cipher key
distribution, identification of malicious actors, source authentication, physical-link signature
identification, countermeasures, and so forth has been the major thrust of research efforts with
regard to cyber-security. This article focuses on cryptography, and is the first of a series of three
articles on cryptography and security in communications. Subsequent articles will cover wireless
and IP network security, as well as optical network security, quantum cryptography, and
quantum-key distribution processes specific to optical networks. In antiquity, sending a secret
message with a messenger through a hostile territory was as dangerous as it has been to date. The
messenger was subject to interception, and the message was subject to the integrity of the
messenger. As such, methods were developed to assure that the message would arrive at its
destination safe and untampered with. Although this article does not attempt to provide a
historical treatise on the subject, it is worth mentioning some sound and proven examples. The
ancient Mesopotamians would write a message in cuneiform script on a clay tablet that was
exposed to sun to dry. This tablet was then enclosed in a clay envelope, which was also dried.
Breaking the clay envelope to read the message would forfeit the message, particularly during
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times of war, and thus the message was useless. Similarly, in ancient China, people would hide a
message in a cake (known as a moon cake) in order to get a message past the emperor’s guards;
today’s fortune cookie is an American concoction of the Chinese moon cake. Moreover, secret
messages have been found in hieroglyphs. Until the 1980s and prior to the advent of the Internet,
the communications circuit-switched network was in general inaccessible. Moreover, this
network was not challenged with virus attacks and the like; in fact, cyber-security is a modern
term that did not exist until the spread of the Internet several years ago. Thus, for many years,
network security had not been a priority and had been underemphasized or overlooked.
However, the circuit-switched network was not flexible and cost-effective to newly emerging
data services and was losing its edge to computer networking that met the low-cost but not the
reliability and security requirements. The first Internet protocol did not include security features;
however, its deployment expanded rapidly. n fact, information on the Internet network takes a
complex route, which is not under the control of the network itself, as compared with
information on the circuit-switched network. Thus, being a connectionless network with
distributed control, and operated by many small and medium-size network providers,
information was easily accessible and vulnerable to eavesdropping, data harvesting, and attacks.
The possibility that a third party may be able to harvest credit card information and health and
other personal records or misrepresenting data injected in the network has generated increasing
concern within industry and government alike. Similar to the Internet, the initial cellular wireless
network was very vulnerable to eavesdropping and calling-number mimicking. In fact, accessing
calling numbers and pin codes from the airwaves was extremely easy by an actor on the highway
using a properly converted receiver and a laptop. Since then, cellular wireless technology has
evolved, and new coding methods and protocol versions with enhanced security
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CHAPTER 3
BASICS OF CRYPTOGRAPHY
When Julius Caesar sent messages to his generals, he didn’t trust his messengers. So he
replaced every A in his messages with a D, every B with an E, and so on through the alphabet.
Only someone who knew the “shift by 3” rule could decipher his messages.
And so we begin. Data that can be read and understood without any special measures is called
plaintext or cleartext. The method of disguising plaintext in such a way as to hide its substance is
called encryption. Encrypting plaintext results in unreadable gibberish called ciphertext. You use
encryption to ensure that information is hidden from anyone for whom it is not intended, even
those who can see the encrypted data. The process of reverting ciphertext to its original plaintext
is called decryption.
Cryptography is the science of using mathematics to encrypt and decrypt data.
Cryptography enables you to store sensitive information or transmit it across insecure networks
(like the Internet) so that it cannot be read by anyone except the intended recipient. While
cryptography is the science of securing data, cryptanalysis is the science
of analyzing and breaking secure communication. Classical cryptanalysis involves an interesting
combination of analytical reasoning, application of mathematical tools, pattern finding, patience,
determination, and luck. Cryptanalysts are also called attackers. Cryptology embraces both
cryptography and cryptanalysis.
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CHAPTER 4
SECURITY, CRYPTOGRAPHY AND PRIVACY
Until the 1980s and prior to the advent of the Internet, the communications
circuit-switched network was in general inaccessible; specialized networking know-how was
required to tap a twowire pair and eavesdrop (as shown in “spy” movies such as “James Bond”
and “Mission Impossible”) or to mimic signaling codes using the so-called “blue box” and
bypass-calling billing. Moreover, this network was not challenged with virus attacks and the like;
in fact, cyber-security is a modern term that did not exist until the spread of the Internet several
years ago. Thus, for many years, network security had not been a priority and had been
underemphasized or overlooked. However, the circuit-switched network was not flexible and
cost-effective to newly emerging data services and was losing its edge to computer networking
that met the low-cost but not the reliability and security requirements. The first Internet protocol
did not include security features; however, its deployment expanded rapidly. In fact, information
on the Internet network takes a complex route, which is not under the control of the network
itself, as compared with information on the circuit-switched network. Thus, being a
connectionless network with distributed control, and operated by many small land medium-size
network providers, information was easily accessible and vulnerable to eavesdropping, data
harvesting, and attacks. The possibility that a third party may be able to harvest credit card
information and health and other personal records or misrepresenting data injected in the
network has generated increasing concern within industry and government alike. Similar to the
Internet, the initial cellular wireless network was very vulnerable to eavesdropping and callingnumber mimicking. In fact, accessing calling numbers and pin codes from the airwaves was
extremely easy by an actor on the highway using a properly converted receiver and a laptop.
Since then, cellular wireless technology has evolved, and new coding methods and protocol
versions with enhanced security and authentication procedures have been added.
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The rapid spread of the Internet, and the lack of robust security features has
unleashed the appetite of bad actors for invading the network and its computers with destructive
results. Destructive programs that hide within other programs sneak into computers where they
execute instructions that harvest personal data, open classified files, destroy files, allow them to
clone themselves and propagate to other computers, flood the network and cause denial of
service, enlist personal computers to execute programs secretly, and so on. Current incidents
have placed network security on high national priority and at the forefront of research. For
instance, cyber-attacks, “stealth” attacks (attacks that do not modify data or leave Website
traces), and silent data extraction have been on the rise, as was reported to the “Internet Security
Alliance Briefing to White House Staff and Members of Congress” (M. K. Daly, September 16,
2004). The post-September-11th cyber-attack known as “Code Red” infected 150,000 computers
in just fourteen hours and two months later the attack “NIMDA” infected 86,000 computers. The
Computer Emergency Response Team (CERT) of Carnegie Melon University reported that by
2002, attacks would increase to 110,000 from 3,700 in 1998. Similarly, the Congressional
Research Service Report to Congress (April 2004) reported that, “Estimates of total worldwide
losses attributable to attacks in 2003 range from $13 billion due to viruses and worms only to
$226 billion for all forms of overt attacks.” And a report filed with the Federal Trade
Commission (see USA Today, April 1, 2005, p. D1), stated that electronic heists of credit card
numbers and other personal data account for one third of all complaints over the last three years.
These reports and others have raised serious concerns with government and industry. In response
to this, an Internet Security Alliance was formed between Carnegie Mellon University’s
Software Engineering Institute (SEI) and its CERT Coordination Center (CERT/CC) and
Electronic Industries Alliance (EIA), a federation of trade associations with more than 2,500
members.
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CHAPTER 5
SYMMETRIC KEY CRYPTOGRAPHY
Symmetric-key algorithms are a class of algorithms for cryptography that use
trivially related, often identical, cryptographic keys for both decryption and encryption etc.
The encryption key is trivially related to the decryption key, in that they may be
identical or there is a simple transformation to go between the two keys. The keys, in practice,
represent a shared secret between two or more parties that can be used to maintain a private
information link.
Other terms for symmetric-key encryption are secret-key, single-key, shared-key,
one-key, and private-key encryption. Use of the last and first terms can create ambiguity with
similar terminology used in public-key cryptography.
Symmetric-key algorithms can be divided into stream ciphers and block ciphers.
Stream ciphers encrypt the bits of the message one at a time, and block ciphers take a number of
bits and encrypt them as a single unit. Blocks of 64 bits have been commonly used. The
Advanced Encryption Standard (AES) algorithm approved by NIST in December 2001 uses 128bit blocks.Symmetric ciphers are often used to achieve other cryptographic primitives than just
encryption.
Encrypting a message does not guarantee that this message is not changed while
encrypted. Hence often a message authentication code is added to a ciphertext to ensure that
changes to the ciphertext will be noted by the receiver. Message authentication codes can be
constructed from symmetric ciphers (e.g. CBC-MAC).
However, symmetric ciphers also can be used for non-repudiation purposes by
ISO 13888-2 standard.
Another application is to build hash functions from block ciphers. See one-way
compression function for descriptions of several such methods.
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CHAPTER 6
IMPLEMENTATION TOOL – MATLAB 7.6
Fig. 1. MATLAB Symbol
MATLAB Is a program for doing numerical computation. It was
originally designed for solving linear algebra type problem using matrices. It’s name is derived
from MATrix LABort ary. MATLAB is also a programming language that currently is widely
used as a platform for developing tools for machine Learning.
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MATLAB is a commercial “Matrix Laboratory” package which operates as an
interactive programming environment. It is a mainstay of the Mathematics Department software
lineup and is also available for PC’s and Macintoshes and may be found on the CIRCA VAXes.
MATLAB is well adapted to numerical experiments since the underlying algorithms for Matlab’s
built in functions and supplied m-files on the standard libraries LINPACK and EISPACK.
Matlab program and script files always have filenames ending with “.m” ;the
programming language is exceptionally straightforward since almost every data object is
assumed to be in an array. Graphical output is available to supplement numerical results.
BUILDING MATRICES
Matlab has many types of matrices which are built into the system. You can
generate random matrices of other sizes and get help on the ‘rand’ command within matlab.
Another special matrix called a Hilbert matrix, is a standard example in numerical linear
algebra. A magic square is a matrix whoch has equal sum along rows and columns. You can
build matrices of your own with any entries that you may want.
BASIC FEATURES
MATLAB is case sensitive,that is “a” is not the same as “A.” if this proves to be
an annoyance, the command ‘casesen’ will toggle the case sensitivity off and on. The MATLAB
display only shows 5 digits in the default mode. The fact is that MATLAB always keeps and
computes in a double precision 16 decimal places and rounds the display to 4 digits.
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CHAPTER 7
AES WITH CBC ALGORITHM
The AES algorithm consists of a complex non-linear core function, which is iterated
multiple times starting from the incoming plain text data block. Each iteration is called a
“round”. The round function is slightly modified for the final round and there is an additional
pre-processing round at the start of every encryption. The number of “rounds” required depends
on the selected key size. For a key size of 128-bit there are 10 rounds, for a 192-bit key there are
12 rounds, and for a 256-bit key there are 14 rounds. The consequence of this is that the longer
key sizes do take slightly more time to process. Each round of AES requires a unique 128-bit
round key schedule that is generated from the supplied 128-bit, 192-bit or 256-bit AES key using
a key expansion algorithm. For 128-bit keys one needs 11 key schedules, for 192-bit keys one
needs 13 key schedules and for 256-bit keys one needs 15 key schedules. The key expansion
process can be accomplished in one of two ways. For the encryption the round key schedules can
be generated “on the fly” in real-time when they are required by the encryption algorithm. This is
especially useful if the AES keys need to change on a regular basis. If the AES keys do not get
changed too often then the round key schedules may be generated off-line and stored in internal
RAM for subsequent use.
Each iteration in the AES with CBC algorithm mainly consists of only four cryptographic
algorithm steps. They are as follows :
(i)
Sub Bytes
(ii)
Shift Rows
(iii)
Mix Columns
(iv)
Add Round Key
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CHAPTER 8
HIGH LEVEL DESCRIPTION OF THE
ALGORITHM
8.1 The SubBytes step
In the SubBytes step, each byte in the array is updated using an 8-bit substitution box,
the Rijndael S-box. This operation provides the non-linearity in the cipher. To avoid attacks
based on simple algebraic properties, the S-box is chosen as a 16x16 look up table with
hexadecimal values. The S-box is also chosen to avoid any fixed points.
Fig. 2. SubBytes
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8.2 The ShiftRows step
The ShiftRows step operates on the rows of the state; it cyclically shifts the bytes in each
row by a certain offset. For AES, the first row is left unchanged. Each byte of the second row is
shifted one to the left. Similarly, the third and fourth rows are shifted by offsets of two and three
respectively.
Fig. 3. ShiftRows
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8.3 The MixColumns step
In the MixColumns step, each column is multiplied by the known matrix that for the 128
bit key. The multiplication operation is defined as: multiplication by 1 means leaving unchanged,
multiplication by 2 means shifting byte to the left and multiplication by 3 means shifting to the
left and then performing xor with the initial unshifted value. In more general sense, each column
is treated as a polynomial and is then multiplied modulo x4+1 with a fixed polynomial c(x) =
0x03 · x3 + x2 + x + 0x02. The coefficients are displayed in their hexadecimal equivalent of the
binary representation of bit polynomials.
Fig. 4. MixColumns
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8.4 The AddRoundKey step
In the AddRoundKey step, the subkey is combined with the state. For each round, a
subkey is derived from the main key using Rijndael's key schedule; each subkey is the same size
as the state. The subkey is added by combining each byte of the state with the corresponding byte
of the subkey using bitwise XOR.
Fig. 5. AddRoundKey
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CHAPTER 9
FLOW DIAGRAM
Fig. 6. Flowchart
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CHAPTER 10
BLOCK DIAGRAMS WITH DESCRIPTIONS
10.1 Standard Encryption Core AES
This entity is an AES encryption component that uses an external key expander. The
component processes each round in a single clock cycle. Plain text input, round key schedule
input and cipher text output ports are 128 bits wide.
Fig. 7. Standard Encryption Core AES
When the start signal is asserted, input data is loaded and a new encryption operation is
started. After a latency of 11, 13 or 15 master clock cycles (depending on the key size of 128,
192 or 256 bits) the ready signal is asserted and the cipher text output is valid. The round key
index cycles through all needed values and is valid one clock cycle before the round key
schedule data is required. This allows the use of external synchronous RAM to store the round
key schedules. A new encryption operation can be started whenever the round key index is zero.
One clock cycle later the output of a previous operation becomes available.
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10.2 Standard Decryption Core AES
This entity is an AES decryption component that uses an external key expander. The
component processes each round in a single clock cycle. Cipher text input, round key schedule
input and plain text output ports are 128 bits wide.
Fig. 8. Standard Decryption Core AES
When the start signal is asserted, input data is loaded and a new decryption operation is
started. After a latency of 11, 13 or 15 master clock cycles (depending on the key size of 128,
192 or 256 bits) the ready signal is asserted and the plain text output is valid. The round key
index cycles through all needed values and is valid one clock cycle before the round key
schedule data is required. This allows the use of external synchronous RAM to store the round
key schedules. A new decryption operation can be started whenever the round key index is zero.
One clock cycle later the output of a previous operation becomes available.
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10.3 Standard Encryption/Decryption Core AES
This entity is a combined encryption/decryption component with external key expander.
The component processes each round in a single clock cycle. Plain/cipher text input, round key
schedule input and cipher/plain text output ports are 128 bits wide. When the start signal is
asserted, input data is loaded and a new operation is started. Depending on the state of a select
signal the operation is either encryption or decryption. After a latency of 11, 13 or 15 master
clock cycles (depending on the key size of 128, 192 or 256 bits) the ready signal is asserted and
the plain text output is valid. The round key index cycles through all needed values and is valid
one clock cycle before the round key schedule data is required. This allows the use of external
synchronous RAM to store the round key schedules. A new operation can be started whenever
the round key index is zero. One clock cycle later the output of a previous operation becomes
available.
Fig. 9. Standard Encryption/Decryption Core AES
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CHAPTER 11
INPUTS & OUTPUTS
INPUTS

Data to be encrypted such as text, image, file etc. of any size. Data is treated as matrix
and is encrypted block by block.

User defined Key of any length can be used to encrypt the data matrix.
INTERNALLY GENERATED

A Predefined Key is used to encrypt the User defined Key.

Substitution Box & Inverse Substitution Box for Sub Bytes algorithm.

Polymat & Inverse Polymat matrices for Mix Column algorithm.
OUTPUTS

The result of AES encryption is an encrypted data matrix of size 128 bit larger than the
input data matrix.

The result of AES decryption is a decrypted data matrix of size same as input data matrix.
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CHAPTER 12
KEY EXPANSION ALGORITHM
With AES encryption, the secret key is known to both the sender and the receiver.
The AES algorithm remains secure, the key cannot be determined by any known means, even if
an eavesdropper knows the plaintext and the cipher text. The AES algorithm is designed to use
one of three key sizes (Nk). AES-128, AES-196 and AES-256 use 128 bit (16 bytes, 4 words),
196 bit (24 bytes, 6 words) and 256 bit (32 bytes, 8 words) key sizes respectively. These keys,
unlike DES, have no known weaknesses. All key values are equally secure thus no value will
render one encryption more vulnerable than another. The keys are then expanded via a key
expansion routine for use in the AES cipher algorithm.This key expansion routine can be
performed all at once or ‘on the fly’ calculating words as they are needed. The key expansion
algorithm is shown below :
void KeyExpansion(byte[] key, word[] w, int Nw) {
int Nr = Nk + 6;
w = new byte[4*Nb*(Nr+1)];
int temp;
int i = 0;
while ( i < Nk) {
w[i] = word(key[4*i], key[4*i+1], key[4*i+2],
key[4*i+3]);
i++;
}
i = Nk;
while(i < Nb*(Nr+1)) {
temp = w[i-1];
if (i % Nk == 0)
temp = SubWord(RotWord(temp)) ˆ Rcon[i/Nk];
else if (Nk > 6 && (i%Nk) == 4)
temp = SubWord(temp);}
w[i] = w[i-Nk] ˆ temp;
i++;
}
Table 1. Key Expansion
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12.1 KEY EXPANSION – FLOW DIAGRAM
Fig. 10 Key Expansion -Flow Diagram

The predefined key is expanded into 11 key matrices using a key expansion algorithm.

The generated 11 key matrices are used to perform AES encryption of user defined key.

The encrypted user defined key is again expanded into 11 key matrices using the same
key expansion algorithm.

The final 11 key matrices are used to perform AES encryption of input data.

The same 11 key matrices are used to perform AES decryption of encrypted data in the
reverse order.
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CHAPTER 13
SUBSTITUTION BOX (SBox)
Table 2 SBox

The numbers 0 to 255 are arranged in random in SBox.

Eg: A number 12 is replaced with 13th element of SBox, ie. 254.

This avoids linearity.
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13.1 INVERSE SUBSTITUTION BOX (iSBox)
Table 3 iSBox

Inverse SBox is generated by replacing 0 to 255 numbers by one position greater than the
corresponding position in SBox.

Eg : 254 is replaced by 255th element in iSBox which is 12.
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13.2 MIXCOLUMN MATRICES
Table 4 MixColumn matrices

Matrix multiplication is done between data matrix and polymat in encryption and with
inverse polymath in decryption.

Bitshift and bitxor are used for matrix multiplication and output values are limited within
255 (GF(2^8) – Galois Field).
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CHAPTER 14
AES ENCRYPTION PROCEDURE

Length of Plain Data is made to a multiple of 16 and divided into 4X4 matrices.

Each 4X4 matrix is encrypted separately.

In first round, add round key is performed between first state matrix and first key matrix.
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
In rounds 2 to 10, Sub bytes is performed to state matrix.

It is followed by shift rows to left.
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
Then mixcolumn algorithm is performed followed by add round key.

Keys 2 to 10 are used in add round key for corresponding rounds.
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
In the last round sub bytes algorithm is performed in state matrix.

It is followed by shift rows to left.
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At last, add round key is performed to state matrix with key 11 in last round to obtain the
encrypted data.

Same procedure is repeated in remaining 4X4 blocks.
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CHAPTER 15
AES DECRYPTION PROCEDURE

Each 4X4 encrypted data matrix is decrypted separately.

In first round, add round key is performed between state matrix and last key matrix.
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
In rounds 2 to 10, shift rows to right is performed to state matrix.

It is followed by inverse sub bytes.
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
Then add round key is performed followed by inverse mixcolumn algorithm.

Keys 2 to 10 are used in add round key for corresponding rounds in reverse order.
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
In the last round shift rows to right is performed to state matrix.

It is followed by inverse sub bytes algorithm.
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AES WITH CBC
At last, add round key is performed to state matrix with key 1 in last round to obtain the
decrypted data.

Same procedure is repeated in remaining 4X4 blocks.
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CHAPTER 16
ALGORITHMS
MAIN FUNCTION – CryptAES
Step 1: Start
Step 2: Find length of input data
Step 3: Reshape input data and use integers if possible
Step 4: Encode if the Command starts with 'e' or 'E', decode otherwise
Step 5: Initialize AES parameters, create CBC initial vector for encoding
Step 6: Process the data – encode or decode.
Step 7: Clear secrets
Step 8: Stop
ENCRYPT FUNCTION – EncodeI
Step 1: Start
Step 2: Initialization of parameters to local variables (SBox & Key)
Step 3: Allocate output as an array of zero with length greater than input data length
Step 4: Initialize substitution for cyclical shift to the left
Step 5: Set CBC IV as first block
Step 6: Open wait bar
Step 7: Iterate steps 8 to 18 until DataLen/16 reached.
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Step 8: Copy 4x4 block from input data to the state matrix and apply the CBC mask
Step 9: Perform first round - Add RoundKey.
Step 10: Iterate steps 11 to 15 nine times (Rounds 2 to 10)
Step 11: Perform SubBytes and Shift Rows to left algorithms
Step 12: Perform Mix Columns algorithm
Step 13: Create polynomial matrix for Mix columns
Step 14: Divide msb with irreducible decimal 283 to limit upto 255
Step 15: Perform Add Roundkey.
Step 16: Encrypted data is new CBC mask
Step 17: Perform Round 11 - subbyte, shiftrows, addroundkey
Step 18: Process the wait bar
Step 19: Close the wait bar.
Step 20: Clear secrets
Step 21: Stop
DECRYPT FUNCTION – DecodeI
Step 1: Start
Step 2: Initialization of parameters to local variables (SBox & Key)
Step 3: Allocate output as an array of zero with length less than encrypted data length
Step 4: Initialize substitution for cyclical shift to the right
Step 5: Set first block as initial CBC IV
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Step 6: Open wait bar
Step 7: Iterate steps 8 to 18 until DataLen/16 reached.
Step 8: Use encrypted data block as CBC value for the block
Step 9: Perform last round - Add RoundKey.
Step 10: Iterate steps 11 to 15 nine times (Rounds 10 down 2)
Step 11: Perform Inverse SubBytes and Shift Rows to right algorithms
Step 12: Perform Add Roundkey.
Step 13: Perform Inverse Mix Columns algorithm
Step 14: Create polynomial matrix for inverse Mix columns
Step 15: Divide msb with irreducible decimal 283 to limit upto 255
Step 16: Apply old CBC mask to decrypted data
Step 17: Perform Round 1 – inverse subbyte, shiftrows to right, addroundkey
Step 18: Process the wait bar
Step 19: Close the wait bar.
Step 20: Clear secrets
Step 21: Stop
INITIALISATION OF AES PARAMETERS Function - Init
Step 1: Start
Step 2: Initialize parameters, create CBC IV
Step 3: Create the S-box and the inverse S-box
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Step 4: Call function ExpandCipher for expansion of pre defined key
Step 5: Limit the range of values to 0:255
Step 6: Limit or expand key to 16 bytes
Step 7: Encrypt user defined key with expanded pre defined key
Step 8: Call function ExpandCipher for expansion of encrypted key
Step 9: Stop
ROUND KEY EXPANSION Function – ExpandCipher
Step 1: Start
Step 2: Expand the 16-byte cipher to the 4x4x11 array
Step 3: Copy 16 bytes column-wise
Step 4: Perform RounKey Expansion algorithm
Step 5: Stop
CALLING PROGRAMS (Calls CryptAES in AES GUI)
TEXT DATA ENCRYPTION Function
Step 1: Start
Step 2: Get the input text from Edit Text Box
Step 3: Find the input Data length
Step 4: If it is not a multiple of 16, append it with ‘#’ to make it a multiple of 16
Step 5: Get the key from Edit Text Box
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Step 6: Perform CryptAES speed test
Step 7: Call CryptAES
Step 8: Set encrypted data in the Edit Text Box
Step 9: Stop
TEXT DATA DECRYPTION Function
Step 1: Start
Step 2: Get the key from Edit Text Box
Step 3: Perform CryptAES speed test
Step 4: Call CryptAES.
Step 5: Set encrypted data in the Edit Text Box
Step 6: Stop
IMAGE DATA READ Function
Step 1: Start
Step 2: Read the image by browsing it
Step 3: Show the image on the respective axis
Step 4: Find the size of image in 3 dimensions
Step 5: Make all dimensions multiple of 16 by appending with ‘0’
Step 6: Stop
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IMAGE DATA ENCRYPTION Function
Step 1: Start
Step 2: Get the key from Edit Text Box
Step 3: Perform CryptAES speed test
Step 4: Call CryptAES.
Step 5: Show the encrypted image on respective axis
Step 6: Stop
IMAGE DATA DECRYPTION Function
Step 1: Start
Step 2: Get the key from Edit Text Box
Step 3: Perform CryptAES spee test
Step 4: Call CryptAES
Step 5: Show the decrypted image on respective axis
Step 6: Stop
FILE ENCRYPTION Function
Step 1: Start
Step 2: Open the clear file
Step 3: Set the pathname and filename in the Edit Text Box.
Step 4: If File ID less than ‘0’, Cannot read the file
Step 5: Read the file in binary
Step 6: Close the file
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Step 7: Find data length and if data length is not a multiple of 16, append it with ‘0’ to make it
multiple of 16.
Step 8: Append dummy data to check correct key.
Step 9: Get the key from Edit Text Box
Step 10: Call CryptAES
Step 11: Save the cipher file by writing in binary.
Step 12: Close the file
Step 13: Stop.
FILE DECRYPTION Function
Step 1: Start
Step 2: Open the cipher file
Step 3: Set the pathname and filename in the Edit Text Box.
Step 4: If File ID less than ‘0’, Cannot read the file
Step 5: Read the file in binary
Step 6: Close the file
Step 7: Get the key from Edit Text Box
Step 8: Call CryptAES
Step 9: Find data length
Step 10: Check correct key by comparing with dummy key appended.
Step 11: Save the clear file by writing in binary.
Step 12: Close the file
Step 13: Stop.
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CHAPTER 17
BLOCK CIPHER: MODES OF OPERATION
In cryptography, modes of operation enable the repeated and secure use of a block
cipher under a single key. A block cipher by itself allows encryption only of a single data block
of the cipher's block length. When targeting a variable-length message, the data must first be
partitioned into separate cipher blocks. Typically, the last block must also be extended to match
the cipher's block length using a suitable padding scheme. A mode of operation describes the
process of encrypting each of these blocks, and generally uses randomization based on an
additional input value, often called an initialization vector, to allow doing so safely.
Modes of operation have primarily been defined for encryption and
authentication. Historically, encryption modes have been studied extensively in regard to their
error propagation properties under various scenarios of data modification. Later development
regarded integrity protection as an entirely separate cryptographic goal from encryption. Some
modern modes of operation combine encryption and authentication in an efficient way, and are
known as authenticated encryption modes.
While modes of operation are commonly associated with symmetric encryption,
they may also be applied to public-key encryption primitives such as RSA in principle (though in
practice public-key encryption of longer messages is generally realized using hybrid encryption).
17.1 INITIALIZATION VECTOR (IV)
An initialization vector (IV) is a block of bits that is used by several modes to
randomize the encryption and hence to produce distinct cipher texts even if the same plaintext is
encrypted multiple times, without the need for a slower re-keying process.
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An initialization vector has different security requirements than a key, so the IV
usually does not need to be secret. However, in most cases, it is important that an initialization
vector is never reused under the same key. For CBC (Cipher Block Chaining), reusing an IV
leaks some information about the first block of plaintext, and about any common prefix shared
by the two messages. In CBC mode, the IV must, in addition, be unpredictable at encryption
time; in particular, the (previously) common practice of re-using the last cipher text block of a
message as the IV for the next message is insecure (for example, this method was used by SSL
2.0). If an attacker knows the IV (or the previous block of cipher text) before he specifies the
next plaintext, he can check his guess about plaintext of some block that was encrypted with the
same key before (this is known as the TLS CBC IV attack).
17.2 PADDING
A block cipher works on units of a fixed size (known as a block size), but
messages come in a variety of lengths. So some modes (namely ECB and CBC) require that the
final block be padded before encryption. Several padding schemes exist. The simplest is to add
null bytes to the plaintext to bring its length up to a multiple of the block size, but care must be
taken that the original length of the plaintext can be recovered; this is so, for example, if the
plaintext is a C style string which contains no null bytes except at the end. Slightly more
complex is the original AES method, which is to add a single one bit, followed by enough zero
bits to fill out the block; if the message ends on a block boundary, a whole padding block will be
added. Most sophisticated are CBC-specific schemes such as cipher text stealing or residual
block termination, which do not cause any extra cipher text, at the expense of some additional
complexity. Schneier and Ferguson suggest two possibilities, both simple: append a byte with
value 128 (hex 80), followed by as many zero bytes as needed to fill the last block, or pad the
last block with n bytes all with value n.
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17.3 ELECTRONIC CODE BOOK
The simplest of the encryption modes is the electronic codebook (ECB) mode.
The message is divided into blocks and each block is encrypted separately.
Fig. 11 Electronic Code Book
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17.4 CIPHER BLOCK CHAINING
CBC mode of operation was invented by IBM in 1976. In the cipher-block
chaining (CBC) mode, each block of plaintext is XORed with the previous ciphertext block
before being encrypted. This way, each ciphertext block is dependent on all plaintext blocks
processed up to that point. Also, to make each message unique, an initialization vector must be
used in the first block.
Fig. 12 Cipher Block Chaining
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If the first block has index 1, the mathematical formula for CBC encryption is
while the mathematical formula for CBC decryption is
CBC has been the most commonly used mode of operation. Its main drawbacks
are that encryption is sequential (i.e., it cannot be parallelized), and that the message must be
padded to a multiple of the cipher block size. One way to handle this last issue is through the
method known as ciphertext stealing.
Note that a one-bit change in a plaintext affects all following ciphertext blocks. A
plaintext can be recovered from just two adjacent blocks of ciphertext. As a consequence,
decryption can be parallelized, and a one-bit change to the ciphertext causes complete corruption
of the corresponding block of plaintext, and inverts the corresponding bit in the following block
of plaintext.
The disadvantage of this method is that identical plaintext blocks are encrypted into identical
ciphertext blocks; thus, it does not hide data patterns well. In some senses, it doesn't provide
serious message confidentiality, and it is not recommended for use in cryptographic protocols at
all. A striking example of the degree to which ECB can leave plaintext data patterns in the
ciphertext is shown below; a pixel-map version of the image on the left was encrypted with ECB
mode to create the center image, versus a non-ECB mode for the right image.
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Fig 13. Original Image
AES WITH CBC
Fig. 14 Encrypted using ECB
mode
Fig. 15 Encrypted using CBC
results in pseudo-randomness
The image on the right is how the image might appear encrypted with CBC, —
indistinguishable from random noise. Note that the random appearance of the image on the right
does not ensure that the image has been securely encrypted; many kinds of insecure encryption
have been developed which would produce output just as 'random-looking'.
ECB mode can also make protocols without integrity protection even more
susceptible to replay attacks, since each block gets decrypted in exactly the same way. For
example, the Phantasy Star Online: Blue Burst online video game uses Blowfish in ECB mode.
Before the key exchange system was cracked leading to even easier methods, cheaters repeated
encrypted "monster killed" message packets, each an encrypted Blowfish block, to illegitimately
gain experience points quickly.
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CHAPTER 18
COMPARISON WITH PREVIOUS STANDARDS
This is impressive and is true because the size of the AES key is exponentially
larger than the DES key. However, DES was broken earlier than expected in part because CPU
processing speeds have also been increasing exponentially. If we assume that CPU’s will
continue to increase exponentially, per Moore’s Law which states CPU processor speed will
double every 18 months, AES-128 will still remain secure for 109.5 years, AES-196 will remain
secure for 211.5 years and AES-256 will remain secure for 301.5 years. While the demise of
Moore’s Law has been considered imminent for the past 20 years or so, and there is no true end
in sight,
it seems unlikely Moore’s Law will continue for another 300 years.
Also the
assumption in these statements is attacks more efficient than brute force will not be found.
Key Length
(Nk words)
4
6
8
AES-128
AES-192
AES-256
Rijndael - 128
Rijndael - 192
Rijndael - 256
DES
*
Expanded
Key Length
(words)
Block Size Number of
(Nb words) Rounds Nr
44
4
10
52
4
12
60
4
14
4
10
4
44
6
12
8
14
4
12
6
52
6
12
8
14
4
14
8
60
6
14
8
14
*
256
2
16
2
of 64 bits, only 56 are used
Table 5 AES, DES, Rijndael Comparisons
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18.1 AES vs DES AT A GLANCE
Table 6. AES vs DES
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CHAPTER 19
ATTACKS AND SECURITY
There are several known methods for attacking block ciphers. The Rijndael
algorithm was designed to be resistant to all the known methods.
The four most common are
linear, differential, XSL and Side Channel Attacks.
19.1 DIFFERENTIAL CRYPTANALYSIS
Differential Cryptanalysis is the study of how differences in input affect
differences in output. Certain values for keys in DES made the encryption algorithm vulnerable
to linear analysis. Increasing the number of rounds greatly reduces the success of differential
attacks. The authors estimated that 5 rounds would make the difficulty of differential analysis
about as hard as a brute force attack on the key. They then added a more rounds as a buffer for
added security.
19.2 LINEAR CRYPTANALYSIS
Linear Crytanalysis is the study of correlations between input and output.
SubBytes & MixColumns are designed to frustrate Linear Analysis.
19.3 XSL CRYPTANALYSIS
Linear Cryptanalysis is an attack method developed in 2002. There is a lot of
debate over how successful XSL can be. The developers claim it will be able to break AES. But
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many mathematicians and cryptography experts have their doubts. Their feeling is that even if
XSL can break AES, it cannot do so well, instead of taking 149 trillion years to brute force an
attack on the key, XSL might be able to find the key in just 140 trillion years, not much of a
benefit.
XSL is a detailed study of the mathematics behind an encryption algorithm. Its run
time is hard to calculate as it uses complex heuristics. Essentially it tries to reduce the cipher to a
series of mathematical equations and then solves those equations. Right now XSL has reduced
AES to a series of 8000 non-linear equations and 1600 unknowns. If a method can be found to
solve these equations AES can be broken.
19.4 SIDE CHANNEL ATTACKS
Side channel Attacks are attacks on the implementation of AES, not on the input
or the AES cipher text. It attempts to correlate various measurements of the encrypting tool with
time in an attempt to guess the key. A professor at MIT,9 encoded an AES algorithm on his
computer, an 850MHz, Pentium III running FreeBSD 4.8 and by measuring time delays between
the CPU and memory was able to successfully guess the key in under 100 minutes. There is a
correlation between the index of an array and the time it takes to get the results back. This is due
to the physical location of the data in the memory device. Data closer to the output lead will not
take as much time to be retrieved as data further away, because it will not take as long for the
signal to propagate its way out of the chip.
He feels he can improve on this time. After running a few thousand encryptions
he spent about an hour studying the results of his measurements. After studying the data, there
were many repetitions to avoid noise, he concluded the key was one of several possibilities. By
trying each one, he was able to find the key.
He believes this analysis process can be
programmed, cutting the time down to just a few minutes.
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The method of measuring time delays in memory requests are called timing attacks.
Power attacks attempt to measure power consumption by the CPU.
It takes more power to
switch 8 bits than it takes to switch 1 bit. Some are also now measuring radiation levels from
CPU’s and gaining knowledge of its inner workings.
There are several techniques which can greatly frustrate side channel attacks. 1)
Avoid use of arrays. Compute values of SBOX and rCon to avoid timing attacks. 2) Design
algorithms and devices to work with constant time intervals. (independent of key and plaintext.)
Study your device spec sheets, and insist on accurate data. For example you should know which
takes longer, XOR or shift operations. 3) Use same memory throughout, remember, cache is
faster than DRAM. 4) Compute Key Expansion on the fly. Don’t compute the Key Expansion
and then reference it from memory. 5) Utilize pipelining to stabilize CPU power consumption.
6) Use specialized chips whenever possible, right now they are significantly faster than CPU’s
and require extremely expensive equipment for side channel attack measurements.
NIST was aware of side channel attacks when evaluating all the finalists.
Comparing the Rijndael algorithm security against side channel attacks to the other four finalists
considered by NIST they concluded:

Rijndael and Serpent use only Boolean operations, table lookups, and fixed
shifts/rotations. These operations are the easiest to defend against attacks.

Twofish uses addition, which is somewhat more difficult to defend against attacks.

MARS and RC6 use multiplication/division/squaring and/or variable shift/rotation. These
operations are the most difficult to defend.
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CHAPTER 20
ADVANTAGES

High data security.

Unclassified.

Publicly disclosed.

Available royalty-free, worldwide.

Capable of handling a block size of at least 128 bits.

At a minimum, capable of handling key sizes of 128, 192, and 256 bits.

Computational efficiency and memory requirements on a variety of software and
hardware including smart cards.

Flexibility, simplicity and ease of implementation.

AES is extremely fast compared to other block ciphers. (though there are tradeoff
between size and speed)

The round transformation is parallel by design. This is important in dedicated hardware
as it allows even faster execution.

AES was designed to be amenable to pipelining.

The cipher does not use arithmetic operations so has no bias towards big or little endian
architectures.

AES is fully self-supporting. Does not use SBoxes of other ciphers, bits from Rand
tables, digits of  or any other such jokes. (Their quote, not mine)

AES is not based on obscure or not well understood processes.

The tight cipher and simple design does not leave enough room to hide a trap door.
The AES will be the government's designated encryption cipher. The expectation is that
the AES will suffice to protect sensitive (unclassified) government information until at least the
next century. It is also expected to be widely adopted by businesses and financial institutions.
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CHAPTER 21
APPLICATIONS

AES (256-bit) is used to encrypt 900 MHz and 2.4 GHz data communications on
MaxStream Radio Modems.

AES is used to encrypt video games on the Sony PSP.

AES is an encryption algorithm used by the IEEE 802.11i (WPA2) standard.

AES in CBC mode is the default cipher used in OpenSSH protocol 2 connections.

AES is employed in WinRAR when encryption is used.

AES is used by Apple's(TM) later OS's using 128-bit encryption

AES is used by Winzip 9.0. and Winrar
.
CHAPTER 22
LIMITATIONS

The inverse cipher is less suited to smart cards, as it takes more codes and cycles.

The cipher and inverse cipher make use of different codes and/or tables.

In hardware, The inverse cipher can only partially re-use circuitry which implements the
cipher.
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CHAPTER 23
CONCLUSION
The security of communications networks is of extreme importance, as it impacts
the privacy of client data as well as national security.
Unlike DES, which has no parameterization and hence no flexibility, AES permits
upgrades as necessary. As technology improves, and as the true strength of AES algorithms
becomes better understood through analysis, the most appropriate parameter values can be
chosen.
The new AES will certainly become the de facto standard for encrypting
all forms of electronic information, replacing DES. AES-encrypted data is unbreakable in the
sense that no known cryptanalysis attack can decrypt the AES cipher text without using a bruteforce search through all possible 256-bit keys.
The major obstacle we found when implementing an AES was that the official
specification document was written from a mathematician's point of view rather than from a
software developer's point of view. In particular, the specification assumes that the reader is
fairly familiar with the GF(28) field and it leaves out a few key facts regarding GF(28)
multiplication that are necessary to correctly implement AES. We have tried here to remove the
mystery from AES, especially surrounding GF(28) field multiplication.
However, having this code in your skill set will remain valuable for a number of
reasons. This implementation is particularly simple and will have low resource overhead. In
addition, access to and an understanding of the source code will enable you to customize the
AES class and use any implementation of it more effectively.
Security is no longer an afterthought in anyone's software design and
development process. AES is an important advance and using and understanding it will greatly
increase the reliability and safety of your software systems.
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CHAPTER 24
REFERENCES
1. AES page available via http://www.nist.gov/CryptoToolkit.
2. Computer Security Objects Register (CSOR): http://csrc.nist.gov/csor/.
3. J. Daemen and V. Rijmen, AES Proposal: Rijndael, AES Algorithm Submission,
September 3, 1999, available at http://www.nist.gov/.
4. J. Daemen and V. Rijmen, The block cipher Rijndael, Smart Card research and
Applications, LNCS 1820, Springer-Verlag, pp. 288-296.
5. B. Gladman’s AES related home page
http://fp.gladman.plus.com/cryptography_technology/.
6. A. Lee, NIST Special Publication 800-21, Guideline for Implementing Cryptography in
the Federal Government, National Institute of Standards and Technology, November
1999.
7. A. Menezes, P. van Oorschot, and S. Vanstone, Handbook of Applied Cryptography,
CRC Press, New York, 1997, p. 81-83.
8. J. Nechvatal, et. al., Report on the Development of the Advanced Encryption Standard
(AES), National Institute of Standards and Technology, October 2, 2000, available at
http://www.nist.gov/.
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CHAPTER 25
APPENDIX
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