From ITU-T Rec. X.680 (02/21):


IA5String [is] mapped into UniversalString by mapping each character into the UniversalString character that has the identical (32-bit) value in the BER encoding of UniversalString as the (8-bit) value of the BER encoding of IA5String[.]


The characters which can appear in the UniversalString type are any of the characters allowed by ISO/IEC 10646 [aka Unicode].


For IA5String, [the] entire character set contains precisely 128 characters[.]

So it would seem that IA5String really is identical to ASCII, considering that all 128 ASCII characters, including non-printable ones, are identical to the Unicode characters of the same index. (This can be tested via Python by assert all(struct.pack('!B', i).decode('ascii') == chr(i) for i in range(128)))

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