There's a few ways to do this. One way is to sign your request with Signature V4 and POST your file to S3.
First, create a policy helper:
import 'dart:convert';
import 'package:amazon_cognito_identity_dart/sig_v4.dart';
class Policy {
String expiration;
String region;
String bucket;
String key;
String credential;
String datetime;
int maxFileSize;
Policy(this.key, this.bucket, this.datetime, this.expiration, this.credential,
this.maxFileSize,
{this.region = 'us-east-1'});
factory Policy.fromS3PresignedPost(
String key,
String bucket,
String accessKeyId,
int expiryMinutes,
int maxFileSize, {
String region,
}) {
final datetime = SigV4.generateDatetime();
final expiration = (DateTime.now())
.add(Duration(minutes: expiryMinutes))
.toUtc()
.toString()
.split(' ')
.join('T');
final cred =
'$accessKeyId/${SigV4.buildCredentialScope(datetime, region, 's3')}';
final p = Policy(key, bucket, datetime, expiration, cred, maxFileSize,
region: region);
return p;
}
String encode() {
final bytes = utf8.encode(toString());
return base64.encode(bytes);
}
@override
String toString() {
return '''
{ "expiration": "${this.expiration}",
"conditions": [
{"bucket": "${this.bucket}"},
["starts-with", "\$key", "${this.key}"],
{"acl": "public-read"},
["content-length-range", 1, ${this.maxFileSize}],
{"x-amz-credential": "${this.credential}"},
{"x-amz-algorithm": "AWS4-HMAC-SHA256"},
{"x-amz-date": "${this.datetime}" }
]
}
''';
}
}
Then, sign your request with your policy helper and upload via http.MultipartRequest:
import 'dart:convert';
import 'dart:io';
import 'package:path/path.dart' as path;
import 'package:async/async.dart';
import 'package:http/http.dart' as http;
import 'package:test/test.dart';
import 'package:amazon_cognito_identity_dart/sig_v4.dart';
import './policy.dart';
void main() {
const _accessKeyId = 'AKXXXXXXXXXXXXXXXXXX';
const _secretKeyId = 'xxxxxxxxxxxxxxxxxxxxxxxx/xxxxxxxxxxxxxxx';
const _region = 'ap-southeast-1';
const _s3Endpoint =
'https://bucketname.s3-ap-southeast-1.amazonaws.com';
final file = File(path.join('/path/to/file', 'square-cinnamon.jpg'));
final stream = http.ByteStream(DelegatingStream.typed(file.openRead()));
final length = await file.length();
final uri = Uri.parse(_s3Endpoint);
final req = http.MultipartRequest("POST", uri);
final multipartFile = http.MultipartFile('file', stream, length,
filename: path.basename(file.path));
final policy = Policy.fromS3PresignedPost('uploaded/square-cinnamon.jpg',
'bucketname', _accessKeyId, 15, length,
region: _region);
final key =
SigV4.calculateSigningKey(_secretKeyId, policy.datetime, _region, 's3');
final signature = SigV4.calculateSignature(key, policy.encode());
req.files.add(multipartFile);
req.fields['key'] = policy.key;
req.fields['acl'] = 'public-read';
req.fields['X-Amz-Credential'] = policy.credential;
req.fields['X-Amz-Algorithm'] = 'AWS4-HMAC-SHA256';
req.fields['X-Amz-Date'] = policy.datetime;
req.fields['Policy'] = policy.encode();
req.fields['X-Amz-Signature'] = signature;
try {
final res = await req.send();
await for (var value in res.stream.transform(utf8.decoder)) {
print(value);
}
} catch (e) {
print(e.toString());
}
}
Please note that this method requires you to provide your Access Key and Secret Key. If you use a service like Cognito, it is recommended to get a temporary Access Key and Secret Key. Examples using temporary access found here.
Disclaimer: I am the original author of the Signature V4 package.
