I'm trying to calculate the content_hash of a Dropbox file, following their algorithm, Dropbox docs here, however, I cannot get the overall hash (result of Step 4) correct.
I can produce the correct hashes on each block, so I suspect the issue starts with the conversion to the concantenated binary string (Step 3).
Per Dropbox's docs, there are 4 steps involved:
They provide an example, with the resulting hash values, which I can only replicate up to step 2 (unfortunately, they do not provide an example of the single binary string for Step 3)
My code is as follows:
Future<void> getDropContentHash() async {
String getHash(List<int> bytes) => sha256.convert(bytes).toString();
String toBinary(String string) {
/// converting string to binary string, following this SO Answer
/// https://stackoverflow.com/a/25906926/12132021
return string.codeUnits.map((x) => x.toRadixString(2).padLeft(8, '0')).join();
}
/// Nasa Milky Way Example:
final response = await http.get(Uri.parse('https://www.dropbox.com/static/images/developers/milky-way-nasa.jpg'));
final bytes = response.bodyBytes;
/// Step 1:
/// Split the file into blocks of 4 MB (4,194,304 or 4 * 1024 * 1024 bytes). The last block (if any) may be smaller than 4 MB.
final chunks = bytes.slices(4 * 1024 * 1024);
print(chunks.length); // 3, as expected in the Nasa example
/// Step 2:
/// Compute the hash of each block using SHA-256.
final chunkHashes = <String>[];
for(final chunk in chunks) {
final blockHash = getHash(chunk);
chunkHashes.add(blockHash);
}
print(chunkHashes);
/*
Per the docs, the chunkHashes are correct:
[
2a846fa617c3361fc117e1c5c1e1838c336b6a5cef982c1a2d9bdf68f2f1992a,
c68469027410ea393eba6551b9fa1e26db775f00eae70a0c3c129a0011a39cf9,
7376192de020925ce6c5ef5a8a0405e931b0a9a8c75517aacd9ca24a8a56818b
]
*/
/// Step 3:
/// Concatenate the hash of all blocks in the binary format to form a single binary string.
/// I suspect the issue with the `toBinary` method, converting to a single binary string:
final concatenatedString = chunkHashes.map((chunkHash) => toBinary(chunkHash)).join();
print(concatenatedString);
/// Step 4:
/// Compute the hash of the concatenated string using SHA-256. Output the resulting hash in hexadecimal format.
final contentHash = getHash(concatenatedString.codeUnits);
print(contentHash);
/*
Does NOT yield the correct value of:
485291fa0ee50c016982abbfa943957bcd231aae0492ccbaa22c58e3997b35e0
Instead, it yields:
0f63574f6c7cf29d1735e8f5ec4ef63abb3bc5c1b8618c6455a2c554bc844396
*/
}
Can anyone help me get the correct overall hash of 485291fa0ee50c016982abbfa943957bcd231aae0492ccbaa22c58e3997b35e0?
I figured it out. I did not need to convert to binary string. I just needed to continue working with bytes, and only convert to a string after the overall hash (step 4)
The resulting code (simplified & refactored) to calculate the Dropbox content hash locally:
Future<void> hashNasaImage() async {
final response = await http.get(Uri.parse('https://www.dropbox.com/static/images/developers/milky-way-nasa.jpg'));
final bytes = response.bodyBytes;
final contentHash = getDropContentHash(bytes);
print(contentHash);
// 485291fa0ee50c016982abbfa943957bcd231aae0492ccbaa22c58e3997b35e0
}
String getDropContentHash(List<int> bytes) {
List<int> getHash(List<int> bytes) => sha256.convert(bytes).bytes;
final chunks = bytes.slices(4 * 1024 * 1024);
final chunkHashes = chunks.fold(<int>[], (buffer, chunk) => buffer..addAll(getHash(chunk)));
return Digest(getHash(chunkHashes)).toString();
}