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Share signature verification across public-key and host-based auth paths #161

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Merged
David Obando (DavidObando) merged 1 commit into from
Aug 24, 2026
Merged

Share signature verification across public-key and host-based auth paths #161

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5 changes: 5 additions & 0 deletions src/cs/Ssh/Messages/Authentication/PublicKeyRequestMessage.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -92,6 +92,10 @@ protected override void OnRead(ref SshDataReader reader)

if (MethodName != AuthenticationMethods.PublicKey)
{
// Host-based authentication requests are intentionally rejected here. The
// host-based branch below is therefore currently unreachable, but it is kept
// correct so that relaxing this check cannot silently break the signature
// verification performed by AuthenticationService.
throw new ArgumentException($"Method name {MethodName} is not valid.");
}

Expand All @@ -102,6 +106,7 @@ protected override void OnRead(ref SshDataReader reader)
ClientHostname = reader.ReadString(Encoding.ASCII);
ClientUsername = reader.ReadString(Encoding.UTF8);
Signature = reader.ReadBinary();
PayloadWithoutSignature = RawBytes.Slice(0, reader.Position - Signature.Count - 4);
}
else
{
Expand Down
79 changes: 59 additions & 20 deletions src/cs/Ssh/Services/AuthenticationService.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -146,6 +146,14 @@ await HandleAuthenticationFailureAsync(
SshAuthenticatingEventArgs args;
if (message.MethodName == AuthenticationMethods.HostBased)
{
// Verify the client host signature before allowing the application to authorize
// the request. (RFC 4252 Section 9)
if (!await VerifySignatureAsync(message, algorithm, publicKey, cancellation)
.ConfigureAwait(false))
{
return;
}

args = new SshAuthenticatingEventArgs(
SshAuthenticationType.ClientHostBased,
username: message.Username,
Expand All @@ -162,27 +170,12 @@ await HandleAuthenticationFailureAsync(
}
else
{
// Verify that the signature matches the public key.
var signature = algorithm.ReadSignatureData(message.Signature);

var sessionId = Session.SessionId;
if (sessionId == null)
// Verify that the signature matches the public key, proving that the client
// possesses the corresponding private key.
if (!await VerifySignatureAsync(message, algorithm, publicKey, cancellation)
.ConfigureAwait(false))
{
throw new InvalidOperationException("Session ID not initialized.");
}

var writer = new SshDataWriter();
writer.WriteBinary(sessionId);
writer.Write(message.PayloadWithoutSignature);
var signedData = writer.ToBuffer();

var verifier = algorithm.CreateVerifier(publicKey);
var verified = verifier.Verify(signedData, signature);
if (!verified)
{
await HandleAuthenticationFailureAsync(
"Client authentication failed due to invalid signature.",
cancellation).ConfigureAwait(false);
return;
}

args = new SshAuthenticatingEventArgs(
Expand All @@ -196,6 +189,52 @@ await HandleAuthenticationFailureAsync(
await HandleAuthenticatingAsync(args, cancellation).ConfigureAwait(false);
}

/// <summary>
/// Verifies the signature on a public-key or host-based authentication request, proving
/// that the sender possesses the private key corresponding to the presented public key.
/// </summary>
/// <returns>True if the signature is valid. False if it is missing or invalid, in which
/// case an authentication failure has already been sent and the caller MUST NOT proceed
/// to raise the <see cref="SshSession.Authenticating" /> event.</returns>
private async Task<bool> VerifySignatureAsync(
PublicKeyRequestMessage message,
PublicKeyAlgorithm algorithm,
IKeyPair publicKey,
CancellationToken cancellation)
{
if (!message.HasSignature)
{
await HandleAuthenticationFailureAsync(
"Client authentication failed due to missing signature.",
cancellation).ConfigureAwait(false);
return false;
}

var sessionId = Session.SessionId;
if (sessionId == null)
{
throw new InvalidOperationException("Session ID not initialized.");
}

var signature = algorithm.ReadSignatureData(message.Signature);

var writer = new SshDataWriter();
writer.WriteBinary(sessionId);
writer.Write(message.PayloadWithoutSignature);
var signedData = writer.ToBuffer();

var verifier = algorithm.CreateVerifier(publicKey);
if (!verifier.Verify(signedData, signature))
{
await HandleAuthenticationFailureAsync(
"Client authentication failed due to invalid signature.",
cancellation).ConfigureAwait(false);
return false;
}

return true;
}

private async Task HandleMessageAsync(
PasswordRequestMessage message, CancellationToken cancellation)
{
Expand Down
13 changes: 13 additions & 0 deletions src/ts/ssh/messages/authenticationMessages.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -55,6 +55,19 @@ export class PublicKeyRequestMessage extends AuthenticationRequestMessage {
protected onRead(reader: SshDataReader): void {
super.onRead(reader);

if (this.methodName === AuthenticationMethod.hostBased) {
this.keyAlgorithmName = reader.readString('ascii');
this.publicKey = reader.readBinary();
this.clientHostname = reader.readString('ascii');
this.clientUsername = reader.readString('ascii');
this.signature = reader.readBinary();
this.payloadWithoutSignature = this.rawBytes!.slice(
0,
this.rawBytes!.length - this.signature.length - 4,
);
return;
}

const hasSignature = reader.readBoolean();
this.keyAlgorithmName = reader.readString('ascii');
this.publicKey = reader.readBinary();
Expand Down
80 changes: 58 additions & 22 deletions src/ts/ssh/services/authenticationService.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -159,6 +159,12 @@ export class AuthenticationService extends SshService {

let args: SshAuthenticatingEventArgs;
if (message.methodName === AuthenticationMethod.hostBased) {
// Verify the client host signature before allowing the application to authorize
// the request. (RFC 4252 Section 9)
if (!(await this.verifySignature(message, publicKeyAlg, publicKey, cancellation))) {
return;
}

args = new SshAuthenticatingEventArgs(SshAuthenticationType.clientHostBased, {
username: message.username ?? '',
publicKey: publicKey,
Expand All @@ -171,28 +177,10 @@ export class AuthenticationService extends SshService {
publicKey: publicKey,
});
} else {
// Verify that the signature matches the public key.
const signature = publicKeyAlg.readSignatureData(message.signature!);

const sessionId = this.session.sessionId;
if (sessionId == null) {
throw new Error('Session ID not initialized.');
}

const writer = new SshDataWriter(
Buffer.alloc(sessionId.length + message.payloadWithoutSignature!.length + 20),
);
writer.writeBinary(sessionId);
writer.write(message.payloadWithoutSignature!);

const signedData = writer.toBuffer();
const verifier = publicKeyAlg.createVerifier(publicKey);
const verified = await verifier.verify(signedData, signature);
if (!verified) {
await this.handleAuthenticationFailure(
'Public key authentication failed: invalid signature.',
cancellation,
);
// Verify that the signature matches the public key, proving that the client
// possesses the corresponding private key.
if (!(await this.verifySignature(message, publicKeyAlg, publicKey, cancellation))) {
return;
}

args = new SshAuthenticatingEventArgs(SshAuthenticationType.clientPublicKey, {
Expand All @@ -206,6 +194,54 @@ export class AuthenticationService extends SshService {
await this.handleAuthenticating(args, cancellation);
}

/**
* Verifies the signature on a public-key or host-based authentication request, proving
* that the sender possesses the private key corresponding to the presented public key.
*
* @returns True if the signature is valid. False if it is missing or invalid, in which case
* an authentication failure has already been sent and the caller MUST NOT proceed to raise
* the `Authenticating` event.
*/
private async verifySignature(
message: PublicKeyRequestMessage,
publicKeyAlg: PublicKeyAlgorithm,
publicKey: KeyPair,
cancellation?: CancellationToken,
): Promise<boolean> {
if (!message.hasSignature || !message.payloadWithoutSignature) {
await this.handleAuthenticationFailure(
'Public key authentication failed: missing signature.',
cancellation,
);
return false;
}

const sessionId = this.session.sessionId;
if (sessionId == null) {
throw new Error('Session ID not initialized.');
}

const signature = publicKeyAlg.readSignatureData(message.signature!);

const writer = new SshDataWriter(
Buffer.alloc(sessionId.length + message.payloadWithoutSignature.length + 20),
);
writer.writeBinary(sessionId);
writer.write(message.payloadWithoutSignature);

const signedData = writer.toBuffer();
const verifier = publicKeyAlg.createVerifier(publicKey);
if (!(await verifier.verify(signedData, signature))) {
await this.handleAuthenticationFailure(
'Public key authentication failed: invalid signature.',
cancellation,
);
return false;
}

return true;
}

private async handlePasswordRequestMessage(
message: PasswordRequestMessage,
cancellation?: CancellationToken,
Expand Down
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