 So, let us begin the session, learning outcome at the end of this session students will be able to explain public key cryptography as well as they are able to differentiate between private and public cryptosystems. So, as we have known there are two types of encryption, the one is private which is also known as symmetric and another one is public which is also called as asymmetric cryptosystem. So, this is the pictorial representation of private key cryptography, Alice and Bob and there is a communication between Alice and Bob. Now, what happens in private key cryptography, it is a traditional private which is also called as secret or single key cryptography why, because it uses a single key or one key. So, that particular key is shared by both sender and receiver. So, our Bob and Alice will use a single key for communication, if that particular key is disclosed the communication can be compromised. It is also called as symmetric why, because the parties are equal and it does not protect sender from receiver by forcing a message and claiming it is sent by sender. That is important that forcing of a message cannot be protected. Now, this is public key cryptography. Another one thing public key cryptography it is not complement, it is not replaces private key cryptography, it is just a complement to that private key cryptography. Now, what are the characteristics of public key cryptography, probably the most significant advance in the 3000 year history of a cryptography, it uses two keys the one is public, another one is private. It is also called as asymmetric why, because parties are not equal, it uses clever application of number theoretic concept to the function and this is the one of the most characteristic or one of the most favorable characteristic of a public key cryptography. It uses clever application of number theoretic concept to the function. This is the pictorial way of a representation Bob and Alice, so communication between Alice and Bob. So, Alice uses a Bob's public key to encrypt a message while Bob will use his own private key to get a message, so use of two keys private and public for encryption and decryption. So, why public key cryptography? How it complement to the private key cryptography? It is developed to address two key issues, remember one thing or listen carefully, the one is key distribution and another one is digital signature. Now, what happens in key distribution? What is the issue with a key distribution? How to secure or how to achieve a secure communication in general without having to trust a key distribution center with your key? And the second one is digital signature, how to verify a message comes in fact from the claimed sender, so these two issues are addressed with the help of public key cryptosystem, so public invention due to Diffie and Hellman at Stanford University in 1976 and known earlier in classified communicative only. So, we will see here in detail how these public and private keys are used, so this cryptography involves the use of two keys, the one is public and another one is private, so as name suggests the public key, key which may be known by anybody and can be used to encrypt message as well as to verify signatures. And what happens with private key, private key which is known only to the recipient and used to decrypt messages and sign signatures or use of digital signatures. It is infeasible to determine private key from public key, remember one thing, it is infeasible to determine private key from public key and it is asymmetric because those who encrypt message or verify signature cannot decrypt message or create signature that is why it is called as asymmetric. Now look at here, this happens in public key cryptosystem, so plain text after applying encryption algorithm with public key converted into cipher text and that cipher text at receiver side by using the decryption algorithm with that receiver's own private key get back to the original plain text, so public key and private key. Now let us check out what are the differences in private and public key, so what happens in private key cryptography, the same algorithm with the same key is used at both side but in public key one algorithm is used for encryption and decryption with pair of keys. The sender and receiver must share the algorithm and key, the sender and receiver must each have one means must have their own key, the key must be secret here one of these two keys only single key will be secret, this is one of the use of public key cryptosystem where it is possible to achieve authentication and confidentiality. So how, so start with a source, so message source X with private key and encryption algorithm authentication is possible and by applying public key and again algorithm confidentiality is possible and at receiver side, now the private key and use of a public key to get original message. So authentication and confidentiality both things are possible with public key cryptosystem that is the important thing or important characteristic of a public key cryptography. Now dear student please pause the video and try to answer the question, what is the question? Public key encryption or decryption is not preferred, why not preferred, is it slow, is it hardware or software sensitive, it has a high computational load or all of because of these reasons, just try or just recall to answer yes, public key encryption and decryption is not preferred because it is slow, it has hardware and it is hardware and software intensive, it has high computational load all are these the disadvantage of public key cryptography. But no matter how many disadvantage it has, this is the main thing authentication and confidentiality achieve is possible because of public key cryptosystem. Lots of public key algorithms are there like RSA, Diffie-Hellman which makes security more feasible as compared to private key cryptography. Remember one thing, public key cryptography is not a replacement, it is just a compliment to the private key cryptosystem. So it can be classified depending upon it uses in a three categories encryption, decryption to provide a secrecy, digital signature to provide authentication and key exchange for a session key, key exchange for a session key. So public key algorithm can be used along with a private key algorithm to provide a security of that secret key or a private key. So lots of applications of asymmetric encryption SSL that is secure socket layer, SSH, PGP, pretty good privacy and most important bit coin. So bit coin which is a latest or hot topic nowadays which is based on asymmetric encryption. So this is the reference, thank you.