Question

NC3A -

• 3.6 What are the principal ingredients of a public-key cryptosystem?

• 3.7 List and briefly define three uses of a public-key cryptosystem.

• 3.8 What is the difference between a private key and a secret key?

• 3.9 What is a digital signature?

Answer 1

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3.6)

+ Plaintext
+ Encryption algorithm
+ Public and private keys
+ Ciphertext
+ Decryption algorithm

3.7)

+ Encryption/decryption: The sender encrypts a message with the recipient's public key.
+ Digital signature: The sender "signs" a message with its private key.
+ Key exchange: Two sides cooperate to exchange as session key. Several different approaches are possible, involving the private key(s) of one or both parties

3.8)

The key used in conventional encryption is typically referred to as a secret key. The two keys used for public-key encryption are referred to as the public key and the private key.

3.9)

A mechanism for authenticating a message. Bob uses a secure hash function, such as SHA-512, to generate a hash value for the message and then encrypts the hash code with his private key, creating a digital signature. Bob sends the message with the signature attached. When Alice receives the message she calculates a hash value for the message, decrypts the signature using Bob's public key and compares the calculated hash value to the decrypted hash value. If the two hash values match, Alice is assured that the message must have been signed by Bob. It is important to emphasize that the digital signature does not provide confidentiality.

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