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.