The Private key is used to decrypt messages from other users. A digital certificate is basically a unique piece of code or a large number that says that the Web server is trusted by an independent source known as a certificate authority. Requirement of Public key Cryptography 1. Public-key cryptography, or asymmetric cryptography, is an encryption scheme that uses two mathematically related, but not identical, keys - a public key and a private key. There are two specific requirements of key management for public key cryptography. There are legitimate interoperability requirements for these. It is computationally easy for a party B to generate a pair [KU b, KR b]. It is computationally easy for a party B to generate a pair of key (public key PUb, private key PRb). •Requirements -It must be computationally easy to encipher or decipher a message given the appropriate key -It must be computationally infeasible to derive the private key from the public key -It must be computationally infeasible to determine the private key from a chosen plaintext attack  2. Requirements for Public-Key Cryptography  1. Unlike symmetric key algorithms that rely on one key to both encrypt and decrypt, each key performs a unique function. It is computationally easy for a sender A, knowing the public key and the message to be encrypted, M, to generate the corresponding ciphertext:  C = E (PUb, M)  3. This is how asymmetric encryption or public-key cryptography works. Assurance of public keys. Requirements for Public-Key Cryptography The cryptosystem illustrated in Figures 9.2 through 9.4 depends on a cryptographic algorithm based on two related keys. It is easy for party B to generate a pair of keys (public key PUb, Mr. Gopal Sakarkar Private key PRb). For n individuals to communicate, number of keys required = 2 x n = 2n keys. Secrecy of private keys. The key parameters given to the vendor or which will get into the POS are those in the public part of the RSA key: $(N,e)$. Asymmetric Encryption Algorithms- These keys are mathematically linked. It is slower than symmetric key cryptography. Throughout the key lifecycle, secret keys must remain secret from all parties except those who are owner and are authorized to use them. For every user, there is 1 Private key and 1 Public key. It involves high computational requirements. If you have 5 users, there are 5 Private keys and 5 Public keys. It is computationally easy for a sender A, knowing the public key and the message to be encrypted M, to generate the corresponding ciphertext: C=EKU b (M). It is easy for a sender A, knowing the public key and message to be encrypt. The Public key is used by everyone else to encrypt messages for that user. Diffie and Hellman postulated this system without demonstrating that such algorithms exist. To implement public-key encryption on a large scale, such as a secure Web server might need, requires a different approach. 2 Requirements for public key cryptography . This is where digital certificates come in. However, they did lay out the conditions that such algorithms must fulfill [DIFF76b]. There are legitimate interoperability requirements for these. 2. it establishes a communication which is one way where the center which is a public key user can encrypt the data in a message and can also check the authenticity of the digital signature where is on the second hand the recipient who is a private key user can decrypt the message. Number of Keys Required- To use public key cryptography, Each individual requires two keys- one public key and one private key. 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