WGU D334 Cryptography / ITEC 2141
Introduction to Cryptography
Exam Set Questions With Reviewed
Correct Detailed Answers
New Update!!
1. Describe with two characters (Alice and Bob) the goal of cryptography.
- ANSWER Alice encrypts a plaintext m into a ciphertext c transmitted to Bob
who decrypts it with a secret decryption key k.
2. What is the property of secret keys ? - ANSWER
3. Describe with Alice and Bob the concept of asymmetric methods of encryption.
- ANSWER Alice encrypts m using Bob's public key pk. Bob decrypts the
ciphertext using his secret key sk.
4. What is a personal key ? - ANSWER
,5. What is the upside of symmetric methods? - ANSWER
They are way faster.
6. What is the upside of asymmetric methods - ANSWER
They do not require us to dangerously share the key with the ones we want to
communicate with.
7. What is the other name for asymmetric methods? - ANSWER
Public-key methods.
8. How can we have the advantages of both? - ANSWER
We can use an asymmetric method to safely share the key, and then use the
symmetric method with this key.
9. What is data integrity? - ANSWER
It is when the receiver can check whether the message was modified.
10. What is authentication (not authentification)? - ANSWER It is when the
receiver can verify the origin of the message.
11. What is non-repudiation? - ANSWER It is when the sender cannot deny
having sent a message.
12. What is a medium? - ANSWER It is the support of the messages (computer,
paper). They do not provide safety.
13. What is a MAC? - ANSWER A Message Authentication Code is a sequence
of bits m̄ generated from the message m. We say that m has been augmented,
and we call (m, m̄ ) the augmented message.
,14. How can the receiver check an augmented message (m, m̄ )? - ANSWER
The receiver checks whether this property holds
15. Which safety properties are ensured using digital signatures? - ANSWER
Authentication and non-repudiation
16. Which method (between symmetric and assymetric) is used in order to sign a
message? - ANSWER Public-key methods are used.
17. Describe with Alice and Bob how to sign a message m. - ANSWER
Alice applies an algorithm Sign with her secret key and gets the
signature s=Sign(sk, m). Bob recieves s for the message m. He can check the
signature with a publicly known verification algorithm Verify by doing :
18. Do we sign before or after the "hashing" (seen later)? - ANSWER
We sign after the hashing. This is generally why we use a hashing: We want to
sign a big document, but that would take too long. Therefore, we sign the hash
value of the document
, 19. What is Cryptanalysis? - ANSWER It is the study of attacks against
cryptographic schemes.
20. What is Cryptology? - ANSWER
It is a concept containing cryptography and cryptanalysis.
21. What is Kerckhoff's Principle? - ANSWER The adversary knows all the
details of the cryptosystem, including the algorithms and their implementations.
22. What are passive attacks? - ANSWER
It is an attack that is just trying to get information. It does not try to modify the
message. The one making an attack (Eve) is called an eavesdropper.
23. What are Ciphertext-only attacks? - ANSWER Eve has the ability to obtain
ciphertexts. She wants to deduce the message of a specific ciphertext.
24. What are Known-plaintext attacks? - ANSWER Eve has a list of plaintext-
ciphertext. She wants to deduce the message of a specific ciphertext (which is not
in the list).
25. What are chosen-plaintext attacks? - ANSWER Eve chooses a list of
plaintexts, and obtains a list of corresponding ciphertexts. Then, she looses the
power to get new ciphertexts, and attempts to decrypt an unseen cyphertext.
26. What are adaptively-chosen-plaintext attack? - ANSWER It is like chosen-
plaintext attacks, except that Eve does not loose the power of obtaining
ciphertexts.
27. What are chosen-ciphertext attacks? - ANSWER Eve chooses a list of
ciphertexts, and obtains a list of corresponding plaintexts. Then, she looses the
power to get new plaintexts, and attempts to encrypt an unseen plaintext.
Introduction to Cryptography
Exam Set Questions With Reviewed
Correct Detailed Answers
New Update!!
1. Describe with two characters (Alice and Bob) the goal of cryptography.
- ANSWER Alice encrypts a plaintext m into a ciphertext c transmitted to Bob
who decrypts it with a secret decryption key k.
2. What is the property of secret keys ? - ANSWER
3. Describe with Alice and Bob the concept of asymmetric methods of encryption.
- ANSWER Alice encrypts m using Bob's public key pk. Bob decrypts the
ciphertext using his secret key sk.
4. What is a personal key ? - ANSWER
,5. What is the upside of symmetric methods? - ANSWER
They are way faster.
6. What is the upside of asymmetric methods - ANSWER
They do not require us to dangerously share the key with the ones we want to
communicate with.
7. What is the other name for asymmetric methods? - ANSWER
Public-key methods.
8. How can we have the advantages of both? - ANSWER
We can use an asymmetric method to safely share the key, and then use the
symmetric method with this key.
9. What is data integrity? - ANSWER
It is when the receiver can check whether the message was modified.
10. What is authentication (not authentification)? - ANSWER It is when the
receiver can verify the origin of the message.
11. What is non-repudiation? - ANSWER It is when the sender cannot deny
having sent a message.
12. What is a medium? - ANSWER It is the support of the messages (computer,
paper). They do not provide safety.
13. What is a MAC? - ANSWER A Message Authentication Code is a sequence
of bits m̄ generated from the message m. We say that m has been augmented,
and we call (m, m̄ ) the augmented message.
,14. How can the receiver check an augmented message (m, m̄ )? - ANSWER
The receiver checks whether this property holds
15. Which safety properties are ensured using digital signatures? - ANSWER
Authentication and non-repudiation
16. Which method (between symmetric and assymetric) is used in order to sign a
message? - ANSWER Public-key methods are used.
17. Describe with Alice and Bob how to sign a message m. - ANSWER
Alice applies an algorithm Sign with her secret key and gets the
signature s=Sign(sk, m). Bob recieves s for the message m. He can check the
signature with a publicly known verification algorithm Verify by doing :
18. Do we sign before or after the "hashing" (seen later)? - ANSWER
We sign after the hashing. This is generally why we use a hashing: We want to
sign a big document, but that would take too long. Therefore, we sign the hash
value of the document
, 19. What is Cryptanalysis? - ANSWER It is the study of attacks against
cryptographic schemes.
20. What is Cryptology? - ANSWER
It is a concept containing cryptography and cryptanalysis.
21. What is Kerckhoff's Principle? - ANSWER The adversary knows all the
details of the cryptosystem, including the algorithms and their implementations.
22. What are passive attacks? - ANSWER
It is an attack that is just trying to get information. It does not try to modify the
message. The one making an attack (Eve) is called an eavesdropper.
23. What are Ciphertext-only attacks? - ANSWER Eve has the ability to obtain
ciphertexts. She wants to deduce the message of a specific ciphertext.
24. What are Known-plaintext attacks? - ANSWER Eve has a list of plaintext-
ciphertext. She wants to deduce the message of a specific ciphertext (which is not
in the list).
25. What are chosen-plaintext attacks? - ANSWER Eve chooses a list of
plaintexts, and obtains a list of corresponding ciphertexts. Then, she looses the
power to get new ciphertexts, and attempts to decrypt an unseen cyphertext.
26. What are adaptively-chosen-plaintext attack? - ANSWER It is like chosen-
plaintext attacks, except that Eve does not loose the power of obtaining
ciphertexts.
27. What are chosen-ciphertext attacks? - ANSWER Eve chooses a list of
ciphertexts, and obtains a list of corresponding plaintexts. Then, she looses the
power to get new plaintexts, and attempts to encrypt an unseen plaintext.
