Hybrid Cryptograph

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A Hybrid Cryptosystem Solution

Cryptograhy is the science of keeping data secure. Encryption is the process of using

cryptography to encode data so that it’s meaning is not immediately obvious. There are

two main types of cryptography that are used, symmetric and asymmetric. It is also

possible to use a hybrid of the two; Hybrid Cryptosystem Solution is an example of this.

Symmetric and asymmetric cryptography both have advantages and disadvantages that

will be discussed in this paper. Hybrid Cryptosystem Solution brings the best of each

together and also works to minimize the disadvantages. This will also be discussed.

Alice and Bob are often used as examples when cryptography is explained, therefore I

will also use them for most examples.

Cryptographic Terms

Cryptography: process of making and using codes to secure transmission of

information

Encryption: converting original message into a form unreadable by unauthorized

individuals

Every security system must provide a bundle of security functions that can assure the

secrecy of the system. These functions are usually referred to as the goals of the security

system. These goals can be listed under the following five main categories:

Authentication: This means that before sending and receiving data using the

system, the receiver and sender identity should be verified.

Secrecy or Confidentiality: Usually this function (feature) is how most people

identify a secure system. It means that only the authenticated people are able to interpret

the message (date) content and no one else.

Integrity: Integrity means that the content of the communicated data is assured to

be free from any type of modification between the end points (sender and receiver). The basic form of integrity is packet check sum in IPv4 packets.

Non-Repudiation: This function implies that neither the sender nor the receiver

can falsely deny that they have sent a certain message.



Service Reliability and Availability: Since secure systems usually get attacked by

intruders, which may affect their availability and type of service to their users. Such

systems should provide a way to grant their users the quality of service they expect.

Symmetric Encryption

Symmetric cryptography, also known as conventional or secret encryption, is

when the encryption and decryption keys are the same value. An example of symmetric

cryptography is the Caesar Cipher. When Alice and Bob want to exchange data, they

must first decide upon an algorithm such as the Caesar Cipher. Alice first writes her

message in plaintext, then shifts each letter of her message 3 characters to the left. So, A

becomes D, B becomes E, and so on. Bob knows in order to decrypt the data, he shifts

each ciphertext letter back 3 characters, this then reveals the plaintext. Both Alice and

Bob know that shifting each letter the appropriate direction is the key. Symmetric

encryption is also known as a shared secret cryptography or shared key cryptography because it uses a single “shared” key that is used to encrypt and decrypt the message.

Symmetric encryption: uses same “secret key” to encipher and decipher message



Encryption methods can be extremely efficient, requiring minimal

processing

Both sender and receiver must possess encryption key

If either copy of key is compromised, an intermediate can decrypt and readmessages

Advantages to using symmetric encryption:

Data is encrypted very quickly.

Provides authentication, as long as the key stays secret

Symmetry of key allows encryption and decryption with the

same key

Disadvantages of symmetric encryption:

If the key is revealed (lost, stolen, guessed, etc…) theinterceptors can immediately decrypt anything that was

encrypted using the key. An imposter using an intercepted key

can produce bogus messages by impersonating the legitimate

sender.

Distribution of keys becomes a problem, especially if keys

change frequently. Keys must be transmitted with extreme

security because they allow access to all the information

encrypted with them. For applications that extend throughout

the world, this can be a very complex task.

Asymmetric Encryption

Asymmetric encryption (public-key encryption)

Uses two different but related keys; either key can encrypt or decrypt

message

If Key A encrypts message, only Key B can decrypt

Highest value when one key serves as private key and the other serves as

public key

Asymmetric encrypt ion, also known as public key encryption, is encryption that uses

two different keys for encrypt ion and decryption. One key is a public key that can be

distributed to anyone. The other is a mathematically related key called a private key or

secret key. This is a key that should be kept secret from the world. Only the owner



should have access to the private key or any back - up copies of it. One should protect

his/her secret key in the same manner that they would treat their bank PIN or credit card

information.

Advantages to using asymmetric encryption:

For any number n users (represented as “n”), only 2*n, (N=2 *n)

keys are required instead of n*(n - 1)/ 2 as with symmetric

encryption. In other words when n=3, N=6, when n=4, N=8, and

when n=10, N=20. In asymmetric encryption, each user only has

1 key pair. This means only a person’s public key is exchanged

and each group of people do not need separate keys.

The problem of distributing keys is solved because a user’s

public key can be shared by anyone.

Disadvantages of using asymmetric encryption:

Only a few public key algorithms are both secure and practical

Some algorithms are only suitable for key distribution

Slow: 100 - 1000 times slower than symmetric algorithms (RSA

vs. DES)

Only three algorithms work well for both key distribution and

encryption: RSA, ElGamal, Rabin.



Hybrid Encryption SolutionIn cryptography, public-key cryptosystems are convenient in that they do not

require the sender and receiver to share a common secret in order to communicate

securely (among other useful properties). However, they often rely on complicated

mathematical computations and are thus generally much more inefficient than

comparable symmetric-key cryptosystems. In many applications, the high cost of

encrypting long messages in a public-key cryptosystem can be prohibitive. A hybrid

cryptosystem is one which combines the convenience of a public-key cryptosystem with

the efficiency of a symmetric-key cryptosystem.



A hybrid cryptosystem can be constructed using any two separate cryptosystems:

a key encapsulation scheme, which is a public-key cryptosystem, and

a data encapsulation scheme, which is a symmetric-key cryptosystem.

The hybrid cryptosystem is itself a public-key system, whose public and private keys

are the same as in the key encapsulation scheme.

Note that for very long messages the bulk of the work in encryption/decryption is

done by the more efficient symmetric-key scheme, while the inefficient public-key

scheme is used only to encrypt/decrypt a short key value.



Example for H ybr id System

To encrypt a message addressed to Alice in a hybrid cryptosystem, Bob does the

following:

1. Obtains Alice's public key.

2. Generates a fresh symmetric key for the data encapsulation scheme.

3. Encrypts the message under the data encapsulation scheme, using the symmetric

key just generated.

4. Encrypt the symmetric key under the key encapsulation scheme, using Alice's

public key.

5. Send both of these encryptions to Alice.

To decrypt this hybrid ciphertext, Alice does the following:

1. uses her private key to decrypt the symmetric key contained in the key

encapsulation segment.

2. uses this symmetric key to decrypt the message contained in the data

encapsulation segment.



Advantages to using Hybrid

The combination of these two encryption methods combines the convenience of

public - key encryption with the speed of conventional encryption.

Using Conventional encryption is about 100 to 1,000 times faster than public - key

encryption, which solves the problem of slow encryption with asymmetric

algorithms.

Public- key encryption provides a solution to key distribution and data

transmission issues when using symmetric encryption.

When used together, performance and key distribution are improved without any

sacrifice in security.

It ties together the advantages of public key and symmetric cryptography, while

also providing a feasible solution to the disadvantages of both.

Hybrid Cryptosystem is a powerful hybrid cryptosystem that combines the

advantages of both symmetric and asymmetric cryptography. At the same time,

Hybrid Cryptosystem minimizes the disadvantages of each system.

Hybrid Cryptograph

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