json.Marshal custom type as base64 string

Question

I have a custom type (Hash [64]byte) and I'm trying to implement MarshalJSON/UnmarshalJSON for it to have it encoded/decoded in JSON as a base64 string. Instead I'm getting an error about an invalid character in the beginning.

package main

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

type Hash [64]byte

func FromString(data string) (Hash, error) {
    decoded, err := base64.StdEncoding.DecodeString(string(data))
    if err != nil {
        return Hash{}, err
    }

    hash := Hash{}
    for index := 0; index < 64; index++ {
        hash[index] = decoded[index]
    }

    return hash, nil
}

func (hash Hash) String() string {
    return base64.StdEncoding.EncodeToString([]byte(hash[:64]))
}

func (hash Hash) MarshalJSON() ([]byte, error) {
    return []byte(hash.String()), nil
}

func (hash *Hash) UnmarshalJSON(data []byte) error {
    decoded, err := FromString(string(data))
    if err != nil {
        return err
    }

    for index := 0; index < 64; index++ {
        hash[index] = decoded[index]
    }

    return nil
}


func main() {
    type TestStructure struct {
        Hash Hash
        Type string
    }

    object := TestStructure{
        Hash: Hash{0xbd, 0xfe, 0xe0, 0xb1, 0x6c, 0xff, 0xb4, 0x51, 0x4c, 0x7b, 0xed, 0x33, 0xc1, 0x6d, 0xac, 0x5e, 0x80, 0x51, 0xec, 0xcb, 0x31, 0x21, 0x8c, 0x54, 0xb, 0xec, 0xbc, 0x7e, 0xbf, 0x4a, 0xce, 0x92, 0x3b, 0xcb, 0xf8, 0xdd, 0x82, 0x45, 0x34, 0xae, 0x58, 0x5, 0x3a, 0x7b, 0x18, 0xdd, 0x30, 0x5c, 0x7e, 0xed, 0xc9, 0xaa, 0x1e, 0x3a, 0x9a, 0x95, 0x30, 0xc3, 0x6b, 0xf8, 0xf9, 0x92, 0x43, 0xc6},
        Type: "I'm a type",
    }

    data, err := json.Marshal(object)
    fmt.Println(data, err)
}

I got the following error:

$ go run hash.go 
[] json: error calling MarshalJSON for type main.Hash: invalid character 'v' looking for beginning of value

What am I doing wrong?

Answer 1

Thanks to the comment @Peter wrote about MarshalText/UnmarshalText I found a nice solution. Probably there are better ways than the for loop to copy the values over but at least this works for now.

package main

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

type Hash [64]byte

func FromString(data string) (Hash, error) {
    decoded, err := base64.StdEncoding.DecodeString(string(data))
    if err != nil {
        return Hash{}, err
    }

    hash := Hash{}
    for index := 0; index < 64; index++ {
        hash[index] = decoded[index]
    }

    return hash, nil
}

func (hash Hash) String() string {
    return base64.StdEncoding.EncodeToString([]byte(hash[:]))
}

func (hash Hash) MarshalText() (text []byte, err error) {
    return []byte(hash.String()), nil
}

func (hash *Hash) UnmarshalText(text []byte) error {
    decoded, err := base64.StdEncoding.DecodeString(string(text))
    if err != nil {
        return err
    }

    for index := 0; index < 64; index++ {
        hash[index] = decoded[index]
    }

    return nil
}

func main() {
    type TestStructure struct {
        Hash Hash
        Type string
    }

    object := TestStructure{
        Hash: Hash{0xbd, 0xfe, 0xe0, 0xb1, 0x6c, 0xff, 0xb4, 0x51, 0x4c, 0x7b, 0xed, 0x33, 0xc1, 0x6d, 0xac, 0x5e, 0x80, 0x51, 0xec, 0xcb, 0x31, 0x21, 0x8c, 0x54, 0xb, 0xec, 0xbc, 0x7e, 0xbf, 0x4a, 0xce, 0x92, 0x3b, 0xcb, 0xf8, 0xdd, 0x82, 0x45, 0x34, 0xae, 0x58, 0x5, 0x3a, 0x7b, 0x18, 0xdd, 0x30, 0x5c, 0x7e, 0xed, 0xc9, 0xaa, 0x1e, 0x3a, 0x9a, 0x95, 0x30, 0xc3, 0x6b, 0xf8, 0xf9, 0x92, 0x43, 0xc6},
        Type: "I'm a type",
    }

    data, err := json.Marshal(object)
    fmt.Println(string(data), err)

    ts := TestStructure{}
    err = json.Unmarshal(data, &ts)
    fmt.Printf("%+v\n", ts)
    fmt.Println(err)

    h, err := FromString(ts.Hash.String())
    fmt.Printf("%+v\n", h)
    fmt.Println(err)
}

Answer 2

Your MarshalJSON method needs to include the enclosing quotation marks in the value it returns, otherwise you end up with invalid JSON. Something like:

func (hash Hash) MarshalJSON() ([]byte, error) {
    return []byte(`"` + hash.String() + `"`), nil
}

should work.

In your error message 'v' is the first character in your base64-encoded text, so the message shows it's finding 'v' when it's looking for a valid JSON type (i.e. a string, number, boolean, object, array, or null), none of which can start with that character.

Making this change and tweaking the type in the final line:

package main

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

type Hash [64]byte

func FromString(data string) (Hash, error) {
    decoded, err := base64.StdEncoding.DecodeString(string(data))
    if err != nil {
        return Hash{}, err
    }

    hash := Hash{}
    for index := 0; index < 64; index++ {
        hash[index] = decoded[index]
    }

    return hash, nil
}

func (hash Hash) String() string {
    return base64.StdEncoding.EncodeToString([]byte(hash[:64]))
}

func (hash Hash) MarshalJSON() ([]byte, error) {
    return []byte(`"` + hash.String() + `"`), nil
}

func (hash *Hash) UnmarshalJSON(data []byte) error {
    decoded, err := FromString(string(data[1 : len(data)-1]))
    if err != nil {
        return err
    }

    for index := 0; index < 64; index++ {
        hash[index] = decoded[index]
    }

    return nil
}

func main() {
    type TestStructure struct {
        Hash Hash
        Type string
    }

    object := TestStructure{
        Hash: Hash{0xbd, 0xfe, 0xe0, 0xb1, 0x6c, 0xff, 0xb4, 0x51, 0x4c, 0x7b, 0xed, 0x33, 0xc1, 0x6d, 0xac, 0x5e, 0x80, 0x51, 0xec, 0xcb, 0x31, 0x21, 0x8c, 0x54, 0xb, 0xec, 0xbc, 0x7e, 0xbf, 0x4a, 0xce, 0x92, 0x3b, 0xcb, 0xf8, 0xdd, 0x82, 0x45, 0x34, 0xae, 0x58, 0x5, 0x3a, 0x7b, 0x18, 0xdd, 0x30, 0x5c, 0x7e, 0xed, 0xc9, 0xaa, 0x1e, 0x3a, 0x9a, 0x95, 0x30, 0xc3, 0x6b, 0xf8, 0xf9, 0x92, 0x43, 0xc6},
        Type: "I'm a type",
    }

    data, err := json.Marshal(object)
    fmt.Println(string(data), err)
}

yields the expected output:

paul@mac:go64$ ./go64
{"Hash":"vf7gsWz/tFFMe+0zwW2sXoBR7MsxIYxUC+y8fr9KzpI7y/jdgkU0rlgFOnsY3TBcfu3Jqh46mpUww2v4+ZJDxg==","Type":"I'm a type"} <nil>
paul@mac:go64$ 

Obviously you'll also need to handle the quotation marks during unmarshal, too.