Which of the following is NOT a characteristic of the ElGamal public key cryptosystem?

A.
It is based on the discrete logarithm problem.
Choose a prime number, p.
Select a random number, j, such that j is relatively prime to p-1. Recall that two numbers are
relatively prime if they have no common factors other than 1.
Select a random number, j, such that j is relatively prime to p-1. The value of j must not be
disclosed. Generate w = g j mod p.
It is required for B2 class systems in order to protect against covert storage channels.

B.
It can be used to generate digital signatures.
Choose two random numbers, g and x (g and x must both be less than p).
Generate w = g j mod p and z = y j M mod p.
Solve for z in the equation M = (xw + jz) mod (p-1). The solution to this equation is beyond the
scope of this coverage. Suffice to say that an algorithm exists to solve for the variable z.
It is an operational assurance requirement that is specified in the Orange Book.

C.
It can perform encryption, but not digital signatures.
Calculate y = g x mod p.
w and z comprise the ciphertext. To decrypt the message, M, in the ElGamal system, calculate M =
z/w xmod p. This can be shown by substituting the values of z and w in the equation as follows: M =
y j M mod p/ g jx mod p Since y j = g xj mod p M = (g xj M / g jx ) mod p To sign a message, M, in the
ElGamal system:
w and z comprise the signature.
It is required for B3 class systems to protect against both covert storage and covert timing
channels.

D.
It can perform encryption.
The private key is x and the public key is y, g, and p. To encrypt a message, M, in the ElGamal
system:
Verification of the signature is accomplished if g M mod p = y w w z mod p.
QUESTION 1649
Which is NOT true about Covert Channel Analysis?
It is required for B2 class systems to protect against covert timing channels.

A.
It is based on the discrete logarithm problem.
Choose a prime number, p.
Select a random number, j, such that j is relatively prime to p-1. Recall that two numbers are
relatively prime if they have no common factors other than 1.
Select a random number, j, such that j is relatively prime to p-1. The value of j must not be
disclosed. Generate w = g j mod p.
It is required for B2 class systems in order to protect against covert storage channels.

B.
It can be used to generate digital signatures.
Choose two random numbers, g and x (g and x must both be less than p).
Generate w = g j mod p and z = y j M mod p.
Solve for z in the equation M = (xw + jz) mod (p-1). The solution to this equation is beyond the
scope of this coverage. Suffice to say that an algorithm exists to solve for the variable z.
It is an operational assurance requirement that is specified in the Orange Book.

C.
It can perform encryption, but not digital signatures.
Calculate y = g x mod p.
w and z comprise the ciphertext. To decrypt the message, M, in the ElGamal system, calculate M =
z/w xmod p. This can be shown by substituting the values of z and w in the equation as follows: M =
y j M mod p/ g jx mod p Since y j = g xj mod p M = (g xj M / g jx ) mod p To sign a message, M, in the
ElGamal system:
w and z comprise the signature.
It is required for B3 class systems to protect against both covert storage and covert timing
channels.

D.
It can perform encryption.
The private key is x and the public key is y, g, and p. To encrypt a message, M, in the ElGamal
system:
Verification of the signature is accomplished if g M mod p = y w w z mod p.
QUESTION 1649
Which is NOT true about Covert Channel Analysis?
It is required for B2 class systems to protect against covert timing channels.

A.
It is based on the discrete logarithm problem.
Choose a prime number, p.
Select a random number, j, such that j is relatively prime to p-1. Recall that two numbers are
relatively prime if they have no common factors other than 1.
Select a random number, j, such that j is relatively prime to p-1. The value of j must not be
disclosed. Generate w = g j mod p.
It is required for B2 class systems in order to protect against covert storage channels.

B.
It can be used to generate digital signatures.
Choose two random numbers, g and x (g and x must both be less than p).
Generate w = g j mod p and z = y j M mod p.
Solve for z in the equation M = (xw + jz) mod (p-1). The solution to this equation is beyond the
scope of this coverage. Suffice to say that an algorithm exists to solve for the variable z.
It is an operational assurance requirement that is specified in the Orange Book.

C.
It can perform encryption, but not digital signatures.
Calculate y = g x mod p.
w and z comprise the ciphertext. To decrypt the message, M, in the ElGamal system, calculate M =
z/w xmod p. This can be shown by substituting the values of z and w in the equation as follows: M =
y j M mod p/ g jx mod p Since y j = g xj mod p M = (g xj M / g jx ) mod p To sign a message, M, in the
ElGamal system:
w and z comprise the signature.
It is required for B3 class systems to protect against both covert storage and covert timing
channels.

A.
It is based on the discrete logarithm problem.
Choose a prime number, p.
Select a random number, j, such that j is relatively prime to p-1. Recall that two numbers are
relatively prime if they have no common factors other than 1.
Select a random number, j, such that j is relatively prime to p-1. The value of j must not be
disclosed. Generate w = g j mod p.
It is required for B2 class systems in order to protect against covert storage channels.

B.
It can be used to generate digital signatures.
Choose two random numbers, g and x (g and x must both be less than p).
Generate w = g j mod p and z = y j M mod p.
Solve for z in the equation M = (xw + jz) mod (p-1). The solution to this equation is beyond the
scope of this coverage. Suffice to say that an algorithm exists to solve for the variable z.
It is an operational assurance requirement that is specified in the Orange Book.

C.
It can perform encryption, but not digital signatures.
Calculate y = g x mod p.
w and z comprise the ciphertext. To decrypt the message, M, in the ElGamal system, calculate M =
z/w xmod p. This can be shown by substituting the values of z and w in the equation as follows: M =
y j M mod p/ g jx mod p Since y j = g xj mod p M = (g xj M / g jx ) mod p To sign a message, M, in the
ElGamal system:
w and z comprise the signature.
It is required for B3 class systems to protect against both covert storage and covert timing
channels.

D.
It can perform encryption.
The private key is x and the public key is y, g, and p. To encrypt a message, M, in the ElGamal
system:
Verification of the signature is accomplished if g M mod p = y w w z mod p.
QUESTION 1649
Which is NOT true about Covert Channel Analysis?
It is required for B2 class systems to protect against covert timing channels.

A.
It is based on the discrete logarithm problem.
Choose a prime number, p.
Select a random number, j, such that j is relatively prime to p-1. Recall that two numbers are
relatively prime if they have no common factors other than 1.
Select a random number, j, such that j is relatively prime to p-1. The value of j must not be
disclosed. Generate w = g j mod p.
It is required for B2 class systems in order to protect against covert storage channels.

B.
It can be used to generate digital signatures.
Choose two random numbers, g and x (g and x must both be less than p).
Generate w = g j mod p and z = y j M mod p.
Solve for z in the equation M = (xw + jz) mod (p-1). The solution to this equation is beyond the
scope of this coverage. Suffice to say that an algorithm exists to solve for the variable z.
It is an operational assurance requirement that is specified in the Orange Book.

C.
It can perform encryption, but not digital signatures.
Calculate y = g x mod p.
w and z comprise the ciphertext. To decrypt the message, M, in the ElGamal system, calculate M =
z/w xmod p. This can be shown by substituting the values of z and w in the equation as follows: M =
y j M mod p/ g jx mod p Since y j = g xj mod p M = (g xj M / g jx ) mod p To sign a message, M, in the
ElGamal system:
w and z comprise the signature.
It is required for B3 class systems to protect against both covert storage and covert timing
channels.

D.
It can perform encryption.
The private key is x and the public key is y, g, and p. To encrypt a message, M, in the ElGamal
system:
Verification of the signature is accomplished if g M mod p = y w w z mod p.
QUESTION 1649
Which is NOT true about Covert Channel Analysis?
It is required for B2 class systems to protect against covert timing channels.

Explanation:

The ElGamal public key cryptosystem can perform both encryption and digital signatures based on
the discrete logarithm problem. These three characteristics are shown in the examples that follow.
To generate a key pair in the ElGamal system:

The correct answer is “It is required for B2 class systems to protect against covert timing channels”.
Orange Book B2 class systems do not need to be protected from covert timing channels. Covert
channel analysis must be performed for B2-level class systems to protect against covert storage
channels only. B3 class systems need to be protected against both covert storage channels and
covert timing channels.