dotnet-strong-name-signing

name

dotnet-strong-name-signing

description

Generate a strong name key (.snk) file for signing .NET assemblies using pure .NET cryptography — no Visual Studio Developer PowerShell or sn.exe required. Works in any terminal. Use this skill when the user wants to create a strong name key, generate an .snk file, sign .NET assemblies, or mentions "strong-name", "snk", "AssemblyOriginatorKeyFile", "SignAssembly", or asks how to sign a .NET library. Also use when scaffolding .NET libraries or NuGet packages that need assembly signing. ALWAYS use this skill when asked to generate or create a strong name key file.

Strong Name Signing for .NET

Generate a strong name key pair (.snk file) for signing .NET assemblies. Uses the .NET runtime's built-in RSACryptoServiceProvider instead of sn.exe, so it works in any PowerShell or terminal — no Visual Studio Developer Command Prompt needed.

Why this matters

The traditional approach requires sn.exe -k MyKey.snk, which is only available in the Visual Studio Developer PowerShell. This is a common pain point — developers outside Visual Studio (using VS Code, Rider, or plain terminals) can't easily generate key files. The pure .NET approach eliminates this dependency entirely.

Strong names in .NET are about identity, not security (Microsoft's guidance). They ensure assembly uniqueness and are recommended for all publicly published NuGet packages because of strong-naming's viral nature — an unsigned library can't be consumed by signed applications.

Workflow

Step 1: Collect Parameters

Read FORMS.md, compute the defaults silently, and present a single summary for confirmation. Only ask follow-up questions for individual fields if the user wants to override a computed or default value. Do not proceed to Step 2 until the user confirms the summary.

Step 2: Generate the Key File

Run this PowerShell command block with pwsh 7+ in the target directory:

$rsa = New-Object System.Security.Cryptography.RSACryptoServiceProvider({KEY_SIZE})
$keyBlob = $rsa.ExportCspBlob($true)
[System.IO.File]::WriteAllBytes("{OUTPUT_PATH}", $keyBlob)
$rsa.Dispose()

Where:

• {KEY_SIZE} — RSA key size from parameters (default: 1024)
• {OUTPUT_PATH} — full path combining {OUTPUT_DIR} and {KEY_NAME}.snk

The ExportCspBlob($true) method exports the full key pair (public + private) in the exact CSP blob format that sn.exe -k produces. The $true parameter includes the private key — essential for signing during builds.

Step 3: Verify and Report

After generating the file, verify it exists and report:

$snkFile = Get-Item "{OUTPUT_PATH}"
Write-Host "✅ Strong name key generated"
Write-Host ""
Write-Host "  File:     $($snkFile.Name)"
Write-Host "  Size:     $($snkFile.Length) bytes"
Write-Host "  Location: $($snkFile.FullName)"
Write-Host "  Key size: {KEY_SIZE}-bit RSA"

Then provide usage guidance based on what was generated:

  Usage in .csproj:
    <PropertyGroup>
      <SignAssembly>true</SignAssembly>
      <AssemblyOriginatorKeyFile>path\to\{KEY_NAME}.snk</AssemblyOriginatorKeyFile>
    </PropertyGroup>

  Or via Directory.Build.props for solution-wide signing.

Step 4: Security Reminder

Remind the user about .snk file handling:

• Open source projects: Microsoft recommends checking in the .snk file — strong names are for identity, not security. This lets contributors build drop-in replacements.
• Closed source / proprietary: Keep the .snk file out of source control. Add *.snk to .gitignore and distribute through secure channels (CI/CD secrets, key vaults).
• Public signing alternative: For open source projects that want signing without distributing the private key, consider public signing with <PublicSign>true</PublicSign>.

Technical Notes

Why RSACryptoServiceProvider over RSA.Create()?

RSACryptoServiceProvider.ExportCspBlob() produces the exact CSP (Cryptographic Service Provider) blob format that MSBuild's SignAssembly task expects. While RSA.Create() is the modern API, its ExportRSAPrivateKey() outputs PKCS#1 DER — a different format that would require conversion. The CSP approach is a direct drop-in replacement for sn.exe output.

Key Size

• 1024-bit (default): Matches sn.exe -k default. Strong names are about identity, not security — this is sufficient for the vast majority of projects.
• 2048-bit: Larger key, no practical benefit for strong naming but available if desired.
• 4096-bit: Largest key. Only needed if organizational policy requires it.

Cross-Platform

This approach works on Windows, macOS, and Linux — anywhere the .NET runtime or PowerShell 7+ is installed. The RSACryptoServiceProvider class is available in both .NET Framework and .NET (Core).