如何在Go中验证RSA密钥长度？

I'm setting up a https server in Go using the following function.

err := http.ListenAndServeTLS(":8080", key, cert, nil)
if err != nil {
    log.Fatal("error...")
}

Where key and cert are respectively self-signed key and certificate files. My problem is that for security I need to validate they self-signed key to have a 2048 bits size (or more). How can I securely and cleanly check for this in Go?

package main

import (
    "crypto/ecdsa"
    "crypto/rsa"
    "crypto/tls"
    "log"
    "net/http"
)

func main() {
    certFile := "/tmp/cert.pem"
    keyFile := "/tmp/key.pem"

    cert, err := tls.LoadX509KeyPair(certFile, keyFile)
    if err != nil {
        log.Fatal(err)
    }

    var bitLen int
    switch privKey := cert.PrivateKey.(type) {
    case *rsa.PrivateKey:
        bitLen = privKey.N.BitLen()
    case *ecdsa.PrivateKey:
        bitLen = privKey.Curve.Params().BitSize
    default:
        log.Fatal("unsupported private key")
    }

    if bitLen < 2048 {
        log.Fatalf("private key length is too small (size: %d)
", bitLen)
    }

    tlsConfig := tls.Config{
        Certificates: []tls.Certificate{cert},
    }

    server := http.Server{
        Addr:      ":8080",
        TLSConfig: &tlsConfig,
    }
    if err := server.ListenAndServeTLS("", ""); err != nil {
        log.Fatal(err)
    }
}