go-pwdhash

go-pwdhash is a Go-first password hashing helper that embraces the PHC (Password Hashing Competition) format, wraps Argon2id with safe defaults, and surfaces a minimal API for hashing, verification, and upgrades.

Argon2id Only

pwdhash intentionally supports Argon2id only. Algorithms that have already been superseded by Argon2id will not be added, reducing the chance of accidentally selecting outdated primitives. If a superior successor to Argon2id emerges, pwdhash will adopt it behind the same API surface.

Password Policies

pwdhash ships with opinionated Argon2id policies so applications can select a strength profile without touching raw parameters:

• Interactive – user login flows where latency matters most.
• Moderate – API keys, service-to-service calls, and privileged automation.
• Sensitive – infrastructure secrets, root accounts, and long-lived credentials.

Policies prevent insecure configurations by clamping the underlying Argon2id memory, iteration, and parallelism values to vetted presets.

Highlights

• PHC-compliant output – hashes look like $argon2id$v=19$... and parse cleanly across ecosystems.
• Deterministic upgrade path – NeedsRehash compares stored parameters to the active policy so callers know exactly when to re-encode.
• Extensible registry – advanced users may inject tuned Argon2id instances or alternate hashers via the option system.
• Constant-time verification – comparisons use helpers under internal/subtle to avoid timing leaks.

Zeroization

pwdhash zeroizes password bytes, salts, and derived keys as soon as they are no longer needed. Due to Go runtime behavior this is a best-effort mitigation, but it significantly shortens the memory exposure window for sensitive material.

Installation

go get github.com/allisson/go-pwdhash

The module targets Go 1.24, depends on golang.org/x/crypto, and uses stretchr/testify solely for tests.

Quick Start

package main

import (
"fmt"

"github.com/allisson/go-pwdhash"
)

func main() {
hasher, err := pwdhash.New(
pwdhash.WithPolicy(pwdhash.PolicyInteractive),
)
if err != nil {
panic(err)
}

encoded, err := hasher.Hash([]byte("s3cret"))
if err != nil {
panic(err)
}

ok, err := hasher.Verify([]byte("s3cret"), encoded)
if err != nil {
panic(err)
}
fmt.Println("password matches?", ok)

rehash, err := hasher.NeedsRehash(encoded)
if err != nil {
panic(err)
}
fmt.Println("should upgrade hash?", rehash)
}

Usage Examples

Login Handler with Rehashes

func login(ctx context.Context, email, candidate string) error {
storedHash := loadHashFromDB(ctx, email)

hasher, err := pwdhash.New(pwdhash.WithPolicy(pwdhash.PolicyModerate))
if err != nil {
return fmt.Errorf("pwdhash init: %w", err)
}

ok, err := hasher.Verify([]byte(candidate), storedHash)
if err != nil {
return fmt.Errorf("pwdhash verify: %w", err)
}
if !ok {
return errInvalidCredentials
}

needsUpgrade, err := hasher.NeedsRehash(storedHash)
if err != nil {
return fmt.Errorf("pwdhash rehash check: %w", err)
}
if needsUpgrade {
upgraded, err := hasher.Hash([]byte(candidate))
if err != nil {
return fmt.Errorf("pwdhash rehash: %w", err)
}
persistHash(ctx, email, upgraded)
}

return nil
}

Role-Based Policies

type account struct {
Email string
Role  string
}

func policyForRole(role string) pwdhash.Policy {
switch role {
case "sre", "root":
return pwdhash.PolicySensitive
case "service", "api":
return pwdhash.PolicyModerate
default:
return pwdhash.PolicyInteractive
}
}

func hashForAccount(acct account, password []byte) (string, error) {
policy := policyForRole(acct.Role)

hasher, err := pwdhash.New(pwdhash.WithPolicy(policy))
if err != nil {
return "", err
}

return hasher.Hash(password)
}

Verifying External Hashes

When migrating from another service, you may only have PHC-formatted hashes. You can still apply the pwdhash registry to validate them while planning a gradual rehash:

func verifyLegacy(hash string, password []byte) (bool, error) {
hasher, err := pwdhash.New()
if err != nil {
return false, err
}

ok, err := hasher.Verify(password, hash)
if err != nil {
return false, err
}

// Decide later whether to rehash with NeedsRehash.
return ok, nil
}

Configuration

pwdhash.New accepts functional options:

• pwdhash.WithPolicy selects one of the built-in presets.
• pwdhash.WithHasher installs a custom pwdhash.Hasher (useful for bespoke Argon2id tuning or for experimenting with future algorithms).

Example of injecting custom parameters:

import (
"github.com/allisson/go-pwdhash"
"github.com/allisson/go-pwdhash/argon2"
)

func tunedHasher() (*pwdhash.PasswordHasher, error) {
argon := &argon2.Argon2idHasher{
Memory:      128 * 1024,
Iterations:  4,
Parallelism: 2,
SaltLength:  16,
KeyLength:   32,
}

return pwdhash.New(pwdhash.WithHasher(argon))
}

PHC Encoding

pwdhash serializes encoding.EncodedHash values using the canonical PHC layout:

$argon2id$v=19$m=65536,t=3,p=4$<base64(salt)>$<base64(hash)>

The parser validates algorithm identifiers, versions, parameter pairs, and Base64 payloads before handing them to the active hasher.

Testing & Tooling

go test ./...

Or use the convenience targets:

make lint   # golangci-lint run -v --fix
make test   # go test -covermode=count -coverprofile=count.out -v ./...

Contributing

1. Fork and clone the repo.
2. Run go test ./... (and make lint) before sending patches.
3. Keep exports documented, prefer table-driven tests, and stick to gofmt/goimports.
4. Argon2id is the focus; proposals for new algorithms should include rationale plus end-to-end tests.

License

MIT licensed. See LICENSE for details.