17 Questions
What is the purpose of Bob's public key in Public-Key Cryptography?
To encrypt messages from Alice
What is the role of Alice's private key in digital signatures?
Creating the signature
In Public-Key Cryptography, what does Bob use to decrypt an encrypted message from Alice?
Bob's private key
What property should a cryptographic hash function possess to be considered secure?
It should be one-way and hard to reverse
What is the purpose of a digital certificate in secure communication?
Authentication of public keys
How does Public-Key Cryptography differ from symmetric-key cryptography?
It requires a separate key for encryption and decryption
What is the purpose of a Message Authentication Code (MAC)?
To provide data integrity for messages between Alice and Bob.
In public-key cryptography, what is the role of a digital certificate authority (CA)?
To sign the binding between an identity and a public key.
Which cryptographic hash function is mentioned in the text examples?
SHA-256
What makes a password strong according to the text?
Including uppercase characters, special characters, and numbers.
How does Alice ensure data integrity when sending a message to Bob using a shared secret key?
By computing a hash of the concatenation of the key and message.
In a symmetric cryptosystem, what is used for both encryption and decryption?
Shared key
Which type of cryptosystem requires each pair of communicating parties to share a separate secret key?
Symmetric Key Distribution
What is the main advantage of public-key cryptography over symmetric-key cryptography?
Eliminates the need for key exchange
Which component of a cryptosystem defines the set of possible encryption keys?
Set of Encryption Keys
What does the Caesar Cipher do to each letter in the plaintext?
Moves it three places forward in the alphabet
Which cryptosystem involves sharing a public key for encryption and a private key for decryption?
Public-Key Cryptography
Study Notes
Message Authentication Codes (MACs)
- MACs ensure data integrity and allow Alice and Bob to detect tampering if they share a secret key.
- The process involves computing a hash (H) of the message (M) and the shared secret key (K), and sending the message and the hash to Bob.
Digital Certificates
- A digital certificate is a binding between an identity and a public key, digitally signed by a Certificate Authority (CA).
Passwords
- A password is a short sequence of characters used to authenticate someone via a secret they know.
- A strong password includes:
- Upper and lower case characters
- Special characters
- Numbers
Password Storage
- A password is typically stored in a password file, encrypted using a hash function.
Public-Key Cryptography
- Bob has two keys: a private key (SB) and a public key (PB), which he broadcasts widely.
- Alice uses Bob's public key to encrypt her message, and Bob uses his private key to decrypt the message.
- Separate keys are used for encryption and decryption.
Public Key Distribution
- Only one key pair is needed for each recipient, with the private key kept secret and the public key shared widely.
Digital Signatures
- Public-key encryption provides a method for digital signatures.
- To sign a message, Alice encrypts it with her private key, creating a digital signature that can be decrypted by anyone using her public key.
Cryptographic Hash Functions
- A cryptographic hash function is a checksum on a message that is one-way and collision-resistant.
- It is easy to compute the hash, but hard to find the original message given only the hash.
Encryption and Decryption
- Encryption and decryption algorithms are chosen so that it is infeasible for someone other than Alice and Bob to determine the plaintext from the ciphertext.
- Ciphertext can be transmitted over an insecure channel, while keeping the plaintext secure.
Cryptosystem
- A cryptosystem consists of:
- The set of possible plaintexts
- The set of possible ciphertexts
- The set of encryption keys
- The set of decryption keys
- The correspondence between encryption keys and decryption keys
- The encryption algorithm
- The decryption algorithm
Caesar Cipher
- A simple substitution cipher where each letter is replaced with the one "three over" in the alphabet.
Symmetric Cryptosystems
- Alice and Bob share a secret key, used for both encryption and decryption.
- Symmetric key distribution requires each pair of communicating parties to share a separate secret key.
Test your knowledge on message authentication codes (MAC) used to provide data integrity between communicators. Understand how Alice and Bob can verify the authenticity of their messages using shared secret keys and hashing functions like SHA-1 and SHA-256.
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