api/internal/auth/token.go

package auth

import (
	"context"
	"errors"
	"fmt"
	"net/http"
	"strings"
	"time"

	jwt "github.com/dgrijalva/jwt-go"
)

var contextKey = &struct{ data string }{"Token Context Key"}

// ErrNoKid is returned if the key id is missing from the jwt token header,
var ErrNoKid = errors.New("Missing \"kid\" field in token")

// ErrKeyNotFound is returned if the key wasn't found.
var ErrKeyNotFound = errors.New("Key not found")

// ErrInvalidClaims is returned by parseClaims if the claims cannot be parsed
var ErrInvalidClaims = errors.New("Invalid claims in token")

// ErrExpired is returned by parseClaims if the expiry date is in the past
var ErrExpired = errors.New("Claims have already expired")

// ErrWrongUser is returned by CheckToken if the key cannot represent this user
var ErrWrongUser = errors.New("Key is not valid for this user")

// ErrWrongPermissions is returned by CheckToken if the key cannot claim one or more of its permissions
var ErrWrongPermissions = errors.New("Key is not valid for this user")

// ErrDeletedUser is returned by CheckToken if the key can represent this user, but the user doesn't exist.
var ErrDeletedUser = errors.New("User was not found")

// A Token contains the parsed results from an bearer token. Its methods are safe to use with a nil receiver, but
// the userID should be checked.
type Token struct {
	UserID      string
	Permissions []string
}

// Authenticated returns true if the token is non-nil and parsed
func (token *Token) Authenticated() bool {
	return token != nil && token.UserID != ""
}

// Permitted returns true if the token is non-nil and has the given permission or the "admin" permission
func (token *Token) Permitted(permissions ...string) bool {
	if token == nil {
		return false
	}

	for _, tokenPermission := range token.Permissions {
		if tokenPermission == "admin" {
			return true
		}

		for _, permission := range permissions {
			if permission == tokenPermission {
				return true
			}
		}
	}

	return false
}

// PermittedUser checks the first permission if the user matches, the second otherwise. This is a common
// pattern.
func (token *Token) PermittedUser(userID, permissionIfUser, permissionOtherwise string) bool {
	if token == nil {
		return false
	}

	if token.UserID == userID {
		return token.Permitted(permissionIfUser)
	}

	return token.Permitted(permissionOtherwise)
}

// TokenFromContext gets the token from context.
func TokenFromContext(ctx context.Context) *Token {
	token, ok := ctx.Value(contextKey).(*Token)
	if !ok {
		return nil
	}

	return token
}

// RequestWithToken either returns the request, or the request with a new context that
// has the token.
func RequestWithToken(r *http.Request) *http.Request {
	header := r.Header.Get("Authorization")
	if header == "" {
		return r
	}

	if !strings.HasPrefix(header, "Bearer ") {
		return r
	}

	token, err := CheckToken(header[7:])
	if err != nil {
		return r
	}

	return r.WithContext(context.WithValue(r.Context(), contextKey, &token))
}

// CheckToken reads the token string and returns a token if everything is kosher.
func CheckToken(tokenString string) (token Token, err error) {
	var key Key

	jwtToken, err := jwt.Parse(tokenString, func(jwtToken *jwt.Token) (interface{}, error) {
		if _, ok := jwtToken.Method.(*jwt.SigningMethodHMAC); !ok {
			return nil, fmt.Errorf("Unexpected signing method: %v", jwtToken.Header["alg"])
		}

		kid, ok := jwtToken.Header["kid"].(string)
		if !ok {
			return nil, ErrNoKid
		}

		key, err = FindKey(kid)
		if err != nil || key.ID == "" {
			return nil, ErrKeyNotFound
		}

		return []byte(key.Secret), nil
	})
	if err != nil {
		return Token{}, err
	}

	userid, permissions, err := parseClaims(jwtToken.Claims)
	if err != nil {
		return Token{}, err
	}

	if !key.ValidForUser(userid) {
		return Token{}, ErrWrongUser
	}

	user, err := FindUser(userid)
	if err != nil {
		return Token{}, ErrDeletedUser
	}

	for _, permission := range permissions {
		found := false

		for _, userPermission := range user.Permissions {
			if permission == userPermission {
				found = true
				break
			}
		}

		if !found {
			return Token{}, ErrWrongPermissions
		}
	}

	return Token{UserID: token.UserID, Permissions: permissions}, nil
}

func parseClaims(jwtClaims jwt.Claims) (userid string, permissions []string, err error) {
	mapClaims, ok := jwtClaims.(jwt.MapClaims)
	if !ok {
		return "", nil, ErrInvalidClaims
	}

	if !mapClaims.VerifyExpiresAt(time.Now().Unix(), true) {
		return "", nil, ErrExpired
	}

	if userid, ok = mapClaims["user"].(string); !ok {
		return "", nil, ErrInvalidClaims
	}

	if claimedPermissions, ok := mapClaims["permissions"].([]interface{}); ok {
		for _, permission := range claimedPermissions {
			if permission, ok := permission.(string); ok {
				permissions = append(permissions, permission)
			}
		}
	}

	if len(permissions) == 0 {
		return "", nil, ErrInvalidClaims
	}

	return
}