How to do asn1 marshal/unmarshal and omit fields?

php editor Apple introduces you how to perform asn1 marshal/unmarshal and omit fields. When dealing with asn1 encoding, we often need to encode (marshal) or decode (unmarshal) the data, but sometimes we only care about some of the fields without parsing the entire structure. To achieve this goal, we can use some techniques to omit unnecessary fields, thereby improving processing efficiency. Next, we'll detail how to accomplish this.

Question content

type bearer struct {
    CreatedAt time.Time     `asn1:"generalized"`
    ExpiresAt time.Time     `asn1:"generalized"`
    Nonce     string
    Signature []byte        `asn1:"-"`
    TTL       time.Duration `asn1:"-"`
    Frequency int           `asn1:"-"`
}

c := &bearer{
  CreatedAt: time.Now()
  ExpiresAt: time.Now().Add(1*time.Minute())
  Nonce: "123456789abcdefghijklmnop"
  Frequency: 1
}

b, err := asn1.Marshal(*c)
os.WriteFile("b64.txt", b, 0777)

The structure will be successfully marshaled, however, when inspecting the structure using Bash base64 -d b64.txt > b64.txt.der I still see asn1:"-" The fields are actually marshaled and written to the file, and fields with no values ​​get Error: Object length is zero.. Why doesn't asn1:"-" work like json?

Solution

Because the encoding/json package is implemented to support the - option, and encoding/asn1 No. As for why, this is not the place. The main goal of accepting encoding/asn1 is to support reading and writing X.509 certificates, it is not meant to be a "Swiss Army Knife" of ASN1 implementations.

If you want to exclude certain fields, create a structure type that excludes these fields. To avoid duplication, you can embed these "stripped" structures into your own structures that include additional fields, for example:

type bearerAsn1 struct {
    CreatedAt time.Time `asn1:"generalized"`
    ExpiresAt time.Time `asn1:"generalized"`
    Nonce     string
}

type bearer struct {
    bearerAsn1
    Signature []byte
    TTL       time.Duration
    Frequency int
}

Only marshal/unmarshal bearer.bearerAsn1, so other fields of bearer will naturally be excluded.

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