Introduction to Cryptography
Cryptography is a word from Greek where crpyto
means "hidden, secret"; and graphy means
"writing" or "study“ . It is the practice and study of
techniques for secure communication in the
presence of third parties .
More generally, it is about constructing and
analyzing protocols that overcome the influence of
adversaries and which are related to various
aspects in information security such as data
confidentiality, data integrity and authentication.
Applications of Cryptography
Applications of cryptography include ATM
cards, computer passwords, and electronic
German Lorenz cipher
machine, used in
World War II to encrypt
general staff messages
ALICE , BOB and EVE
In cryptography, it is customary to use three
Characters in an information exchange.
ALICE:- who Needs to send the DATA.
BOB:- recipient of the DATA
EVE:- person who somehow disturbs the
communication between Alice and Bob
Categories of cryptography
In symmetric-key cryptography, theIn symmetric-key cryptography, the
same key is used by the sendersame key is used by the sender
(for encryption)(for encryption)
and the receiver (for decryption).and the receiver (for decryption).
The key is shared.The key is shared.
Keys used in cryptography
Comparison between two categories of cryptography
SECURITY SERVICESSECURITY SERVICES
Network security can provide five services. Four of theseNetwork security can provide five services. Four of these
services are related to the message exchanged using theservices are related to the message exchanged using the
network. The fifth service provides entity authenticationnetwork. The fifth service provides entity authentication
or identification.or identification.
Message ConfidentialityMessage Confidentiality
Message IntegrityMessage Integrity
Message AuthenticationMessage Authentication
Message NonrepudiationMessage Nonrepudiation
Entity AuthenticationEntity Authentication
The concept of how to achieve message confidentiality or privacyThe concept of how to achieve message confidentiality or privacy
has not changed for thousands of years. The message must behas not changed for thousands of years. The message must be
encrypted at the sender site and decrypted at the receiver site.encrypted at the sender site and decrypted at the receiver site.
This can be done using either symmetric-key cryptography orThis can be done using either symmetric-key cryptography or
asymmetric-key cryptography.asymmetric-key cryptography.
Message confidentiality using symmetric keys in two directions
Message confidentiality using asymmetric keys
MESSAGE INTEGRITYMESSAGE INTEGRITY
Encryption and decryption provide secrecy, or confidentiality,Encryption and decryption provide secrecy, or confidentiality,
but not integrity. However, on occasion we may not even needbut not integrity. However, on occasion we may not even need
secrecy, but instead must have integrity.secrecy, but instead must have integrity.
To preserve the integrity of a document,To preserve the integrity of a document,
both the document and the fingerprint areboth the document and the fingerprint are
Message and message digest
The message digest needs to be kept secret.The message digest needs to be kept secret.
MESSAGE AUTHENTICATIONMESSAGE AUTHENTICATION
A hash function per se cannot provide authentication. TheA hash function per se cannot provide authentication. The
digest created by a hash function can detect any modificationdigest created by a hash function can detect any modification
in the message, but not authentication.in the message, but not authentication.
DIGITAL SIGNATUREDIGITAL SIGNATURE
When Alice sends a message to Bob, Bob needs to check theWhen Alice sends a message to Bob, Bob needs to check the
authenticity of the sender; he needs to be sure that theauthenticity of the sender; he needs to be sure that the
message comes from Alice and not Eve. Bob can ask Alice tomessage comes from Alice and not Eve. Bob can ask Alice to
sign the message electronically. In other words, an electronicsign the message electronically. In other words, an electronic
signature can prove the authenticity of Alice as the sender ofsignature can prove the authenticity of Alice as the sender of
the message. We refer to this type of signature as a digitalthe message. We refer to this type of signature as a digital
A digital signature needs a public-key system.A digital signature needs a public-key system.
Entity authentication is a technique designedEntity authentication is a technique designed
to let one party prove the identity of anotherto let one party prove the identity of another
party. An entity can be a person, a process, aparty. An entity can be a person, a process, a
client, or a server. The entity whose identityclient, or a server. The entity whose identity
needs to be proved is called the claimant; theneeds to be proved is called the claimant; the
party that tries to prove the identity of theparty that tries to prove the identity of the
claimant is called the verifier.claimant is called the verifier.
In challenge-response authentication,
the claimant proves that she knows a secret
without revealing it.
KEY MANAGEMENTKEY MANAGEMENT
We never discussed how secret keys inWe never discussed how secret keys in
symmetric-key cryptography and how publicsymmetric-key cryptography and how public
keys in asymmetric-key cryptography arekeys in asymmetric-key cryptography are
distributed and maintained. In this section,distributed and maintained. In this section,
we touch on these two issues. We firstwe touch on these two issues. We first
discuss the distribution of symmetric keys;discuss the distribution of symmetric keys;
we then discuss the distribution ofwe then discuss the distribution of
asymmetric keys.asymmetric keys.
A session symmetric key between two parties isA session symmetric key between two parties is
used only once.used only once.
K. J. SOMAIYA COLLEGE OF ENGINEERING