Review Paper on Cryptography
Vaishnavi Kannan, Smita jhajharia , Dr.Seema Verma
Abstract--In the present era digital communication is by far
the most important means of communication. The information
being transmitted is prone to various attacks (active and
passive). Hence, information security is a very important part
of the communication process. Cryptography is the branch of
information security that provides an excellent method to
protect data. Over the years many encryption techniques have
been provided and used. This paper discusses different
encryption algorithms (symmetric and asymmetric) and newer
areas that are being explored in cryptography.
Index Terms—Encryption, Decryption, Ciphers, Secret Key
I INTRODUCTION
Cryptography- Cryptography is the study of methods for
secret writing and message hiding. The field of
cryptography includes algorithms, methods and protocols
for the encryption of a message and its safe traversal over a
network (i.e without delay and safe from the hands of an
intruder) [1]. Applications of cryptography include its use
for confidentiality( to make the message inaccessible to the
unauthorized receivers or attackers), integrity( to ensure
that the message received is in its original form and not
metamorphosed), authenticity(the receiver should be able to
verify the sender of the message , its origin and/ or the path
travelled by it), non repudiation(confirm the receipt of a
message and identity of the sender sending it) and to
control/limit the availability , validity or recipient in time or
space.
Encryption-It is the act of obscuring a message by
mystifying its contents. Encryption can also be used to
create digital signatures, which helps in the authentication
of a message.
Decryption - It is the inverse process of encryption. Usually
the cipher or algorithm used for encryption is the one used
for decryption.
Cipher - A cipher is an algorithm, function, method or
process for performing encryption or decryption. A cipher
generally is a set of well defined steps which can be
followed to encrypt and decrypt a data.
Plaintext - Any information before being encrypted or in its
original undisturbed form.
Ciphertext - It is the output or the incomprehensible code
obtained by encryption of the plain text.
Cryptanalysis- It is the act of trying to decipher an
encrypted message without the knowledge of the actual keys
or the algorithm used during encryption. A cryptanalytic attack
can have two possible goals-The cryptanalyst may have
some part of the plaintext and is interested in the rest of it
or he might have the cipher text and may want to know
about the key and algorithms used.
Cryptographic Attacks - They are classified as
1)Passive Attack- The goal here might be to only
read the contents of a message and not change anything.
2) Active Attack- The goal here is to change the contents of
the message to disable future use or understanding of the
message.
Cryptanalytic Techniques –
1)Cipher Text Only - The attacker tries to decipher the
message without having the knowledge about the kind of
information contained in the message and must conjecture
from the cipher text only.
2)Known Plaintext Attack - The attacker knows or can
guess some parts of the plaintext.
3)Man – in –the-Middle Attack - The idea behind this
attack is to intervene the communication between two
communicating parties. The attacker can access the traffic,
information, modify the original form and then forward it to
the receiver. Such attacks can be prevented by public key
encryption.
4) Correlation - The correlation (any relation that cannot be
deduced and that is not expected on the basis of chance
alone) between the secret key and the cipher text are the
main source of information in the communication and
hence enable the attackers [2].
5) Attack Against or Using the Underlying HardwareThese attacks make use of the data of the very fine
measurements of the cryptographic device to compute the
key and the encryption information.
6) Faults in the Cryptosystem- These can even lead to the
discovery of the secret key.
II TYPES OF CRYPTOGRAPHY
Cryptography can be broadly classified into
Symmetric/Secret
Symmetric-key cryptography refers to cryptographic methods
where both the sender and receiver use the same key. The
sender would use this key and cipher to encrypt data and
the receiver uses the same key and a decryption technique
to get back the data. Symmetric key ciphers can be
implemented as either block cipher or stream cipher. Also
called as a one-key cipher or a shared key cipher they have
the drawback of having to manage keys and to use them
securely.
Asymmetric/Public
In asymmetric or public-key cryptography, each participant
uses a private key and a public key (i.e. asymmetric
encryption requires 2 unique keys). The private key is kept
secret whereas the public key is announced to the public.
Hash Functions
Hash Functions are also called message digests or one-way
encryption. Unlike in public and private key cryptography
they use no key and are one way functions. . A fixed-length
hash value is calculated for the given plaintext to make it
extremely difficult to retrieve either the contents or length
of the plaintext.A hash function can be defined by the
equation f(message)=hash .A given hash value cannot feasibly
be reversed to get a message that produced that hash value.
There is no inverse hash function f’(hash)=message.
A good hash algorithm [3]
1).Converts a message of any size into a fixed "hash length"
digitalstring.
2).Given a hash H it should be near impossible to find an M
such that H= hash (M). This property of hash functions is
also called preimage resistance.
3).hash functions also obey the second preimage resistance
property which states that Given an input M1 it should be
hard to find another m2 such that hash(m1)=hash(m2).
Some famous Hash functions in use today are Hashed
Message Authentication Code (HMAC), Message Digest,
MD4, MD5, and Secure Hash Algorithm
They can be used for digital signatures, in challengeresponse protocols and key derivation functions, for
message authentication and integrity functions [3].
Fig I shows a typical Hash algorithm in work.
Fig I Hash algorithms
III SUBSTITUTION CIPHERS
They involve the simple substitution of one character with
another. For e.g., If the symbols in the plain text are
alphabetical we replace one character with another. For e.g.,
A can be replaced with D and T with Z.
They can be categorized as monoalphabetic or polyalphabetic.
In monoalphabetic a character in the plaintext is always
substituted by the same character regardless of its position
in the text.
In polyalphabetic each occurrence of a character can be
substituted with a different symbol. The most common
example is the shift cipher.
Shift/Caesar Cipher-In this cipher alphabets are substituted
by another alphabet found some fixed number of positions
down in the alphabet set. Correspondingly the decryption
algorithm substitutes each alphabet by the alphabet found
key places up in the alphabet set. For e.g. A can be made E
when key is 4. The shift cipher is called the Caesar cipher
when the key is 13.Fig II shows the working of a Caesar
cipher where latter A is replaced with D , B with E and so
on.
Other types of substitution ciphers include homophonic
polygraphic, mechanical substitution cipher and the one
time pad.
IV.TRANSPOSITION CIPHER
There is no substitution of characters; instead their positions
are changed without any change in their value. For e.g. a
character in the tenth position may appear in the fifteenth
position in the cipher. A transposition cipher reorders
(permutes) symbols one block at a time. Transposition
ciphers may include Rail Fence Cipher, Route Cipher,
Double Transposition, Myszkowski and Disrupted
Transposition Cipher and Grille Cipher. Some of the
substitution and transposition ciphers are discussed in Table
I.Transposition ciphers are quite weak on their own but can
be combined with other ciphers to provide a stronger
encryption method.for e.g., transposition cipher can be
performed before and after applying a rotating substitution
cipher on a plain text.
FigI I -CaesarCipher
Table I Symmetric Encryption Ciphers
S.No
NAME
YEAR
DEVELOPER
1
Hill Cipher
1929
Lester S. Hill
Converts plain text into matrix based on ASCII values.
2
Vignere Cipher
1553
Giovan
Bellaso
This cipher works by replacing each letter by another letter
specified number of positions further in the alphabet.
3
Vernam Cipher
1917
Gilbert
Vernam
4
Columnar
Transposition
In a columnar transposition, the message is written out in rows
of fixed length, and then read out randomly column by column.
5
Book Cipher
A book cipher is a cipher in which the key is some aspect of a
book or other piece of text in a book. Users of book ciphers
take advantage that the details of the key are sufficiently well
hidden from attackers in practice. However, This can have
problems; if a word appears in the plaintext but not in the book,
it cannot be encoded.
6
Mirror writing
The cipher text is the plaintext as reflected by a mirror. It
appears normal when it is reflected in a mirror.
Standford
DESCRIPTION
The plain text is XOR’ed with the key bits.
7
Pigpen Cipher
Freemasons
18 century
Each letter is substituted by a symbol. The alphabet is written
in grids, and then each letter is enciphered by replacing it with
a symbol that depends on the portion of the grid that contains
the letter.
8
Morse Code
Samuel Morse
mid-1830’s
It is a code that uses “dots” and “dashes” for substitution.
9
10
th
Scytale
The original plaintext message is written on the paper which is
wrapped around a cylinder. While it is wound around the
cylinder. The Cypher text is obtained by reading the
unwounded paper. To decipher the message, the paper can be
wrapped around another stick of the same dimensions.
Polybius Square
A Polybius Square is a table that is used to translate letters into
numbers. For encryption, this table(5x5) can be shared with the
recipient. To accommodate all the 26 letters into the 25 spots of
the table, the letters i and j are written together or are
combined. To encode a message each letter is replaced with the
row and column in which it appears.
V. MODERN ROUND CIPHERS
The ciphers used today involve multiple rounds, each round
being a complex cipher made up of several simple ciphers
and accordingly have been named round ciphers. The key
used in each round is derived from the main key (the round
key) using a key. There are two famous modern symmetrickey ciphers: DES and AES. These ciphers are referred to as
block ciphers because they divide the plaintext into blocks
and use the same key to encrypt and decrypt the blocks.
DES has been the de facto standard until recently.
However, AES is the formal standard now.
VI. PUBLIC KEY ALGORITHMS
RSA(rivest shamir adleman algorithm)
The RSA public key cryptosystem relies on the property
that finding two large prime numbers is relatively easy
compared to factorizing their product to provide an efficient
encryption algorithm. Each user of RSA creates and then
distributes the product of two large prime numbers, along
with a public key. A participant creates his public and
private keys using the following steps—
1. Select two large random prime numbers p and q (not
equal).(NO SPACE)
2. Calculate n=pq.
3. Select an odd integer e that is relatively prime to f(n)=(p1)(q-1).
4. Compute the multiplicative inverse of e modulo f(n).
5. P=(e,n) is the RSA public key.
6. S=(d,n) is the RSA secret key.
To encrypt a message M use the public key as
P(M)= Me mod n
(1)
To convert the cipher text back to the message use
S(C)= Cd mod n.
(2)
1. Each party i chooses a random number xi from the
interval 0 ≤ xi <l or from some larger interval
2. They publish their xi.
3. The common key of the parties i and j is axixj , which they
both can compute quickly from the published information
and from their own secret numbers.
4. These keys can then be used for encryption and
decryption.
The only use of DH is for secret-key key exchange. It is not
used for authentication or digital signatures.
Digital Signature Algorithm
Just like a signature on a paper which can be used as a
testimony a digital signature can be used by a receiver to
confirm that the message has not been changed during
transmission as well to verify the credibility of the sender’s
identity. Stored data and programs can also be signed with
digital signatures to verify the integrity of the data and
programs at any later time. Digital signatures make use of
asymmetric cryptography and employ three basic
algorithms:
1)A key generation algorithm that selects a private key at
random from a collection of possible private keys. The
algorithm generates a private key and a corresponding public
key.
2)A signing algorithm that produces a signature with
message and private key as the inputs.
3)A signature verifying algorithm that either accepts or
rejects the message's claim to authenticity.
Elliptic Curve Cryptography
ECC is an approach to encryption that utilizes the complex
nature of elliptic curves in finite fields. ECC is very similar
to Diffie-Hellman Key Exchange and RSA algorithms for
Encryption.
If an attacker is able to factor the modulus n in the public
key then he can easily work out the secret key. Therefore if
factorizing large numbers was undemanding, then breaking
the RSA cipher becomes effortless.
Its disadvantages are in its speed, its enormous cost, and the
ease of manipulation.
The advantage is in its ease of use and ease of upgradation
because of flexibility and portability.
Diffie Hellman Algorithm
This is the simplest and the most secure method to
exchange keys. The pre-master secret is established
between the client and server using the diffie hellman
protocol .The diffie hellman half-keys are sent in plain text.
Neither party are known to each other and the key is shared
through an insecure communication channel.
Fig. III Elliptic Curve Cryptography
The difference is that the numbers used in ECC are chosen
from a finite field defined within an elliptic curve
expression(as shown in Fig.III ,P and Q are te two numbers
chosen). An Elliptic curve is a mathematical structure on
which a number of operations which provide a one way
(hash) function can be performed. ECC uses these one way
functions for efficient cryptographic systems. The function
is called Elliptic Curve Discrete logarithm Problem
(ECDLP). ECC provides equal level of security using
smaller key lengths unlike in integer factorization or
discrete logarithm based systems which require a longer
key. Also ECC occupies less space and is faster compared
to RSA.
VII. CURRENT PROGRESS IN CRYPTOGRAPHY
Early cryptography was solely concerned with converting
messages into unreadable figures to protect the message
from colluders during its transmission. It was limited to its
use in concealment of both diplomatic and military secrets
from the enemy. Today cryptography has expanded its
domain and is used widely to protect data by encryption in
a cost effective manner. The earliest forms of cryptography
were transposition and substitution ciphers which deal with
the encryption of a message by permuting the words or
alphabets and substituting them with another alphabet or
number respectively. Such ciphers were easy to decode and
were vulnerable to various attack. In the modern era,
cryptography has grown from basic message confidentiality
to include some phases of message integrity checking,
sender/receiver identity authentication, and digital
signatures, among other things. Modern cryptography dates
back to the late 1960’s when Date Encryption Standards
was adopted by NIST for encrypting overt information. In
1978 RSA public key algorithm (one of the first public key
algorithms) was given by Ron Rivest, Adi Shamir, and
Leonard Adleman at the Massachusetts Institute of Technology.
Its discovery solved the need to exchange keys and it also
proposed the world wide acceptable standard techniques
like authentication and digital signatures. In the 1980’s
Elliptical Curve Cryptography became popular due its
better security and a smaller key. Chaotic cryptography was
introduced in 1993 which utilizes the complex behavior of
chaotic dynamical systems to hide or mask information.
Several variations and types of chaotic cryptography have
been presented since [4]. Another type of cryptography
called the policy based cryptography has been proposed.
Policy based cryptography uses a policy to encrypt
messages in such a way that only the policy holders are able
to decipher the message. The encryption of such may
contain AND’ing or OR’ing of conditions [5]. Current
research in cryptography include the amalgamation of two
or more fields to come up with an efficient encryption
technique. You may use one for help in transmission of
encryption keys that are then used by a second, different
method to encrypt the message. This is known as a hybrid
cryptosystem, which uses the best traits of its components
to make up for the weakness of others. Quantum
cryptography is one such field which combines quantum
theory with cryptography. This area focuses mainly on the
quantum key exchange and the key exchange uses an
invisible photon. These use photons to create a shared bit
string between two parties. The security of QKE rests on
the law that no information about the quantum state of a
particle can be obtained without introducing disturbance.
Also any attempt to eavesdropping may be identified. A
future aim within the reach of current technology is QKE
between the ground and a satellite. Quantum key
cryptography depends on public key cryptography for
integrity and authentication. Hence it can be used for digital
signatures[6].Current challenges in QKE include the
availability of a source for single photons, higher detector
efficiencies, better key generation rates, authentication, and
the integration of a QKE system into a computer network.
Another field is Genetic Algorithm (GA) which is typically
used to obtain solution for optimization and search
problems. They model genetic processes usually of
inheritance and DNA formation and make use of the
crossing over, mutation, selection, population size and
individual fitness theories[7]. Biometric cryptosystems are
also widely in use today. They don’t have the need to
remember passwords or exchange keys and still provide
guaranteed identification and privacy but require the
presence of the user at all times. Also there are fuzziness
issues because of the high variability of biometric trait.
Neural cryptography is another emerging field which deals
with the application of Neural Network algorithms (and
other stochastic ones) for use in encryption and
cryptanalysis. It is based on the fact that neural networks
can synchronize by mutual learning .On the other hand is
Visual cryptography technique which allows visual
information like pictures or text, etc. to be encrypted in
such a way that decryption becomes a manual operation
that does not require the use of a computer. It was proposed
by Naor and Shamir in 1994. Visual Cryptography makes
use of two transparent images. One is a collection of
random pixels and the other image contains the message. It
is impossible to retrieve the message from only one of the
images. Both the images in the correct order are required to
reveal the information. It does not require complex
mathematical calculations for decryption. The two images
are usually printed on a transparent sheet. It has the
disadvantage of wanting information compression in order
to use less disk space[8][9]. Lightweight cryptography aims
to develop more efficient cryptographic techniques keeping
in mind the typical constraints in the hardware of Internet of
Things (IoT)The hardware are most likely be constrained in
computational power, battery, and memory[10] .
Lightweight cryptography is tailored for such constrained
devices, with the goal of balancing the tradeoffs between
low resource requirements, performance, and cryptographic
strength [11]. Techniques used to meet this challenge
include the use of block ciphers, hash functions, and public
key cryptography for lightweight cryptography.
VIII. CRYPTOGRAPHY IN NETWORK SECURITY
Computer and network security is an ever expanding area.
Security issues and incidents rise at an alarming rate every
year. As the complexity in the network rises the need for
security also rises. Many applications, softwares and
companies have the need to hide details from the users.
Some of the most common security wants of a company are
Methods for
1) Authentication
2) Access control
3) Data confidentiality
4) Non-Repudiation
Cryptography is one such technique that provides a system
to meet the said terms. Apart from security cryptography
also provides1) Proxy signature mechanism-this mechanism allows
a proxy signer to sign on behalf of an individual or
organization.
2) Monitoring Communication
3) Fractional observing of data-used when the sender
wants only some part of the message to be monitored
and not all. In this case translucent cryptography is
used that works on the space between strongly
encrypted and weak/no encryption areas.
4) Certificates-A certificate is an electronic document
that identifies an individual, organization or a server.
5) Key distribution- Diffie Hellman and Quantum Key
Exchange were designed specifically for this
purpose.
Security in most networks is ensured by applying
cryptographic methods in various protocols.
The level at which cryptography is applied decides the level
of transparency to the user. Some of the protocols which
use encryption in some form area) SSL- It uses an encrypted tunnel for the exchange of
data. It provides all the basic security services except
for access control.
b) SSH- It is very similar and also provides a channel for
secure exchange of data. However it also provides
access control.
c) KERBEROS-It is used to provide a way to
authenticate both the sender and the receiver.it uses
only symmetric key for encryption. Apart from nonrepudiation all other services are provided.
d) SET-It is specially designed for the security in credit
card transactions.
e) PGP-It is used to encrypt the data of an email with the
use of asymmetric key encryption. It provides
authentication, confidentiality and data integrity.
While cryptography can ensure security against some types
of attacks there are other factors which can degrade the
quality of the network to a great extent.
IX. CONCLUSION
Computer and network security is a new but fast moving
technology still needs to be properly defined. Cryptography
plays vital role in explosive growth of digital data storage
and communication. It is used to achieve the mains of
security goals like confidentiality,integrity, authentication,
no-repudiation. In order to achieve these goals, various
cryptographic algorithms are developed. This paper
presents a review of various symmetric and asymmetric
ciphers and their algorithms(classical and modern), which
are used in cryptography for various security purposes.
With the help of these cipher’s and algorithms one can
generates his own cipher’s by making slight modifications
to the existing cipher algorithms. Also performance
evaluation of various ciphers can be done and be improved
upon in the future.
X. ACKNOWLEDGMENT
I would like to take this opportunity to thank and express
my deep respect and regard for DR. S. K. PAL (SC ’F’
DRDO, Delhi), DR. Seema Verma (Associate professor,
Banasthali University) for their exemplary guidance,
constant encouragement and valuable feedback throughout
the preperation of this paper.Their valuable suggestions
were immensly helpful and working under them was a
knowledgeable experience.
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