Code Example

Imports System
Imports System.IO
Imports System.Net
Imports System.Security.Cryptography
Imports System.IdentityModel.Tokens.Jwt
Imports Microsoft.IdentityModel.Tokens
Imports Org.BouncyCastle.OpenSsl
Imports Org.BouncyCastle.Crypto.Parameters
Imports Org.BouncyCastle.Security

Module Program

    '=== YOUR APP CONFIGURATION ===
    Const AppId As String = "2203300" ' GitHub App ID (numeric)
    Const InstallationId As String = "92200261" ' Installation ID
    Const PemFileName As String = "myworksandbox.2025-10-29.private-key.pem" ' PEM file in output folder

    Sub Main()
        Try
            Console.WriteLine("Checking if tag exists...")

            Dim owner As String = "your-owner-here" ' e.g. "elync"
            Dim repo As String = "your-repo-here"   ' e.g. "GitHubAppTest"
            Dim tagName As String = "v1.0.0"        ' e.g. "v1.0.0"

            Dim exists As Boolean = TagExists(owner, repo, tagName)
            Console.WriteLine($"Tag '{tagName}' exists on {owner}/{repo}: {exists}")

        Catch ex As Exception
            Console.WriteLine("ERROR: " & ex.Message)
        End Try

        Console.WriteLine(vbCrLf & "Done. Press any key to exit.")
        Console.ReadKey()
    End Sub

    '=====================
    '   MAIN FUNCTION
    '=====================
    Public Function TagExists(owner As String, repo As String, tagName As String) As Boolean
        Dim token As String = GetInstallationToken()

        Dim url As String = $"https://api.github.com/repos/{owner}/{repo}/git/ref/tags/{tagName}"
        Dim request = CType(WebRequest.Create(url), HttpWebRequest)
        request.Method = "GET"
        request.UserAgent = "GitHubAppTest"
        request.Headers.Add("Authorization", "Bearer " & token)
        request.Accept = "application/vnd.github+json"

        Try
            Using response As HttpWebResponse = CType(request.GetResponse(), HttpWebResponse)
                If response.StatusCode = HttpStatusCode.OK Then
                    Return True
                End If
            End Using
        Catch ex As WebException
            If ex.Response IsNot Nothing Then
                Dim httpResp = CType(ex.Response, HttpWebResponse)
                If httpResp.StatusCode = HttpStatusCode.NotFound Then
                    Return False ' Tag does not exist
                ElseIf httpResp.StatusCode = HttpStatusCode.Unauthorized Then
                    Throw New Exception("Unauthorized. Check token permissions or app installation.")
                End If
            End If
            Throw
        End Try

        Return False
    End Function

    '=====================
    ' INSTALLATION TOKEN
    '=====================
    Private Function GetInstallationToken() As String
        Dim pem As String = File.ReadAllText(PemFileName)
        Using rsa As RSA = LoadRsaFromPem(pem)
            Dim jwt As String = GenerateJwt(AppId, rsa)

            Dim url As String = $"https://api.github.com/app/installations/{InstallationId}/access_tokens"
            Dim request = CType(WebRequest.Create(url), HttpWebRequest)
            request.Method = "POST"
            request.UserAgent = "GitHubAppTest"
            request.Headers.Add("Authorization", "Bearer " & jwt)
            request.Accept = "application/vnd.github+json"

            Using response As HttpWebResponse = CType(request.GetResponse(), HttpWebResponse)
                Using reader As New StreamReader(response.GetResponseStream())
                    Dim json = reader.ReadToEnd()
                    Dim token = Newtonsoft.Json.Linq.JObject.Parse(json)("token")?.ToString()
                    If token Is Nothing Then Throw New Exception("Could not retrieve installation token.")
                    Return token
                End Using
            End Using
        End Using
    End Function

    '=====================
    ' JWT GENERATOR
    '=====================
    Private Function GenerateJwt(appId As String, rsa As RSA) As String
        Dim securityKey As New RsaSecurityKey(rsa)
        Dim credentials As New SigningCredentials(securityKey, SecurityAlgorithms.RsaSha256)
        Dim handler As New JwtSecurityTokenHandler()

        Dim now = DateTimeOffset.UtcNow
        Dim claims = New List(Of System.Security.Claims.Claim) From {
            New System.Security.Claims.Claim(JwtRegisteredClaimNames.Iat, now.ToUnixTimeSeconds().ToString(), System.Security.Claims.ClaimValueTypes.Integer64)
        }

        Dim token As New JwtSecurityToken(
            issuer:=appId,
            audience:=Nothing,
            claims:=claims,
            notBefore:=now.UtcDateTime,
            expires:=now.AddMinutes(9).UtcDateTime,
            signingCredentials:=credentials
        )

        Return handler.WriteToken(token)
    End Function

    '=====================
    ' PEM PARSING
    '=====================
    Private Function LoadRsaFromPem(pem As String) As RSA
        Using reader As New StringReader(pem)
            Dim pemReader As New PemReader(reader)
            Dim keyObject = pemReader.ReadObject()

            If TypeOf keyObject Is Org.BouncyCastle.Crypto.AsymmetricCipherKeyPair Then
                Dim privateKeyParams = CType(keyObject, Org.BouncyCastle.Crypto.AsymmetricCipherKeyPair).Private
                Return CreateRsa(CType(privateKeyParams, RsaPrivateCrtKeyParameters))
            ElseIf TypeOf keyObject Is RsaPrivateCrtKeyParameters Then
                Return CreateRsa(CType(keyObject, RsaPrivateCrtKeyParameters))
            Else
                Throw New Exception("Unsupported PEM format.")
            End If
        End Using
    End Function

    Private Function CreateRsa(privateKey As RsaPrivateCrtKeyParameters) As RSA
        Dim rsaParams As RSAParameters = DotNetUtilities.ToRSAParameters(privateKey)
        Dim rsa As RSA = RSA.Create()
        rsa.ImportParameters(rsaParams)
        Return rsa
    End Function

End Module