restic/vendor/github.com/elithrar/simple-scrypt/README.md

simple-scrypt

simple-scrypt provides a convenience wrapper around Go's existing scrypt package that makes it easier to securely derive strong keys ("hash user passwords"). This library allows you to:

• Generate a scrypt derived key with a crytographically secure salt and sane default parameters for N, r and p.
• Upgrade the parameters used to generate keys as hardware improves by storing them with the derived key (the scrypt spec. doesn't allow for this by default).
• Provide your own parameters (if you wish to).

The API closely mirrors Go's bcrypt library in an effort to make it easy to migrate—and because it's an easy to grok API.

Example

simple-scrypt doesn't try to re-invent the wheel or do anything "special". It wraps the scrypt.Key function as thinly as possible, generates a crytographically secure salt for you using Go's crypto/rand package, and returns the derived key with the parameters prepended:

package main

import(
    "fmt"
    "log"

    "github.com/elithrar/simple-scrypt"
)

func main() {
    // e.g. r.PostFormValue("password")
    passwordFromForm := "prew8fid9hick6c"

    // Generates a derived key of the form "N$r$p$salt$dk" where N, r and p are defined as per
    // Colin Percival's scrypt paper: http://www.tarsnap.com/scrypt/scrypt.pdf
    // scrypt.Defaults (N=16384, r=8, p=1) makes it easy to provide these parameters, and
    // (should you wish) provide your own values via the scrypt.Params type.
    hash, err := scrypt.GenerateFromPassword([]byte(passwordFromForm), scrypt.DefaultParams)
    if err != nil {
        log.Fatal(err)
    }

    // Print the derived key with its parameters prepended.
    fmt.Printf("%s\n", hash)

    // Uses the parameters from the existing derived key. Return an error if they don't match.
    err := scrypt.CompareHashAndPassword(hash, []byte(passwordFromForm))
    if err != nil {
        log.Fatal(err)
    }
}

Upgrading Parameters

Upgrading derived keys from a set of parameters to a "stronger" set of parameters as hardware improves, or as you scale (and move your auth process to separate hardware), can be pretty useful. Here's how to do it with simple-scrypt:

func main() {
    // SCENE: We've successfully authenticated a user, compared their submitted
    // (cleartext) password against the derived key stored in our database, and
    // now want to upgrade the parameters (more rounds, more parallelism) to
    // reflect some shiny new hardware we just purchased. As the user is logging
    // in, we can retrieve the parameters used to generate their key, and if
    // they don't match our "new" parameters, we can re-generate the key while
    // we still have the cleartext password in memory
    // (e.g. before the HTTP request ends).
    current, err := scrypt.Cost(hash)
    if err != nil {
        log.Fatal(err)
    }

    // Now to check them against our own Params struct (e.g. using reflect.DeepEquals)
    // and determine whether we want to generate a new key with our "upgraded" parameters.
    slower := scrypt.Params{
        N: 32768,
        R: 8,
        P: 2,
        SaltLen: 16,
        DKLen: 32,
    }

    if !reflect.DeepEqual(current, slower) {
        // Re-generate the key with the slower parameters
        // here using scrypt.GenerateFromPassword
    }
}

TO-DO:

The following features are planned. PRs are welcome.

• Tag a release build.
• Automatically calculate "optimal" values for N, r, p similar to the Ruby scrypt library e.g. func Calibrate(duration int, mem int, fallback Params) (Params, error)
  • contributed thanks to @tgulacsi.

License

MIT Licensed. See LICENSE file for details.