How to convert .crt file to .pem file in c#

Question

I have the crt file and I need to convert it to pem by code in C#, how can I do that?

command in openssl:

openssl x509 -inform der -in file.crt -out file.pem

Answer 1

On modern versions of .NET you can use the PemEncoding class:

.NET 7:

string pem = PemEncoding.WriteString("CERTIFICATE", der);

.NET 5:

string pem = new string(PemEncoding.Write("CERTIFICATE", der));

But, in "stone-aged" .NET Framework you have to write the fairly easy transform yourself ($"-----BEGIN {label}-----\n{Convert.ToBase64String(der, Base64FormattingOptions.IncludeNewLines)}\n-----END {label}-----\n" being the short form).

Borrowed/tweaked from the Remarks section of https://learn.microsoft.com/en-us/dotnet/api/system.security.cryptography.x509certificates.certificaterequest.createsigningrequest?view=net-6.0#system-security-cryptography-x509certificates-certificaterequest-createsigningrequest:

private static string PemEncode(string label, byte[] der)
{
    StringBuilder builder = new StringBuilder();

    builder.Append("-----BEGIN ");
    builder.Append(label);
    builder.AppendLine("-----");

    string base64 = Convert.ToBase64String(pkcs10);

    int offset = 0;
    const int LineLength = 64;

    while (offset < base64.Length)
    {
        int lineEnd = Math.Min(offset + LineLength, base64.Length);
        builder.AppendLine(base64.Substring(offset, lineEnd - offset));
        offset = lineEnd;
     }

     builder.Append("-----END ");
     builder.Append(label);
     builder.AppendLine("-----");
     return builder.ToString();
}