CompTIA Security+ Question J-100

Which of the following BEST describes part of the PKI process?

A. User1 decrypts data with User2’s private key
B. User1 hashes data with User2’s public key
C. User1 hashes data with User2’s private key
D. User1 encrypts data with User2’s public key

Answer: D

Explanation:
In a PKI the sender encrypts the data using the receiver’s public key. The receiver decrypts the data using his own private key. PKI is a two-key, asymmetric system with four main components: certificate authority (CA), registration authority (RA), RSA (the encryption algorithm), and digital certificates. Messages are encrypted with a public key and decrypted with a private key. A PKI example: You want to send an encrypted message to Jordan, so you request his public key. Jordan responds by sending you that key. You use the public key he sends you to encrypt the message. You send the message to him.

Jordan uses his private key to decrypt the message.

CompTIA Security+ Question J-55

In order to securely communicate using PGP, the sender of an email must do which of the following when sending an email to a recipient for the first time?

A. Import the recipient’s public key
B. Import the recipient’s private key
C. Export the sender’s private key
D. Export the sender’s public key

Answer: A

Explanation:
See step 4 below.

1.

When a user encrypts plaintext with PGP, PGP first compresses the plaintext.

2.

PGP then creates a session key, which is a one-time-only secret key.

3.

This session key works with a very secure, fast conventional encryption algorithm to encrypt the plaintext; the result is ciphertext.

4.

Once the data is encrypted, the session key is then encrypted to the recipient’s public key. This public key-encrypted session key is transmitted along with the ciphertext to the recipient.

CompTIA Security+ Question H-70

Company A sends a PGP encrypted file to company B. If company A used company B’s public key to encrypt the file, which of the following should be used to decrypt data at company B?

A. Registration
B. Public key
C. CRLs
D. Private key

Answer: D

Explanation:
In a PKI the sender encrypts the data using the receiver’s public key. The receiver decrypts the data using his own private key.

PKI is a two-key, asymmetric system with four main components: certificate authority (CA), registration authority (RA), RSA (the encryption algorithm), and digital certificates. Messages are encrypted with a public key and decrypted with a private key. A PKI example: You want to send an encrypted message to Jordan, so you request his public key. Jordan responds by sending you that key. You use the public key he sends you to encrypt the message. You send the message to him. Jordan uses his private key to decrypt the message.

CompTIA Security+ Question G-27

Which of the following is true about PKI? (Select TWO).

A. When encrypting a message with the public key, only the public key can decrypt it.
B. When encrypting a message with the private key, only the private key can decrypt it.
C. When encrypting a message with the public key, only the CA can decrypt it.
D. When encrypting a message with the public key, only the private key can decrypt it.
E. When encrypting a message with the private key, only the public key can decrypt it.

Answer: D,E

Explanation:
E: You encrypt data with the private key and decrypt with the public key, though the opposite is much more frequent. Public-key cryptography, also known as asymmetric cryptography, is a class of cryptographic protocols based on algorithms that require two separate keys, one of which is secret (or private) and one of which is public. Although different, the two parts of this key pair are mathematically linked.

D: In a PKI the sender encrypts the data using the receiver’s public key. The receiver decrypts the data using his own private key. PKI is a two-key, asymmetric system with four main components: certificate authority (CA), registration authority (RA), RSA (the encryption algorithm), and digital certificates. Messages are encrypted with a public key and decrypted with a private key. A PKI example: You want to send an encrypted message to Jordan, so you request his public key. Jordan responds by sending you that key. You use the public key he sends you to encrypt the message. You send the message to him.

Jordan uses his private key to decrypt the message.

CompTIA Security+ Question E-80

Peter must send Ann a message and provide Ann with assurance that he was the actual sender. Which of the following will Peter need to use to BEST accomplish the objective?

A. A pre-shared private key
B. His private key
C. Ann’s public key
D. His public key

Answer: B

Explanation:
To achieve both authentication and confidentiality, Peter should include Ann’s name in the message, sign it using his private key, and then encrypt both the message and the signature using Ann’s public key.