Can't unmarshal json with base64 encoded byte slice in go

Question

I'm having a hard time unmarshalling a struct that is sent to me from an AWS lambda function.

All of the pkidata fields are empty no matter what I do. I created this code sample to isolate the process :

package main

import (
    "encoding/base64"
    "encoding/json"
    "fmt"
)

func main() {
    mystring := `{"error":{"errormessage": "errormessagetest", "errortype": "errortypetest"}, "pkidata":{"certificate":"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","privatekey":"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","certificatechain":"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"}}`
// yeah the string is long lol 
    var body ResponseBody
    if err := json.Unmarshal([]byte(mystring), &body); err != nil {
        fmt.Println(fmt.Errorf("failed unmarshaling response | %s", err.Error()))
    }

    fmt.Println(body.Error.ErrorMessage)

    certChain, err := base64.StdEncoding.DecodeString(mystring)
    if err != nil {
        fmt.Println(err.Error())
    }
    fmt.Println(certChain)
}

type PkiGenerationInput struct {
    TeacherUsername string `json:"teacherusername"`
}

type Response struct {
    Headers ResponseHeaders `json:"headers"`
    Body    ResponseBody    `json:"body"`
}

type ResponseHeaders struct {
    ContentType string `json:"Content-Type"`
}

type ResponseBody struct {
    PkiData ParsablePkiData `json:"pkidata"`
    Error   struct {
        ErrorMessage string `json:"errormessage"`
        ErrorType    string `json:"errortype"`
    } `json:"error"`
}

type ParsablePkiData struct {
    Certificate      string `json:"certificate"`
    PrivateKey       string `json:"privatekey"`
    CertificateChain string `json:"certificatechain"`
}

Here is the output

errormessagetest
illegal base64 data at input byte 0
[]

the fact I can read the errormessagetest shows that the unmarshalling kinda works, so that's weird.

I checked for any unexported fields but we can clearly see they are all capitalized. I also reduced the string size "just to see" and it didn't change anything.

Answer 1

Answer to synthetize all of the comments :

First of all, my "dumb mistake" was because I tried to decode the wrong variable :

// mystring needs to be replaced by body.PkiData.Certificate
base64.StdEncoding.DecodeString(mystring) 

But the research led us to discover that the json.Unmarshal() function decodes base64 by itself when the result type matches what the declared type. Given the fact the encoded data was a []byte, I just replaced the string type by a []byte in the struct.

That allows to create a much more simple version of the previous code :

type ResponseBody struct {
    PkiData PkiData `json:"pkidata"`
}

type PkiData struct {
    Certificate      []byte
    PrivateKey       []byte
    CertificateChain []byte
}

var body ResponseBody
if err := json.Unmarshal(rawResp.Payload, &body); err != nil {
    return PkiData{}, fmt.Errorf("failed unmarshaling response | %s", err.Error())
}