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Replace System.Random with cryptographically secure PRNG in Program.Code() #44

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Merged
tuduce merged 4 commits into from
Dec 4, 2025
Merged

Replace System.Random with cryptographically secure PRNG in Program.Code() #44

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f7306bc
Initial plan
Copilot Dec 4, 2025
aacaeb2
Replace System.Random with cryptographically secure HMACSHA256-based …
Copilot Dec 4, 2025
99c3d02
Address code review: Simplify using statements and add modulo bias note
Copilot Dec 4, 2025
9fb5f9c
Simplify SeededCryptoRandom: Remove SHA256 layer, use HMACSHA256 dire…
Copilot Dec 4, 2025
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36 changes: 35 additions & 1 deletion JoinFS.Tests/ProgramCode.cs
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Original file line number Diff line number Diff line change
@@ -1,5 +1,6 @@
using System;
using System.Text;
using System.Security.Cryptography;

namespace JoinFS.Tests
{
Expand All @@ -9,6 +10,39 @@ namespace JoinFS.Tests
/// </summary>
public static class ProgramCode
{
/// <summary>
/// Cryptographically secure random number generator seeded with a key
/// </summary>
private class SeededCryptoRandom
{
private readonly byte[] seed;
private int counter;

public SeededCryptoRandom(int key)
{
seed = BitConverter.GetBytes(key);
counter = 0;
}

/// <summary>
/// Generate a deterministic but cryptographically derived random number in range [0, maxValue)
/// </summary>
public int Next(int maxValue)
{
if (maxValue <= 0)
throw new ArgumentOutOfRangeException(nameof(maxValue));

// Use HMACSHA256 to derive random value from seed and counter
using (var hmac = new HMACSHA256(seed))
{
byte[] counterBytes = BitConverter.GetBytes(counter++);
byte[] hash = hmac.ComputeHash(counterBytes);
uint randomValue = BitConverter.ToUInt32(hash, 0);
return (int)(randomValue % (uint)maxValue);
}
}
}

public static string? Code(string? s, bool bToCode, int nKey)
{
if (s == null)
Expand Down Expand Up @@ -70,7 +104,7 @@ public static class ProgramCode
int nInc = 0;
int k;
const int nPasses = 11;
Random rnd = new Random(nKey);
SeededCryptoRandom rnd = new SeededCryptoRandom(nKey);
for (int j = 0; j < nPasses; ++j)
{
k = (bToCode) ? j : nPasses - j - 1;
Expand Down
36 changes: 35 additions & 1 deletion JoinFS/Program.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -12,6 +12,7 @@
using System.Configuration;
using System.Globalization;
using System.Reflection;
using System.Security.Cryptography;
using JoinFS.Properties;

namespace JoinFS
Expand Down Expand Up @@ -1935,6 +1936,39 @@ public void MonitorAircraftDetails()

static class Program
{
/// <summary>
/// Cryptographically secure random number generator seeded with a key
/// </summary>
private class SeededCryptoRandom
{
private readonly byte[] seed;
private int counter;

public SeededCryptoRandom(int key)
{
seed = BitConverter.GetBytes(key);
counter = 0;
}

/// <summary>
/// Generate a deterministic but cryptographically derived random number in range [0, maxValue)
/// </summary>
public int Next(int maxValue)
{
if (maxValue <= 0)
throw new ArgumentOutOfRangeException(nameof(maxValue));

// Use HMACSHA256 to derive random value from seed and counter
using (var hmac = new HMACSHA256(seed))
{
byte[] counterBytes = BitConverter.GetBytes(counter++);
byte[] hash = hmac.ComputeHash(counterBytes);
uint randomValue = BitConverter.ToUInt32(hash, 0);
return (int)(randomValue % (uint)maxValue);
}
}
}

public static string Code(string s, bool bToCode, int nKey)
{
if (s == null)
Expand Down Expand Up @@ -1996,7 +2030,7 @@ public static string Code(string s, bool bToCode, int nKey)
int nInc = 0;
int k;
const int nPasses = 11;
Random rnd = new Random(nKey);
SeededCryptoRandom rnd = new SeededCryptoRandom(nKey);
for (int j = 0; j < nPasses; ++j)
{
k = (bToCode) ? j : nPasses - j - 1;
Expand Down