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public class PasswordGen
{
// Define default min and max password lengths.
private static int DEFAULT_MIN_PASSWORD_LENGTH = 8;
private static int DEFAULT_MAX_PASSWORD_LENGTH = 10;

// Define supported password characters divided into groups.
// You can add (or remove) characters to (from) these groups.
private static string PASSWORD_CHARS_LCASE = "abcdefgijkmnopqrstwxyz";
private static string PASSWORD_CHARS_UCASE = "ABCDEFGHJKLMNPQRSTWXYZ";
private static string PASSWORD_CHARS_NUMERIC = "23456789";
private static string PASSWORD_CHARS_SPECIAL = "()!";

/// <summary>
/// Generates a random password.
/// </summary>
/// <returns>
/// Randomly generated password.
/// </returns>
/// <remarks>
/// The length of the generated password will be determined at
/// random. It will be no shorter than the minimum default and
/// no longer than maximum default.
/// </remarks>
public static string Generate() {
return Generate(DEFAULT_MIN_PASSWORD_LENGTH,
DEFAULT_MAX_PASSWORD_LENGTH);
}

/// <summary>
/// Generates a random password of the exact length.
/// </summary>
/// <param name="length">
/// Exact password length.
/// </param>
/// <returns>
/// Randomly generated password.
/// </returns>
public static string Generate(int length) {
return Generate(length, length);
}

/// <summary>
/// Generates a random password.
/// </summary>
/// <param name="minLength">
/// Minimum password length.
/// </param>
/// <param name="maxLength">
/// Maximum password length.
/// </param>
/// <returns>
/// Randomly generated password.
/// </returns>
/// <remarks>
/// The length of the generated password will be determined at
/// random and it will fall with the range determined by the
/// function parameters.
/// </remarks>
public static string Generate(int minLength,
int maxLength) {
// Make sure that input parameters are valid.
if (minLength <= 0 || maxLength <= 0 || minLength > maxLength)
return null;

// Create a local array containing supported password characters
// grouped by types. You can remove character groups from this
// array, but doing so will weaken the password strength.
char[][] charGroups = new char[][]
{
PASSWORD_CHARS_LCASE.ToCharArray(),
PASSWORD_CHARS_UCASE.ToCharArray(),
PASSWORD_CHARS_NUMERIC.ToCharArray(),
PASSWORD_CHARS_SPECIAL.ToCharArray()
};

// Use this array to track the number of unused characters in each
// character group.
int[] charsLeftInGroup = new int[charGroups.Length];

// Initially, all characters in each group are not used.
for (int i = 0; i < charsLeftInGroup.Length; i++)
charsLeftInGroup = charGroups.Length;

// Use this array to track (iterate through) unused character groups.
int[] leftGroupsOrder = new int[charGroups.Length];

// Initially, all character groups are not used.
for (int i = 0; i < leftGroupsOrder.Length; i++)
leftGroupsOrder = i;

// Because we cannot use the default randomizer, which is based on the
// current time (it will produce the same "random" number within a
// second), we will use a random number generator to seed the
// randomizer.

// Use a 4-byte array to fill it with random bytes and convert it then
// to an integer value.
byte[] randomBytes = new byte[4];

// Generate 4 random bytes.
RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider();
rng.GetBytes(randomBytes);

// Convert 4 bytes into a 32-bit integer value.
int seed = (randomBytes[0] & 0x7f) << 24 |
randomBytes[1] << 16 |
randomBytes[2] << 8 |
randomBytes[3];

// Now, this is real randomization.
Random random = new Random(seed);

// This array will hold password characters.
char[] password = null;

// Allocate appropriate memory for the password.
if (minLength < maxLength)
password = new char[random.Next(minLength, maxLength + 1)];
else
password = new char[minLength];

// Index of the next character to be added to password.
int nextCharIdx;

// Index of the next character group to be processed.
int nextGroupIdx;

// Index which will be used to track not processed character groups.
int nextLeftGroupsOrderIdx;

// Index of the last non-processed character in a group.
int lastCharIdx;

// Index of the last non-processed group.
int lastLeftGroupsOrderIdx = leftGroupsOrder.Length - 1;

// Generate password characters one at a time.
for (int i = 0; i < password.Length; i++) {
// If only one character group remained unprocessed, process it;
// otherwise, pick a random character group from the unprocessed
// group list. To allow a special character to appear in the
// first position, increment the second parameter of the Next
// function call by one, i.e. lastLeftGroupsOrderIdx + 1.
if (lastLeftGroupsOrderIdx == 0)
nextLeftGroupsOrderIdx = 0;
else
nextLeftGroupsOrderIdx = random.Next(0,
lastLeftGroupsOrderIdx);

// Get the actual index of the character group, from which we will
// pick the next character.
nextGroupIdx = leftGroupsOrder[nextLeftGroupsOrderIdx];

// Get the index of the last unprocessed characters in this group.
lastCharIdx = charsLeftInGroup[nextGroupIdx] - 1;

// If only one unprocessed character is left, pick it; otherwise,
// get a random character from the unused character list.
if (lastCharIdx == 0)
nextCharIdx = 0;
else
nextCharIdx = random.Next(0, lastCharIdx + 1);

// Add this character to the password.
password = charGroups[nextGroupIdx][nextCharIdx];

// If we processed the last character in this group, start over.
if (lastCharIdx == 0)
charsLeftInGroup[nextGroupIdx] =
charGroups[nextGroupIdx].Length;
// There are more unprocessed characters left.
else {
// Swap processed character with the last unprocessed character
// so that we don't pick it until we process all characters in
// this group.
if (lastCharIdx != nextCharIdx) {
char temp = charGroups[nextGroupIdx][lastCharIdx];
charGroups[nextGroupIdx][lastCharIdx] =
charGroups[nextGroupIdx][nextCharIdx];
charGroups[nextGroupIdx][nextCharIdx] = temp;
}
// Decrement the number of unprocessed characters in
// this group.
charsLeftInGroup[nextGroupIdx]--;
}

// If we processed the last group, start all over.
if (lastLeftGroupsOrderIdx == 0)
lastLeftGroupsOrderIdx = leftGroupsOrder.Length - 1;
// There are more unprocessed groups left.
else {
// Swap processed group with the last unprocessed group
// so that we don't pick it until we process all groups.
if (lastLeftGroupsOrderIdx != nextLeftGroupsOrderIdx) {
int temp = leftGroupsOrder[lastLeftGroupsOrderIdx];
leftGroupsOrder[lastLeftGroupsOrderIdx] =
leftGroupsOrder[nextLeftGroupsOrderIdx];
leftGroupsOrder[nextLeftGroupsOrderIdx] = temp;
}
// Decrement the number of unprocessed groups.
lastLeftGroupsOrderIdx--;
}
}

// Convert password characters into a string and return the result.
return new string(password);
}
}