feat(spv): verify the chain identity of a server on connect

After the handshake the wallet checks server.features genesis_hash and,
when the profile has checkpoints, the header at the highest checkpoint
not above the server's tip (mainnet shares Bitcoin's genesis, so the
checkpoint is what tells the chains apart). A server on another chain is
refused with WrongChainException and the next candidate is tried. Used by
the GUI, peer discovery, the CLI and the server probe. See ADR 0036.

tests/PalladiumWallet.Tests/Spv/ChainVerifierTests.cs

@@ -0,0 +1,162 @@
+using NBitcoin.Crypto;
+using PalladiumWallet.Core.Chain;
+using PalladiumWallet.Core.Net;
+using PalladiumWallet.Core.Spv;
+using PalladiumWallet.Tests.Net;
+
+namespace PalladiumWallet.Tests.Spv;
+
+/// <summary>Chain-identity check after the handshake (genesis + highest checkpoint ≤ tip).</summary>
+public class ChainVerifierTests
+{
+    private static readonly TimeSpan Timeout = TimeSpan.FromSeconds(10);
+
+    // 80 arbitrary bytes are a valid "header" for hashing purposes; seed makes them distinct.
+    private static string HeaderHex(int seed) =>
+        Convert.ToHexString(Hashes.SHA256(BitConverter.GetBytes(seed))
+            .Concat(Hashes.SHA256(BitConverter.GetBytes(seed + 1_000)))
+            .Concat(new byte[16]).ToArray()).ToLowerInvariant();
+
+    private static Checkpoint CheckpointFor(int height, int seed) =>
+        new(height, BlockHeaderInfo.Parse(HeaderHex(seed)).Hash.ToString(), 0x1d00ffff);
+
+    private static ChainProfile Profile(params Checkpoint[] checkpoints) =>
+        ChainProfiles.Mainnet with { Checkpoints = checkpoints };
+
+    private static async Task<ElectrumClient> ConnectAsync(FakeElectrumServer s) =>
+        await ElectrumClient.ConnectAsync(s.Host, s.Port, useSsl: false);
+
+    private static void Tip(FakeElectrumServer s, int height) =>
+        s.Handle("blockchain.headers.subscribe", _ => new { height, hex = HeaderHex(0) });
+
+    [Fact]
+    public async Task Accepts_a_server_with_the_expected_genesis_and_checkpoint_header()
+    {
+        await using var server = new FakeElectrumServer();
+        Tip(server, 150);
+        server.Handle("blockchain.block.header", p => p[0].GetInt32() == 100 ? HeaderHex(7) : throw new FakeElectrumError(-1, "no"));
+        await using var client = await ConnectAsync(server);
+
+        await client.VerifyChainAsync(Profile(CheckpointFor(100, 7)), default).WaitAsync(Timeout);
+    }
+
+    [Fact]
+    public async Task Uses_the_highest_checkpoint_not_above_the_tip()
+    {
+        await using var server = new FakeElectrumServer();
+        Tip(server, 150);
+        var asked = new List<int>();
+        server.Handle("blockchain.block.header", p => { asked.Add(p[0].GetInt32()); return HeaderHex(asked[^1]); });
+        await using var client = await ConnectAsync(server);
+
+        await client.VerifyChainAsync(
+            Profile(CheckpointFor(50, 50), CheckpointFor(100, 100), CheckpointFor(200, 200)), default).WaitAsync(Timeout);
+
+        Assert.Equal([100], asked);
+    }
+
+    [Fact]
+    public async Task Rejects_a_different_header_at_the_checkpoint_height()
+    {
+        await using var server = new FakeElectrumServer();
+        Tip(server, 150);
+        server.Handle("blockchain.block.header", _ => HeaderHex(999)); // other chain
+        await using var client = await ConnectAsync(server);
+
+        var ex = await Assert.ThrowsAsync<WrongChainException>(
+            () => client.VerifyChainAsync(Profile(CheckpointFor(100, 7)), default).WaitAsync(Timeout));
+        Assert.Equal(server.Port, ex.Port);
+        Assert.Contains("checkpoint", ex.Message);
+    }
+
+    [Fact]
+    public async Task Rejects_a_different_genesis_hash_before_asking_for_headers()
+    {
+        await using var server = new FakeElectrumServer();
+        server.Handle("server.features", _ => new { genesis_hash = ChainProfiles.Testnet.GenesisHash });
+        await using var client = await ConnectAsync(server);
+
+        await Assert.ThrowsAsync<WrongChainException>(
+            () => client.VerifyChainAsync(Profile(), default).WaitAsync(Timeout));
+        Assert.Equal(0, server.CallCount("blockchain.headers.subscribe"));
+    }
+
+    [Fact]
+    public async Task Genesis_comparison_is_case_insensitive()
+    {
+        await using var server = new FakeElectrumServer();
+        server.Handle("server.features", _ => new { genesis_hash = ChainProfiles.Mainnet.GenesisHash.ToUpperInvariant() });
+        Tip(server, 10);
+        await using var client = await ConnectAsync(server);
+
+        await client.VerifyChainAsync(Profile(), default).WaitAsync(Timeout);
+    }
+
+    [Theory]
+    [InlineData("error")]
+    [InlineData("no-genesis")]
+    [InlineData("not-an-object")]
+    public async Task Rejects_a_server_that_does_not_report_a_genesis_hash(string mode)
+    {
+        await using var server = new FakeElectrumServer();
+        server.Handle("server.features", _ => mode switch
+        {
+            "error" => throw new FakeElectrumError(-32601, "unknown method"),
+            "no-genesis" => new { hash_function = "sha256" },
+            _ => "nope",
+        });
+        await using var client = await ConnectAsync(server);
+
+        await Assert.ThrowsAsync<WrongChainException>(
+            () => client.VerifyChainAsync(Profile(), default).WaitAsync(Timeout));
+    }
+
+    [Fact]
+    public async Task A_tip_below_the_first_checkpoint_is_accepted_on_the_genesis_alone()
+    {
+        await using var server = new FakeElectrumServer();
+        Tip(server, 50);
+        await using var client = await ConnectAsync(server);
+
+        await client.VerifyChainAsync(Profile(CheckpointFor(100, 7)), default).WaitAsync(Timeout);
+        Assert.Equal(0, server.CallCount("blockchain.block.header"));
+    }
+
+    [Fact]
+    public async Task A_profile_without_checkpoints_relies_on_the_genesis_alone()
+    {
+        await using var server = new FakeElectrumServer();
+        Tip(server, 5_000_000);
+        await using var client = await ConnectAsync(server);
+
+        await client.VerifyChainAsync(Profile(), default).WaitAsync(Timeout);
+        Assert.Equal(0, server.CallCount("blockchain.block.header"));
+    }
+
+    [Theory]
+    [InlineData("server-error")]
+    [InlineData("short-header")]
+    [InlineData("not-hex")]
+    public async Task A_missing_or_malformed_checkpoint_header_is_a_wrong_chain(string mode)
+    {
+        await using var server = new FakeElectrumServer();
+        Tip(server, 150);
+        server.Handle("blockchain.block.header", _ => mode switch
+        {
+            "server-error" => throw new FakeElectrumError(-1, "no such block"),
+            "short-header" => "00ff",
+            _ => "zz",
+        });
+        await using var client = await ConnectAsync(server);
+
+        await Assert.ThrowsAsync<WrongChainException>(
+            () => client.VerifyChainAsync(Profile(CheckpointFor(100, 7)), default).WaitAsync(Timeout));
+    }
+
+    [Fact]
+    public void The_shipped_mainnet_profile_has_checkpoints_the_check_can_use()
+    {
+        Assert.NotEmpty(ChainProfiles.Mainnet.Checkpoints);
+        Assert.All(ChainProfiles.Mainnet.Checkpoints, c => Assert.Equal(64, c.BlockHash.Length));
+    }
+}