Cryptography Using .NET: Exercises
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Exercises. From the previous sections, you learned about
encryption, hashing, and digital signatures for use with C# and the .NET
Framework. In this section you will be presented with a number of
exercises requiring you to use the knowledge you learned from the
previous sections along with a number of questions.
If you are submitting your work include your name in the header of
each program for problem 1 - 7. Each program must be named
according to its associated problem, e.g., Problem1.cs. . . Problem7.cs.
Zip all the files into one file and upload it onto the class FTP server.
You must include printouts of each program and answers to each question.
Problem 1.
Encrypt a plaintext with the following options: -AES-CBC mode with PKCS#7
padding mode -128-bit key size and block size.
Key = { 0xFC, 0x02, 0xC6, 0x04,
0x05, 0x06, 0x07, 0x08,
0x27, 0x10, 0x83, 0x12,
0x3B, 0x14, 0x15, 0xAF }
IV = { 0x99, 0x5C, 0x8F, 0x04,
0x05, 0x06, 0x07, 0x08,
0x09, 0x8E, 0x11, 0x12,
0x13, 0x14, 0x15, 0x99 }
Plaintext:
Identification and Two-factor Authentication
What is the Base64-encoded ciphertext?
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Consider the
Convert.ToBase64String(byte[] array) and Convert.FromBase64String(string
s) methods.
Problem 2. Write a decryption program to verify your
answer in Problem 1.
Hints:
- You can add an additional decryption method inside the CryptoClass
class.
- Create a byte array to save the decrypted message, such as
byte[] array1 = new byte[x_cipher_rx.Length];,where x_cipher_rx is the byte array obtained after using
FromBase64String to decode
the received Base64-encoded ciphertext.
- You need to create an instance of MemoryStream initialized with
the byte array x_cipher_rx (we no longer use the default constructor of
MemoryStream here).
- Use the CreateDecryptormethod similar to the CreateEncryptor
method in the encryption example, and also use Readinstead of Write
here.
Problem 3. Given ciphertext and plaintext use the
information provided in Problem 1 except for the key to find the last
missing three bytes of the key.
Base64-encoded ciphertext:
T6+b7B9/iXa5ennZ+dpG/mmCURaBiTthLHitnmZN5vY=
Plaintext:
Central Intelligence Agency
Partial key (first 13 bytes):
Partial_Key = { 0xFC, 0x02, 0xC6, 0x04,
0x05, 0x06, 0x07, 0x08,
0x27, 0x10, 0x83, 0x12,
0x3B }
What are the last three bytes of the key in Hexadecimal form?
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Problem 4. Given a message you must compute the hash.
Message:
Programming .NET Security Part I
What is the hash code?
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Comment on the results.
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Change the message to:
Programming .NET Security Part V
What is the hash code?
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Comment on the results.
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Problem 5: Keyed Hashing Algorithm. Compute the
HMACSHA1 hash code using the given message.
Message: Martin Luther King, Jr.
Key: I Have a Dream
What is the output hash code?
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Problem 6: DSA Digital Signature. Write a DSA digital
signature program to show the following processes.
1. Alice signs the message:
Digital Signature and Electronic Authentication Law
2. She exports her public parameters using the ExportParameters
method.
3. Bob receives the signature and imports the public parameters
4. Bob verifies the signature.
The output printout must show both the signature and the byte arrays of
the random number P and Q, which are Alice’s public parameters.
Problem 7: RSA Digital Signature. Write a RSA (with SHA-1 hashing algorithm) digital signature program to
show the following processes:
1. Alice signs the message:
Public-Key Cryptography Standards (PKCS)
2. Do the same as in Problem 6, but use the XML parameter
exporting/importing instead.
The output must show both the signature and its associated XML
string of public keys. You will see the Modulus and Exponent values
inside this XML string.
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