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tilte: information security
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* ### 1. [5%] What is the difference between passive and active security threats?
Answer:
* A passive attack attempts to learn or make use of information from the system but does not affect system resources.
* Passive attacks are in the nature of eavesdropping on, or monitoring of, transmissions. The goal of the opponent is to obtain information that is being transmitted. Two types of passive attacks are the release of message contents and traffic analysis.
* Passive attacks are very difficult to detect, because they do not involve any alteration of the data. Typically, the message traffic is sent and received in an apparently normal fashion, and neither the sender nor receiver is aware that a third party has read the messages or observed the traffic pattern. However, it is feasible to prevent the success of these attacks, usually by means of encryption. Thus, the emphasis in dealing with passive attacks is on prevention rather than detection.

* An active attack attempts to alter system resources or affect their operation.
* Active attacks involve some modification of the data stream or the creation of a false stream and can be subdivided into four categories: masquerade, replay, modification of messages, and denial of service
* Active attacks present the opposite characteristics of passive attacks. Whereas passive attacks are difficult to detect, measures are available to prevent their success. On the other hand, it is quite difficult to prevent active attacks absolutely because of the wide variety of potential physical, software, and network vulnerabilities. Instead, the goal is to detect active attacks and to recover from any disruption or delays caused by them. If the detection has a deterrent effect, it may also contribute to prevention.

* ### 2. [5%] What are the essential ingredients of a symmetric cipher?
Answer:
* A symmetric cipher has five ingredients:
* Plaintext: This is the original intelligible message or data that is fed into the algorithm as input.
* Encryption algorithm: The encryption algorithm performs various substitutions and transformations on the plaintext.
* Secret key: The secret key is also input to the encryption algorithm. The key is a value independent of the plaintext and of the algorithm. The algorithm will produce a different output depending on the specific key being used at the time. The exact substitutions and transformations performed by the algorithm depend on the key.
* Ciphertext: This is the scrambled message produced as output. It depends on the plaintext and the secret key. For a given message, two different keys will produce two different ciphertexts. The ciphertext is an apparently random stream of data and, as it stands, is unintelligible.
* Decryption algorithm: This is essentially the encryption algorithm run in reverse. It takes the ciphertext and the secret key and produces the original plaintext.

* ### 3. There are several types of attacks on encrypted messages, such as ciphertext-only, chosen-ciphertext, known-plaintext, chosen-plaintext, and chosen-text attacks.
* (a) [5%] Which type of attacks is the easiest to defend? Why?
* (b) [5%] If you want to design a system, which type of attacks do you have to consider making the system more secure? Please describe the definition of the type you choose and show the reasons.

Answer:
* (a) The ciphertext-only attack is the easiest to defend against because the opponent has the least amount of information to work with.
* (b) If you want to make the system more secure, you should consider the attack for "Chosen Text".
* In Chosen Text attack, Cryptanalyst know:
* Encryption algorithm
* Ciphertext
* Plaintext message chosen by cryptanalyst, together with its corresponding ciphertext generated with the secret key
* Ciphertext chosen by cryptanalyst, together with its corresponding decrypted plaintext generated with the secret key

With so many resources, if the Cryptanalyst still can not crack the encryption, then we can make sure this system have a good ability to secure our information.

* ### 4. Please answer the questions below:
* (a) [5%] Please describe the advantages and disadvantages of using a symmetric encryption and an asymmetric encryption.
* (b) [5%] In what kind of scenarios can we use these two encryptions mentioned above? Why?

Answer:
(a)
ADVANTAGES AND DISADVANTAGES OF SYMMETRIC CRYPTOSYSTEMS
ADVANTAGES
• A symmetric cryptosystem is faster.
• In Symmetric Cryptosystems, encrypted data can be transferred on the link
even if there is a possibility that the data will be intercepted. Since there is no
key transmiited with the data, the chances of data being decrypted are null.
• A symmetric cryptosystem uses password authentication to prove the
receiver’s identity.
• A system only which possesses the secret key can decrypt a message.
DISADVANTAGES
• Symmetric cryptosystems have a problem of key transportation. The secret
key is to be transmitted to the receiving system before the actual message is to
be transmitted. Every means of electronic communication is insecure as it is
impossible to guarantee that no one will be able to tap communication
channels. So the only secure way of exchanging keys would be exchanging
them personally.
• Cannot provide digital signatures that cannot be repudiated
ADVANTAGES AND DISADVANTAGES OF ASYMMETRIC
CRYPTOSYSTEM
ADVANTAGES
• In asymmetric or public key, cryptography there is no need for exchanging
keys, thus eliminating the key distribution problem.
• The primary advantage of public-key cryptography is increased security: the
private keys do not ever need to be transmitted or revealed to anyone.
• Can provide digital signatures that can be repudiated
DISADVANTAGES
• A disadvantage of using public-key cryptography for encryption is speed:
there are popular secret-key encryption methods which are significantly faster
than any currently available public-key encryption method.

symmetric：保護訊息
asymmetric：保護訊息、金鑰交換、數位簽章

對稱加密
鑑於其具有更快的運算速度，對稱加密在現代計算機系統中被廣泛用於保護信息。 例如，美國政府使用高級加密標準（AES）來加密和分類和感信息。 AES取代了之前的數據加密標準（DES），後者是在20世紀70年代開發的，一直作為對稱加密的標準。

非對稱加密
非對稱加密通常用於大量用戶需要同時加密和解密消息或數據的系統中，尤其是在運算速度和計算資源充足的情況下。該系統的一個常用案例就是加密電子郵件，其中公鑰可以用於加密消息，私鑰可以用於解密。

### 5. [5%] What is the message authentication code? [5%] In what ways can a hash value be secured to provide message authentication?
Answer:
message authentication code (MAC), also known as a keyed hash function. Typically, MACs are used between two parties that share a secret key to authenticate information exchanged between those parties. A MAC function takes as input a secret key and a data block and produces a hash value, referred to as the MAC, which is associated with the protected message. If the integrity of the message needs to be checked, the MAC function can be applied to the message and the result compared with the associated MAC value. An attacker who alters the message will be unable to alter the associated MAC value without knowledge of the secret key. Note that the verifying party also knows who the sending party is because no one else knows the secret key.

Hash codes can be secured to become a MAC in various ways: HMAC, CBC-MAC and
CMAC are examples. HMAC is MACs based on hash function.

page. 343

### 6. [10%] Let 𝑝=233,𝑞=173, and 𝑛=𝑝𝑞=40309. Please find the smallest positive integer 𝑑 such that (10$^23)$)𝑑=10 (mod 𝑛). You should write down the details of the calculation step by step.
Answer:
page. 295

### 7. [10%] A and B want to communicate with each other. How can A send a message to B but only B can see it, and B can confirm that the message was sent by A? Please explain why your method can achieve these requirements.
Answer：
一開始我跟小明先交換公開金鑰(這樣我們都有對方的金鑰，遇到非對稱式問題，先以這當第一步思考比較簡單)，然後我用小明的公鑰把訊息加密，再用我自己的私鑰對該封訊息簽名(數位簽章)，小明收到後，先用我的公鑰確認這個簽名是我的沒錯，再用他的私鑰解開內容，便完成了訊息只有小明能看到，且確認是我本人寄給他的雙重保障!!

### 8. [10%] Please describe how to use public-key cryptosystems to distribute a symmetric secret key between A and B. You should ensure confidentiality and authentication in your scheme and explain why they can be achieved.
Answer:
page. 453

### 9. [10%] With DSS, since the value of k is generated for each signature, even if the same message is signed twice on different occasions, the signatures will differ. This is not true in the RSA signature scheme. What are the practical implications of the difference?

### 10. Please answer the questions below:
* (a) [5%] What is an Adaptive Chosen Message attack on a digital signature?
* (b) [5%] What is an Existential Forgery in a digital signature scheme?
* (c) [5%] What is ElGamal digital signature scheme?
* (d) [5%] What is Diffie-Hellman key exchange protocol?

Answer:
* (a) Adaptive chosen message attack: C is allowed to use A as an “oracle.” This means that C may request from A signatures of messages that depend on previously obtained message-signature pairs.
* (b) C forges a signature for at least one message. C has no control over the message. Consequently, this forgery may only be a minor nuisance to A.
* (c) page. 424
* (d) page. 332