steward/internal/auth/auth.go

// Package auth handles passwords, API tokens, sessions and permissions.
package auth

import (
	"crypto/rand"
	"crypto/sha256"
	"crypto/subtle"
	"encoding/base32"
	"encoding/base64"
	"encoding/hex"
	"errors"
	"fmt"
	"sort"
	"strconv"
	"strings"

	"golang.org/x/crypto/argon2"

	"steward/internal/model"
)

// Argon2id parameters (OWASP: m=19 MiB, t=2, p=1).
const (
	argonTime    = 2
	argonMemory  = 19 * 1024
	argonThreads = 1
	argonKeyLen  = 32
	saltLen      = 16
)

// MinPasswordLength is the shortest accepted password.
const MinPasswordLength = 8

// HashPassword returns an argon2id hash in PHC string format.
func HashPassword(password string) (string, error) {
	if len(password) < MinPasswordLength {
		return "", fmt.Errorf("a password needs at least %d characters", MinPasswordLength)
	}
	salt := make([]byte, saltLen)
	if _, err := rand.Read(salt); err != nil {
		return "", err
	}
	key := argon2.IDKey([]byte(password), salt, argonTime, argonMemory, argonThreads, argonKeyLen)
	return fmt.Sprintf("$argon2id$v=%d$m=%d,t=%d,p=%d$%s$%s", argon2.Version, argonMemory, argonTime, argonThreads,
		base64.RawStdEncoding.EncodeToString(salt), base64.RawStdEncoding.EncodeToString(key)), nil
}

// dummyHash is checked when a user doesn't exist, so a login takes as
// long whether or not the username is right.
var dummyHash, _ = HashPassword("not-a-real-password")

// CheckPassword reports whether password matches hash. An empty hash never
// matches (in the same time).
func CheckPassword(hash, password string) bool {
	if hash == "" {
		hash = dummyHash
		password = "\x00" + password
	}
	parts := strings.Split(hash, "$")
	if len(parts) != 6 || parts[1] != "argon2id" {
		return false
	}
	var m, t uint32
	var p uint8
	for _, kv := range strings.Split(parts[3], ",") {
		k, v, _ := strings.Cut(kv, "=")
		n, err := strconv.ParseUint(v, 10, 32)
		if err != nil {
			return false
		}
		switch k {
		case "m":
			m = uint32(n)
		case "t":
			t = uint32(n)
		case "p":
			p = uint8(n)
		}
	}
	salt, err1 := base64.RawStdEncoding.DecodeString(parts[4])
	want, err2 := base64.RawStdEncoding.DecodeString(parts[5])
	if err1 != nil || err2 != nil || m == 0 || t == 0 || p == 0 {
		return false
	}
	got := argon2.IDKey([]byte(password), salt, t, m, p, uint32(len(want)))
	return subtle.ConstantTimeCompare(got, want) == 1
}

var b32 = base32.NewEncoding("abcdefghijklmnopqrstuvwxyz234567").WithPadding(base32.NoPadding)

// RandomString returns a random string with nbytes of entropy.
func RandomString(nbytes int) string {
	b := make([]byte, nbytes)
	if _, err := rand.Read(b); err != nil {
		panic(err)
	}
	return b32.EncodeToString(b)
}

// TokenPrefix starts every API token, so leaked ones are easy to spot.
const TokenPrefix = "steward_"

// NewToken makes a token: the whole string to give out, its public ID
// (stored in clear) and the hash of its secret.
func NewToken() (token, publicID, secretHash string) {
	publicID = RandomString(10) // 16 characters
	secret := RandomString(20)  // 32 characters
	return TokenPrefix + publicID + "_" + secret, publicID, HashSecret(secret)
}

// ParseToken splits a token into its public ID and secret.
func ParseToken(token string) (publicID, secret string, err error) {
	rest, ok := strings.CutPrefix(strings.TrimSpace(token), TokenPrefix)
	if !ok {
		return "", "", errors.New("not a Steward token (they start with " + TokenPrefix + ")")
	}
	publicID, secret, ok = strings.Cut(rest, "_")
	if !ok || len(publicID) != 16 || len(secret) != 32 {
		return "", "", errors.New("malformed token")
	}
	return publicID, secret, nil
}

// HashSecret hashes a high-entropy secret (token secrets, session IDs).
func HashSecret(secret string) string {
	h := sha256.Sum256([]byte(secret))
	return hex.EncodeToString(h[:])
}

// CheckSecret compares a secret with a stored hash in constant time.
func CheckSecret(secret, hash string) bool {
	return subtle.ConstantTimeCompare([]byte(HashSecret(secret)), []byte(hash)) == 1
}

// NewSessionID returns a session ID and its CSRF token.
func NewSessionID() (id, csrf string) {
	return RandomString(32), RandomString(32)
}

// Resources grants apply to.
const (
	ResConfig = "config" // settings, subnets, reservations, history
	ResLeases = "leases" // leases and DHCP activity
	ResAudit  = "audit"  // the audit log
	ResAll    = "*"
)

// Resources lists them in display order.
var Resources = []string{ResConfig, ResLeases, ResAudit}

// Access levels.
const (
	Read  = "read"
	Write = "write"
)

// Roles are ready-made sets of grants.
var Roles = map[string][]model.Grant{
	"viewer":   {{Resource: ResConfig, Access: Read}, {Resource: ResLeases, Access: Read}},
	"operator": {{Resource: ResConfig, Access: Write}, {Resource: ResLeases, Access: Write}, {Resource: ResAudit, Access: Read}},
	"leases":   {{Resource: ResLeases, Access: Read}},
}

// RoleNames lists the roles.
func RoleNames() []string {
	var out []string
	for n := range Roles {
		out = append(out, n)
	}
	sort.Strings(out)
	return out
}

// ParseGrant reads "resource:access" ("leases:read", "*:write").
func ParseGrant(s string) (model.Grant, error) {
	res, acc, ok := strings.Cut(strings.ToLower(strings.TrimSpace(s)), ":")
	g := model.Grant{Resource: res, Access: acc}
	if !ok {
		return g, fmt.Errorf("grant %q: write it as resource:access, e.g. leases:read", s)
	}
	return g, ValidateGrant(&g)
}

// ValidateGrant checks a grant.
func ValidateGrant(g *model.Grant) error {
	g.Resource = strings.ToLower(strings.TrimSpace(g.Resource))
	g.Access = strings.ToLower(strings.TrimSpace(g.Access))
	ok := g.Resource == ResAll
	for _, r := range Resources {
		ok = ok || g.Resource == r
	}
	if !ok {
		return fmt.Errorf("unknown resource %q (use %s or *)", g.Resource, strings.Join(Resources, ", "))
	}
	if g.Access != Read && g.Access != Write {
		return fmt.Errorf("access must be read or write, not %q", g.Access)
	}
	return nil
}

// FormatGrant writes a grant as "resource:access".
func FormatGrant(g model.Grant) string { return g.Resource + ":" + g.Access }

// Principal is who makes a request.
type Principal struct {
	User   *model.User
	Token  *model.Token
	Method string // session, token, socket
	IP     string
	Agent  string
}

// Name is the user's name.
func (p *Principal) Name() string {
	if p == nil || p.User == nil {
		return ""
	}
	return p.User.Username
}

// IsAdmin reports whether the principal has full rights: an
// administrator, not through a restricted token.
func (p *Principal) IsAdmin() bool {
	return p.User != nil && p.User.Admin && (p.Token == nil || p.Token.Grants == nil)
}

// Can reports whether the principal may read (write false) or change
// (write true) a resource.
func (p *Principal) Can(resource string, write bool) bool {
	if p == nil || p.User == nil {
		return false
	}
	allowed := p.User.Admin || covers(p.User.Grants, resource, write)
	if p.Token != nil && p.Token.Grants != nil {
		allowed = allowed && covers(p.Token.Grants, resource, write)
	}
	return allowed
}

func covers(gs []model.Grant, resource string, write bool) bool {
	for _, g := range gs {
		if (g.Resource == ResAll || g.Resource == resource) && (!write || g.Access == Write) {
			return true
		}
	}
	return false
}

// Permissions lists what the principal may do, for display.
func (p *Principal) Permissions() []string {
	var out []string
	for _, r := range Resources {
		switch {
		case p.Can(r, true):
			out = append(out, r+":write")
		case p.Can(r, false):
			out = append(out, r+":read")
		}
	}
	if p.IsAdmin() {
		out = append(out, "users:write")
	}
	return out
}

// Narrower reports whether every grant in a is covered by b.
func Narrower(a, b []model.Grant) bool {
	for _, g := range a {
		if !covers(b, g.Resource, g.Access == Write) {
			if g.Resource != ResAll {
				return false
			}
			for _, r := range Resources {
				if !covers(b, r, g.Access == Write) {
					return false
				}
			}
		}
	}
	return true
}