There is nothing perfectly 100% secure computer system. There will always be bugs and security experts know that. So, system architects employer strategy is called defensive depth which uses many layers of varying security mechanisms to frustrate attackers. We have one of the most common forms of computer security cryptography. The word cryptography comes from the roots of crypto and Griffey roughly translating to secret writing. In order to make information secret, you use a cypher. An algorithm that converts plaintext into ciphertext which is gibberish unless you have a key that lets you undo the cypher. The process of making text secret is called encryption and the reverse process is called decryption.
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Cypher have been used long before computers showed up Julius Caesar use what is now called a season cypher to encrypt private correspondence. He would shift the letters in a message forward by three places. So, A became D. The message recipients had to know both the algorithm and the numbers shift by which acted as the key. The Caesar cypher is one example of a larger class of techniques called substitution cyphers. These replace every letter in a message with something else according to a translation. A big drawback of basic substitution cyphers is that letter frequencies are preserved. For example, A is the most common letter in English. So, if cypher translates A into an X then X will show up the most frequently in the cipher-text. Skilled cryptanalyst can work backwards from these kinds of statistics to figure out the message. Fundamental class of techniques are permutation cyphers. let’s look at the simple example called a common are transposition cypher. here we take a message and fill the letters into a grid. in this case we’ve chosen 5 by 5 to encrypt town message we read out the characters in a different order. let’s say from the bottom left working up was one column to time than you letter ordering what’s called a permutation is the encrypted message. the ordering direction as well as the five-by-five grid size serves as the key. like before if the ciphering key unknown recipient can reverse the process through the original message by the 1900s cryptography was mechanised in the form of encryption machines.
Cryptography is important because it allows you to securely protect data that you do not want anyone else to have access to. It is used to protect corporate secrets, secure classified information, and protect personal information to guard against things like identity theft. Let us get started, with the help of an example of a scenario to explain what cryptography is. Let us assume we have a person called Anne. Suppose Anne sends a message to his friend Sam who is on the other side of the world.
Now obviously he wants this message to be private and nobody else should have access to the message. Now he uses a public forum, for example, the Internet for sending this message. The goal is to secure this communication and of course, we must be secured against someone. Now let us say, there is a smart guy called eve who is secretly got access to a communication channel. Since this guy has access to your communication, he can do much more than just eavesdrop. For example, he can try to change the message. Now, this is just a small example, what if eve gets access to your private information, well that could result in a big catastrophe.
So how can Andy be sure that nobody in the middle could access the message sent to Sam. The goal here is to make communication secure and that is where cryptography comes in. So, what exactly is cryptography, well cryptography is a practise and the study of techniques for securing communication and data in the presence of adversaries.
So let me take a moment to explain how that happens, well first we have a message, this message is firstly converted into a numeric form and then this numeric form is applied to the key called an encryption key and this encryption key is used in the encryption algorithm. So once the numeric message and the encryption key has been applied an encryption algorithm. What we get is called a cipher-text. Now, this cipher-text is sent over the network to the other side of the world where the other person to who the message is intended, will use a decryption key cipher-text as a parameter of the decryption algorithm and then he will get what we send as a message.
So let us see how cryptography can help secure the connection between Anne and Sam. So, to protect his message Anny first converts his readable message to an unreadable form. Here he converts a message to some random numbers and after that he uses a key to encrypt his message after applying this key to the numerical form of his message, he gets a new value in cryptography we call this cipher-text.
So now if Anne sends the cipher-text or encrypted message over communication channel. He won’t have to worry about somebody in the middle of discovering the private message. Even if somebody manages to discover the message, he won’t be able to decrypt the message without having a proper key to unlock this message. So, suppose eve here discovers the message and he somehow manages to tamper with the message and message finally reach. Sam would need a key to decrypt the message to recover the original plaintext so using the key you could convert a cipher-text to numerical value corresponding to the plaintext. Now after using the key for decryption or will come out as the original plaintext message or an error. Now this error is very important. It is the way Sam knows that message sent by Anne is not the same as a message that he received. So, the error in a sense tells us that eve has tampered with the message. Now the important thing to note here is that in modern cryptography the security of the system purely relies on keeping the encryption and decryption key secret. Based on the type of keys encryption algorithms cryptography is classified under the following categories. now cryptography is broadly classified under 2 categories namely symmetric key cryptography and asymmetric key cryptography popularly also known as public key cryptography.