v0.0.1 alpha

src/CaptchaKit.AspNetCore/CaptchaKitService.cs

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+using System.Security.Cryptography;
+using System.Text;
+using CaptchaKit;
+using Microsoft.Extensions.Caching.Distributed;
+using Microsoft.Extensions.DependencyInjection;
+using SkiaSharp;
+
+namespace CaptchaKit.AspNetCore;
+
+public sealed class CaptchaKitOptions
+{
+    public int CodeLength { get; set; } = 5;
+    public TimeSpan Lifetime { get; set; } = TimeSpan.FromMinutes(2);
+    public string CacheKeyPrefix { get; set; } = "captchakit:";
+}
+
+public interface ICaptchaKitService
+{
+    Task<CaptchaChallenge> CreateAsync(CancellationToken cancellationToken = default);
+    Task<bool> VerifyAsync(string id, string answer, CancellationToken cancellationToken = default);
+}
+
+public sealed class CaptchaKitService(
+    ICaptchaGenerator generator,
+    ICaptchaChallengeStore store,
+    CaptchaKitOptions options) : ICaptchaKitService
+{
+    public async Task<CaptchaChallenge> CreateAsync(CancellationToken cancellationToken = default)
+    {
+        var id = Convert.ToHexString(RandomNumberGenerator.GetBytes(24));
+        var expiresAt = DateTimeOffset.UtcNow.Add(options.Lifetime);
+        var generated = generator.Generate(new CaptchaGenerationRequest(id, expiresAt, options.CodeLength));
+        if (!string.Equals(generated.Challenge.Id, id, StringComparison.Ordinal))
+            throw new InvalidOperationException("The captcha generator must preserve the requested challenge ID.");
+        await store.StoreAsync(id, Hash(id, Normalize(generated.Answer)), expiresAt, cancellationToken);
+        return generated.Challenge;
+    }
+
+    public async Task<bool> VerifyAsync(string id, string answer, CancellationToken cancellationToken = default)
+    {
+        if (string.IsNullOrWhiteSpace(id) || string.IsNullOrWhiteSpace(answer))
+            return false;
+        var expected = await store.TakeAsync(id, cancellationToken);
+        return expected is not null && FixedEquals(expected, Hash(id, Normalize(answer)));
+    }
+
+    private static string Normalize(string value) => value.Trim().ToUpperInvariant();
+    private static string Hash(string id, string answer) =>
+        Convert.ToHexString(SHA256.HashData(Encoding.UTF8.GetBytes($"{id}:{answer}")));
+    private static bool FixedEquals(string left, string right) =>
+        CryptographicOperations.FixedTimeEquals(Encoding.UTF8.GetBytes(left), Encoding.UTF8.GetBytes(right));
+}
+
+public sealed class DistributedCaptchaChallengeStore(
+    IDistributedCache cache,
+    CaptchaKitOptions options) : ICaptchaChallengeStore
+{
+    public Task StoreAsync(string id, string verificationValue, DateTimeOffset expiresAt, CancellationToken cancellationToken = default) =>
+        cache.SetStringAsync(Key(id), verificationValue, new DistributedCacheEntryOptions
+        {
+            AbsoluteExpiration = expiresAt
+        }, cancellationToken);
+
+    public async Task<string?> TakeAsync(string id, CancellationToken cancellationToken = default)
+    {
+        var key = Key(id);
+        var answer = await cache.GetStringAsync(key, cancellationToken);
+        await cache.RemoveAsync(key, cancellationToken);
+        return answer;
+    }
+
+    private string Key(string id) => options.CacheKeyPrefix + id;
+}
+
+public sealed class SkiaTextCaptchaGenerator : ICaptchaGenerator
+{
+    private const string Alphabet = "ABCDEFGHJKLMNPQRSTUVWXYZ23456789";
+
+    public GeneratedCaptcha Generate(CaptchaGenerationRequest request)
+    {
+        var answer = string.Concat(Enumerable.Range(0, request.CodeLength)
+            .Select(_ => Alphabet[RandomNumberGenerator.GetInt32(Alphabet.Length)]));
+        using var bitmap = new SKBitmap(160, 54);
+        using var canvas = new SKCanvas(bitmap);
+        canvas.Clear(new SKColor(244, 248, 252));
+        DrawNoise(canvas, 160, 54, 13);
+        using var paint = new SKPaint { IsAntialias = true };
+        using var font = new SKFont(SKTypeface.FromFamilyName("Arial", SKFontStyle.Bold), 30);
+        for (var index = 0; index < answer.Length; index++)
+        {
+            var x = 14 + index * 29 + RandomNumberGenerator.GetInt32(-2, 3);
+            var y = 38 + RandomNumberGenerator.GetInt32(-4, 5);
+            paint.Color = RandomColor(35, 145);
+            canvas.Save();
+            canvas.RotateDegrees(RandomNumberGenerator.GetInt32(-20, 21), x, y);
+            canvas.DrawText(answer[index].ToString(), x, y, SKTextAlign.Left, font, paint);
+            canvas.Restore();
+        }
+        DrawNoise(canvas, 160, 54, 8);
+        using var image = SKImage.FromBitmap(bitmap);
+        using var data = image.Encode(SKEncodedImageFormat.Png, 100);
+        var challenge = new CaptchaChallenge(request.Id, "image/png", data.ToArray(), request.ExpiresAt);
+        return new GeneratedCaptcha(challenge, answer);
+    }
+
+    private static void DrawNoise(SKCanvas canvas, int width, int height, int count)
+    {
+        using var paint = new SKPaint { IsAntialias = true, StrokeWidth = 1 };
+        for (var index = 0; index < count; index++)
+        {
+            paint.Color = new SKColor(RandomByte(), RandomByte(), RandomByte(), (byte)RandomNumberGenerator.GetInt32(50, 130));
+            canvas.DrawLine(RandomNumberGenerator.GetInt32(width), RandomNumberGenerator.GetInt32(height), RandomNumberGenerator.GetInt32(width), RandomNumberGenerator.GetInt32(height), paint);
+        }
+    }
+
+    private static SKColor RandomColor(int min, int max) => new(
+        (byte)RandomNumberGenerator.GetInt32(min, max),
+        (byte)RandomNumberGenerator.GetInt32(min, max),
+        (byte)RandomNumberGenerator.GetInt32(min, max));
+    private static byte RandomByte() => RandomNumberGenerator.GetBytes(1)[0];
+}
+
+public static class CaptchaKitServiceCollectionExtensions
+{
+    public static IServiceCollection AddCaptchaKit(
+        this IServiceCollection services,
+        Action<CaptchaKitOptions>? configure = null)
+    {
+        var options = new CaptchaKitOptions();
+        configure?.Invoke(options);
+        if (options.CodeLength is < 4 or > 8 ||
+            options.Lifetime < TimeSpan.FromSeconds(30) ||
+            options.Lifetime > TimeSpan.FromMinutes(10))
+            throw new ArgumentOutOfRangeException(nameof(configure), "Code length must be 4–8 and lifetime must be 30 seconds–10 minutes.");
+        services.AddSingleton(options);
+        services.AddSingleton<ICaptchaGenerator, SkiaTextCaptchaGenerator>();
+        services.AddSingleton<ICaptchaChallengeStore, DistributedCaptchaChallengeStore>();
+        services.AddSingleton<ICaptchaKitService, CaptchaKitService>();
+        return services;
+    }
+}