UNIT – 1
Computer security is information security as applied to computers and networks.
The field covers all the processes and mechanisms by which computer-based equipment,
information and services are protected from unintended or unauthorized access, change or
destruction. This included not only protection from unauthorized activities or untrustworthy
individuals, but also from unplanned events and natural disasters.
A Taxonomy of Computer Security
Computer security is frequently associated with three core areas, which can be conveniently
summarized by the acronym "CIA":



Confidentiality -- Ensuring that information is not accessed by unauthorized persons
Integrity -- Ensuring that information is not altered by unauthorized persons in a way
that is not detectable by authorized users
Authentication -- Ensuring that users are the persons they claim to be
1. Confidentiality
You may find the notion of confidentiality to be straightforward: Only authorized people or
systems can access protected data. However, as we see in later chapters, ensuring confidentiality
can be difficult. For example, who determines which people or systems are authorized to access
the current system? By "accessing" data, do we mean that an authorized party can access a single
bit? the whole collection? pieces of data out of context? Can someone who is authorized disclose
those data to other parties?
Confidentiality is the security property we understand best because its meaning is narrower than
the other two. We also understand confidentiality well because we can relate computing
examples to those of preserving confidentiality in the real world.
2. Integrity
Integrity is much harder to pin down. As Welke and Mayfield [WEL90, MAY91, NCS91b] point
out, integrity means different things in different contexts. When we survey the way some people
use the term, we find several different meanings. For example, if we say that we have preserved
the integrity of an item, we may mean that the item is









precise
accurate
unmodified
modified only in acceptable ways
modified only by authorized people
modified only by authorized processes
consistent
internally consistent
meaningful and usable
3. Availability
Availability applies both to data and to services (that is, to information and to information
processing), and it is similarly complex. As with the notion of confidentiality, different people
expect availability to mean different things. For example, an object or service is thought to be
available if




It is present in a usable form.
It has capacity enough to meet the service's needs.
It is making clear progress, and, if in wait mode, it has a bounded waiting time.
The service is completed in an acceptable period of time.
We can construct an overall description of availability by combining these goals. We say a data
item, service, or system is available if





There is a timely response to our request.
Resources are allocated fairly so that some requesters are not favored over others.
The service or system involved follows a philosophy of fault tolerance, whereby hardware or
software faults lead to graceful cessation of service or to work-arounds rather than to crashes
and abrupt loss of information.
The service or system can be used easily and in the way it was intended to be used.
Concurrency is controlled; that is, simultaneous access, deadlock management, and exclusive
access are supported as required.
Computer security is not restricted to these three broad concepts. Additional ideas that are
often considered part of the taxonomy of computer security include:

Access control -- Ensuring that users access only those resources and services that they
are entitled to access and that qualified users are not denied access to services that they
legitimately expect to receive



Nonrepudiation -- Ensuring that the originators of messages cannot deny that they in
fact sent the messages2
Availability -- Ensuring that a system is operational and functional at a given moment,
usually provided through redundancy; loss of availability is often referred to as "denialof-service"
Privacy -- Ensuring that individuals maintain the right to control what information is
collected about them, how it is used, who has used it, who maintains it, and what purpose
it is used for
Functional View
Computer security can also be analyzed by function. It can be broken into five distinct functional
areas:3





Risk avoidance -- A security fundamental that starts with questions like: Does my
organization or business engage in activities that are too risky? Do we really need an
unrestricted Internet connection? Do we really need to computerize that secure business
process? Should we really standardize on a desktop operating system with no access
control intrinsics?
Deterrence -- Reduces the threat to information assets through fear. Can consist of
communication strategies designed to impress potential attackers of the likelihood of
getting caught. See Rule 5: The Fear of Getting Caught is the Beginning of Wisdom.
Prevention -- The traditional core of computer security. Consists of implementing
safeguards like the tools covered in this book. Absolute prevention is theoretical, since
there's a vanishing point where additional preventative measures are no longer costeffective.
Detection -- Works best in conjunction with preventative measures. When prevention
fails, detection should kick in, preferably while there's still time to prevent damage.
Includes log-keeping and auditing activities
Recovery -- When all else fails, be prepared to pull out backup media and restore from
scratch, or cut to backup servers and net connections, or fall back on a disaster recovery
facility. Arguably, this function should be attended to before the others
Computer Criminals
It involve the use of information technology to gain an illegal or an unauthorized access to a
computer system with intent of damaging, deleting or altering computer data. Computer crimes
also include the activities such as electronic frauds, misuse of devices, identity theft and data as
well as system interference.
Marcus Rogers has identified eight types of cyber-criminals, distinguished by their skill levels and
motivations. Rogers is an associate professor at Purdue University in West Lafayette, Ind., where he
heads cyberforensics research in the university's department of computer technology.
NOVICE
• Limited computer and programming skills.
• Rely on toolkits to conduct their attacks.
• Can cause extensive damage to systems since they don't understand how the attack works.
• Looking for media attention.
CYBER PUNKS
• Capable of writing their own software.
• Have an understanding of the systems they are attacking.
• Many are engaged in credit card number theft and
telecommunications fraud.
• Have a tendency to brag about their exploits.
INTERNALS
a) Disgruntled employees or ex-employees
• May be involved in technology-related jobs.
• Aided by privileges they have or had been assigned as part of their
job function.
• Pose largest security problem.
b) Petty thieves
• Include employees, contractors, consultants .
• Computer literate.
• Opportunistic: take advantage of poor internal security.
• Motivated by greed or necessity to pay off other habits, such as
drugs or gambling.
CODERS
• Act as mentors to the newbies. Write the scripts and automated
tools that others use.
• Motivated by a sense of power and prestige.
• Dangerous — have hidden agendas, use Trojan horses.
OLD GUARD HACKERS
• Appear to have no criminal intent.
• Alarming disrespect for personal property.
• Appear to be interested in the intellectual endeavor.
PROFESSIONAL CRIMINALS
• Specialize in corporate espionage.
• Guns for hire.
• Highly motivated, highly trained, have access to state-of-the-art
equipment.
INFORMATION WARRIORS / CYBER-TERRORISTS
• Increase in activity since the fall of many Eastern Bloc intelligence
agencies.
• Well funded.
• Mix political rhetoric with criminal activity.Political activist
• Possible emerging category.
• Engage in hacktivism.
Computer crimes
involve activities of software theft, wherein the privacy of the
users is hampered. These criminal activities involve the breach of human and information
privacy, as also the theft and illegal alteration of system critical information. The different types
of computer crimes have necessitated the introduction and use of newer and more effective
security measures.
Types of computer crime
Hacking Currently defined as to gain illegal or unautorized access to a file, computer or
network.
Identity Theft Various crimes in which a criminal or large group uses the identity of an
unknowing, innocent person.


Phishing: E-mail fishing for personal and financial information disguised as legitimate business email.
Pharming: False websites that fish for personal and financial information by planting false URLs
in Domain Name Users.
Credit Card Fraud A wide-ranging term for theft and fraud committed using a credit card or any
similar payment mechanism as a fraudulent source of funds in a transaction.
Forgery The process of making, adapting, or imitating objects, statistics, or documents, with the
intent to deceive.

Digital Forgery: New technologies are used to create fake checks, passports, visas, birth
certificates with little skill or investments.
Scams A confidence game or other fraudulent scheme,especially for making a quick profit,to
cheat or swindle.



Auctions: Some sellers do not sell items or send inferior products.
Stock Fraud: Common method is to buy a stock low, send out email urging others to buy and
then, sell when the price goes up.
Click Fraud: Repeated clicking on an ad to either increase a site's revenue or to use up a
competitors advertising budget.
Hacking
Originally, the word, Hacking, was not used as defined above. Hacking was clever piece of code
constructed by hackers who were smart and creative programmers. In 1970s to 1990s, the
definition of hacking changed as many people started using computers and abused computer
terminologies. By 1980s, hacking behavior included spreading viruses, pranks, thefts, and phone
phreaking.
The difference between hackers and other criminals is the purpose of crime. Hackers commonly
try to benefit not only themselves but also other computer users. Therefore, they have some
ethics for their action. They believe sharing computer programs is important in development of
new softwares.Openness will help people to access anything they need and use it for their
personal demand. Decentralization will prevent authority from abusing information and
controlling people. Hands On Imperative/Free access to computersItalic text is indispensable to
break barriers between people and new technology. Those ethics will slowly change as the
demand for computer changes.
Identity Theft
Today, threats of identify theft come in many forms. It is important that you learn how to
recognize fraudulent activity to protect yourself from identity theft. Identity theft occurs when
someone uses your personally identifying information, like your name, Social Security number,
or credit card number, without your permission, to commit fraud or other crimes.
For identity thieves, this information is as good as gold. Skilled identity thieves may use a
variety of methods such as Old-Fashion Stealing, Skimming, Phishing, or Dumpster Diving to
get hold of your information. Once they have your personal information, identity thieves use it in
a variety of ways, for instance Credit Card Fraud, Bank/Account Fraud, or Government
Document Fraud.
To prevent any such identity theft is to monitor you personal accounts, bank statements and
check your credit report on a regular basis. If you check your credit report regularly, you may be
able to limit the damage caused by identity theft. Filing a police report, checking your credit
reports, notifying creditors, and disputing any unauthorized transactions are some of the steps
you must take immediately to restore your good name.
Scams
As of today there are so many ways to get lured to online scams. People who scam others are
often referred as scammers, cheaters and swindlers. Online scams are everywhere they can be
fake auctions and promotions. When people read great deals online they believe what they see
but in reality it is a game of deceit. The frauds can also happen with health care, credit card,
vacation and lottery. The internet changed the way we operate from research, leisure and work.
Every day people are cheated by these frauds. People get scammed by fake photos, damaged
goods, misleading information, and false advertising. People are tricked into providing their
private information like social security numbers, address, phone numbers and credit card
information.
Forgery
Digital forgery has become a big problem with the boom of the internet. Many businesses need
proof of identity to perform a service, and with identity fraud being a larger goal for criminals
this proof is difficult to accept as truthful. Criminals have access to much more advanced
technology and are willing to go to further lengths to steal people's information. A social security
number, credit card number, or bank account number are not strong enough proof to show who
someone is anymore. Many companies ask for copies of a social security card, birth certificate,
or a monthly bill with your name and address on it for further verification. Even going to these
lengths is not enough. Digital forgery is taken one step further with software to recreate and
manipulate these private documents and proceed with the scam intended.
Unfortunately these scams are being made even more accessible to even the least educated of
internet criminals. It is to the point where a thief can obtain your credit card information and
recreate your birth certificate for less than it costs to fill up his gas tank. This is frightening
because you will never even know if it is happening to you. Everyone must be aware that there
are always cyber-criminals on the loose, and no information is sacred on the internet.
Methods of Defense
Security Components
1.Confidentiality: The assets are accessible only by authorized parties.
– Keeping data and resources hidden
2.Integrity: The assets are modified only by authorized parties, and only in authorized ways.
– Data integrity (integrity)
– Origin integrity (authentication)
3.Availability: Assets are accessible to authorized parties.
– Enabling access to data and resources
Methods to safe computer crime(Methods of Defense)
1.Encryption
2. Software controls
3. Hardware controls
4.Policies
5.Physical controls
1. Encryption
at the heart of all security methods
Confidentiality of data
Some protocols rely on encryption to ensure availability of resources.
Encryption does not solve all computer security problems.
2. Software controls
Internal program controls
OS controls
Development controls
Software controls are usually the 1st aspects of computer security that come to mind.
3. Policies and Mechanisms
Policy says what is, and is not, allowed
– This defines “security” for the site/system/etc.
Mechanisms enforce policies
Mechanisms can be simple but effective
– Example: frequent changes of passwords
Composition of policies
– If policies conflict, discrepancies may create security vulnerabilities
Legal and ethical controls
– Gradually evolving and maturing
4. Overlapping Controls
Several different controls may apply to one potential exposure.
H/w control + S/w control + Data control
Goals of Security
1.Prevention
– Prevent attackers from violating security policy
2.Detection
– Detect attackers’ violation of security policy
3.Recovery
– Stop attack, assess and repair damage
– Continue to function correctly even if attack succeeds
Cryptography is the science of information security. The word is derived from the
Greek kryptos, meaning hidden. Cryptography is closely related to the disciplines of cryptology
and cryptanalysis. Cryptography includes techniques such as microdots, merging words with
images, and other ways to hide information in storage or transit. However, in today's computercentric world, cryptography is most often associated with scrambling plaintext (ordinary text,
sometimes referred to as cleartext) into ciphertext (a process called encryption), then back again
(known as decryption). Individuals who practice this field are known as cryptographers.
Modern cryptography concerns itself with the following four objectives:
1) Confidentiality (the information cannot be understood by anyone for whom it was
unintended)
2) Integrity (the information cannot be altered in storage or transit between sender and intended
receiver without the alteration being detected)
3) Non-repudiation (the creator/sender of the information cannot deny at a later stage his or her
intentions in the creation or transmission of the information)
4) Authentication (the sender and receiver can confirm each other?s identity and the
origin/destination of the information)
Types of Cryptography
There are two main types of cryptography:




Secret key cryptography
Public key cryptography
Hash function
Trust Models
1. Secret key cryptography(Symmetric Key)
Symmetric-key algorithms[1] are a class of algorithms for cryptography that use the same
cryptographic keys for both encryption of plaintext and decryption of ciphertext. The keys may
be identical or there may be 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.[2] This requirement that both parties have access to the secret key is
one of the main drawbacks of symmetric key encryption, in comparison to public-key
encryption.
Types of symmetric-key algorithms
Symmetric-key encryption can use either stream ciphers or block ciphers.[citation needed]

Stream ciphers encrypt the digits (typically bits) of a message one at a time.

Block ciphers take a number of bits and encrypt them as a single unit, padding the plaintext so
that it is a multiple of the block size. Blocks of 64 bits have been commonly used. The Advanced
Encryption Standard (AES) algorithm approved by NIST in December 2001 uses 128-bit blocks.
2. Asymmetric Key (public-key cryptography)
Asymmetric cryptography or public-key cryptography is cryptography in which a pair of keys is
used to encrypt and decrypt a message so that it arrives securely. Initially, a network user
receives a public and private key pair from a certificate authority. Any other user who wants to
send an encrypted message can get the intended recipient's public key from a public directory.
They use this key to encrypt the message, and they send it to the recipient. When the recipient
gets the message, they decrypt it with their private key, which no one else should have access to.
Witfield Diffie & Martin Hellman, researchers at Stanford University, first publicly proposed
asymmetric encryption in their 1977 paper, New Directions In Cryptography. (The concept had
been independently and covertly proposed by James Ellis several years before when he was
working for the British Government Communications Headquarters.) An asymmetric algorithm,
as outlined in the Diffie-Hellman paper, is a trap door or one-way function. Such a function is
easy to perform in one direction, but difficult or impossible to reverse. For example, it is easy to
compute the product of two given numbers, but it is computationally much harder to find the two
factors given only their product. Given both the product and one of the factors, it is easy to
compute the second factor, which demonstrates the fact that the hard direction of the
computation can be made easy when access to some secret key is given. The function used, the
algorithm, is known universally. This knowledge does not enable the decryption of the message.
The only added information that is necessary and sufficient for decryption is the recipient's secret
key.
Asymmetric key encryption uses different keys for encryption and decryption. These two
keys are mathematically related and they form a key pair. One of these two keys should be
kept private, called private-key, and the other can be made public (it can even be sent in
mail), called public-key. Hence this is also called Public Key Encryption.
A private key is typically used for encrypting the message-digest; in such an application
private-key algorithm is called message-digest encryption algorithm. A public key is
typically used for encrypting the secret-key; in such a application private-key algorithm is
called key encryption algorithm.
Popular private-key algorithms are RSA (invented by Rivest, Shamir and Adlemen) and
DSA (Digital Signature Algorithm). While for an ordinary use of RSA, a key size of 768 can
be used, but for corporate use a key size of 1024 and for extremely valuable information a
key size of 2048 should be used.
Asymmetric key encryption is much slower than symmetric key encryption and hence they
are only used for key exchanges and digital signatures.
3. Hash Function
A cryptographic hash function is a hash function; that is, an algorithm that takes an arbitrary
block of data and returns a fixed-size bit string, the (cryptographic) hash value, such that an
(accidental or intentional) change to the data will (with very high probability) change the hash
value. The data to be encoded are often called the "message," and the hash value is sometimes
called the message digest or simply digest.
The ideal cryptographic hash function has four main properties:




it is easy to compute the hash value for any given message
it is infeasible to generate a message that has a given hash
it is infeasible to modify a message without changing the hash
it is infeasible to find two different messages with the same hash
Cryptographic hash functions have many information security applications, notably in digital
signatures, message authentication codes (MACs), and other forms of authentication. They can
also be used as ordinary hash functions, to index data in hash tables, for fingerprinting, to detect
duplicate data or uniquely identify files, and as checksums to detect accidental data corruption.
Indeed, in information security contexts, cryptographic hash values are sometimes called
(digital) fingerprints, checksums, or just hash values, even though all these terms stand for
functions with rather different properties and purposes.
There are several well-known hash functions in use today:





Hashed Message Authentication Code (HMAC): Combines authentication via a shared
secret with hashing.
Message Digest 2 (MD2): Byte-oriented, produces a 128-bit hash value from an
arbitrary-length message, designed for smart cards.
MD4: Similar to MD2, designed specifically for fast processing in software.
MD5: Similar to MD4 but slower because the data is manipulated more. Developed after
potential weaknesses were reported in MD4.
Secure Hash Algorithm (SHA): Modeled after MD4 and proposed by NIST for the
Secure Hash Standard (SHS), produces a 160-bit hash value.
Substitution cipher
Substitution cipher
In cryptography, a substitution cipher is a method of encryption by which units of plaintext are
replaced with ciphertext, according to a regular system; the "units" may be single letters (the
most common), pairs of letters, triplets of letters, mixtures of the above, and so forth. The
receiver deciphers the text by performing an inverse substitution.
Substitution ciphers can be compared with transposition ciphers. In a transposition cipher, the
units of the plaintext are rearranged in a different and usually quite complex order, but the units
themselves are left unchanged. By contrast, in a substitution cipher, the units of the plaintext are
retained in the same sequence in the ciphertext, but the units themselves are altered.
There are a number of different types of substitution cipher. If the cipher operates on single
letters, it is termed a simple substitution cipher; a cipher that operates on larger groups of
letters is termed polygraphic. A monoalphabetic cipher uses fixed substitution over the entire
message, whereas a polyalphabetic cipher uses a number of substitutions at different positions
in the message, where a unit from the plaintext is mapped to one of several possibilities in the
ciphertext and vice versa.
For example:
Plain Alphabet: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Cipher Alphabet: Z Y X W V U T S R Q P O N M L K J I H G F E D C B A
With the above, the Plain text "This is a sample" would encrypt to "Gsrh rh z hznkov."
With Substitution Ciphers, the secret is in the mapping between the plain and cipher alphabets.
However, there are several analytical techniques to help break these ciphers with only the
ciphertext. See Frequency analysis
Types of substitution
1. Homophonic substitution
2. Polyalphabetic substitution
3. Polygraphic substitution
4. Mechanical substitution ciphers
Transposition cipher
It is a simple data encryption scheme in which plaintext characters are shifted in some regular
pattern to form ciphertext.
In manual systems transpositions are generally carried out with the aid of an easily remembered
mnemonic. For example, a popular schoolboy cipher is the “rail fence,” in which letters of the
plaintext are written alternating between rows and the rows are then read sequentially to give the
cipher. In a depth-two rail fence (two rows) the message WE ARE DISCOVERED SAVE
YOURSELF would be written
Simple frequency counts on the ciphertext would reveal to the cryptanalyst that letters occur with
precisely the same frequency in the cipher as in an average plaintext and, hence, that a simple
rearrangement of the letters is probable.
The rail fence is the simplest example of a class of transposition ciphers, known as route ciphers,
that enjoyed considerable popularity in the early history of cryptology. In general, the elements
of the plaintext (usually single letters) are written in a prearranged order (route) into a geometric
array (matrix)—typically a rectangle—agreed upon in advance by the transmitter and receiver
and then read off by following another prescribed route through the matrix to produce the cipher.
The key in a route cipher consists of keeping secret the geometric array, the starting point, and
the routes. Clearly both the matrix and the routes can be much more complex than in this
example; but even so, they provide little security. One form of transposition (permutation) that
was widely used depends on an easily remembered key word for identifying the route in which
the columns of a rectangular matrix are to be read. For example, using the key word AUTHOR
and ordering the columns by the lexicographic order of the letters in the key word
In decrypting a route cipher, the receiver enters the ciphertext symbols into the agreed-upon
matrix according to the encryption route and then reads the plaintext according to the original
order of entry. A significant improvement in cryptosecurity can be achieved by reencrypting the
cipher obtained from one transposition with another transposition. Because the result (product)
of two transpositions is also a transposition, the effect of multiple transpositions is to define a
complex route in the matrix, which in itself would be difficult to describe by any simple
mnemonic.
In the same class also fall systems that make use of perforated cardboard matrices called grilles;
descriptions of such systems can be found in most older books on cryptography. In contemporary
cryptography, transpositions serve principally as one of several encryption steps in forming a
compound or product cipher.
Making "Good" Encryption Algorithms
So far, the encryption algorithms we have seen have been trivial, intended primarily to
demonstrate the concepts of substitution and permutation. At the same time, we have examined
several approaches cryptanalysts use to attack encryption algorithms. Now we examine
algorithms that are widely used in the commercial world. Unlike the previous sections, this
section does not delve deeply into the details either of the inner workings of an algorithm or its
cryptanalysis. (We save that investigation for Chapter 10.)
What Makes a "Secure" Encryption Algorithm?
There are many kinds of encryption, including many techniques beyond those we discuss in this
book. Suppose you have text to encrypt. How do you choose an encryption algorithm for a
particular application? To answer this question, reconsider what we have learned so far about
encryption. We looked at two broad classes of algorithms: substitutions and transpositions.
Substitutions "hide" the letters of the plaintext, and multiple substitutions dissipate high letter
frequencies to make it harder to determine how the substitution is done. By contrast,
transpositions scramble text so that adjacent-character analysis fails.
Sidebar 2-4 Soviet Encryption During World War II
Kahn [KAH96] describes a system that the Soviet Union thought unbreakable during
World War II. It combined substitution with a one-time pad. The basic idea was to
diffuse high-frequency letters by mapping them to single digits. This approach kept the
length of cryptograms small and thus reduced the on-air time as the message was
transmitted.
To see how the encryption worked, consider the eight most common letters of the
English language: ASINTOER, arranged as in "a sin to er(r)" to make them easy to
remember. These letters were assigned to single digits, 0 to 7. To encode a message, an
analyst would begin by selecting a keyword that became the first row of a matrix.
Then, the remaining letters of the alphabet were listed in rows underneath, as shown
below. Moving vertically through the matrix, the digits 0 to 7 were assigned to the
eight common letters, and then the two-digit groups from 80 to 99 were mapped to the
remaining letters of the alphabet plus any symbols. In our example, the keyword is
SUNDAY:
S
U
N
0
83 2
90 6
97
B
C
F
H
E
D
A
Y
G
80 84 3
91 94 98
I
J
L
1
85 88 92 95 7
P
Q
K
M
O
R
T
V
81 86 4
5
96 99
X
/
Z
.
W
82 87 89 93
Then the message "whereis/456/ airborne " would be encoded as
w
h
e
99 98 3
r b
r e
4
o r n e
4 80 7
4 2 3
i s
3
1
/ 4 5
0
6 /
a
i
93 44 55 66 93 6
1
or 99983431 09344556 69361480 7423. (Digits of plaintext numbers were repeated.)
Finally, the numerical message was encrypted with a one-time pad from a common
reference book with numerical tables ”one that would not arouse suspicion, such as a
navigator's book of tables.
For each type of encryption we considered , we described the advantages and disadvantages. But
there is a broader question: What does it mean for a cipher to be "good"? The meaning of good
depends on the intended use of the cipher. A cipher to be used by military personnel in the field
has different requirements from one to be used in a secure installation with substantial computer
support. In this section, we look more closely at the different characteristics of ciphers.
Shannon's Characteristics of "Good" Ciphers
In 1949, Claude Shannon [SHA49] proposed several characteristics that identify a good cipher.
1. The amount of secrecy needed should determine the amount of labor appropriate for
the encryption and decryption.
Principle 1 is a reiteration of the principle of timeliness from Chapter 1 and of the earlier
observation that even a simple cipher may be strong enough to deter the casual interceptor or to
hold off any interceptor for a short time.
2. The set of keys and the enciphering algorithm should be free from complexity.
This principle implies that we should restrict neither the choice of keys nor the types of plaintext
on which the algorithm can work. For instance, an algorithm that works only on plaintext having
an equal number of As and Es is useless. Similarly, it would be difficult to select keys such that
the sum of the values of the letters of the key is a prime number. Restrictions such as these make
the use of the encipherment prohibitively complex. If the process is too complex, it will not be
used. Furthermore, the key must be transmitted, stored, and remembered , so it must be short.
3. The implementation of the process should be as simple as possible.
Principle 3 was formulated with hand implementation in mind: A complicated algorithm is prone
to error or likely to be forgotten. With the development and popularity of digital computers,
algorithms far too complex for hand implementation became feasible . Still, the issue of
complexity is important. People will avoid an encryption algorithm whose implementation
process severely hinders message transmission, thereby undermining security. And a complex
algorithm is more likely to be programmed incorrectly.
4. Errors in ciphering should not propagate and cause corruption of further
information in the message.
Principle 4 acknowledges that humans make errors in their use of enciphering algorithms. One
error early in the process should not throw off the entire remaining ciphertext . For example,
dropping one letter in a columnar transposition throws off the entire remaining encipherment.
Unless the receiver can guess where the letter was dropped, the remainder of the message will be
unintelligible. By contrast, reading the wrong row or column for a polyalphabetic substitution
affects only one character ”remaining characters are unaffected.
5. The size of the enciphered text should be no larger than the text of the original
message.
The idea behind principle 5 is that a ciphertext that expands dramatically in size cannot possibly
carry more information than the plaintext, yet it gives the cryptanalyst more data from which to
infer a pattern. Furthermore, a longer ciphertext implies more space for storage and more time to
communicate.
These principles were developed before the ready availability of digital computers, even though
Shannon was aware of computers and the computational power they represented. Thus, some of
the concerns he expressed about hand implementation are not really limitations on computerbased implementation. For example, a cipher's implementation on a computer need not be
simple, as long as the time complexity of the implementation is tolerable.
Properties of "Trustworthy" Encryption Systems
Commercial users have several requirements that must be satisfied when they select an
encryption algorithm. Thus, when we say that encryption is "commercial grade," we mean that it
meets these constraints:



It is based on sound mathematics . Good cryptographic algorithms are not just invented;
they are derived from solid principles.
It has been analyzed by competent experts and found to be sound . Even the best
cryptographic experts can think of only so many possible attacks, and the developers may
become too convinced of the strength of their own algorithm. Thus, a review by critical
outside experts is essential.
It has stood the "test of time." As a new algorithm gains popularity, people continue to
review both its mathematical foundations and the way it builds upon those foundations.
Although a long period of successful use and analysis is not a guarantee of a good
algorithm, the flaws in many algorithms are discovered relatively soon after their release.
Three algorithms are popular in the commercial world: DES (data encryption standard), RSA
(Rivest “Shamir “Adelman, named after the inventors), and AES (advanced encryption
standard). The DES and RSA algorithms (as well as others) meet our criteria for commercialgrade encryption; AES, which is quite new, meets the first two and is starting to achieve
widespread adoption.
Symmetric and Asymmetric Encryption Systems
Recall that the two basic kinds of encryptions are symmetric (also called "secret key") and
asymmetric (also called "public key"). Symmetric algorithms use one key, which works for both
encryption and decryption. Usually, the decryption algorithm is closely related to the encryption
one. (For example, the Caesar cipher with a shift of 3 uses the encryption algorithm "substitute
the character three letters later in the alphabet" with the decryption "substitute the character three
letters earlier in the alphabet.")
The symmetric systems provide a two-way channel to their users: A and B share a secret key,
and they can both encrypt information to send to the other as well as decrypt information from
the other. As long as the key remains secret, the system also provides authentication, proof that a
message received was not fabricated by someone other than the declared sender. Authenticity is
ensured because only the legitimate sender can produce a message that will decrypt properly
with the shared key.
The symmetry of this situation is a major advantage of this type of encryption, but it also leads to
a problem: key distribution. How do A and B obtain their shared secret key? And only A and B
can use that key for their encrypted communications. If A wants to share encrypted
communication with another user C, A and C need a different shared key. Key distribution is the
major difficulty in using symmetric encryption. In general, n users who want to communicate in
pairs need n * ( n “ 1)/2 keys. In other words, the number of keys needed increases at a rate
proportional to the square of the number of users! So a property of symmetric encryption
systems is that they require a means of key distribution .
Public key systems, on the other hand, excel at key management. By the nature of the public key
approach, you can send a public key in an e-mail message or post it in a public directory. Only
the corresponding private key, which presumably is kept private, can decrypt what has been
encrypted with the public key.
But for both kinds of encryption, a key must be kept well secured. Once the symmetric or private
key is known by an outsider, all messages written previously or in the future can be decrypted
(and hence read or modified) by the outsider. So, for all encryption algorithms, key management
is a major issue. It involves storing, safeguarding, and activating keys.
Stream and Block Ciphers
Most of the ciphers studied in this chapter are stream ciphers ; that is, they convert one symbol of
plaintext immediately into a symbol of ciphertext. (The exception is the columnar transposition
cipher.) The transformation depends only on the symbol, the key, and the control information of
the encipherment algorithm. A model of stream enciphering is shown in Figure 2-6.
Figure 2-6. Stream Encryption.
Some kinds of errors, such as skipping a character in the key during encryption, affect the
encryption of all future characters. However, such errors can sometimes be recognized during
decryption because the plaintext will be properly recovered up to a point, and then all following
characters will be wrong. If that is the case, the receiver may be able to recover from the error by
dropping a character of the key on the receiving end. Once the receiver has successfully
recalibrated the key with the ciphertext, there will be no further effects from this error.
To address this problem and make it harder for a cryptanalyst to break the code, we can use
block ciphers. A block cipher encrypts a group of plaintext symbols as one block. The columnar
transposition and other transpositions are examples of block ciphers. In the columnar
transposition, the entire message is translated as one block. The block size need not have any
particular relationship to the size of a character. Block ciphers work on blocks of plaintext and
produce blocks of ciphertext, as shown Figure 2-7. In the figure, the central box represents an
encryption machine: The previous plaintext pair is converted to po , the current one being
converted is IH , and the machine is soon to convert ES .
Figure 2-7. Block Cipher Systems.
Table 2-3 compares the advantages and disadvantages of stream and block encryption
algorithms.
Table 2-3. Comparing Stream and Block Algorithms.
Stream Encryption Algorithms
Advantages


Disadvantages

Block Encryption Algorithms
Speed of transformation . Because
each symbol is encrypted without
regard for any other plaintext
symbols, each symbol can be
encrypted as soon as it is read.
Thus, the time to encrypt a symbol
depends only on the encryption
algorithm itself, not on the time it
takes to receive more plaintext.
Low error propagation . Because
each symbol is separately encoded,
an error in the encryption process
affects only that character.

Low diffusion . Each symbol is
separately enciphered. Therefore,


High diffusion . Information
from the plain-text is
diffused into several
ciphertext symbols. One
ciphertext block may depend
on several plaintext letters.
Immunity to insertion of
symbols . Because blocks of
symbols are enciphered, it is
impossible to insert a single
symbol into one block. The
length of the block would
then be incorrect, and the
decipherment would quickly
reveal the insertion.
Slowness of encr y ption .
The person or machine using
Stream Encryption Algorithms

all the information of that symbol is
contained in one symbol of the
ciphertext.
Susceptibility to malicious
insertions and modifications .
Because each symbol is separately
enciphered, an active interceptor
who has broken the code can splice
together pieces of previous
messages and transmit a spurious
new message that may look
authentic .
Block Encryption Algorithms

a block cipher must wait
until an entire block of
plaintext symbols has been
received before starting the
encryption process.
Error propagation . An error
will affect the
transformation of all other
characters in the same block.
Data Encryption Standard (DES)
It is a is a widely-used method of data encryption using a private (secret) key that was judged so
difficult to break by the U.S. government that it was restricted for exportation to other countries.
There are 72,000,000,000,000,000 (72 quadrillion) or more possible encryption keys that can be
used. For each given message, the key is chosen at random from among this enormous number of
keys. Like other private key cryptographic methods, both the sender and the receiver must know
and use the same private key.
DES applies a 56-bit key to each 64-bit block of data. The process can run in several modes and
involves 16 rounds or operations. Although this is considered "strong" encryption, many
companies use "triple DES", which applies three keys in succession. This is not to say that a
DES-encrypted message cannot be "broken." Early in 1997, Rivest-Shamir-Adleman, owners of
another encryption approach, offered a $10,000 reward for breaking a DES message. A
cooperative effort on the Internet of over 14,000 computer users trying out various keys finally
deciphered the message, discovering the key after running through only 18 quadrillion of the 72
quadrillion possible keys! Few messages sent today with DES encryption are likely to be subject
to this kind of code-breaking effort.
DES originated at IBM in 1977 and was adopted by the U.S. Department of Defense. It is
specified in the ANSI X3.92 and X3.106 standards and in the Federal FIPS 46 and 81 standards.
Concerned that the encryption algorithm could be used by unfriendly governments, the U.S.
government has prevented export of the encryption software. However, free versions of the
software are widely available on bulletin board services and Web sites. Since there is some
concern that the encryption algorithm will remain relatively unbreakable, NIST has indicated
DES will not be recertified as a standard and submissions for its replacement are being accepted.
The next standard will be known as the Advanced Encryption Standard (AES).
As a practical matter, anyone today who wants high security uses a more powerful
version of DES called Triple-DES.
To start encrypting with Triple-DES, two 56-bit keys are selected. Data is encrypted via
DES three times, the first time by the first key, the second time by the second key and
the third time by the first key once more. This process creates an encrypted datastream
that is unbreakable with today's code-breaking techniques and available computing
power, while being compatible with DES.
The Feistel function (F function) of DES
General
Designers
IBM
First published
1977 (standardized in January 1979)
Derived from
Lucifer
Successors
Triple DES, G-DES, DES-X, LOKI89, ICE
Cipher detail
Key sizes
56 bits
Block sizes
64 bits
Structure
Balanced Feistel network
Rounds
16
Best public cryptanalysis
DES is now considered insecure because a brute force attack is possible (see EFF DES cracker).
As of 2008, the best analytical attack is linear cryptanalysis, which requires 243 known
plaintexts and has a time complexity of 239–43 (Junod, 2001).
Modes of DES
FIPS 81 describes four approved modes of DES: Electronic Codebook (ECB) mode,
Cipher Block Chaining (CBC) mode, Cipher Feedback (CFB) mode, and Output
Feedback (OFB) mode. 5 The National Institute of Standards and Technology (NIST)
Special Publication 800-38A describes a 5th method, Counter (CTR).6 These modes can
be used with both DES and Triple DES.
Key differences in each mode are error propagation and block vs. stream ciphers.
1.Error propagation means an error in a step of encryption or decryption (such as a
bit flipped from 0 to 1) propagates to subsequent steps, which causes further
errors.
2.A block cipher encrypts a set block size of data (64 bits for ECB and CBC
modes).
3.A stream cipher encrypts bits or groups of bits (1–64 bits in CFB, OFB, and CTR
modes). Although DES is a block cipher, it emulates stream ciphers in these
modes.
1.Electronic Codebook (ECB) Mode
Electronic Codebook is the DES native mode, “a direct application of the DES algorithm
to encrypt and decrypt data.”7 In this mode, each block of plaintext is independently
encrypted into a respective block of ciphertext. This is done via a Feistel (named after
Horst Feistel, one of the creators of Lucifer) cipher, which creates 16 subkeys based on
the symmetric key and encrypts the plaintext via 16 rounds of transformation. The same
process is used (with the symmetric key) to convert ciphertext back into plaintext; the
difference is the 16 subkeys are supplied in reverse order.
Repeated blocks of identical plaintext result in repeated blocks of ciphertext, which can
aid cryptanalysis of the ciphertext.
This effect is best illustrated. The first image is the SANS logo (bitmap format); the
second image is the SANS logo bitmap encrypted via DES ECB mode. Although the
bitmap data is encrypted, the original pattern is clearly visible.
The pattern is visible because repeated blocks of plaintext pixels in the bitmap are
encrypted into repeated blocks of respective ciphertext pixels.
In this mode, errors do not propagate, as each block is encrypted independently.
The term Codebook refers to cryptographic code books, which contain dictionaries of
words or phrases (such as “Attack has begun”) with a coded equivalent (“The eagle has
flown”).
2. Cipher Block Chaining (CBC) Mode
Cipher Block Chaining Mode is a block cipher which XORs (exclusive OR) each new
block of plaintext with the previous block of ciphertext (they are “chained” together).
This means repeated blocks of plaintext do not result in repeated blocks of ciphertext.
CBC also uses an initialization vector, which is a random initial block used to ensure that
two identical plaintexts result in different ciphertexts (due to different initialization
vectors).
Here is the same SANS logo bitmap data, encrypted with DES CBC mode:
No pattern is visible. This is true for all DES modes other than ECB.
In this mode, errors propagate, as each previous step’s encrypted output is XORed
(“chained”) with the new block of plaintext.
3.Cipher Feedback (CFB) Mode
Cipher Feedback mode is a stream cipher that encrypts plaintext by breaking it into units
of X (from 1 to 64) bits. This allows bit or byte-level encryption.
CFB mode uses a random initialization vector, and previous units of ciphertext are
XORed with subsequent units of plaintext (the cipher is “fed back” to the plaintext). As
with CBC, errors propagate.
4.Output Feedback (OFB) Mode
Like CFB mode, Output Feedback mode uses a random initialization vector and encrypts
plaintext by breaking it down into a stream by encrypting units of X (from 1 to 64) bits of
plaintext.
OFB mode differs from CFB mode by creating a pseudo-random stream of bits (called
“output”), which is XORed with the plaintext during each step (the “output” is “fed back”
to the plaintext).
Because the output (and not ciphertext) is XORed to the plaintext, errors do not
propagate.
5.Counter (CTR) Mode
Counter mode is a stream cipher like OFB mode; the key difference is the addition of
counter blocks. The counter can be added or concatenated to a nonce (a random value
that is used once), and then incremented for each unit of plaintext that is encrypted. The
first counter block acts as an initialization vector. In each round, the counter blocks are
XORed with plaintext.
Advanced Encryption Standard
AES stands for Advanced Encryption Standard. AES is a symmetric key encryption technique
which will replace the commonly used Data Encryption Standard (DES).
It was the result of a worldwide call for submissions of encryption algorithms issued by the US
Government's National Institute of Standards and Technology (NIST) in 1997 and completed in
2000.
The winning algorithm, Rijndael, was developed by two Belgian cryptologists, Vincent Rijmen
and Joan Daemen.
AES provides strong encryption and has been selected by NIST as a Federal Information
Processing Standard in November 2001 (FIPS-197), and in June 2003 the U.S. Government
(NSA) announced that AES is secure enough to protect classified information up to the TOP
SECRET level, which is the highest security level and defined as information which would
cause "exceptionally grave damage" to national security if disclosed to the public.
The AES algorithm uses one of three cipher key strengths: a 128-, 192-, or 256-bit encryption
key (password). Each encryption key size causes the algorithm to behave slightly differently, so
the increasing key sizes not only offer a larger number of bits with which you can scramble the
data, but also increase the complexity of the cipher algorithm.
BitZipper supports 128- and 256-bit encryption keys, which is the two key strengths supported
by WinZip 9. Both key strengths provide significantly better security than standard ZIP 2.0
encryption. It is slightly faster to encrypt and decrypt data protected with 128-bit AES, but with
today's fast PCs the time difference is barely notable.
Steps in AES
1. SubBytes
2. ShiftRows
3. AddRoundKey
1.The SubBytes step
In the SubBytes step, each byte in the state is replaced with its entry in a fixed 8-bit lookup table, S; bij
= S(aij).
In the SubBytes step, each byte in the state matrix is replaced with a SubByte using an 8-bit
substitution box, the Rijndael S-box. This operation provides the non-linearity in the cipher. The
S-box used is derived from the multiplicative inverse over GF(28), known to have good nonlinearity properties. To avoid attacks based on simple algebraic properties, the S-box is
constructed by combining the inverse function with an invertible affine transformation. The Sbox is also chosen to avoid any fixed points (and so is a derangement), and also any opposite
fixed points.
2.The ShiftRows step
In the ShiftRows step, bytes in each row of the state are shifted cyclically to the left. The number of
places each byte is shifted differs for each row.
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. For blocks of sizes 128 bits and 192 bits, the shifting pattern is the same. Row n is
shifted left circular by n-1 bytes. In this way, each column of the output state of the ShiftRows
step is composed of bytes from each column of the input state. (Rijndael variants with a larger
block size have slightly different offsets). For a 256-bit block, the first row is unchanged and the
shifting for the second, third and fourth row is 1 byte, 3 bytes and 4 bytes respectively—this
change only applies for the Rijndael cipher when used with a 256-bit block, as AES does not use
256-bit blocks.
3.The MixColumns step
In the MixColumns step, each column of the state is multiplied with a fixed polynomial c(x).
In the MixColumns step, the four bytes of each column of the state are combined using an
invertible linear transformation. The MixColumns function takes four bytes as input and outputs
four bytes, where each input byte affects all four output bytes. Together with ShiftRows,
MixColumns provides diffusion in the cipher.
During this operation, each column is multiplied by the known matrix that for the 128-bit key is
The multiplication operation is defined as: multiplication by 1 means no change, multiplication
by 2 means shifting to the left, and multiplication by 3 means shifting to the left and then
performing xor with the initial unshifted value. After shifting, a conditional xor with 0x11B
should be performed if the shifted value is larger than 0xFF.
In more general sense, each column is treated as a polynomial over GF(28) 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 from
GF(2)[x]. The MixColumns step can also be viewed as a multiplication by a particular MDS
matrix in a finite field. This process is described further in the article Rijndael mix columns.
Difference between DES and AES
DES is really old while AES is relatively new
DES is breakable while AES is still unbreakable
DES uses a much smaller key size compared to AES
DES uses a smaller block size compared to AES
DES uses a balanced Feistel structure while AES uses substitution-permutation


DES was originally designed to run in specialized hardware and is
considered “computationally expensive” on general-purpose processors.
AES was designed to run efficiently on a variety of processors, including
general-purpose ones.
AES is more secure when compared to DES as it uses large blocks for
encryption and the algorithm is relatively complicated than AES.
Possible number of AES 128-bit keys are 1021 times greater than DES 56bit keys, hence it is found that a machine that could recover a DES key
in a second (I.e. try 255keys per second) , it takes 149 trillion years to
crack a 128-bit AES key.
RSA
RSA is an algorithm for public-key cryptography that is based on the presumed difficulty of factoring
large integers, the factoring problem. RSA stands for Ron Rivest, Adi Shamir and Leonard Adleman, who
first publicly described it in 1977. Clifford Cocks, an English mathematician, had developed an equivalent
system in 1973, but it was classified until 1997. A user of RSA creates and then publishes the product of
two large prime numbers, along with an auxiliary value, as their public key. The prime factors must be
kept secret. Anyone can use the public key to encrypt a message, but with currently published methods,
if the public key is large enough, only someone with knowledge of the prime factors can feasibly decode
the message.[1] Whether breaking RSA encryption is as hard as factoring is an open question known as
the RSA problem.
The RSA algorithm was publicly described in 1977 by Ron Rivest, Adi Shamir, and Leonard
Adleman at MIT; the letters RSA are the initials of their surnames, listed in the same order as on
the paper.
RSA Operation
The RSA algorithm involves three steps:
key generation,
encryption and
decryption.
1.Key generation
RSA involves a public key and a private key. The public key can be known to everyone and is
used for encrypting messages. Messages encrypted with the public key can only be decrypted
using the private key. The keys for the RSA algorithm are generated the following way:
1. Choose two distinct prime numbers p and q.
o For security purposes, the integers p and q should be chosen at random, and should be
of similar bit-length. Prime integers can be efficiently found using a primality test.
2. Compute n = pq.
o n is used as the modulus for both the public and private keys
3. Compute φ(n) = (p – 1)(q – 1), where φ is Euler's totient function.
4. Choose an integer e such that 1 < e < φ(n) and greatest common divisor of (e, φ(n)) = 1; i.e., e
and φ(n) are coprime.
o e is released as the public key exponent.
o e having a short bit-length and small Hamming weight results in more efficient
encryption - most commonly 0x10001 = 65,537. However, small values of e (such as 3)
have been shown to be less secure in some settings.[4]
5. Determine d as:
i.e., d is the multiplicative inverse of e mod φ(n).



This is more clearly stated as solve for d given (de) = 1 mod φ(n)
This is often computed using the extended Euclidean algorithm.
d is kept as the private key exponent.
By construction, d*e= 1 mod φ(n). The public key consists of the modulus n and the public (or
encryption) exponent e. The private key consists of the modulus n and the private (or
decryption) exponent d which must be kept secret. (p, q, and φ(n) must also be kept secret
because they can be used to calculate d.)


An alternative, used by PKCS#1, is to choose d matching de ≡ 1 mod λ with λ = lcm(p − 1, q − 1),
where lcm is the least common multiple. Using λ instead of φ(n) allows more choices for d. λ can
also be defined using the Carmichael function, λ(n).
The ANSI X9.31 standard prescribes, IEEE 1363 describes, and PKCS#1 allows, that p and q match
additional requirements: be strong primes, and be different enough that Fermat factorization
fails.
2.Encryption
Alice transmits her public key
send message M to Alice.
to Bob and keeps the private key secret. Bob then wishes to
He first turns M into an integer m, such that
by using an agreed-upon reversible
protocol known as a padding scheme. He then computes the ciphertext corresponding to
.
This can be done quickly using the method of exponentiation by squaring. Bob then transmits
to Alice.
Note that at least nine values of m could yield a ciphertext c equal to m,[5] but this is very
unlikely to occur in practice.
3.Decryption
Alice can recover
from
by using her private key exponent
via computing
.
Given
, she can recover the original message M by reversing the padding scheme.
(In practice, there are more efficient methods of calculating
below.)
using the pre computed values
DSA
he Digital Signature Algorithm (DSA) is a United States Federal Government standard or FIPS
for digital signatures. It was proposed by the National Institute of Standards and Technology
(NIST) in August 1991 for use in their Digital Signature Standard (DSS), specified in FIPS
186,[1] adopted in 1993. A minor revision was issued in 1996 as FIPS 186-1.[2] The standard was
expanded further in 2000 as FIPS 186-2 and again in 2009 as FIPS 186-3.[3]
DSA is covered by U.S. Patent 5,231,668, filed July 26, 1991, and attributed to David W.
Kravitz,[4] a former NSA employee. This patent was given to "The United States of America as
represented by the Secretary of Commerce, Washington, D.C." and the NIST has made this
patent available worldwide royalty-free.[5] Dr. Claus P. Schnorr claims that his U.S. Patent
4,995,082 (expired) covered DSA; this claim is disputed.[6] DSA is a variant of the ElGamal
Signature Scheme.
DSA is a modification to a signature scheme devised by Taher ElGamal in 1984. In a single paper,
ElGamal presented both a public-key encryption scheme, and a distinct signature scheme.
RSA is used for signing, by 'encrypting' the message (normally, the hash of the message) using
the secret key, and the signature is verified by 'decrypting' using the public key. This reverses
the roles of the keys, when RSA is used for confidentiality. RSA uses the same operations for
encryption and decryption, and the public and secret keys have a reciprocal relationship to one
another.
However, the ElGamal cryptosystem uses quite different operations for encryption and
decryption, so the trick used to convert RSA into a signature scheme doesn't work. This explains
why ElGamal presented a separate (but mathematically related) scheme for signing.
The ElGamal signature scheme uses a randomly chosen number in the signing procedure, so that
a given message can have a vast number of different signatures. It is not designed to allow
computation of the signed message (or signed hash) -- only verification.
So ElGamal gave the world a public-key cryptosystem that can't be used for signing, and a
public-key signature scheme that can't be used for encryption. To my understanding, he didn't
do this to prove that these things can be done; rather, these properties fell out from the
arithmetic he used.
The ElGamal signature scheme in its original form was never adopted as a widely-used standard,
mainly because of size: the signatures are large, and the computations time-consuming. The
security of the scheme is believed to be based on the difficulty of the discrete logarithm
problem (DLP). For DLP to be secure in multiplication groups (the most commonly used setting),
the groups must be very large: orginally, 1024 bits was recommended; now, probably 2048 or
4096 bits. (As a side note, in practice, signatures must be "harder" than cryptosystems, because
secrets usually lose value in 1-30 years, whereas the secure verification of a signature for legal
purposes may be necessary after 50+ years).
The ElGamal signing scheme thus leads to signatures of at least 1024 bits, which are rather
cumbersome. Today, most signatures are between 128 and 256 bits in length, with 160 being
the most common.
The wizards at NSA made a very clever improvement to ElGamal, to create DSA. DSA is set in
large group (1024 bits max, in the original standard; extensible to much larger sizes in revisions
to the standard), but the computations for signature and verification are performed in a much
smaller subgroup, chosen to match the size of the hash function. So, for example, you could
choose a 2048 bit group, but use a 160-bit subgroup compatible with SHA-1. In this subgroup,
the signature is the same size as the hash, and the computations are quicker -- but the security
is believed to be based on the size of the main group.
APPENDED CORRECTION: All of the ElGamal algorithms: ElGamal encryption, ElGamal signature,
and its derivative the DSA, result in two numbers, each the size of the modulus (or in the case of
DSA, the subgroup). So with a 1024-bit modulus, and ElGamal signature occupies 2048 bits,
making it even more cumbersome than I described above. For DSA with a 160-bit subgroup
(chosen for SHA-1 compatibility), the signature is 320 bits in length. This compares favorably
with RSA signatures, which must be modulus-length, so today RSA signatures must be at least
1024 bits long, and for long-term security much larger than that.
What is Data Encryption?
Encryption uses a mathematical algorithm to scramble readable text that cannot be read unless
the reader has the key to "unlock," or convert, the information back to its readable form. This
means that your sensitive data cannot be accessed without you providing a password.
Types of Data Encryption
There are many different types of data encryption, but not all are reliable. In the beginning, 64bit encryption was thought to be strong, but was proven wrong with the introduction of 128-bit
solutions. AES (Advanced Encryption Standard) is the new standard and permits a maximum of
256-bits. In general, the stronger the computer, the better chance it has at breaking a data
encryption scheme.
Data encryption schemes generally fall in two categories:
1.symmetric and
AES, DES
2. Asymmetric.
DSA , RSA