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Add X509AuthorityKeyIdentifier to generated leaf certificates #1899

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feda483
Add X509AuthorityKeyIdentifier to generated leaf certificates
AronUJVARY Sep 29, 2026
4fcb5ab
Regenerate root certificate without SubjectKeyIdentifier or KeyCertSi…
AronUJVARY Sep 30, 2026
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144 changes: 142 additions & 2 deletions DevProxy.Proxy.Kestrel.Tests/CertificateAuthorityTests.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -54,6 +54,8 @@ public void FirstRun_CreatesValidRoot_AndPersistsToDisk()
Assert.Equal("CN=Dev Proxy CA", root.Subject);
Assert.True(root.HasPrivateKey);
Assert.True(IsCertificateAuthority(root));
Assert.True(HasSubjectKeyIdentifier(root));
Assert.True(HasKeyCertSignUsage(root));
Assert.True(root.NotAfter > DateTime.Now);
Assert.True(File.Exists(_rootPath));
}
Expand Down Expand Up @@ -105,6 +107,36 @@ public void NonCaRoot_Regenerates()
Assert.True(IsCertificateAuthority(ca.RootCertificate));
}

[Fact]
public void CaRootWithoutSki_Regenerates()
{
File.WriteAllBytes(_rootPath, CreateCaWithoutSkiPfx());

using var ca = new CertificateAuthority(_rootPath, _leafDir);

Assert.True(HasSubjectKeyIdentifier(ca.RootCertificate));
}

[Fact]
public void CaRootWithoutKeyCertSignUsage_Regenerates()
{
File.WriteAllBytes(_rootPath, CreateCaWithoutKeyCertSignUsagePfx());

using var ca = new CertificateAuthority(_rootPath, _leafDir);

Assert.True(HasKeyCertSignUsage(ca.RootCertificate));
}

[Fact]
public void CaRootWithoutKeyUsageExtension_Regenerates()
{
File.WriteAllBytes(_rootPath, CreateCaWithoutKeyUsageExtensionPfx());

using var ca = new CertificateAuthority(_rootPath, _leafDir);

Assert.True(HasKeyCertSignUsage(ca.RootCertificate));
}

[Fact]
public void GetCertificateForHost_MintsLeafSignedByRoot_AndPersists()
{
Expand All @@ -116,6 +148,7 @@ public void GetCertificateForHost_MintsLeafSignedByRoot_AndPersists()
Assert.Equal("CN=example.com", leaf.Subject);
Assert.Equal(ca.RootCertificate.Subject, leaf.Issuer);
Assert.Contains("example.com", GetSanText(leaf), StringComparison.Ordinal);
Assert.True(DoesAuthorityKeyIdentifierMatchCA(leaf, ca.RootCertificate));
Assert.True(File.Exists(Path.Combine(_leafDir, "example.com.pfx")));
}

Expand Down Expand Up @@ -195,21 +228,111 @@ public void GetCertificateForHost_LeafDoesNotOutliveRoot()
Assert.True(leaf.NotAfter <= ca.RootCertificate.NotAfter);
}

[Fact]
public void GetCertificateForHost_LeafWithoutAki_Regenerates()
{
using var ca = new CertificateAuthority(_rootPath, _leafDir);

_ = Directory.CreateDirectory(_leafDir);
File.WriteAllBytes(Path.Combine(_leafDir, "example.com.pfx"), CreateLeafPfx(ca.RootCertificate, null));

var leaf = ca.GetCertificateForHost("example.com");

Assert.True(DoesAuthorityKeyIdentifierMatchCA(leaf, ca.RootCertificate));
}

[Fact]
public void GetCertificateForHost_LeafWithMismatchedAki_Regenerates()
{
var rootNotAfter = DateTimeOffset.UtcNow.AddDays(10);
var other_ca = CreateRootCertificate(rootNotAfter);
using var ca = new CertificateAuthority(_rootPath, _leafDir);

_ = Directory.CreateDirectory(_leafDir);
File.WriteAllBytes(Path.Combine(_leafDir, "example.com.pfx"), CreateLeafPfx(ca.RootCertificate, other_ca));

var leaf = ca.GetCertificateForHost("example.com");

Assert.True(DoesAuthorityKeyIdentifierMatchCA(leaf, ca.RootCertificate));
}

private static bool IsCertificateAuthority(X509Certificate2 cert) =>
cert.Extensions.OfType<X509BasicConstraintsExtension>().FirstOrDefault()?.CertificateAuthority == true;

private static bool HasSubjectKeyIdentifier(X509Certificate2 cert) =>
cert.Extensions.OfType<X509SubjectKeyIdentifierExtension>().Any();

private static bool HasKeyCertSignUsage(X509Certificate2 cert) =>
(cert.Extensions
.OfType<X509KeyUsageExtension>()
.FirstOrDefault()?.KeyUsages & X509KeyUsageFlags.KeyCertSign) == X509KeyUsageFlags.KeyCertSign;

private static bool DoesAuthorityKeyIdentifierMatchCA(X509Certificate2 cert, X509Certificate2 ca)
{
var ski = ca.Extensions
.OfType<X509SubjectKeyIdentifierExtension>()
.FirstOrDefault()?.SubjectKeyIdentifierBytes;

var aki = cert.Extensions
.OfType<X509AuthorityKeyIdentifierExtension>()
.FirstOrDefault()?.KeyIdentifier;

return aki.HasValue
&& ski.HasValue
&& aki.Value.Span.SequenceEqual(ski.Value.Span);
}

private static string GetSanText(X509Certificate2 cert)
{
var san = cert.Extensions.FirstOrDefault(e => e.Oid?.Value == "2.5.29.17");
return san?.Format(false) ?? string.Empty;
}

private static byte[] CreateRootPfx(DateTimeOffset NotAfter)
private static X509Certificate2 CreateRootCertificate(DateTimeOffset NotAfter)
{
using var rsa = RSA.Create(2048);
var req = new CertificateRequest("CN=Old Root", rsa, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
req.CertificateExtensions.Add(new X509BasicConstraintsExtension(true, false, 0, true));
using var cert = req.CreateSelfSigned(NotAfter.AddDays(-10), NotAfter);
req.CertificateExtensions.Add(new X509SubjectKeyIdentifierExtension(req.PublicKey, false));
req.CertificateExtensions.Add(new X509KeyUsageExtension(
X509KeyUsageFlags.KeyCertSign | X509KeyUsageFlags.CrlSign, true));
return req.CreateSelfSigned(NotAfter.AddDays(-10), NotAfter);
}

private static byte[] CreateRootPfx(DateTimeOffset NotAfter)
{
using var cert = CreateRootCertificate(NotAfter);
return cert.Export(X509ContentType.Pkcs12, string.Empty);
}

private static byte[] CreateCaWithoutSkiPfx()
{
using var rsa = RSA.Create(2048);
var req = new CertificateRequest("CN=CA Root Without SKI", rsa, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
req.CertificateExtensions.Add(new X509BasicConstraintsExtension(true, false, 0, true));
req.CertificateExtensions.Add(new X509KeyUsageExtension(
X509KeyUsageFlags.KeyCertSign | X509KeyUsageFlags.CrlSign, true));
using var cert = req.CreateSelfSigned(DateTimeOffset.UtcNow.AddDays(-1), DateTimeOffset.UtcNow.AddYears(1));
return cert.Export(X509ContentType.Pkcs12, string.Empty);
}

private static byte[] CreateCaWithoutKeyCertSignUsagePfx()
{
using var rsa = RSA.Create(2048);
var req = new CertificateRequest("CN=CA Root Without KeyCertSign", rsa, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
req.CertificateExtensions.Add(new X509BasicConstraintsExtension(true, false, 0, true));
req.CertificateExtensions.Add(new X509KeyUsageExtension(
X509KeyUsageFlags.CrlSign, true));
using var cert = req.CreateSelfSigned(DateTimeOffset.UtcNow.AddDays(-1), DateTimeOffset.UtcNow.AddYears(1));
return cert.Export(X509ContentType.Pkcs12, string.Empty);
}

private static byte[] CreateCaWithoutKeyUsageExtensionPfx()
{
using var rsa = RSA.Create(2048);
var req = new CertificateRequest("CN=CA Root Without KeyUsageExtension", rsa, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
req.CertificateExtensions.Add(new X509BasicConstraintsExtension(true, false, 0, true));
using var cert = req.CreateSelfSigned(DateTimeOffset.UtcNow.AddDays(-1), DateTimeOffset.UtcNow.AddYears(1));
return cert.Export(X509ContentType.Pkcs12, string.Empty);
}

Expand All @@ -221,4 +344,21 @@ private static byte[] CreateNonCaPfx()
using var cert = req.CreateSelfSigned(DateTimeOffset.UtcNow.AddDays(-1), DateTimeOffset.UtcNow.AddYears(1));
return cert.Export(X509ContentType.Pkcs12, string.Empty);
}

private static byte[] CreateLeafPfx(X509Certificate2 ca, X509Certificate2? aki_source)
{
using var rsa = RSA.Create(2048);
var req = new CertificateRequest("CN=Leaf", rsa, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
req.CertificateExtensions.Add(new X509BasicConstraintsExtension(false, false, 0, true));
if (aki_source is not null) {
req.CertificateExtensions.Add(
X509AuthorityKeyIdentifierExtension.CreateFromCertificate(
aki_source, includeKeyIdentifier: true, includeIssuerAndSerial: false));
}
var serialNumber = new byte[8];
RandomNumberGenerator.Fill(serialNumber);
using var cert = req.Create(ca, DateTimeOffset.UtcNow.AddDays(-1), DateTimeOffset.UtcNow.AddDays(10), serialNumber);
return cert.Export(X509ContentType.Pkcs12, string.Empty);
}

}
30 changes: 27 additions & 3 deletions DevProxy.Proxy.Kestrel/Internal/CertificateAuthority.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -148,7 +148,15 @@ private X509Certificate2 LoadOrCreateRoot()
.OfType<X509BasicConstraintsExtension>()
.FirstOrDefault()?.CertificateAuthority == true;

if (cert.HasPrivateKey && isCa && cert.NotAfter > DateTime.Now)
var hasSKI = cert.Extensions
.OfType<X509SubjectKeyIdentifierExtension>()
.Any();

var hasKeyCertSign = (cert.Extensions
.OfType<X509KeyUsageExtension>()
.FirstOrDefault()?.KeyUsages & X509KeyUsageFlags.KeyCertSign) == X509KeyUsageFlags.KeyCertSign;

if (cert.HasPrivateKey && isCa && hasSKI && hasKeyCertSign && cert.NotAfter > DateTime.Now)
{
return cert;
}
Expand Down Expand Up @@ -214,7 +222,7 @@ private X509Certificate2 LoadOrCreateLeaf(string host)
return leaf;
}

private static X509Certificate2? TryLoadLeaf(string path)
private X509Certificate2? TryLoadLeaf(string path)
{
if (!File.Exists(path))
{
Expand All @@ -228,7 +236,19 @@ private X509Certificate2 LoadOrCreateLeaf(string host)
string.Empty,
X509KeyStorageFlags.Exportable);

if (cert.HasPrivateKey && cert.NotAfter > DateTime.Now)
var ski = _ca.Extensions
.OfType<X509SubjectKeyIdentifierExtension>()
.FirstOrDefault()?.SubjectKeyIdentifierBytes;

var aki = cert.Extensions
.OfType<X509AuthorityKeyIdentifierExtension>()
.FirstOrDefault()?.KeyIdentifier;

var akiMatchesCA = aki.HasValue
&& ski.HasValue
&& aki.Value.Span.SequenceEqual(ski.Value.Span);

if (cert.HasPrivateKey && akiMatchesCA && cert.NotAfter > DateTime.Now)
{
return cert;
}
Expand Down Expand Up @@ -326,6 +346,10 @@ private X509Certificate2 CreateLeafCertificate(string host)
request.CertificateExtensions.Add(sanBuilder.Build());
request.CertificateExtensions.Add(new X509SubjectKeyIdentifierExtension(request.PublicKey, false));

request.CertificateExtensions.Add(
X509AuthorityKeyIdentifierExtension.CreateFromCertificate(
_ca, includeKeyIdentifier: true, includeIssuerAndSerial: false));

var serialNumber = new byte[8];
RandomNumberGenerator.Fill(serialNumber);

Expand Down