Add sutil CLI in cmd/sutil with internal/cli command tree

internal/cli/cbk.go

@@ -0,0 +1,82 @@
+package cli
+
+import (
+	"fmt"
+	"io"
+
+	"github.com/spf13/cobra"
+
+	"sneak.berlin/go/x/pkg/cbk"
+)
+
+// maxMnemonicInputBytes bounds how much of stdin `cbk derive` will read; a
+// 24-word mnemonic is well under 300 bytes.
+const maxMnemonicInputBytes = 64 << 10
+
+// newCBKCommand returns `sutil cbk`, the parent of the crypto_box
+// keypair commands.
+func newCBKCommand() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "cbk",
+		Short: "crypto_box keypairs derived from BIP39 mnemonics",
+	}
+
+	cmd.AddCommand(newCBKDeriveCommand())
+
+	return cmd
+}
+
+// newCBKDeriveCommand returns `sutil cbk derive`.
+func newCBKDeriveCommand() *cobra.Command {
+	var publicOnly bool
+
+	cmd := &cobra.Command{
+		Use:   "derive",
+		Short: "Derive a crypto_box keypair from a mnemonic read on stdin",
+		Long: `Reads a BIP39 mnemonic from stdin, in any letter case and with any
+whitespace between words, and prints the crypto_box keypair derived from it
+as hex. The same mnemonic always yields the same keypair.
+
+The mnemonic is read from stdin rather than taken as an argument so that it
+does not end up in shell history or process listings:
+
+    sutil generate mnemonic > mnemonic.txt
+    sutil cbk derive < mnemonic.txt`,
+		Args: cobra.NoArgs,
+		RunE: func(cmd *cobra.Command, _ []string) error {
+			input := io.LimitReader(cmd.InOrStdin(), maxMnemonicInputBytes)
+
+			mnemonic, err := io.ReadAll(input)
+			if err != nil {
+				return fmt.Errorf("reading mnemonic from stdin: %w", err)
+			}
+
+			keypair, err := cbk.New(cbk.Params{Mnemonic: string(mnemonic)})
+			if err != nil {
+				return err
+			}
+
+			return writeKeypair(cmd.OutOrStdout(), keypair, publicOnly)
+		},
+	}
+
+	cmd.Flags().BoolVar(&publicOnly, "public", false, "print only the public key")
+
+	return cmd
+}
+
+// writeKeypair prints the keypair as labeled hex lines, or just the bare
+// public key when publicOnly is set.
+func writeKeypair(w io.Writer, keypair cbk.CryptoBoxKeypair, publicOnly bool) error {
+	if publicOnly {
+		_, err := fmt.Fprintln(w, keypair.PublicKeyHex())
+
+		return err
+	}
+
+	_, err := fmt.Fprintf(
+		w, "public: %s\nprivate: %s\n", keypair.PublicKeyHex(), keypair.PrivateKeyHex(),
+	)
+
+	return err
+}