Deconding base64 API response throws a FormatException - dart

Question

An API I'm working on returns the following Base64, UTF8 Data.
H4sIAAAAAAAEAIXS3WvCMBAA8H8l5MmB9Wtf2idtJ86HDrHKNsYeYntrgzUpl2RQpf/7UsExsXZvCcf97ri7A0XQBsUT04y6BzoGRIlAXUrbFDIQkZx+g9D8GBR4+tChjY8NZgvOtGZfEnc2n6Za58rtdk3EOwnoQhqEKqMjRcYFVGb1ldT9sByPf7l+xcWgIuR7LkXVwKDXH5G1PyfByiOvErOY+Cnb5TasUp4rskBQNh1isilIABiBBRwPxJ60Aik0CRm3YTIRAhx/8nJDy/Z51duGqkvJ4tqyrXeJW/tEcGbe8hK9a0DXuGGiXvVTEElsHP+5ptH7OnPQO5p+EWWS+CiVqpMvrIcr1vkEj4OXRmg7Q+LxLZxsk1+Ijw3decEbCU1u7b8N1iDDBmSFLNr+fwMrrpTUpOUB2mtzZtOa7Ywaylxf+YQnGTgBQ80TUZBwPbfyZ1mWP1eCbD1AAwAA
I wrote the following test to convert it to a UTF8 Json string.
void main() {
const data = "H4sIAAAAAAAEAIXS3WvCMBAA8H8l5MmB9Wtf2idtJ86HDrHKNsYeYntrgzUpl2RQpf/7UsExsXZvCcf97ri7A0XQBsUT04y6BzoGRIlAXUrbFDIQkZx+g9D8GBR4+tChjY8NZgvOtGZfEnc2n6Za58rtdk3EOwnoQhqEKqMjRcYFVGb1ldT9sByPf7l+xcWgIuR7LkXVwKDXH5G1PyfByiOvErOY+Cnb5TasUp4rskBQNh1isilIABiBBRwPxJ60Aik0CRm3YTIRAhx/8nJDy/Z51duGqkvJ4tqyrXeJW/tEcGbe8hK9a0DXuGGiXvVTEElsHP+5ptH7OnPQO5p+EWWS+CiVqpMvrIcr1vkEj4OXRmg7Q+LxLZxsk1+Ijw3decEbCU1u7b8N1iDDBmSFLNr+fwMrrpTUpOUB2mtzZtOa7Ywaylxf+YQnGTgBQ80TUZBwPbfyZ1mWP1eCbD1AAwAA";
test('Base64 decoder', () {
final bytes = base64.decode(data);
String jsonString;
try {
jsonString = utf8.decode(bytes);
} catch (e) {
// ignore: avoid_print
print(e);
}
// ignore: avoid_print
print("$jsonString\n");
final decoded = json.decode(jsonString);
// ignore: avoid_print
print("$decoded\n");
expect(decoded, !null);
});
}
The bytes that get decoded from the base64 string look good to me, but when I try to utf8.decode() the bytes. I get a FormatException, or, if I add allowMalformed = true, a non-sense string.
How can I decode the Json string correctly?

Answer 1

Your BASE64 input is a GZip compressed JSON String. The following program will work:
import 'dart:convert';
import 'dart:io';
void main() {
const data = "H4sIAAAAAAAEAIXS3WvCMBAA8H8l5MmB9Wtf2idtJ86HDrHKNsYeYntrgzUpl2RQpf/7UsExsXZvCcf97ri7A0XQBsUT04y6BzoGRIlAXUrbFDIQkZx+g9D8GBR4+tChjY8NZgvOtGZfEnc2n6Za58rtdk3EOwnoQhqEKqMjRcYFVGb1ldT9sByPf7l+xcWgIuR7LkXVwKDXH5G1PyfByiOvErOY+Cnb5TasUp4rskBQNh1isilIABiBBRwPxJ60Aik0CRm3YTIRAhx/8nJDy/Z51duGqkvJ4tqyrXeJW/tEcGbe8hK9a0DXuGGiXvVTEElsHP+5ptH7OnPQO5p+EWWS+CiVqpMvrIcr1vkEj4OXRmg7Q+LxLZxsk1+Ijw3decEbCU1u7b8N1iDDBmSFLNr+fwMrrpTUpOUB2mtzZtOa7Ywaylxf+YQnGTgBQ80TUZBwPbfyZ1mWP1eCbD1AAwAA";
final bytes = base64.decode(data);
String jsonString;
try {
jsonString = utf8.decode(GZipCodec().decode(bytes));
} catch (e) {
// ignore: avoid_print
print(e);
}
// ignore: avoid_print
print("$jsonString\n");
final dynamic decoded = json.decode(jsonString);
// ignore: avoid_print
print("$decoded\n");
}
Returns:
{"returnData":{"#errore":"","elencoEventi":{"#nrEventi":"8","#urlPiattaforma":"https://uci.getyourevent.online","evento":[{"#idEventi":"1","#descrizione":"2019 UCI MTB World Championships Presented by Mercedes-Benz (Mont Sainte Anne-CAN)"},{"#idEventi":"3","#descrizione":"2019 UCI Road World Championships (Yorkshire-GBR)"},{"#idEventi":"4","#descrizione":"2019 UCI Urban World Championships (Chengdu-CHN)"},{"#idEventi":"5","#descrizione":"2020 UCI Cyclo Cross World Championships"},{"#idEventi":"6","#descrizione":"2020 Mercedes-Benz UCI Mountain Bike World Cup"},{"#idEventi":"7","#descrizione":"2020 UCI BMX Supercross World Cup"},{"#idEventi":"8","#descrizione":"2020 UCI Track World Championships Presented by Tissot (Berlin-GER)"},{"#idEventi":"9","#descrizione":"2020 UCI Road World Championships (Aigle-Martigny SUI)"}]}}}
{returnData: {#errore: , elencoEventi: {#nrEventi: 8, #urlPiattaforma: https://uci.getyourevent.online, evento: [{#idEventi: 1, #descrizione: 2019 UCI MTB World Championships Presented by Mercedes-Benz (Mont Sainte Anne-CAN)}, {#idEventi: 3, #descrizione: 2019 UCI Road World Championships (Yorkshire-GBR)}, {#idEventi: 4, #descrizione: 2019 UCI Urban World Championships (Chengdu-CHN)}, {#idEventi: 5, #descrizione: 2020 UCI Cyclo Cross World Championships}, {#idEventi: 6, #descrizione: 2020 Mercedes-Benz UCI Mountain Bike World Cup}, {#idEventi: 7, #descrizione: 2020 UCI BMX Supercross World Cup}, {#idEventi: 8, #descrizione: 2020 UCI Track World Championships Presented by Tissot (Berlin-GER)}, {#idEventi: 9, #descrizione: 2020 UCI Road World Championships (Aigle-Martigny SUI)}]}}}