Go unmarshal asn failing

Question

package main

import (
    "encoding/asn1"
    "fmt"
)

type SimpleStruct struct {
    Value int
}

func main() {
    berBytes := []byte{0x02, 0x01, 0x05}

    var simpleStruct SimpleStruct
    _, err := asn1.Unmarshal(berBytes, &simpleStruct)
    if err != nil {
        fmt.Println("Error:", err)
        return
    }

    fmt.Printf("Decoded value: %d\n", simpleStruct.Value)
}

I was trying to unmarshal but getting below error:

Error: asn1: structure error: tags don't match (16 vs {class:0 tag:2 length:1 isCompound:false}) {optional:false explicit:false application:false private:false defaultValue: tag: stringType:0 timeType:0 set:false omitEmpty:false} SimpleStruct @2

Could someone please help? Thanks

Answer 1

0x020105 encodes integer 5 (see https://lapo.it/asn1js/#AgEF), so it should be unmarshaled into an integer instead of a struct which has an integer field:

package main

import (
    "encoding/asn1"
    "fmt"
)

func main() {
    berBytes := []byte{0x02, 0x01, 0x05}

    var v int
    _, err := asn1.Unmarshal(berBytes, &v)
    if err != nil {
        fmt.Println("Error:", err)
        return
    }

    fmt.Printf("Decoded value: %d\n", v)
    // Output:
    //   Decoded value: 5
}

And SimpleStruct{Value: 5} is marshaled into 0x3003020105:

package main

import (
    "encoding/asn1"
    "fmt"
)

type SimpleStruct struct {
    Value int
}

func main() {
    simpleStruct := SimpleStruct{Value: 5}
    buf, err := asn1.Marshal(simpleStruct)
    if err != nil {
        fmt.Println("Error:", err)
        return
    }

    fmt.Printf("Encoded value: 0x%x\n", buf)
    // Output:
    //   Encoded value: 0x3003020105
}