### What are the essential ingredients of a symmetric cipher?

+ Plaintext

+ Encryption algorithm

+ Secret key

+ Ciphertext

+ Decryption algorithm

### How many keys are required for two people to communicate via a symmetric cipher?

one - it's used for both encryption and decryption

### What are the two principal requirements for the secure use of symmetric encryption?

+ A strong encryption algorithm. The opponent should be unable to decrypt ciphertext or discover the key even if he or she is in possession of a number of ciphertexts together with the plaintext that produced each ciphertext.

+ Sender and receiver must have obtained copies of the secret key in a secure fashion and must keep the key secure.

### List three approaches to message authentication.

+ Using conventional encryption

+ Using public-key encryption

+ Using a secret value

### What is a message authentication code?

A small block of data, that is appended to a message to assure that the sender is authentic and that the message is unaltered.

### What properties must a hash function have to be useful for message authentication?

+ H can be applied to a block of data of any size.

+ H produces a fixed-length output.

+ H(x) is relatively easy to compute for any given x, making both hardware and software

implementations practical

+ For any given value h, it is computationally infeasible to find x such that H(x) = h (one-way

property).

+ For any given block x, it is computationally infeasible to find y =ΜΈ x with H(y) = H(x) (weak collision resistant)

+ It is computationally infeasible to find any pair (x, y) such that H(x) = H(y) (strong

collision resistant)

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

+ Plaintext

+ Encryption algorithm

+ Public and private keys

+ Ciphertext

+ Decryption algorithm

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

+ 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

### What is the difference between a private key and a secret key?

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.

### What is a digital signature?

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.

### What is a public-key certificate?

A certificate consists of a public key plus a user ID of the key owner, with the whole block signed by a trusted third party (= certificate authority CA). The user can then publish the certificate and anyone needing this user's public key can obtain the certificate and verify that it is valid by means of the attached signature.

### How can public-key encryption be used to distribute a secret key for symmetric encryption?

Digital Envelope - Bob does the following:

+ Prepare a message

+ Generate a random symmetric key that will be used this time only.

+ Encrypt that message using symmetric key encryption with the one-time key.Encrypt the one-time key using public-key encryption with Alice's public key. - Attach the encrypted one-time key to the encrypted message and send it to Alice.