CompTIA Security+ Question K-71

Ann wants to send a file to Peter using PKI. Which of the following should Ann use in order to sign the file?

A. Peter’s public key
B. Peter’s private key
C. Ann’s public key
D. Ann’s private key

Answer: D

Explanation:
The sender uses his private key, in this case Ann’s private key, to create a digital signature. The message is, in effect, signed with the private key. The sender then sends the message to the receiver. The receiver uses the public key attached to the message to validate the digital signature. If the values match, the receiver knows the message is authentic. The receiver uses a key provided by the sender—the public key—to decrypt the message. Most digital signature implementations also use a hash to verify that the message has not been altered, intentionally or accidently, in transit.

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 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 H-58

Protecting the confidentiality of a message is accomplished by encrypting the message with which of the following?

A. Sender’s private key
B. Recipient’s public key
C. Sender’s public key
D. Recipient’s private key

Answer: B

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

Topic 7, Mixed Questions

CompTIA Security+ Question G-56

An SSL/TLS private key is installed on a corporate web proxy in order to inspect HTTPS requests.
Which of the following describes how this private key should be stored so that it is protected from theft?

A. Implement full disk encryption
B. Store on encrypted removable media
C. Utilize a hardware security module
D. Store on web proxy file system

Answer: C

Explanation:
Hardware Security Module (HSM) hardware-based encryption solution that is usually used in conjunction with PKI to enhance security with certification authorities (CAs). It is available as an expansion card and can cryptographic keys, passwords, or certificates.

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 F-14

Which of the following is true about an email that was signed by User A and sent to User B?

A. User A signed with User B’s private key and User B verified with their own public key.
B. User A signed with their own private key and User B verified with User A’s public key.
C. User A signed with User B’s public key and User B verified with their own private key.
D. User A signed with their own public key and User B verified with User A’s private key.

Answer: B

Explanation:
The sender uses his private key, in this case User A’s private key, to create a digital signature. The message is, in effect, signed with the private key. The sender then sends the message to the receiver. The receiver (User B) uses the public key attached to the message to validate the digital signature. If the values match, the receiver knows the message is authentic. The receiver uses a key provided by the sender—the public 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.