To be fair, if you don't need to support anything other than Unicode, then this is not an advantage, and over time we're all going to need non-Unicode less and less. That said I'm a big fan of ASN.1 (see my comment history).
I'm still confused how these ISO 2022 strings even work, and the ASN1 docs discourage using the UniversalString and GraphicString types. All these different string types are intimidating if I just want unicode/ascii, and even if I were using an obscure encoding, I'd use generic bytes instead of wanting asn1 to care about it.
GeneralString relies on control characters to "load" character sets into the C0 and C1 registers. This is madness -- specifically it's pre-Unicode madness, but before Unicode it made sense.
> I'm still confused how these ISO 2022 strings even work
There is C0, G0, C1, and G1 sets (C0 and C1 are control characters and G0 and G1 are graphic characters), and escape sequences are used to select the C or G set for bytes with or without the high bit set. Graphic string does not allow control characters and General string does allow control characters.
You probably do not need all control characters; your schema should probably restrict which control characters are allowed in each context (although the ASN.1 schema format does not seem to have any way to do this). This way, you will only handle the control characters which are appropriate for your use.
This is messy, although canonical form simplifies it by adding some restrictions (this is one of the reasons why DER is better than BER, in my opinion). TRON code is better and is much simpler than the working of ISO 2022. (Unicode has a different kind of mess; although decoding is simpler, actually handling the decoded characters in text is its own big mess for many reasons. Unicode is a stateful character set, even though the encoding is stateless; TRON code is the other way around (and with a significantly simpler stateful encoding than ISO 2022).)
> the ASN1 docs discourage using the UniversalString and GraphicString types
UniversalString is UTF-32BE and GraphicString is ISO 2022 without control characters. By knowing what they are, you should know in which circumstances they should be considered useful or not useful; I think that they should not be discouraged in general (although usually if you want Unicode, you would use UTF-8 rather than UTF-32, there are some circumstances where you might want to use UTF-32, such as if the data or program is already UTF-32 for other reasons).
(The data type which probably should be avoided is the UTC time type, which is not Y2K compliant.)
> All these different string types are intimidating if I just want unicode/ascii
If you only want ASCII, use the IA5 type (or Visible if you do not want control characters); if you only want Unicode, use the UTF-8 string type (or Universal if you want UTF-32 instead for some reason). ("IA5" is another name for ASCII that as far as I can tell hardly anyone other than ITU uses.)
However, Unicode is not a very good character set, and they should not force or expect you to use it.
As I had mentioned before, you do not need to use or implement all of the ASN.1 data types; only use the ones appropriate for your application (so, if you do not like most of the types, then don't use those types). I also made up some additional nonstandard ASN.1 types (called ASN.1X), which also might be useful for some applications; you are not required to use or implement these either.
> However, Unicode is not a very good character set, and they should not force or expect you to use it.
Unicode is an excellent character set, and for 99% of cases (much more probably) it's absolutely the best choice. So one should choose Unicode (and UTF-8) in all cases unless there is an excellent reason to do otherwise. As time passes there will be fewer and fewer cases where Unicode is not sufficient, so really we are asymptotically approaching the point at which Unicode is the only good choice to make.
This is all independent of ASN.1. But it is true that ASN.1 has built-in types for Unicode and non-Unicode strings that many other protocols lack.
Have you written up anything about ASN.1X anywhere? I'd love to take a look.
> Have you written up anything about ASN.1X anywhere? I'd love to take a look.
ASN1_BCD_STRING (64): Represents a string with the following characters:
"0123456789*#+-. " (excluding the quotation marks). Each octet encodes
two characters, where the high nybble corresponds to the first character
and the low nybble corresponds to the second character.
ASN1_PC_STRING (65): Represents a string of characters in the PC
character set. Note that the control characters can also be used as
graphic characters.
ASN1_TRON_STRING (66): Represents a string of characters in the TRON
character set, encoded as TRON-8.
ASN1_KEY_VALUE_LIST (67): Represents a set of keys (with no duplicate
keys) and with a value associated with each key. The encoding is the same
as for a SET of the keys, but with the corresponding value immediately
after each key (when they are sorted, only the keys are sorted and the
values are kept with the corresponding keys).
ASN1_UTC_TIMESTAMP (68): Represents a number of UTC seconds (and
optionally fractions of seconds), excluding leap seconds, relative to
epoch.
ASN1_SI_TIMESTAMP (69): Represents a number of SI seconds (and
optionally fractions of seconds), including leap seconds, relative to
epoch.
ASN1_UTC_TIME_INTERVAL (70): Represents a time interval as a number
of UTC seconds. The number of seconds does not include leap seconds.
ASN1_SI_TIME_INTERVAL (71): Represents a time interval as a number
of SI seconds (which may include fractions).
ASN1_OUT_OF_BAND (72): This type is not for use for general-purpose
data. It represents something which is transmitted out of band (e.g. a
file descriptor) with whatever transport mechanism is being used. The
transport mechanism defines how a value of this type is supposed to be
encoded with whatever ASN.1 encoding is being used.
ASN1_MORSE_STRING (73): Represents a string of characters in the
Morse character set. The encoding is like a relative object identifier,
where 0 means an empty space, and others is like bijective base 2 with
1 for dots and 2 for dashes, with the high bit for the first dot/dash,
e.g. 4 means A and 8 means U.
ASN1_REFERENCE (74): A reference to another node within the same file.
(Not all implementations will support this feature.) The encoding is like
a Relative Object Identifier; the first number is how many times to go to
the parent node (where 0 means the reference itself), and then the rest of
the numbers specify which child node of the current node to go to where 0
means the first child, 1 means the second child, etc. It can reference a
primitive or constructed node of a BER file, but you cannot specify a
child index for a child of a primitive node, since primitive nodes cannot
have child nodes. At least one number (how many levels of parents) is
required, but any number of numbers is potentially possible.
ASN1_IDENTIFIED_DATA (75): Data which has a format and/or meaning which
is identified within the data. The encoding is always constructed and
consists of two or three items. The first item is a set of object
identifiers, object descriptors (used only for display), and/or sequences
where the first item of the sequence is a object identifier. The receiver
ignores any items in this set that it does not understand. The second
item in a ASN1_IDENTIFIED_DATA can be any single item of any type; it is
interpreted according to the object identifiers in the first set that the
receiver understands. The third item is optional, and if it is present it
is a key/value list of extensions; the keys are object identifiers and
the values are of any type according to the object identifiers. The default
value of this key/value list is an empty key/value list.
ASN1_RATIONAL (76): Stored as constructed, containing two integers, being
the numerator and the denominator. The denominator must be greater than
zero. If it is canonical form, then it must be lowest terms.
ASN1_TRANSLATION_LIST (77): A key/value list where the keys identify
languages. If the key is null then it means the default in case no language
present in this list is applicable. The types of the values depends on the
application (usually they will be some kind of character strings).
In addition, the same number for the BMP string type can also be used for a UTF-16 string type, and there is a "OBJECT IDENTIFIER RELATIVE TO" type which encodes a OID as either relative or absolute (in canonical form, it is always relative when possible) in order to save space; the schema will specify what it is relative to. ANY and ANY DEFINED BY are allowed despite being removed from the most recent versions of standard ASN.1. (The schema format for these extensions is not defined, since I am not using the ASN.1 schema format; however, someone who does use it might do so if they need it.)
There is also SDER, which is a superset of DER but a subset of BER, in case you do not want the mess of BER but do not want to require strictly canonical form either; and also SDSER which uses the same encoding for types and values than SDER but but length works differently in order to support streaming better.
As is usual, you do not have to use any or all of these types, but someone might find them useful for some uses. I have used some of them in my own stuff.
