pansy/internal/service/password.go

package service

import (
	"crypto/rand"
	"crypto/subtle"
	"encoding/base64"
	"errors"
	"fmt"
	"strings"

	"golang.org/x/crypto/argon2"
)

// argon2id parameters: RFC 9106's second recommended profile (m=64 MiB, t=3,
// p=4) — the memory-constrained option, appropriate for a self-hosted box where
// the 2 GiB first profile is too heavy. They are encoded into every stored hash,
// so raising them later leaves old hashes verifiable.
const (
	argonMemKiB  = 64 * 1024 // 64 MiB
	argonTime    = 3
	argonThreads = 4
	argonKeyLen  = 32
	argonSaltLen = 16
)

// errBadHash marks a stored hash string that could not be parsed — a data or
// programming error, not a wrong password. Callers treat it as an auth failure
// and log it (Login does).
var errBadHash = errors.New("service: malformed password hash")

// b64 is the padding-free base64 used inside the PHC-style hash string.
var b64 = base64.RawStdEncoding

// timingHash is a valid argon2id hash used to equalize login response time when
// an email is unknown (see Service.dummyHash). It uses a fixed salt rather than
// crypto/rand so it can never fail to be produced at startup — an all-important
// property, since a missing timing hash would silently re-open account
// enumeration. It guards nothing, so a static salt is safe; deriving it from the
// live argon parameters keeps its cost matched to a real verify.
func timingHash() string {
	salt := []byte("pansy-timing-eq!") // exactly argonSaltLen (16) bytes
	key := argon2.IDKey([]byte("x"), salt, argonTime, argonMemKiB, argonThreads, argonKeyLen)
	return fmt.Sprintf("$argon2id$v=%d$m=%d,t=%d,p=%d$%s$%s",
		argon2.Version, argonMemKiB, argonTime, argonThreads,
		b64.EncodeToString(salt), b64.EncodeToString(key),
	)
}

// hashPassword returns a self-describing argon2id hash in the PHC string format
// "$argon2id$v=19$m=...,t=...,p=...$salt$hash" (all base64, no padding).
func hashPassword(password string) (string, error) {
	salt := make([]byte, argonSaltLen)
	if _, err := rand.Read(salt); err != nil {
		return "", fmt.Errorf("service: generate salt: %w", err)
	}
	key := argon2.IDKey([]byte(password), salt, argonTime, argonMemKiB, argonThreads, argonKeyLen)
	return fmt.Sprintf("$argon2id$v=%d$m=%d,t=%d,p=%d$%s$%s",
		argon2.Version, argonMemKiB, argonTime, argonThreads,
		b64.EncodeToString(salt), b64.EncodeToString(key),
	), nil
}

// verifyPassword reports whether password matches the encoded argon2id hash. The
// comparison is constant-time. A malformed encoded value returns errBadHash.
func verifyPassword(encoded, password string) (bool, error) {
	mem, t, threads, salt, want, err := decodeHash(encoded)
	if err != nil {
		return false, err
	}
	got := argon2.IDKey([]byte(password), salt, t, mem, threads, uint32(len(want)))
	return subtle.ConstantTimeCompare(got, want) == 1, nil
}

// decodeHash parses a PHC-format argon2id string into its parameters, salt, and
// derived key. Any structural problem returns errBadHash (with zero values) so
// the caller never has to distinguish parse failures.
func decodeHash(encoded string) (mem, t uint32, threads uint8, salt, key []byte, err error) {
	fail := func() (uint32, uint32, uint8, []byte, []byte, error) {
		return 0, 0, 0, nil, nil, errBadHash
	}

	parts := strings.Split(encoded, "$")
	// ["", "argon2id", "v=19", "m=..,t=..,p=..", "<salt>", "<hash>"]
	if len(parts) != 6 || parts[1] != "argon2id" {
		return fail()
	}

	var version int
	if _, e := fmt.Sscanf(parts[2], "v=%d", &version); e != nil || version != argon2.Version {
		return fail()
	}
	if _, e := fmt.Sscanf(parts[3], "m=%d,t=%d,p=%d", &mem, &t, &threads); e != nil {
		return fail()
	}
	if salt, err = b64.DecodeString(parts[4]); err != nil {
		return fail()
	}
	if key, err = b64.DecodeString(parts[5]); err != nil || len(key) == 0 {
		return fail()
	}
	return mem, t, threads, salt, key, nil
}