Diffie-Hellman Key Exchange Algorithm
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Questions and Answers

What is the formula to calculate the session key for User A?

  • SKA = R_A^XB mod p
  • SKA = R_A^XB mod g
  • SKA = R_B^XA mod g
  • SKA = R_B^XA mod p (correct)

    • What is the value of the session key calculated by Alice in the simple example?

  • 20
  • 19
  • 17
  • 18 (correct)

    • What is the purpose of the session key in the e-commerce example?

  • To encrypt the payment information
  • To encrypt the order details (correct)
  • To authenticate the user
  • To decrypt the order details

    • What is the value of g in the simple example?

    <p>7</p> Signup and view all the answers

    What is the formula to calculate Bob's public key in the simple example?

    <p>RB = 7^XB mod p</p> Signup and view all the answers

    What is the ideal number of bits for the integer p?

    <p>1024 bits</p> Signup and view all the answers

    What is the purpose of Alice sending her public key to Bob?

    <p>To enable Bob to calculate the session key</p> Signup and view all the answers

    What is the value of the session key calculated by Bob in the simple example?

    <p>18</p> Signup and view all the answers

    What is the role of the entity labeled 'CR' in the public-key certificate?

    <p>Certificate authority</p> Signup and view all the answers

    What is the purpose of a public-key certificate?

    <p>To authenticate the identity of an entity</p> Signup and view all the answers

    What is the relationship between 'E-' and 'K' in the diagram?

    <p>'E-' is the public key and 'K' is the private key</p> Signup and view all the answers

    What is the function of the component labeled 'A' in the diagram?

    <p>Authentication</p> Signup and view all the answers

    What is the purpose of the 'ID' component in the diagram?

    <p>To identify the entity requesting the certificate</p> Signup and view all the answers

    What is the significance of the 'S' component in the diagram?

    <p>It represents the signature algorithm used</p> Signup and view all the answers

    What is the role of the 'T' component in the diagram?

    <p>To issue the certificate</p> Signup and view all the answers

    What is the relationship between 'R' and 'E-' in the diagram?

    <p>'R' is the public key and 'E-' is the private key</p> Signup and view all the answers

    What is a digital certificate comparable to?

    <p>A digital driver license</p> Signup and view all the answers

    What is a key feature of a digital certificate?

    <p>It is cryptographically signed and practically impossible for anyone to forge</p> Signup and view all the answers

    What binds together in a digital certificate?

    <p>Identity, authority, public key, and other information</p> Signup and view all the answers

    What is the primary purpose of a digital certificate in E-Commerce applications?

    <p>To provide both confidentiality and authentication</p> Signup and view all the answers

    What is encrypted in a digital certificate?

    <p>Communication link between the website and consumer</p> Signup and view all the answers

    What is the ultimate goal of using digital certificates in E-Commerce applications?

    <p>To build the consumer's trust</p> Signup and view all the answers

    Who proposed the first public-key type scheme in 1976?

    <p>W. Diffie and M.E. Hellman</p> Signup and view all the answers

    What is the main advantage of using a hybrid system in public-key encryption?

    <p>Faster key exchange and symmetric key algorithm usage</p> Signup and view all the answers

    What is a characteristic of the Diffie-Hellman key exchange algorithm?

    <p>It allows group of users to agree on a secret key over an insecure channel</p> Signup and view all the answers

    What is the role of the prime number p in the Diffie-Hellman key exchange algorithm?

    <p>It is used as a modulus in the calculation of the public key</p> Signup and view all the answers

    What is the purpose of the public key RA in the Diffie-Hellman key exchange algorithm?

    <p>To share with the other user</p> Signup and view all the answers

    What is the main difference between the Diffie-Hellman key exchange algorithm and public-key encryption algorithms?

    <p>The Diffie-Hellman algorithm cannot be used to encrypt and decrypt messages</p> Signup and view all the answers

    What is the purpose of the private random number XA in the Diffie-Hellman key exchange algorithm?

    <p>To keep secret and private</p> Signup and view all the answers

    Why is the Diffie-Hellman key exchange algorithm important in public-key cryptosystems?

    <p>It is a practical method for public exchange of a secret key</p> Signup and view all the answers

    What is the main issue with the public key announcement approach?

    <p>Anyone can forge a public announcement and read encrypted messages</p> Signup and view all the answers

    What happens when Darth intercepts Bob's public key in the key management problem?

    <p>Darth sends his own public key to Alice and calculates a shared key with Bob</p> Signup and view all the answers

    What is the purpose of a public key certificate?

    <p>To verify the authenticity of a public key</p> Signup and view all the answers

    What happens when Alice receives Darth's second public key?

    <p>She calculates a shared key with Darth</p> Signup and view all the answers

    What is the limitation of the publicly available directory approach?

    <p>It is not secure because the directory can be compromised</p> Signup and view all the answers

    What is the main advantage of the public key certificate solution?

    <p>It is more secure than the other solutions</p> Signup and view all the answers

    Study Notes

    Diffie-Hellman Key Exchange Algorithm

    • Proposed by Diffie and Hellman in 1976, it is the first public-key type scheme and a practical method for public exchange of a secret key.
    • Used in a number of commercial products.

    Key Exchange Application

    • In a hybrid system, Alice uses Bob's public key to send him a secret shared session key.
    • Alice and Bob use the session key to exchange information using a symmetric key algorithm (like DES).

    Diffie-Hellman Symmetric Key Agreement

    • Allows a group of users to agree on a secret key over an insecure channel.
    • Cannot be used to encrypt and decrypt messages.

    Steps in the Diffie-Hellman Key Exchange Algorithm

    • Select a large prime number p and an integer g that is a primitive root of p.
    • User A selects a private random number XA, then calculates his public key RA as RA = g^XA mod p.
    • User B does the same to calculate his public key RB as RB = g^XB mod p.
    • Each side keeps the X value private and makes the R value public to the other side.
    • User A, B calculates their session key Sk as Sk_A = RB^XA mod p and Sk_B = RA^XB mod p.

    Simple Example

    • Uses small numbers, but in a real situation, the numbers are very large.
    • Alice chooses Xa = 3 and calculates her public key RA = 7^3 mod 23 = 21.
    • Bob chooses Xb = 6 and calculates his public key RB = 7^6 mod 23 = 4.
    • Alice and Bob exchange their public keys.
    • Alice calculates the session key SKa = 4^3 mod 23 = 18.
    • Bob calculates the session key SKb = 21^6 mod 23 = 18.

    Realistic Example

    • Uses a program to create a random integer of 512 bits (the ideal is 1024 bits).
    • The integer p is a 159-digit number.
    • g, x, and y are chosen as shown in the example.

    Using the Session Key in E-commerce

    • Bob wants to order brownies from Alice and keep the entire transaction private.
    • Bob arranges with Alice to generate a session key.
    • Bob uses the session key to encrypt his order and sends it to Alice.

    Man-in-the-Middle Attack

    • Bob transmits his public key to Alice, but Darth intercepts this and transmits his second public key to Alice.
    • Alice receives Darth's second public key and calculates the shared key with Darth instead of Bob.

    Key Management Problem & The Solution

    • Any user could pretend to be user A and broadcast a public key, reading all encrypted messages intended for A.
    • The solution has three types: public key announcement, public key directory, and public-key certificate.

    Public-key Certificate

    • A digital certificate is like a digital driver's license that binds identity, authority, public key, and other information to a user.
    • It is cryptographically signed and practically impossible to forge.
    • It provides confidentiality and authentication, ensuring a secure communication between the website and the consumer.

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    Description

    This quiz covers the basics of the Diffie-Hellman Key Exchange Algorithm, a public-key type scheme proposed by Diffie and Hellman in 1976. Learn about its application in public key cryptosystems.

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