Add DeviceTokenService (random token, SHA-256 hash, fixed-time verify)

backend.tests/DeviceTokenServiceTests.cs

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+using backend.Devices;
+using FluentAssertions;
+using Xunit;
+
+namespace backend.tests;
+
+public class DeviceTokenServiceTests
+{
+    private readonly DeviceTokenService _svc = new();
+
+    [Fact]
+    public void Generate_returns_url_safe_token_of_expected_length()
+    {
+        var (plaintext, hash) = _svc.Generate();
+        plaintext.Should().MatchRegex("^[A-Za-z0-9_-]+$");
+        plaintext.Length.Should().BeGreaterThanOrEqualTo(40);
+        hash.Should().NotBeNullOrEmpty();
+        hash.Should().NotBe(plaintext);
+    }
+
+    [Fact]
+    public void Hash_is_deterministic_for_same_input()
+    {
+        var (plaintext, hash) = _svc.Generate();
+        _svc.Hash(plaintext).Should().Be(hash);
+    }
+
+    [Fact]
+    public void Verify_accepts_matching_plaintext_and_rejects_others()
+    {
+        var (plaintext, hash) = _svc.Generate();
+        _svc.Verify(plaintext, hash).Should().BeTrue();
+        _svc.Verify(plaintext + "x", hash).Should().BeFalse();
+        _svc.Verify("totally-different", hash).Should().BeFalse();
+    }
+}

backend/Devices/DeviceTokenService.cs

@@ -0,0 +1,36 @@
+using System.Security.Cryptography;
+using System.Text;
+
+namespace backend.Devices;
+
+public class DeviceTokenService
+{
+    public (string Plaintext, string Hash) Generate()
+    {
+        Span<byte> bytes = stackalloc byte[32];
+        RandomNumberGenerator.Fill(bytes);
+        var plaintext = Base64UrlEncode(bytes);
+        return (plaintext, Hash(plaintext));
+    }
+
+    public string Hash(string plaintext)
+    {
+        Span<byte> hash = stackalloc byte[32];
+        SHA256.HashData(Encoding.UTF8.GetBytes(plaintext), hash);
+        return Convert.ToHexString(hash);
+    }
+
+    public bool Verify(string plaintext, string expectedHash)
+    {
+        var actual = Hash(plaintext);
+        return CryptographicOperations.FixedTimeEquals(
+            Encoding.UTF8.GetBytes(actual),
+            Encoding.UTF8.GetBytes(expectedHash));
+    }
+
+    private static string Base64UrlEncode(ReadOnlySpan<byte> bytes)
+        => Convert.ToBase64String(bytes)
+            .TrimEnd('=')
+            .Replace('+', '-')
+            .Replace('/', '_');
+}