feat: separate future and expired checks

As the RFC says that proofs signed in the future MAY be accepted, the default behavior should then be that a proof signed in the future should NOT be accepted. [See section 11.1-4](https://datatracker.ietf.org/doc/html/rfc9449#section-11.1-4)

This PR adds a new default value for allowed proof age, `DEFAULT_ALLOWED_PROOF_AGE`, that is set to a more generous 5 minutes. It also changes the existing default time window into the future to 0 seconds.

Now if a `TimeWindow` is not set in the `ParseOptions`, future proofs will all be rejected. The functionality to control how long a proof is allowed to live is controlled by the new parse option `AllowedProofAge`.

parse.go

@@ -34,7 +34,8 @@ const (
  CONNECT HTTPVerb = "CONNECT"
 )
 
-const DEFAULT_TIME_WINDOW = time.Second * 30
+const DEFAULT_ALLOWED_PROOF_AGE = time.Minute * 5
+const DEFAULT_ALLOWED_TIME_WINDOW = time.Second * 0
 
 // ParseOptions and its contents are optional for the Parse function.
 type ParseOptions struct {
@@ -45,9 +46,12 @@ type ParseOptions struct {
  // If set to true the authorization server has to validate the nonce timestamp itself.
  NonceHasTimestamp bool
 
- // The allowed clock-skew on the `iat` of the proof. Defaults to 1 minute if not specified.
+ // The allowed clock-skew (into the future) on the `iat` of the proof. If not set proof is rejected if issued in the future.
  TimeWindow *time.Duration
 
+ // The allowed age of a proof. If not set the default is 5 minutes.
+ AllowedProofAge *time.Duration
+
  // dpop_jkt parameter that is optionally sent by the client to the authorization server on token request.
  // If set the proof proof-of-possession public key needs to match or the proof is rejected.
  JKT string
@@ -107,16 +111,16 @@ func Parse(
  // This satisfies point 11 in https://datatracker.ietf.org/doc/html/rfc9449#section-4.3
  if !opts.NonceHasTimestamp {
  // Check that `iat` is not too far into the past.
- past := DEFAULT_TIME_WINDOW
- if opts.TimeWindow != nil {
- past = *opts.TimeWindow
+ past := DEFAULT_ALLOWED_PROOF_AGE
+ if opts.AllowedProofAge != nil {
+ past = *opts.AllowedProofAge
  }
  if claims.IssuedAt.Before(time.Now().Add(-past)) {
  return nil, errors.Join(ErrInvalidProof, ErrExpired)
  }
 
  // Check that `iat` is not too far into the future.
- future := DEFAULT_TIME_WINDOW
+ future := DEFAULT_ALLOWED_TIME_WINDOW
  if opts.TimeWindow != nil {
  future = *opts.TimeWindow
  }
@@ -130,6 +134,7 @@ func Parse(
  // without the need for extracting and hashing it again.
  jwk, ok := dpopToken.Header["jwk"].(map[string]interface{})
  if !ok {
+ // keyFunc used with parseWithClaims should ensure that this can not happen but better safe than sorry.
  return nil, ErrMissingJWK
  }
  jwkJSONbytes, err := getThumbprintableJwkJSONbytes(jwk)