如何在 ASN.1 Golang 中从 PrintableString 到 GeneralString？

Question

I have Golang structure:

type Dog struct {
    Name string
    UUID *big.Int
}

Then I asn1.Marshal it:

dog := Dog{Name: "Rex", UUID: new(big.Int).SetBytes([]byte{0, 0, 7})}
dogAsn, _ := asn1.Marshal(dog)

When I look at the ASN.1 structure of "dogAsn" (using dumpasn1 Linux utility)I see:

SEQUENCE {
    PrintableString 'Rex'
    INTEGER
        00 00 00 07
    }

I wish NOT to have "PrintableString" there, but instead "GeneralString":

(Desired output):

SEQUENCE {
    GeneralString 'Rex'
    INTEGER
        00 00 00 07
}

Adding asn1:"tag:27" to the "Name" field:

type Dog struct {
    Name string `asn1:"tag:27"`
    ...
}

Doesn't solve my issue. Any good ideas? Thanks in advance!

Answer 1

> Solution has been found!

==============================

If you need to create an ASN.1 GeneralString in a Golang, then do this:

1.

type Dog struct {
    Name asn1.RawValue // previously was "string"
    UUID *big.Int
}

dogGeneralStr := asn1.RawValue{Tag: asn1.TagGeneralString, Bytes: []byte("Rex")}
dog := Dog{Name: dogGeneralStr, UUID: new(big.Int).SetBytes([]byte{0, 0,7})}
dogAsn, _ := asn1.Marshal(dog)

3. SEQUENCE { GeneralString 'Rex' INTEGER 00 00 00 07 }

PS Then inside your program, whenever you need this dog.Name in a []byte or string form you just do: dogGeneralStr.Bytes or string(dogGeneralStr.Bytes) respectively.

Answer 2

You can't do this with the built in library. The marshalling logic allows you to pick between IA5String , PrintableString , NumericString or UTF8String .

I found an alternative ASN.1 library: https://github.com/Logicalis/asn1 But is is no longer maintained and also encodes all strings as OctetString

So you best bet is to copy the source code of encoding/asn1 and add a custom tag to indicate GeneralString while encoding. Perhaps you could even make a PR out of it.