Specify the key statement encoding and key log chain (crypto.md 7.1, 7.2)

Section 7 always required "a canonical, length-prefixed encoding" for signatures without ever specifying one. That gap had to be closed before enrollment could exist: the client hashes the key statement and uses the result as an OIDC nonce, so the provider signs over those exact bytes. Two implementations disagreeing by one byte produce two nonces and an enrollment nobody can verify -- and it only shows up against a real provider, never in a local test. JSON cannot be the hashed form. Property order, number formatting, Unicode escaping and whitespace all vary between serialisers. So the statement is transmitted as JSON and hashed as a fixed binary encoding, and the two are independent by construction. Three details are load-bearing rather than stylistic: - The presence byte before each string is what makes the encoding injective. Without it an absent email and an empty one encode identically, and two different statements share a binding. - Timestamps truncate to milliseconds. PostgreSQL stores microseconds, so a statement that has been through the database must still hash to what the client hashed. The same applies to the key log, where an entry that cannot reproduce its own hash after being read back makes the chain unverifiable. - The key log entry hash deliberately excludes the database sequence. It is unknown until the insert runs, and order already follows the hash links -- so a renumbered or gapped sequence column cannot silently reorder history. KeyStatementFields is separate from Contracts.KeyStatement on purpose: one may gain JSON fields freely, the other cannot change without invalidating every stored binding, and Crypto must not depend on the contract assembly. KeyStatementDriftTests makes a field added to one and not the other a build failure, because a wire field outside the binding is unauthenticated data the server can change undetected. 54 new tests and two new golden vector sections. The vectors pin the absent-versus-empty email case and confirm that an offset-bearing sub-millisecond timestamp encodes identically to its truncated UTC form. Only additions to vectors.json; nothing existing moved.
2026-07-28 16:06:11 +02:00
parent e6673f0bf2
commit d2a2ed8a29
13 changed files with 1363 additions and 1 deletions
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+namespace DodoSSH.Crypto.Tests;
+
+/// <summary>The key log hash chain. See docs/crypto.md §7.2.</summary>
+public sealed class KeyLogChainTests
+{
+    private static readonly Guid Alice = Guid.Parse("0192f0c8-1a2b-7c3d-8e4f-5a6b7c8d9e0f");
+    private static readonly Guid Bob = Guid.Parse("0192f0c8-1a2b-7c3d-8e4f-5a6b7c8d9e10");
+    private static readonly DateTimeOffset CreatedAt = DateTimeOffset.FromUnixTimeMilliseconds(1_750_000_000_123);
+
+    [Fact]
+    public void GenesisPreviousHash_IsThirtyTwoZeroBytes()
+    {
+        var genesis = KeyLogChain.CreateGenesisPreviousHash();
+
+        genesis.Length.ShouldBe(CryptoSpec.DigestSize);
+        genesis.ShouldAllBe(b => b == 0);
+    }
+
+    [Fact]
+    public void GenesisPreviousHash_IsANewArrayEachCall()
+    {
+        // Shared mutable state in a hash input would be a spectacular way to corrupt a chain.
+        var first = KeyLogChain.CreateGenesisPreviousHash();
+        first[0] = 0xFF;
+
+        KeyLogChain.CreateGenesisPreviousHash()[0].ShouldBe((byte)0);
+    }
+
+    [Fact]
+    public void ComputeEntryHash_IsDeterministic()
+    {
+        Hash().ShouldBe(Hash());
+    }
+
+    [Fact]
+    public void ChangingThePreviousHash_ChangesTheEntryHash()
+    {
+        // The link itself. If this held, a server could reorder or drop entries undetectably.
+        var linked = Hash(previousHash: Hash());
+
+        linked.ShouldNotBe(Hash());
+    }
+
+    [Theory]
+    [InlineData("user")]
+    [InlineData("generation")]
+    [InlineData("encryptionKey")]
+    [InlineData("signingKey")]
+    [InlineData("signature")]
+    [InlineData("createdAt")]
+    public void ChangingAnyField_ChangesTheEntryHash(string field)
+    {
+        var baseline = Hash();
+
+        var altered = field switch
+        {
+            "user" => Hash(userId: Bob),
+            "generation" => Hash(generation: 2),
+            "encryptionKey" => Hash(encryptionPublicKey: TestKeys.Alternate),
+            "signingKey" => Hash(signingPublicKey: TestKeys.Alternate),
+            "signature" => Hash(signature: AlternateSignature),
+            "createdAt" => Hash(createdAt: CreatedAt.AddMilliseconds(1)),
+            _ => throw new ArgumentOutOfRangeException(nameof(field), field, "Unknown field."),
+        };
+
+        altered.ShouldNotBe(baseline);
+    }
+
+    [Fact]
+    public void SubMillisecondPrecision_IsTruncatedAway()
+    {
+        // The stored column round-trips through PostgreSQL's microseconds. If the hash used finer
+        // precision than the storage, no entry could ever reproduce its own hash after being read.
+        Hash(createdAt: CreatedAt.AddTicks(9_999)).ShouldBe(Hash(createdAt: CreatedAt));
+    }
+
+    [Fact]
+    public void TruncateTimestamp_MatchesWhatTheHashUses()
+    {
+        var truncated = KeyLogChain.TruncateTimestamp(CreatedAt.AddTicks(9_999));
+
+        truncated.ToUnixTimeMilliseconds().ShouldBe(CreatedAt.ToUnixTimeMilliseconds());
+        truncated.Ticks.ShouldBe(truncated.Ticks / TimeSpan.TicksPerMillisecond * TimeSpan.TicksPerMillisecond);
+        truncated.Offset.ShouldBe(TimeSpan.Zero);
+    }
+
+    [Theory]
+    [InlineData(0)]
+    [InlineData(31)]
+    [InlineData(33)]
+    public void ComputeEntryHash_RejectsAWrongLengthPreviousHash(int length)
+    {
+        Should.Throw<ArgumentException>(() => Hash(previousHash: new byte[length]));
+    }
+
+    [Fact]
+    public void ComputeEntryHash_RejectsAWrongLengthSignature()
+    {
+        Should.Throw<ArgumentException>(() => Hash(signature: new byte[32]));
+    }
+
+    [Fact]
+    public void ComputeEntryHash_RejectsAGenerationBelowOne()
+    {
+        Should.Throw<ArgumentOutOfRangeException>(() => Hash(generation: 0));
+    }
+
+    private static byte[] AlternateSignature { get; } =
+        [.. Enumerable.Range(0, CryptoSpec.SignatureSize).Select(i => (byte)(0xC0 + i))];
+
+    private static byte[] Signature { get; } =
+        [.. Enumerable.Range(0, CryptoSpec.SignatureSize).Select(i => (byte)(0x80 + i))];
+
+    private static byte[] Hash(
+        byte[]? previousHash = null,
+        Guid? userId = null,
+        int generation = 1,
+        byte[]? encryptionPublicKey = null,
+        byte[]? signingPublicKey = null,
+        byte[]? signature = null,
+        DateTimeOffset? createdAt = null) =>
+        KeyLogChain.ComputeEntryHash(
+            previousHash ?? KeyLogChain.CreateGenesisPreviousHash(),
+            userId ?? Alice,
+            generation,
+            encryptionPublicKey ?? TestKeys.Encryption,
+            signingPublicKey ?? TestKeys.Signing,
+            signature ?? Signature,
+            createdAt ?? CreatedAt);
+}