age/cmd/age-plugin-batchpass/plugin-batchpass.go

package main

import (
	"errors"
	"flag"
	"fmt"
	"io"
	"log"
	"os"
	"strconv"
	"strings"

	"filippo.io/age"
	"filippo.io/age/plugin"
)

const usage = `age-plugin-batchpass is an age plugin that enables non-interactive
passphrase-based encryption and decryption using environment variables.

WARNING:

This functionality is not built into the age CLI because most applications
should use native keys instead of scripting passphrase-based encryption.

Humans are notoriously bad at remembering and generating strong passphrases.
age uses scrypt to partially mitigate this, which is necessarily very slow.

If a computer will be doing the remembering anyway, you can and should use
native keys instead. There is no need to manage separate public and private
keys, you encrypt directly to the private key:

    $ age-keygen -o key.txt
    $ age -e -i key.txt file.txt > file.txt.age
    $ age -d -i key.txt file.txt.age > file.txt

Likewise, you can store a native identity string in an environment variable
or through your CI secrets manager and use it to encrypt and decrypt files
non-interactively:

    $ export AGE_SECRET=$(age-keygen)
    $ age -e -i <(echo "$AGE_SECRET") file.txt > file.txt.age
    $ age -d -i <(echo "$AGE_SECRET") file.txt.age > file.txt

The age CLI also natively supports passphrase-encrypted identity files, so you
can use that functionality to non-interactively encrypt multiple files such that
you will be able to decrypt them later by entering the same passphrase:

    $ age-keygen -pq | age -p -o encrypted-identity.txt
    Public key: age1pq1cd[... 1950 more characters ...]
    Enter passphrase (leave empty to autogenerate a secure one):
    age: using autogenerated passphrase "eternal-erase-keen-suffer-fog-exclude-huge-scorpion-escape-scrub"
    $ age -r age1pq1cd[... 1950 more characters ...] file.txt > file.txt.age
    $ age -d -i encrypted-identity.txt file.txt.age > file.txt
    Enter passphrase for identity file "encrypted-identity.txt":

Finally, when using this plugin care should be taken not to let the password be
persisted in the shell history or leaked to other users on multi-user systems.

Usage:

    $ AGE_PASSPHRASE=password age -e -j batchpass file.txt > file.txt.age

    $ AGE_PASSPHRASE=password age -d -j batchpass file.txt.age > file.txt

Alternatively, you can use AGE_PASSPHRASE_FD to read the passphrase from
a file descriptor. Trailing newlines are stripped from the file contents.

When encrypting, you can set AGE_PASSPHRASE_WORK_FACTOR to adjust the scrypt
work factor (between 1 and 30, default 18). Higher values are more secure
but slower.

When decrypting, you can set AGE_PASSPHRASE_MAX_WORK_FACTOR to limit the
maximum scrypt work factor accepted (between 1 and 30, default 30). This can
be used to avoid very slow decryptions.`

func main() {
	flag.Usage = func() { fmt.Fprintf(os.Stderr, "%s\n", usage) }

	p, err := plugin.New("batchpass")
	if err != nil {
		log.Fatal(err)
	}
	p.HandleIdentityAsRecipient(func(data []byte) (age.Recipient, error) {
		if len(data) != 0 {
			return nil, fmt.Errorf("batchpass identity does not take any payload")
		}
		pass, err := passphrase()
		if err != nil {
			return nil, err
		}
		r, err := age.NewScryptRecipient(pass)
		if err != nil {
			return nil, fmt.Errorf("failed to create scrypt recipient: %v", err)
		}
		if envWorkFactor := os.Getenv("AGE_PASSPHRASE_WORK_FACTOR"); envWorkFactor != "" {
			workFactor, err := strconv.Atoi(envWorkFactor)
			if err != nil {
				return nil, fmt.Errorf("invalid AGE_PASSPHRASE_WORK_FACTOR: %v", err)
			}
			if workFactor > 30 || workFactor < 1 {
				return nil, fmt.Errorf("AGE_PASSPHRASE_WORK_FACTOR must be between 1 and 30")
			}
			r.SetWorkFactor(workFactor)
		}
		return r, nil
	})
	p.HandleIdentity(func(data []byte) (age.Identity, error) {
		if len(data) != 0 {
			return nil, fmt.Errorf("batchpass identity does not take any payload")
		}
		pass, err := passphrase()
		if err != nil {
			return nil, err
		}
		maxWorkFactor := 0
		if envMaxWorkFactor := os.Getenv("AGE_PASSPHRASE_MAX_WORK_FACTOR"); envMaxWorkFactor != "" {
			maxWorkFactor, err = strconv.Atoi(envMaxWorkFactor)
			if err != nil {
				return nil, fmt.Errorf("invalid AGE_PASSPHRASE_MAX_WORK_FACTOR: %v", err)
			}
			if maxWorkFactor > 30 || maxWorkFactor < 1 {
				return nil, fmt.Errorf("AGE_PASSPHRASE_MAX_WORK_FACTOR must be between 1 and 30")
			}
		}
		return &batchpassIdentity{password: pass, maxWorkFactor: maxWorkFactor}, nil
	})
	os.Exit(p.Main())
}

type batchpassIdentity struct {
	password      string
	maxWorkFactor int
}

func (i *batchpassIdentity) Unwrap(stanzas []*age.Stanza) ([]byte, error) {
	for _, s := range stanzas {
		if s.Type == "scrypt" && len(stanzas) != 1 {
			return nil, errors.New("an scrypt recipient must be the only one")
		}
	}
	if len(stanzas) != 1 || stanzas[0].Type != "scrypt" {
		// Don't fallback to other identities, this plugin should mostly be used
		// in isolation, from the CLI.
		return nil, fmt.Errorf("file is not passphrase-encrypted")
	}
	ii, err := age.NewScryptIdentity(i.password)
	if err != nil {
		return nil, err
	}
	if i.maxWorkFactor != 0 {
		ii.SetMaxWorkFactor(i.maxWorkFactor)
	}
	fileKey, err := ii.Unwrap(stanzas)
	if errors.Is(err, age.ErrIncorrectIdentity) {
		// ScryptIdentity returns ErrIncorrectIdentity to make it possible to
		// try multiple passphrases from the API. If a user is invoking this
		// plugin, it's safe to say they expect it to be the only mechanism to
		// decrypt a passphrase-protected file.
		return nil, fmt.Errorf("incorrect passphrase")
	}
	return fileKey, err
}

func passphrase() (string, error) {
	envPASSPHRASE := os.Getenv("AGE_PASSPHRASE")
	envFD := os.Getenv("AGE_PASSPHRASE_FD")
	if envPASSPHRASE != "" && envFD != "" {
		return "", fmt.Errorf("AGE_PASSPHRASE and AGE_PASSPHRASE_FD are mutually exclusive")
	}
	if envPASSPHRASE == "" && envFD == "" {
		return "", fmt.Errorf("either AGE_PASSPHRASE or AGE_PASSPHRASE_FD must be set")
	}

	if envPASSPHRASE != "" {
		return envPASSPHRASE, nil
	}

	fd, err := strconv.Atoi(envFD)
	if err != nil {
		return "", fmt.Errorf("invalid AGE_PASSPHRASE_FD: %v", err)
	}
	f := os.NewFile(uintptr(fd), "AGE_PASSPHRASE_FD")
	if f == nil {
		return "", fmt.Errorf("failed to open file descriptor %d", fd)
	}
	defer f.Close()
	const maxPassphraseSize = 1024 * 1024 // 1 MiB
	b, err := io.ReadAll(io.LimitReader(f, maxPassphraseSize+1))
	if err != nil {
		return "", fmt.Errorf("failed to read passphrase from fd %d: %v", fd, err)
	}
	if len(b) > maxPassphraseSize {
		return "", fmt.Errorf("passphrase from fd %d is too long", fd)
	}
	return strings.TrimRight(string(b), "\r\n"), nil
}