cmd/ethkey: add command to change key passphrase (#16516)

This change introduces 

    ethkey changepassphrase <keyfile>

to change the passphrase of a key file.

cmd/ethkey/changepassphrase.go

@@ -0,0 +1,72 @@
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"strings"
+
+	"github.com/ethereum/go-ethereum/accounts/keystore"
+	"github.com/ethereum/go-ethereum/cmd/utils"
+	"gopkg.in/urfave/cli.v1"
+)
+
+var newPassphraseFlag = cli.StringFlag{
+	Name:  "newpasswordfile",
+	Usage: "the file that contains the new passphrase for the keyfile",
+}
+
+var commandChangePassphrase = cli.Command{
+	Name:      "changepassphrase",
+	Usage:     "change the passphrase on a keyfile",
+	ArgsUsage: "<keyfile>",
+	Description: `
+Change the passphrase of a keyfile.`,
+	Flags: []cli.Flag{
+		passphraseFlag,
+		newPassphraseFlag,
+	},
+	Action: func(ctx *cli.Context) error {
+		keyfilepath := ctx.Args().First()
+
+		// Read key from file.
+		keyjson, err := ioutil.ReadFile(keyfilepath)
+		if err != nil {
+			utils.Fatalf("Failed to read the keyfile at '%s': %v", keyfilepath, err)
+		}
+
+		// Decrypt key with passphrase.
+		passphrase := getPassphrase(ctx)
+		key, err := keystore.DecryptKey(keyjson, passphrase)
+		if err != nil {
+			utils.Fatalf("Error decrypting key: %v", err)
+		}
+
+		// Get a new passphrase.
+		fmt.Println("Please provide a new passphrase")
+		var newPhrase string
+		if passFile := ctx.String(newPassphraseFlag.Name); passFile != "" {
+			content, err := ioutil.ReadFile(passFile)
+			if err != nil {
+				utils.Fatalf("Failed to read new passphrase file '%s': %v", passFile, err)
+			}
+			newPhrase = strings.TrimRight(string(content), "\r\n")
+		} else {
+			newPhrase = promptPassphrase(true)
+		}
+
+		// Encrypt the key with the new passphrase.
+		newJson, err := keystore.EncryptKey(key, newPhrase, keystore.StandardScryptN, keystore.StandardScryptP)
+		if err != nil {
+			utils.Fatalf("Error encrypting with new passphrase: %v", err)
+		}
+
+		// Then write the new keyfile in place of the old one.
+		if err := ioutil.WriteFile(keyfilepath, newJson, 600); err != nil {
+			utils.Fatalf("Error writing new keyfile to disk: %v", err)
+		}
+
+		// Don't print anything.  Just return successfully,
+		// producing a positive exit code.
+		return nil
+	},
+}