return an error if signature is attempted with json boms

Signed-off-by: Leon <leon@1ux.dev>

Author: Lachstec

CycloneDX/ExitCode.cs

@@ -25,6 +25,7 @@ enum ExitCode
         LocalPackageCacheError,
         DotnetRestoreFailed,
         GitHubLicenseResolutionFailed,
-        UnableToLocateDependencyBomRef
+        UnableToLocateDependencyBomRef,
+        UnsupportedSignatureFormat
     }
 }

CycloneDX/Runner.cs

@@ -445,8 +445,16 @@ public async Task<int> HandleCommandAsync(RunOptions options)
 
             if (!string.IsNullOrEmpty(signingKeyPath))
             {
-                IBomSigner signer = new XmlBomSigner();
-                bomContents = await signer.SignAsync(signingKeyPath, bomContents);
+                if (format == OutputFileFormat.Xml)
+                {
+                    IBomSigner signer = new XmlBomSigner();
+                    bomContents = await signer.SignAsync(signingKeyPath, bomContents);
+                }
+                else
+                {
+                    Console.WriteLine("Signing the BOM is only supported with XML BOMs at the moment.");
+                    return (int)ExitCode.UnsupportedSignatureFormat;
+                }
             }