[Yuren0312]

Fixes #104.

Problem

For character string types that are not known-multiplier types (UTF8String, GeneralString, GraphicString, TeletexString/T61String, VideotexString), a permitted-alphabet (FROM) constraint is not PER-visible (ITU-T X.691 30.6). Such a value must be encoded as an unconstrained length in octets followed by the octets, exactly as if the constraint were absent.

pycrate applied the alphabet anyway: _get_char_dyn() returned the alphabet's bit width (self._const_alpha.rdyn) even though _clen is None for these types, so __to_per_val() and _from_per() evaluated cdyn < self._clen against None and raised TypeError: '<' not supported between instances of 'int' and 'NoneType'. A valid value could neither be encoded nor decoded, in UPER or APER.

Fix

Ignore the alphabet constraint in _get_char_dyn() when _clen is None (i.e. for non-known-multiplier types), so the value is encoded as raw octets with an unconstrained length, matching the spec and the behaviour of the same type without a constraint:/n/n```diff
def _get_char_dyn(self):
# returns the number of bits required to encode a character

•    if self._const_alpha:/n+        if self._const_alpha and self._clen is not None:/n```
  

Verification

Added regression types U8s04 ::= UTF8String (FROM ("013")), Ges01 ::= GeneralString (FROM ("013")) and Tes01 ::= TeletexString (FROM ("013")) to test/res/test_asn1rt_mod.asn, plus encode/decode assertions in test_asn1rt.py. All three now round-trip "13" as 023133 (one length octet 02 + octets 31 33), identical to an unconstrained UTF8String.

test/test_asn1rt.py runs green end-to-end (test_rt_base, test_rrc3g, test_lteran, test_nrran, test_tcap_map, test_tcap_cap, test_x509).

Related: #42 fixed the SIZE half of the same X.691 30.6 rule for UTF8String; this covers the remaining permitted-alphabet case.

[mitshell]

Thanks for the fix and the PR