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add support for reading sk keys
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This adds partial support for "sk" keys that are associated with a
hardware key. We can only read keys but can't use them for signing.

Reference: https://api.libssh.org/rfc/PROTOCOL.u2f
Issue: dlech/KeeAgent#300
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@dlech
dlech committed May 24, 2022
1 parent aaa5ebd commit dd41ac2
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Showing 10 changed files with 324 additions and 44 deletions.
6 changes: 4 additions & 2 deletions SshAgentLib/Agent.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -792,7 +792,8 @@ public void AnswerMessage(Stream messageStream, Process process = null)
{
var publicKeyParams = messageParser.ReadSsh2PublicKeyData(
out var nonce,
out var cert
out var cert,
out var application
);
var privateKeyParams = messageParser.ReadSsh2KeyData(publicKeyParams);
var key = new SshKey(
Expand All @@ -801,7 +802,8 @@ out var cert
privateKeyParams,
"",
nonce,
cert
cert,
application
)
{
Comment = messageParser.ReadString(),
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13 changes: 11 additions & 2 deletions SshAgentLib/AgentClient.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -229,11 +229,20 @@ public ICollection<ISshKey> ListKeys(SshVersion aVersion)
var publicKeyParser = new BlobParser(publicKeyBlob);
var publicKeyParams = publicKeyParser.ReadSsh2PublicKeyData(
out var nonce,
out var cert
out var cert,
out var application
);
var comment = replyParser.ReadString();
keyCollection.Add(
new SshKey(SshVersion.SSH2, publicKeyParams, null, comment, nonce, cert)
new SshKey(
SshVersion.SSH2,
publicKeyParams,
null,
comment,
nonce,
cert,
application
)
);
}
break;
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69 changes: 63 additions & 6 deletions SshAgentLib/BlobParser.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -159,11 +159,13 @@ OpensshCertificateInfo ReadCertificate()
/// <returns>AsymmetricKeyParameter containing the public key</returns>
public AsymmetricKeyParameter ReadSsh2PublicKeyData(
out byte[] nonce,
out OpensshCertificateInfo cert
out OpensshCertificateInfo cert,
out string application
)
{
nonce = null;
cert = null;
application = null;
var algorithm = KeyFormatIdentifier.Parse(ReadString());

switch (algorithm)
Expand All @@ -189,7 +191,11 @@ out OpensshCertificateInfo cert
// we are being called from SSH2_AGENTC_ADD_IDENTITY and this blob
// is the whole certificate, not the nonce
var certParser = new BlobParser(nonceOrPublicKey);
return certParser.ReadSsh2PublicKeyData(out nonce, out cert);
return certParser.ReadSsh2PublicKeyData(
out nonce,
out cert,
out application
);
}
else
{
Expand Down Expand Up @@ -222,7 +228,11 @@ out OpensshCertificateInfo cert
// we are being called from SSH2_AGENTC_ADD_IDENTITY and this blob
// is the whole certificate, not the nonce
var certParser = new BlobParser(nonceOrPublicKey);
return certParser.ReadSsh2PublicKeyData(out nonce, out cert);
return certParser.ReadSsh2PublicKeyData(
out nonce,
out cert,
out application
);
}
else
{
Expand All @@ -242,10 +252,18 @@ out OpensshCertificateInfo cert
case PublicKeyAlgorithm.EcdsaSha2Nistp256:
case PublicKeyAlgorithm.EcdsaSha2Nistp384:
case PublicKeyAlgorithm.EcdsaSha2Nistp521:
case PublicKeyAlgorithm.SkEcdsaSha2Nistp256:
case PublicKeyAlgorithm.SkEcdsaSha2Nistp384:
case PublicKeyAlgorithm.SkEcdsaSha2Nistp521:
{
var curveName = ReadString();
var publicKey = ReadBlob();

if (algorithm.HasApplication())
{
application = ReadString();
}

var x9Params = NistNamedCurves.GetByName(curveName.Replace("nistp", "P-"));
var domainParams = new ECDomainParameters(
x9Params.Curve,
Expand All @@ -260,20 +278,33 @@ out OpensshCertificateInfo cert
case PublicKeyAlgorithm.EcdsaSha2Nistp256CertV1:
case PublicKeyAlgorithm.EcdsaSha2Nistp384CertV1:
case PublicKeyAlgorithm.EcdsaSha2Nistp521CertV1:
case PublicKeyAlgorithm.SkEcdsaSha2Nistp256CertV1:
case PublicKeyAlgorithm.SkEcdsaSha2Nistp384CertV1:
case PublicKeyAlgorithm.SkEcdsaSha2Nistp521CertV1:
{
var nonceOrPublicKey = ReadBlob();

if (nonceOrPublicKey.Length != 32)
{
// we are being called from SSH2_AGENTC_ADD_IDENTITY and this blob
// is the whole certificate, not the nonce
var certParser = new BlobParser(nonceOrPublicKey);
return certParser.ReadSsh2PublicKeyData(out nonce, out cert);
return certParser.ReadSsh2PublicKeyData(
out nonce,
out cert,
out application
);
}
else
{
var curveName = ReadString();
var publicKey = ReadBlob();

if (algorithm.HasApplication())
{
application = ReadString();
}

nonce = nonceOrPublicKey;
cert = ReadCertificate();

Expand All @@ -291,20 +322,28 @@ out OpensshCertificateInfo cert
}

case PublicKeyAlgorithm.SshEd25519:
case PublicKeyAlgorithm.SkSshEd25519:
{
var publicKey = ReadBlob();

if (algorithm.HasApplication())
{
application = ReadString();
}

return new Ed25519PublicKeyParameters(publicKey);
}

case PublicKeyAlgorithm.SshEd25519CertV1:
case PublicKeyAlgorithm.SkSshEd25519CertV1:
{
var nonceOrPublicKey = ReadBlob();
if (nonceOrPublicKey.Length != 32)
{
// we are being called from SSH2_AGENTC_ADD_IDENTITY and this blob
// is the whole certificate, not the nonce
var certParser = new BlobParser(nonceOrPublicKey);
certParser.ReadSsh2PublicKeyData(out nonce, out cert);
certParser.ReadSsh2PublicKeyData(out nonce, out cert, out application);
var publicKey = ReadBlob();

return new Ed25519PublicKeyParameters(publicKey);
Expand All @@ -313,6 +352,11 @@ out OpensshCertificateInfo cert
{
var publicKey = ReadBlob();

if (algorithm.HasApplication())
{
application = ReadString();
}

nonce = nonceOrPublicKey;
cert = ReadCertificate();

Expand All @@ -321,20 +365,28 @@ out OpensshCertificateInfo cert
}

case PublicKeyAlgorithm.SshEd448:
case PublicKeyAlgorithm.SkSshEd448:
{
var publicKey = ReadBlob();

if (algorithm.HasApplication())
{
application = ReadString();
}

return new Ed448PublicKeyParameters(publicKey);
}

case PublicKeyAlgorithm.SshEd448CertV1:
case PublicKeyAlgorithm.SkSshEd448CertV1:
{
var nonceOrPublicKey = ReadBlob();
if (nonceOrPublicKey.Length != 32)
{
// we are being called from SSH2_AGENTC_ADD_IDENTITY and this blob
// is the whole certificate, not the nonce
var certParser = new BlobParser(nonceOrPublicKey);
certParser.ReadSsh2PublicKeyData(out nonce, out cert);
certParser.ReadSsh2PublicKeyData(out nonce, out cert, out application);
var publicKey = ReadBlob();

return new Ed448PublicKeyParameters(publicKey);
Expand All @@ -343,6 +395,11 @@ out OpensshCertificateInfo cert
{
var publicKey = ReadBlob();

if (algorithm.HasApplication())
{
application = ReadString();
}

nonce = nonceOrPublicKey;
cert = ReadCertificate();

Expand Down
3 changes: 2 additions & 1 deletion SshAgentLib/IAgent.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -111,7 +111,8 @@ public static ISshKey AddKeyFromFile(
privateKey.Decrypt(getPassPhraseCallback, progress),
privateKey.PublicKey.Comment,
publicKey.Nonce,
publicKey.Certificate
publicKey.Certificate,
publicKey.Application
);

if (constraints != null)
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11 changes: 11 additions & 0 deletions SshAgentLib/ISshKey.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -62,6 +62,12 @@ public interface ISshKey : IDisposable
/// </summary>
OpensshCertificateInfo Certificate { get; }

/// <summary>
/// Gets the application for keys associated with a hardware key or
/// <c>null</c> for keys that are not associated with a hardware key.
/// </summary>
string Application { get; }

/// <summary>
/// returns true if key does not have private key parameters
/// </summary>
Expand Down Expand Up @@ -217,6 +223,11 @@ public static byte[] GetPublicKeyBlob(this ISshKey key, bool cert = true)
);
}

if (key.Application != null)
{
builder.AddStringBlob(key.Application);
}

if (cert && key.Certificate != null)
{
var principalsBuilder = new BlobBuilder();
Expand Down
7 changes: 6 additions & 1 deletion SshAgentLib/Keys/OpensshPrivateKey.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -204,11 +204,16 @@ public static SshPrivateKey Read(Stream stream)
throw new FormatException("checkint does not match in private key.");
}
var publicKey = privateKeyParser.ReadSsh2PublicKeyData(out var nonce, out var cert);
var publicKey = privateKeyParser.ReadSsh2PublicKeyData(
out var nonce,
out var cert,
out var application
);
var privateKey = privateKeyParser.ReadSsh2KeyData(publicKey);
// var comment = privateKeyParser.ReadString();
// TODO: what to do with comment?
// TODO: should we throw if nonce/cert is not null?
// TODO: do we expect application?
return privateKey;
};
Expand Down
38 changes: 33 additions & 5 deletions SshAgentLib/Keys/SshPublicKey.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -63,7 +63,9 @@ public string AuthorizedKeysString
{
var builder = new StringBuilder();

builder.Append(GetAlgorithmIdentifier(Parameter, Certificate != null));
builder.Append(
GetAlgorithmIdentifier(Parameter, Certificate != null, Application != null)
);
builder.Append(' ');
builder.Append(Convert.ToBase64String(KeyBlob));

Expand All @@ -81,6 +83,12 @@ public string AuthorizedKeysString

public OpensshCertificateInfo Certificate { get; }

/// <summary>
/// Gets the application for hardware security keys or <c>null</c> if
/// this key is not associated with a hardware key.
/// </summary>
public string Application { get; }

/// <summary>
/// Creates a new public key.
/// </summary>
Expand All @@ -101,9 +109,14 @@ public SshPublicKey(SshVersion version, byte[] keyBlob, string comment = "")
Parameter = parser.ReadSsh1PublicKeyData();
break;
case SshVersion.SSH2:
Parameter = parser.ReadSsh2PublicKeyData(out var nonce, out var certificate);
Parameter = parser.ReadSsh2PublicKeyData(
out var nonce,
out var certificate,
out var application
);
Nonce = nonce;
Certificate = certificate;
Application = application;
break;
default:
throw new ArgumentException("unsupported SSH version", nameof(version));
Expand Down Expand Up @@ -136,8 +149,12 @@ public SshPublicKey WithoutCertificate()

// separate the key from the certificate
var parser = new BlobParser(KeyBlob);
var parameters = parser.ReadSsh2PublicKeyData(out var nonce, out var certificate);
var key = new SshKey(Version, parameters);
var parameters = parser.ReadSsh2PublicKeyData(
out var nonce,
out var certificate,
out var application
);
var key = new SshKey(Version, parameters, null, "", null, null, application);

return new SshPublicKey(Version, key.GetPublicKeyBlob(), Comment);
}
Expand Down Expand Up @@ -176,7 +193,8 @@ public static SshPublicKey Read(Stream stream)

private static string GetAlgorithmIdentifier(
AsymmetricKeyParameter parameters,
bool hasCertificate
bool hasCertificate,
bool hasApplication
)
{
var algorithm = GetBaseAlgorithmIdentifier(parameters);
Expand All @@ -186,6 +204,16 @@ bool hasCertificate
algorithm += "[email protected]";
}

if (hasApplication)
{
algorithm = "sk-" + algorithm;

if (!hasCertificate)
{
algorithm += "@openssh.com";
}
}

return algorithm;
}

Expand Down
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