A message is encrypted using a private key and sent to an individual using asymmetric encryption.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2022). The mark or mark type is shown in brackets.

1. [2] One mark for each correct point (Max 2)
2. • A private key is the unpublished/secret key/never transmitted anywhere.
3. • It has a matching public key
4. • It is used to decrypt data that was encrypted with its matching public key.
5. [2] One mark for each correct point (Max 2)
6. • The message to be sent is encrypted using the recipient's public key. // The message to be sent is encrypted using the sender's private key.
7. • The message is decrypted using the recipient's private key. // The message is decrypted using the sender's public key.
8. [4] One mark for each correct point (Max 4)
9. • The message together with the digital signature is decrypted using the receiver's private key
10. • The digital signature received is decrypted with the sender's public key to recover the message digest sent
11. • The decrypted message received is hashed with the agreed hashing algorithm to reproduce the message digest of the message received
12. • The two message digests are compared
13. • ... if both digests are the same the message has not been altered // if they are different the message has been altered.