The use of hashes typically comes with the assumption that they cannot be reversed. As such,
they are the perfect fit for storing passwords. Passwords can be verified by hashing the input
and comparing them to the stored hash. As hashing is deterministic, the same hash will always
be generated. Unlike encryption, hashing is not designed to be reversible. If an attacker obtains
the hash of the password, they should be unable to find the original password. However,
rainbow tables have been created for popular hashing functions and passwords. Thus, hashed
passwords that are part of the rainbow table can be quickly looked up. Furthermore, since
hashing is deterministic, two users with the same password would have the same hash. This
gives additional information to an attacker.
To mitigate these issues, a salt can be added to the password. Typically, a different salt is used
per user, causing users with the same password to have different hashes. Furthermore, salted
passwords are much less likely to be found in a rainbow table, increasing the security of the
password hashes.
In this exercise, you will employ MD5, which is an outdated hashing algorithm. The goal of this
exercise is to find collisions using a birthday attack. As the logic is the same, you will be
evaluated using a hashing function with a smaller output space.
First, write a program in C# to find the MD5 hash of a string. Documentation for MD5 in C# is
available here: MD5 Class (System.Security.Cryptography)
To convert a string to a byte array, use the Encoding.UTF8.GetBytes method. You can test your
code using an online MD5 calculator. Strings converted using that method will produce identical
hashes as an MD5 calculator.
A 1 byte salt will be passed to you as a command line argument. You should append this byte to
the end of your byte array before hashing.
For example, suppose you are hashing the string “Hello World!” and are passed the salt C5.
Bytes before hashing: 48 65 6C 6C 6F 20 57 6F 72 6C 64 21 C5
Bytes after hashing: E6 D9 B0 B9 D1 78 B2 40 02 89 EB EA 33 E8 B8 82
You can compare this result to hashing only “Hello World!” to see the difference:
Bytes before hashing (only “Hello World!”): 48 65 6C 6C 6F 20 57 6F 72 6C 64 21
Bytes after hashing: ED 07 62 87 53 2E 86 36 5E 84 1E 92 BF C5 0D 8C.
You will now perform a birthday attack with a function for calculating a hash with salt. As the
standard version of MD5 is time-consuming to attack, you should modify the hashing result to
make it easier to attack. Instead of using the full hash, only compare the first 5 bytes (in this
case, “Hello World!” with the salt C5 hashes to E6 D9 B0 B9 D1).
You may choose the length of the string, but you may only use alphanumeric characters in your
string [A-Z][a-z][0-9]. Your program should output two strings that hash to the same value with
MD5 and the given salt. These strings should be separated by a comma.
Continuing from the earlier example, suppose that the byte C5 is passed in as the salt. Here is a
sample command and output from the program:
Command: dotnet run “C5”
Sample output: AQJCMW0DGL,I95ORWB1A7
In this example, both values hash to 2B 68 3B 65 7A when salted with C5.
Note: There will be many different possible solutions. Your answers will be verified by hashing
them.
Submission Directions for Project Deliverables
Compress your project folder into a .zip archive and submit that file. The autograder will run the
following commands on your archive for testing:
unzip submission_name.zip
cd submission_name
dotnet run
Try these commands before submitting to ensure your submission is properly set up. You can
add the command line arguments from the provided examples to verify that your program works
correctly.
Important Note: Your folder name must match the submission name. For example, if
Coursera asks you to submit a file named P2.zip, running the unzip command as shown should
produce a folder named P2. If you do not follow this convention, the autograder will be unable to
find your project.
Project 2 "submission_name": P2

Use this as a template:

using System;
using System.Linq;
using System.Security.Cryptography;
using System.Text;
using System.Collections.Generic;

namespace P2
{
class Program
{
// This function will help us get the input from the command line
public static string getInputFromCommandLine(string[] args)
{
// get the input from the command line
string input = "";
if (args.Length == 1)
{
input = args[0]; // Gets the first string after the 'dotnet run' command
}
else
{
Console.WriteLine("Not enough or too many inputs provided after 'dotnet run' ");
}
return input;
}

static void Main(string[] args)
{
// Some helpful hints:
// The main idea is to concateneate the salt to a random string,
// then feed that into the hashFunction,
// then keep track of those salted hashes until you find a matching pair of salted hashes,
// then print the solution which is the two strings that gave the matching salted hashes
// NOTE: When I say salted hashes, I mean that you salted the password and then fed it into the hashFunction. So it is the hash of the password+salt (in this case "+" means concatenated together into one)

// https://docs.microsoft.com/en-us/dotnet/api/system.security.cryptography.md5?view=netcore-3.1
// hint: what does Create() do?

// This code will convert a string to a byte array
string example = "Edward Snowden";
byte[] exampleByteArray = Encoding.UTF8.GetBytes(example);

// passwords have to be made only using alphanumeric characters, so you can make random passwords using any of the characters in the string provided below (note: The starter code doesn't include lowercase just for simplicity but you can include lowercase as well. )
string alphanumeric_characters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";

// optional hint: What data structure can you use to store the salted hashes that has a really fast lookup time of O(1) (constant) ?
// You don't have to use this data structure, but it will make your code run fast. The System.Collections.Generic libary is a good place to start

// TODO: Employ the Birthday Paradox to find a collision in the MD5 hash function

// These were given as en example, you are going to have to find two passwords that have matching salted hashes with your code and then output them for the autograder to see
string password1 = "AQJCMW0DGL";
string password2 = "I95ORWB1A7";
Console.WriteLine(password1 + "," + password2); // This is one way to properly format the output for the autograder
}

}
}

Please comment the steps or break down the process to write this program. 

Answer rating: 100% (QA)

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