oauth2-proxy/pkg/encryption/utils.go

package encryption

import (
	"crypto/hmac"
	"crypto/rand"
	"crypto/sha1" // #nosec G505
	"crypto/sha256"
	"encoding/base64"
	"fmt"
	"hash"
	"io"
	"net/http"
	"strconv"
	"strings"
	"time"

	"github.com/oauth2-proxy/oauth2-proxy/pkg/logger"
)

// SecretBytes attempts to base64 decode the secret, if that fails it treats the secret as binary
func SecretBytes(secret string) []byte {
	b, err := base64.RawURLEncoding.DecodeString(strings.TrimRight(secret, "="))
	if err == nil {
		// Only return decoded form if a valid AES length
		// Don't want unintentional decoding resulting in invalid lengths confusing a user
		// that thought they used a 16, 24, 32 length string
		for _, i := range []int{16, 24, 32} {
			if len(b) == i {
				return b
			}
		}
	}
	// If decoding didn't work or resulted in non-AES compliant length,
	// assume the raw string was the intended secret
	return []byte(secret)
}

// cookies are stored in a 3 part (value + timestamp + signature) to enforce that the values are as originally set.
// additionally, the 'value' is encrypted so it's opaque to the browser

// Validate ensures a cookie is properly signed
func Validate(cookie *http.Cookie, seed string, expiration time.Duration) (value []byte, t time.Time, ok bool) {
	// value, timestamp, sig
	parts := strings.Split(cookie.Value, "|")
	if len(parts) != 3 {
		return
	}
	if checkSignature(parts[2], seed, cookie.Name, parts[0], parts[1]) {
		ts, err := strconv.Atoi(parts[1])
		if err != nil {
			return
		}
		// The expiration timestamp set when the cookie was created
		// isn't sent back by the browser. Hence, we check whether the
		// creation timestamp stored in the cookie falls within the
		// window defined by (Now()-expiration, Now()].
		t = time.Unix(int64(ts), 0)
		if t.After(time.Now().Add(expiration*-1)) && t.Before(time.Now().Add(time.Minute*5)) {
			// it's a valid cookie. now get the contents
			rawValue, err := base64.URLEncoding.DecodeString(parts[0])
			if err == nil {
				value = rawValue
				ok = true
				return
			}
		}
	}
	return
}

// SignedValue returns a cookie that is signed and can later be checked with Validate
func SignedValue(seed string, key string, value []byte, now time.Time) string {
	encodedValue := base64.URLEncoding.EncodeToString(value)
	timeStr := fmt.Sprintf("%d", now.Unix())
	sig := cookieSignature(sha256.New, seed, key, encodedValue, timeStr)
	cookieVal := fmt.Sprintf("%s|%s|%s", encodedValue, timeStr, sig)
	return cookieVal
}

func cookieSignature(signer func() hash.Hash, args ...string) string {
	h := hmac.New(signer, []byte(args[0]))
	for _, arg := range args[1:] {
		_, err := h.Write([]byte(arg))
		// If signing fails, fail closed and return something that won't validate
		if err != nil {
			garbage := make([]byte, 16)
			if _, err := io.ReadFull(rand.Reader, garbage); err != nil {
				logger.Fatal("HMAC & RNG functions both failing. Shutting down for security")
			}
			return base64.URLEncoding.EncodeToString(garbage)
		}
	}
	var b []byte
	b = h.Sum(b)
	return base64.URLEncoding.EncodeToString(b)
}

func checkSignature(signature string, args ...string) bool {
	checkSig := cookieSignature(sha256.New, args...)
	if checkHmac(signature, checkSig) {
		return true
	}

	// TODO: After appropriate rollout window, remove support for SHA1
	legacySig := cookieSignature(sha1.New, args...)
	return checkHmac(signature, legacySig)
}

func checkHmac(input, expected string) bool {
	inputMAC, err1 := base64.URLEncoding.DecodeString(input)
	if err1 == nil {
		expectedMAC, err2 := base64.URLEncoding.DecodeString(expected)
		if err2 == nil {
			return hmac.Equal(inputMAC, expectedMAC)
		}
	}
	return false
}