A failing Diffie-Hellman WIP project in Go. Apparently Go cannot find the "big" package that it comes with?

dh.go

package main

import (
  "fmt"
  cr "crypto/rand"
  "os"
  enc "encoding/ascii85"
  "big"
)

func main() {
  var (
    f *os.File
    s []byte
    b = big.NewInt(0)
  )
  const G = 7
  
  f, _ = os.Open("/dev/urandom")
  defer f.Close()

  var P, _ = cr.Prime(f, 1024)
  s = make( []byte, enc.MaxEncodedLen(len(P.Bytes())) )

  _ = enc.Encode(s, P.Bytes())
  fmt.Printf("Prime: %s", s)

  // var xa = cr.Int(f, 1 << 16)
}