key.go

package main

// "Fork" Geth
// add:
// func DecryptKeyFromFile(keysDirPath string, keyAddr common.Address, auth string) (keyBytes []byte, keyId []byte, err error) {
//  return decryptKeyFromFile(keysDirPath, keyAddr, auth)
// }
// to crypto/key_store_passphrase.go

import (
  "fmt"
  "github.com/ethereum/go-ethereum/crypto"
  "github.com/ethereum/go-ethereum/common"
)

func main() {
  var address = common.HexToAddress("6b2474fb5b7203acc098f5186e6bf5beacde9364")
  var plainText, _, _ = crypto.DecryptKeyFromFile("/Users/david/Library/Ethereum/keystore/", address, "password")
  fmt.Println(crypto.ToECDSA(plainText).D)
}