Unmarshalling asn failed

php editor Xigua is here to share with you a question about "unmarshalling asn failed". In network communications, ASN (Autonomous System Number) is a number used to identify an autonomous system. However, sometimes the ungrouping fails when ungrouping the ASN. This may be caused by incorrect ASN encoding format, corrupted ASN packets, or incompatible parsers. In this article, we will discuss the reasons and solutions for the failure of unmarshalling ASN to help everyone better understand and solve this problem.

Question content

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 tried to unmarshal but got the following error:

<code>
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
</code>

Can anyone help? Thanks

Solution

0x020105 encodes the integer 5 (see https://www.php.cn/link/8ae7733f9bc11275e8d0a0fdabe5be0a), so it should Unmarshal it to an integer instead of a struct with integer fields:

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 marshalled to 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
}

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