The following example is a snippet of a C# code that generates an RSA private key out of a .pem file and uses it to sign a JWT. The privateKey variable, stores the contents of the .pem file minus the "-----BEGIN RSA PRIVATE KEY----" and "-----END RSA PRIVATE KEY-----" lines.
string privateKey = @" MIIEpAIBAA ... y53DdfYA=="; byte[] RSAprivateKey = Convert.FromBase64String(privateKey); RSA rsa = RSA.Create(); rsa.ImportRSAPrivateKey(RSAprivateKey, out _); var jwt = tokenHandler.CreateEncodedJwt( issuer: "...", audience: ..., ... signingCredentials: new SigningCredentials( key: new RsaSecurityKey(rsa), algorithm: SecurityAlgorithms.RsaSha256) );