What is the algorithm to compute the Amazon-S3 Etag for a file larger than 5GB?

Question

Files uploaded to Amazon S3 that are smaller than 5GB have an ETag that is simply the MD5 hash of the file, which makes it easy to check if your local files are the same as what you put on S3.

But if your file is larger than 5GB, then Amazon computes the ETag differently.

For example, I did a multipart upload of a 5,970,150,664 byte file in 380 parts. Now S3 shows it to have an ETag of 6bcf86bed8807b8e78f0fc6e0a53079d-380 . My local file has an md5 hash of 702242d3703818ddefe6bf7da2bed757 . I think the number after the dash is the number of parts in the multipart upload.

I also suspect that the new ETag (before the dash) is still an MD5 hash, but with some meta data included along the way from the multipart upload somehow.

Does anyone know how to compute the ETag using the same algorithm as Amazon S3?

Answer 1

Say you uploaded a 14MB file to a bucket without server-side encryption, and your part size is 5MB. Calculate 3 MD5 checksums corresponding to each part, ie the checksum of the first 5MB, the second 5MB, and the last 4MB. Then take the checksum of their concatenation. MD5 checksums are often printed as hex representations of binary data, so make sure you take the MD5 of the decoded binary concatenation, not of the ASCII or UTF-8 encoded concatenation. When that's done, add a hyphen and the number of parts to get the ETag.

Here are the commands to do it on Mac OS X from the console:

$ dd bs=1m count=5 skip=0 if=someFile | md5 >>checksums.txt
5+0 records in
5+0 records out
5242880 bytes transferred in 0.019611 secs (267345449 bytes/sec)
$ dd bs=1m count=5 skip=5 if=someFile | md5 >>checksums.txt
5+0 records in
5+0 records out
5242880 bytes transferred in 0.019182 secs (273323380 bytes/sec)
$ dd bs=1m count=5 skip=10 if=someFile | md5 >>checksums.txt
2+1 records in
2+1 records out
2599812 bytes transferred in 0.011112 secs (233964895 bytes/sec)

At this point all the checksums are in checksums.txt . To concatenate them and decode the hex and get the MD5 checksum of the lot, just use

$ xxd -r -p checksums.txt | md5

And now append "-3" to get the ETag, since there were 3 parts.

Notes

• If you uploaded with aws-cli via aws s3 cp then you most likely have a 8MB chunksize. According to the docs , that is the default.
• If the bucket has server-side encryption (SSE) turned on, the ETag won't be the MD5 checksum (see the API documentation ). But if you're just trying to verify that an uploaded part matches what you sent, you can use the Content-MD5 header and S3 will compare it for you .
• md5 on macOS just writes out the checksum, but md5sum on Linux/brew also outputs the filename. You'll need to strip that, but I'm sure there's some option to only output the checksums. You don't need to worry about whitespace cause xxd will ignore it.

Code Links

• A Gist I wrote with a working script for macOS .
• The project at s3md5 .

Answer 2

Based on answers here, I wrote a Python implementation which correctly calculates both multi-part and single-part file ETags.

def calculate_s3_etag(file_path, chunk_size=8 * 1024 * 1024):
    md5s = []

    with open(file_path, 'rb') as fp:
        while True:
            data = fp.read(chunk_size)
            if not data:
                break
            md5s.append(hashlib.md5(data))

    if len(md5s) < 1:
        return '"{}"'.format(hashlib.md5().hexdigest())

    if len(md5s) == 1:
        return '"{}"'.format(md5s[0].hexdigest())

    digests = b''.join(m.digest() for m in md5s)
    digests_md5 = hashlib.md5(digests)
    return '"{}-{}"'.format(digests_md5.hexdigest(), len(md5s))

The default chunk_size is 8 MB used by the official aws cli tool, and it does multipart upload for 2+ chunks. It should work under both Python 2 and 3.

Answer 3

bash implementation

python implementation

The algorithm literally is (copied from the readme in the python implementation) :

1. md5 the chunks
2. glob the md5 strings together
3. convert the glob to binary
4. md5 the binary of the globbed chunk md5s
5. append "-Number_of_chunks" to the end of the md5 string of the binary

Answer 4

Not sure if it can help:

We're currently doing an ugly (but so far useful) hack to fix those wrong ETags in multipart uploaded files, which consists on applying a change to the file in the bucket; that triggers a md5 recalculation from Amazon that changes the ETag to matches with the actual md5 signature.

In our case:

File: bucket/Foo.mpg.gpg

1. ETag obtained: "3f92dffef0a11d175e60fb8b958b4e6e-2"
2. Do something with the file ( rename it , add a meta-data like a fake header, among others)
3. Etag obtained: "c1d903ca1bb6dc68778ef21e74cc15b0"

We don't know the algorithm, but since we can "fix" the ETag we don't need to worry about it either.

Answer 5

Same algorithm, java version: (BaseEncoding, Hasher, Hashing, etc comes from the guava library

/**
 * Generate checksum for object came from multipart upload</p>
 * </p>
 * AWS S3 spec: Entity tag that identifies the newly created object's data. Objects with different object data will have different entity tags. The entity tag is an opaque string. The entity tag may or may not be an MD5 digest of the object data. If the entity tag is not an MD5 digest of the object data, it will contain one or more nonhexadecimal characters and/or will consist of less than 32 or more than 32 hexadecimal digits.</p> 
 * Algorithm follows AWS S3 implementation: https://github.com/Teachnova/s3md5</p>
 */
private static String calculateChecksumForMultipartUpload(List<String> md5s) {      
    StringBuilder stringBuilder = new StringBuilder();
    for (String md5:md5s) {
        stringBuilder.append(md5);
    }

    String hex = stringBuilder.toString();
    byte raw[] = BaseEncoding.base16().decode(hex.toUpperCase());
    Hasher hasher = Hashing.md5().newHasher();
    hasher.putBytes(raw);
    String digest = hasher.hash().toString();

    return digest + "-" + md5s.size();
}

Answer 6

In an above answer, someone asked if there was a way to get the md5 for files larger than 5G.

An answer that I could give for getting the MD5 value (for files larger than 5G) would be to either add it manually to the metadata, or use a program to do your uploads which will add the information.

For example, I used s3cmd to upload a file, and it added the following metadata.

$ aws s3api head-object --bucket xxxxxxx --key noarch/epel-release-6-8.noarch.rpm 
{
  "AcceptRanges": "bytes", 
  "ContentType": "binary/octet-stream", 
  "LastModified": "Sat, 19 Sep 2015 03:27:25 GMT", 
  "ContentLength": 14540, 
  "ETag": "\"2cd0ae668a585a14e07c2ea4f264d79b\"", 
  "Metadata": {
    "s3cmd-attrs": "uid:502/gname:staff/uname:xxxxxx/gid:20/mode:33188/mtime:1352129496/atime:1441758431/md5:2cd0ae668a585a14e07c2ea4f264d79b/ctime:1441385182"
  }
}

It isn't a direct solution using the ETag, but it is a way to populate the metadata you want (MD5) in a way you can access it. It will still fail if someone uploads the file without metadata.

Answer 7

According to the AWS documentation the ETag isn't an MD5 hash for a multi-part upload nor for an encrypted object: http://docs.aws.amazon.com/AmazonS3/latest/API/RESTCommonResponseHeaders.html

Objects created by the PUT Object, POST Object, or Copy operation, or through the AWS Management Console, and are encrypted by SSE-S3 or plaintext, have ETags that are an MD5 digest of their object data.

Objects created by the PUT Object, POST Object, or Copy operation, or through the AWS Management Console, and are encrypted by SSE-C or SSE-KMS, have ETags that are not an MD5 digest of their object data.

If an object is created by either the Multipart Upload or Part Copy operation, the ETag is not an MD5 digest, regardless of the method of encryption.

Answer 8

Here's yet another piece in this crazy AWS challenge puzzle.

FWIW, this answer assumes you already have figured out how to calculate the "MD5 of MD5 parts" and can rebuild your AWS Multi-part ETag from all the other answers already provided here.

What this answer addresses is the annoyance of having to "guess" or otherwise "divine" the original upload part size.

We use several different tools for uploading to S3 and they all seem to have different upload part sizes, so "guessing" really wasn't an option. Also, we have a lot of files that were historically uploaded when part sizes seemed to be different. Also, the old trick of using an internal server copy to force the creation of an MD5-type ETag also no longer works as AWS has changed their internal server copies to also use multi-part (just with a fairly large part size).

So... How can you figure out the object's part size?

Well, if you first make a head_object request and detect that the ETag is a multi-part type ETag (includes a '-<partcount>' at the end), then you can make another head_object request, but with an additional part_number attribute of 1 (the first part). This follow-on head_object request will then return you the content_length of the first part. Viola... Now you know the part size that was used and you can use that size to re-create your local ETag which should match the original uploaded S3 ETag created when the object was uploaded.

Additionally, if you wanted to be exact (perhaps some multi-part uploads were to use variable part sizes), then you could continue to call head_object requests with each part_number specified and calculate each part's MD5 from the returned parts content_length.

Hope that helps...

Answer 9

Here is the algorithm in ruby...

require 'digest'

# PART_SIZE should match the chosen part size of the multipart upload
# Set here as 10MB
PART_SIZE = 1024*1024*10 

class File
  def each_part(part_size = PART_SIZE)
    yield read(part_size) until eof?
  end
end

file = File.new('<path_to_file>')

hashes = []

file.each_part do |part|
  hashes << Digest::MD5.hexdigest(part)
end

multipart_hash = Digest::MD5.hexdigest([hashes.join].pack('H*'))
multipart_etag = "#{multipart_hash}-#{hashes.count}"

Thanks to Shortest Hex2Bin in Ruby and Multipart Uploads to S3 ...

Answer 10

And here is a PHP version of calculating the ETag:

function calculate_aws_etag($filename, $chunksize) {
    /*
    DESCRIPTION:
    - calculate Amazon AWS ETag used on the S3 service
    INPUT:
    - $filename : path to file to check
    - $chunksize : chunk size in Megabytes
    OUTPUT:
    - ETag (string)
    */
    $chunkbytes = $chunksize*1024*1024;
    if (filesize($filename) < $chunkbytes) {
        return md5_file($filename);
    } else {
        $md5s = array();
        $handle = fopen($filename, 'rb');
        if ($handle === false) {
            return false;
        }
        while (!feof($handle)) {
            $buffer = fread($handle, $chunkbytes);
            $md5s[] = md5($buffer);
            unset($buffer);
        }
        fclose($handle);

        $concat = '';
        foreach ($md5s as $indx => $md5) {
            $concat .= hex2bin($md5);
        }
        return md5($concat) .'-'. count($md5s);
    }
}

$etag = calculate_aws_etag('path/to/myfile.ext', 8);

And here is an enhanced version that can verify against an expected ETag - and even guess the chunksize if you don't know it!

function calculate_etag($filename, $chunksize, $expected = false) {
    /*
    DESCRIPTION:
    - calculate Amazon AWS ETag used on the S3 service
    INPUT:
    - $filename : path to file to check
    - $chunksize : chunk size in Megabytes
    - $expected : verify calculated etag against this specified etag and return true or false instead
        - if you make chunksize negative (eg. -8 instead of 8) the function will guess the chunksize by checking all possible sizes given the number of parts mentioned in $expected
    OUTPUT:
    - ETag (string)
    - or boolean true|false if $expected is set
    */
    if ($chunksize < 0) {
        $do_guess = true;
        $chunksize = 0 - $chunksize;
    } else {
        $do_guess = false;
    }

    $chunkbytes = $chunksize*1024*1024;
    $filesize = filesize($filename);
    if ($filesize < $chunkbytes && (!$expected || !preg_match("/^\\w{32}-\\w+$/", $expected))) {
        $return = md5_file($filename);
        if ($expected) {
            $expected = strtolower($expected);
            return ($expected === $return ? true : false);
        } else {
            return $return;
        }
    } else {
        $md5s = array();
        $handle = fopen($filename, 'rb');
        if ($handle === false) {
            return false;
        }
        while (!feof($handle)) {
            $buffer = fread($handle, $chunkbytes);
            $md5s[] = md5($buffer);
            unset($buffer);
        }
        fclose($handle);

        $concat = '';
        foreach ($md5s as $indx => $md5) {
            $concat .= hex2bin($md5);
        }
        $return = md5($concat) .'-'. count($md5s);
        if ($expected) {
            $expected = strtolower($expected);
            $matches = ($expected === $return ? true : false);
            if ($matches || $do_guess == false || strlen($expected) == 32) {
                return $matches;
            } else {
                // Guess the chunk size
                preg_match("/-(\\d+)$/", $expected, $match);
                $parts = $match[1];
                $min_chunk = ceil($filesize / $parts /1024/1024);
                $max_chunk =  floor($filesize / ($parts-1) /1024/1024);
                $found_match = false;
                for ($i = $min_chunk; $i <= $max_chunk; $i++) {
                    if (calculate_aws_etag($filename, $i) === $expected) {
                        $found_match = true;
                        break;
                    }
                }
                return $found_match;
            }
        } else {
            return $return;
        }
    }
}

Answer 11

The short answer is that you take the 128bit binary md5 digest of each part, concatenate them into a document, and hash that document. The algorithm presented in this answer is accurate.

Note: the multipart ETAG form with the hyphen will change to the form without the hyphen if you "touch" the blob (even without modifying the content). That is, if you copy, or do an in-place copy of your completed multipart-uploaded object (aka PUT-COPY), S3 will recompute the ETAG with the simple version of the algorithm. ie the destination object will have an etag without the hyphen.

You've probably considered this already, but if your files are less than 5GB, and you already know their MD5s, and upload parallelization provides little to no benefit (eg you are streaming the upload from a slow network, or uploading from a slow disk), then you may also consider using a simple PUT instead of a multipart PUT, and pass your known Content-MD5 in your request headers -- amazon will fail the upload if they don't match. Keep in mind that you get charged for each UploadPart.

Furthermore, in some clients, passing a known MD5 for the input of a PUT operation will save the client from recomputing the MD5 during the transfer. In boto3 (python), you would use the ContentMD5 parameter of the client.put_object() method, for instance. If you omit the parameter, and you already knew the MD5, then the client would be wasting cycles computing it again before the transfer.

Answer 12

node.js implementation -

const fs = require('fs');
const crypto = require('crypto');

const chunk = 1024 * 1024 * 5; // 5MB

const md5 = data => crypto.createHash('md5').update(data).digest('hex');

const getEtagOfFile = (filePath) => {
  const stream = fs.readFileSync(filePath);
  if (stream.length <= chunk) {
    return md5(stream);
  }
  const md5Chunks = [];
  const chunksNumber = Math.ceil(stream.length / chunk);
  for (let i = 0; i < chunksNumber; i++) {
    const chunkStream = stream.slice(i * chunk, (i + 1) * chunk);
    md5Chunks.push(md5(chunkStream));
  }

  return `${md5(Buffer.from(md5Chunks.join(''), 'hex'))}-${chunksNumber}`;
};

Answer 13

A version in Rust:

use crypto::digest::Digest;
use crypto::md5::Md5;
use std::fs::File;
use std::io::prelude::*;
use std::iter::repeat;

fn calculate_etag_from_read(f: &mut dyn Read, chunk_size: usize) -> Result<String> {
    let mut md5 = Md5::new();
    let mut concat_md5 = Md5::new();
    let mut input_buffer = vec![0u8; chunk_size];
    let mut chunk_count = 0;
    let mut current_md5: Vec<u8> = repeat(0).take((md5.output_bits() + 7) / 8).collect();

    let md5_result = loop {
        let amount_read = f.read(&mut input_buffer)?;
        if amount_read > 0 {
            md5.reset();
            md5.input(&input_buffer[0..amount_read]);
            chunk_count += 1;
            md5.result(&mut current_md5);
            concat_md5.input(&current_md5);
        } else {
            if chunk_count > 1 {
                break format!("{}-{}", concat_md5.result_str(), chunk_count);
            } else {
                break md5.result_str();
            }
        }
    };
    Ok(md5_result)
}

fn calculate_etag(file: &String, chunk_size: usize) -> Result<String> {
    let mut f = File::open(file)?;
    calculate_etag_from_read(&mut f, chunk_size)
}

See a repo with a simple implementation: https://github.com/bn3t/calculate-etag/tree/master

Answer 14

Extending Timothy Gonzalez's answer :

Identical files will have different etag when using multipart upload.

It's easy to test it with WinSCP, because it uses multipart upload.

When I upload multiple indentical copies of the same file to S3 via WinSCP then each has different etag. When I download them and calculate md5, then they are still indentical.

So from what I tested different etags doesn't mean that files are different.

I see no alternative way to obtain any hash for S3 files without downloading them first.

This is true for multipart uploads. For not-multipart it should still be possible to calculate etag locally.

Answer 15

I have a solution for iOS and macOS without using external helpers like dd and xxd. I have just found it, so I report it as it is, planning to improve it at a later stage. For the moment, it relies on both Objective-C and Swift code. First of all, create this helper class in Objective-C:

AWS3MD5Hash.h

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface AWS3MD5Hash : NSObject

- (NSData *)dataFromFile:(FILE *)theFile startingOnByte:(UInt64)startByte length:(UInt64)length filePath:(NSString *)path singlePartSize:(NSUInteger)partSizeInMb;

- (NSData *)dataFromBigData:(NSData *)theData startingOnByte:(UInt64)startByte length:(UInt64)length;

- (NSData *)dataFromHexString:(NSString *)sourceString;

@end

NS_ASSUME_NONNULL_END

AWS3MD5Hash.m

#import "AWS3MD5Hash.h"
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define SIZE 256

@implementation AWS3MD5Hash


- (NSData *)dataFromFile:(FILE *)theFile startingOnByte:(UInt64)startByte length:(UInt64)length filePath:(NSString *)path singlePartSize:(NSUInteger)partSizeInMb {


   char *buffer = malloc(length);


   NSURL *fileURL = [NSURL fileURLWithPath:path];
   NSNumber *fileSizeValue = nil;
   NSError *fileSizeError = nil;
   [fileURL getResourceValue:&fileSizeValue
                           forKey:NSURLFileSizeKey
                            error:&fileSizeError];

   NSInteger __unused result = fseek(theFile,startByte,SEEK_SET);

   if (result != 0) {
      free(buffer);
      return nil;
   }

   NSInteger result2 = fread(buffer, length, 1, theFile);

   NSUInteger difference = fileSizeValue.integerValue - startByte;

   NSData *toReturn;

   if (result2 == 0) {
       toReturn = [NSData dataWithBytes:buffer length:difference];
    } else {
       toReturn = [NSData dataWithBytes:buffer length:result2 * length];
    }

     free(buffer);

     return toReturn;
 }

 - (NSData *)dataFromBigData:(NSData *)theData startingOnByte:  (UInt64)startByte length:(UInt64)length {

   NSUInteger fileSizeValue = theData.length;
   NSData *subData;

   if (startByte + length > fileSizeValue) {
        subData = [theData subdataWithRange:NSMakeRange(startByte, fileSizeValue - startByte)];
    } else {
       subData = [theData subdataWithRange:NSMakeRange(startByte, length)];
    }

        return subData;
    }

- (NSData *)dataFromHexString:(NSString *)string {
    string = [string lowercaseString];
    NSMutableData *data= [NSMutableData new];
    unsigned char whole_byte;
    char byte_chars[3] = {'\0','\0','\0'};
    NSInteger i = 0;
    NSInteger length = string.length;
    while (i < length-1) {
       char c = [string characterAtIndex:i++];
       if (c < '0' || (c > '9' && c < 'a') || c > 'f')
           continue;
       byte_chars[0] = c;
       byte_chars[1] = [string characterAtIndex:i++];
       whole_byte = strtol(byte_chars, NULL, 16);
       [data appendBytes:&whole_byte length:1];
    }

        return data;
}


@end

Now create a plain swift file:

AWS Extensions.swift

import UIKit
import CommonCrypto

extension URL {

func calculateAWSS3MD5Hash(_ numberOfParts: UInt64) -> String? {


    do {

        var fileSize: UInt64!
        var calculatedPartSize: UInt64!

        let attr:NSDictionary? = try FileManager.default.attributesOfItem(atPath: self.path) as NSDictionary
        if let _attr = attr {
            fileSize = _attr.fileSize();
            if numberOfParts != 0 {



                let partSize = Double(fileSize / numberOfParts)

                var partSizeInMegabytes = Double(partSize / (1024.0 * 1024.0))



                partSizeInMegabytes = ceil(partSizeInMegabytes)

                calculatedPartSize = UInt64(partSizeInMegabytes)

                if calculatedPartSize % 2 != 0 {
                    calculatedPartSize += 1
                }

                if numberOfParts == 2 || numberOfParts == 3 { // Very important when there are 2 or 3 parts, in the majority of times
                                                              // the calculatedPartSize is already 8. In the remaining cases we force it.
                    calculatedPartSize = 8
                }


                if mainLogToggling {
                    print("The calculated part size is \(calculatedPartSize!) Megabytes")
                }

            }

        }

        if numberOfParts == 0 {

            let string = self.memoryFriendlyMd5Hash()
            return string

        }




        let hasher = AWS3MD5Hash.init()
        let file = fopen(self.path, "r")
        defer { let result = fclose(file)}


        var index: UInt64 = 0
        var bigString: String! = ""
        var data: Data!

        while autoreleasepool(invoking: {

                if index == (numberOfParts-1) {
                    if mainLogToggling {
                        //print("Siamo all'ultima linea.")
                    }
                }

                data = hasher.data(from: file!, startingOnByte: index * calculatedPartSize * 1024 * 1024, length: calculatedPartSize * 1024 * 1024, filePath: self.path, singlePartSize: UInt(calculatedPartSize))

                bigString = bigString + MD5.get(data: data) + "\n"

                index += 1

                if index == numberOfParts {
                    return false
                }
                return true

        }) {}

        let final = MD5.get(data :hasher.data(fromHexString: bigString)) + "-\(numberOfParts)"

        return final

    } catch {

    }

    return nil
}

   func memoryFriendlyMd5Hash() -> String? {

    let bufferSize = 1024 * 1024

    do {
        // Open file for reading:
        let file = try FileHandle(forReadingFrom: self)
        defer {
            file.closeFile()
        }

        // Create and initialize MD5 context:
        var context = CC_MD5_CTX()
        CC_MD5_Init(&context)

        // Read up to `bufferSize` bytes, until EOF is reached, and update MD5 context:
        while autoreleasepool(invoking: {
            let data = file.readData(ofLength: bufferSize)
            if data.count > 0 {
                data.withUnsafeBytes {
                    _ = CC_MD5_Update(&context, $0, numericCast(data.count))
                }
                return true // Continue
            } else {
                return false // End of file
            }
        }) { }

        // Compute the MD5 digest:
        var digest = Data(count: Int(CC_MD5_DIGEST_LENGTH))
        digest.withUnsafeMutableBytes {
            _ = CC_MD5_Final($0, &context)
        }
        let hexDigest = digest.map { String(format: "%02hhx", $0) }.joined()
        return hexDigest

    } catch {
        print("Cannot open file:", error.localizedDescription)
        return nil
    }
}

struct MD5 {

    static func get(data: Data) -> String {
        var digest = [UInt8](repeating: 0, count: Int(CC_MD5_DIGEST_LENGTH))

        let _ = data.withUnsafeBytes { bytes in
            CC_MD5(bytes, CC_LONG(data.count), &digest)
        }
        var digestHex = ""
        for index in 0..<Int(CC_MD5_DIGEST_LENGTH) {
            digestHex += String(format: "%02x", digest[index])
        }

        return digestHex
    }
    // The following is a memory friendly version
    static func get2(data: Data) -> String {

    var currentIndex = 0
    let bufferSize = 1024 * 1024
    //var digest = [UInt8](repeating: 0, count: Int(CC_MD5_DIGEST_LENGTH))

    // Create and initialize MD5 context:
    var context = CC_MD5_CTX()
    CC_MD5_Init(&context)


    while autoreleasepool(invoking: {
        var subData: Data!
        if (currentIndex + bufferSize) < data.count {
            subData = data.subdata(in: Range.init(NSMakeRange(currentIndex, bufferSize))!)
            currentIndex = currentIndex + bufferSize
        } else {
            subData = data.subdata(in: Range.init(NSMakeRange(currentIndex, data.count - currentIndex))!)
            currentIndex = currentIndex + (data.count - currentIndex)
        }
        if subData.count > 0 {
            subData.withUnsafeBytes {
                _ = CC_MD5_Update(&context, $0, numericCast(subData.count))
            }
            return true
        } else {
            return false
        }

    }) { }

    // Compute the MD5 digest:
    var digest = Data(count: Int(CC_MD5_DIGEST_LENGTH))
    digest.withUnsafeMutableBytes {
        _ = CC_MD5_Final($0, &context)
    }

    var digestHex = ""
    for index in 0..<Int(CC_MD5_DIGEST_LENGTH) {
        digestHex += String(format: "%02x", digest[index])
    }

    return digestHex

}
}

Now add:

#import "AWS3MD5Hash.h"

to your Objective-C Bridging header. You should be ok with this setup.

Example usage

To test this setup, you could be calling the following method inside the object that is in charge of handling the AWS connections:

func getMd5HashForFile() {


    let credentialProvider = AWSCognitoCredentialsProvider(regionType: AWSRegionType.USEast2, identityPoolId: "<INSERT_POOL_ID>")
    let configuration = AWSServiceConfiguration(region: AWSRegionType.APSoutheast2, credentialsProvider: credentialProvider)
    configuration?.timeoutIntervalForRequest = 3.0
    configuration?.timeoutIntervalForResource = 3.0

    AWSServiceManager.default().defaultServiceConfiguration = configuration

    AWSS3.register(with: configuration!, forKey: "defaultKey")
    let s3 = AWSS3.s3(forKey: "defaultKey")


    let headObjectRequest = AWSS3HeadObjectRequest()!
    headObjectRequest.bucket = "<NAME_OF_YOUR_BUCKET>"
    headObjectRequest.key = self.latestMapOnServer.key




    let _: AWSTask? = s3.headObject(headObjectRequest).continueOnSuccessWith { (awstask) -> Any? in

        let headObjectOutput: AWSS3HeadObjectOutput? = awstask.result

        var ETag = headObjectOutput?.eTag!
        // Here you should parse the returned Etag and extract the number of parts to provide to the helper function. Etags end with a "-" followed by the number of parts. If you don't see this format, then pass 0 as the number of parts.
        ETag = ETag!.replacingOccurrences(of: "\"", with: "")

        print("headObjectOutput.ETag \(ETag!)")

        let mapOnDiskUrl = self.getMapsDirectory().appendingPathComponent(self.latestMapOnDisk!)

        let hash = mapOnDiskUrl.calculateAWSS3MD5Hash(<Take the number of parts from the ETag returned by the server>)

        if hash == ETag {
            print("They are the same.")
        }

        print ("\(hash!)")

        return nil
    }



}

If the ETag returned by the server does not have "-" at the end of the ETag, just pass 0 to calculateAWSS3MD5Hash. Please comment if you encounter any problems. I am working on a swift only solution, I will update this answer as soon as I finish. Thanks

Answer 16

Regarding chunk size, I noticed that it seems to depend of number of parts. The maximun number of parts are 10000 as AWS documents.

So starting on a default of 8MB and knowing the filesize, chunk size and parts can be calculated as follows:

chunk_size=8*1024*1024
flsz=os.path.getsize(fl)

while flsz/chunk_size>10000:
  chunk_size*=2

parts=math.ceil(flsz/chunk_size)

Parts have to be up-rounded

Answer 17

I just saw that the AWS S3 Console 'upload' uses an unusual part (chunk) size of 17,179,870 - at least for larger files.

Using that part size gave me the correct ETag hash using the methods described earlier. Thanks to @TheStoryCoder for the php version.

Thanks to @hans for his idea to use head-object to see the actual sizes of each part.

I used the AWS S3 Console (on Nov28 2020) to upload about 50 files ranging in size from 190MB to 2.3GB and all of them had the same part size of 17,179,870.

Answer 18

Working algorithm implemented in Node.js (TypeScript).

/**
 * Generate an S3 ETAG for multipart uploads in Node.js 
 * An implementation of this algorithm: https://stackoverflow.com/a/19896823/492325
 * Author: Richard Willis <willis.rh@gmail.com>
 */
import fs from 'node:fs';
import crypto, { BinaryLike } from 'node:crypto';

const defaultPartSizeInBytes = 5 * 1024 * 1024; // 5MB

function md5(contents: string | BinaryLike): string {
  return crypto.createHash('md5').update(contents).digest('hex');
}

export function getS3Etag(
  filePath: string,
  partSizeInBytes = defaultPartSizeInBytes
): string {
  const { size: fileSizeInBytes } = fs.statSync(filePath);
  let parts = Math.floor(fileSizeInBytes / partSizeInBytes);
  if (fileSizeInBytes % partSizeInBytes > 0) {
    parts += 1;
  }
  const fileDescriptor = fs.openSync(filePath, 'r');
  let totalMd5 = '';

  for (let part = 0; part < parts; part++) {
    const skipBytes = partSizeInBytes * part;
    const totalBytesLeft = fileSizeInBytes - skipBytes;
    const bytesToRead = Math.min(totalBytesLeft, partSizeInBytes);
    const buffer = Buffer.alloc(bytesToRead);
    fs.readSync(fileDescriptor, buffer, 0, bytesToRead, skipBytes);
    totalMd5 += md5(buffer);
  }

  const combinedHash = md5(Buffer.from(totalMd5, 'hex'));
  const etag = `${combinedHash}-${parts}`;
  return etag;
}

Answer 19

I liked Emerson's leading answer above - especially the xxd part - but I was too lazy to use dd so I went with split , guessing at an 8M chunk size because I uploaded with aws s3 cp :

$ split -b 8M large.iso XXX
$ md5sum XXX* > checksums.txt
$ sed -i 's/ .*$//' checksums.txt 
$ xxd -r -p checksums.txt | md5sum
99a090df013d375783f0f0be89288529  -
$ wc -l checksums.txt 
80 checksums.txt
$ 

It was immediately obvious that both parts of my S3 etag matched my file's calculated etag.

UPDATE:

This has been working nicely:

$ ll large.iso
-rw-rw-r-- 1 user   user   669134848 Apr 12  2021 large.iso
$ 
$ etag large.iso
99a090df013d375783f0f0be89288529-80
$ 
$ type etag
etag is a function
etag () 
{ 
    split -b 8M --filter=md5sum $1 | cut -d' ' -f1 | pee "xxd -r -p | md5sum | cut -d' ' -f1" "wc -l" | paste -d'-' - -
}
$ 

Answer 20

All the other answers assume a standard and regular part size. But that assumption may not be true. Across the console and various SDKs there are different defaults. And the low-level API does allow a lot of variety.

Complications:

• S3 multi-part uploads can have parts of any size (within a min and max for non-last parts).
• Even the non-last parts can be different sizes.
• When you upload they don't have to be consecutive part numbers.
• If you do a multi-part upload with only 1 part, the etag is the more complicated version, not the simple MD5
• etags tend to be wrapped in double-quotes. I don't know why. But that's just a thing that might trip you up.

So we need find find out how many parts there are, and how big they are.

• You cannot reliably get the part count from boto3's Object.parts_count attribute. I don't know if the same is true of other SDKs.
• The get_object_attributes API documentation claims that it returns a list of parts and sizes. But when I tested those fields were missing. Even for multi-part uploads that were not completed.
• Even if you assume equal part sizes (except the last part), you cannot deduce part size from content length and part count. eg if a 90MB file has 3 parts, was that 30MBx3, or 40MB+40MB+10MB?

Let's assume that you have a local file and you want to check whether it matches the content of the object in S3. (And assume that you've already checked whether the lengths differ, because that's a faster check.)

Here's a python3 script to do that. (I chose python just because that's what I'm familiar with.)

We use head_object to get the e-tag. With the e-tag we can deduce whether it was a single-part upload or multi-part, and how many parts. We use head_object passing in PartNumber , calling that for each part, to get the length of each part. You could use multiprocessing to speed that up. (Noting that boto3 's client should not be passed between processes.)

import boto3
from hashlib import md5

def content_matches(local_path, bucket, key) -> bool:
    client = boto3.client('s3')
    resp = client.head_object(Bucket=bucket, Key=key)
    remote_e_tag = resp['ETag']
    total_length = resp['ContentLength']
    if '-' not in remote_e_tag:
        # it was a single-part upload
        m = md5()
        
        # you could read from the file in chunks to avoid loading the whole thing into memory
        # the chunks would not have to match any SDK standard. It can be whatever you want.
        # (The MD5 library will act as if you hashed in one go)
        with open(file, 'rb') as f:
            local_etag = f'"md5(f.read()).hexdigest()"'
        return local_etag == remote_e_tag
    else:
        # multi-part upload
        # to find the number of parts, get it from the e-tag
        # e.g. 123-56 has 56 parts
        num_parts = int(remote_e_tag.strip('"').split('-')[-1])
        print(f"Assuming {num_parts=} from {remote_e_tag=}")
        
        md5s = []
    
        with open(local_path, 'rb') as f:
            sz_read = 0
            
            for part_num in range(1,num_parts+1):
                resp = client.head_object(Bucket=bucket, Key=key, PartNumber=part_num)
                
                sz_read += resp['ContentLength']
                local_data_part = f.read(resp['ContentLength'])
                assert len(local_data_part) == resp['ContentLength'] # sanity check
                md5s.append(md5(local_data_part))
        assert sz_read == total_length, "Sum of part sizes doesn't equal total file size"
        digests = b''.join(m.digest() for m in md5s)
        digests_md5 = md5(digests)
        local_etag = f'"{digests_md5.hexdigest()}-{len(md5s)}"'
        return remote_e_tag == local_etag

And a script to test it with all those edge cases:

import boto3
from pprint import pprint
from hashlib import md5
from main import content_matches

MB = 2 ** 20

bucket = 'mybucket'
key = 'test-multi-part-upload'
local_path = 'test-data'

# first upload the object
s3 = boto3.resource('s3')
obj = s3.Object(bucket, key)
mpu = obj.initiate_multipart_upload()

parts = []
part_sizes = [6 * MB, 5 * MB, 5] # deliberately non-standard and not consistent
upload_part_nums = [1,3,8] # test non-consecutive part numbers for upload
with open(local_path, 'wb') as fw:
    with open('/dev/random', 'rb') as fr:
        for (part_num, part_size) in zip(upload_part_nums, part_sizes):
            part = mpu.Part(part_num)
            data = fr.read(part_size)
            print(f"Uploading part {part_num}")
            resp = part.upload(Body=data)
            parts.append({
                'ETag': resp['ETag'],
                'PartNumber': part_num
            })
            fw.write(data)

resp = mpu.complete(MultipartUpload={
    'Parts': parts
})

obj.reload()

assert content_matches(local_path, bucket, key)

Answer 21

No,

Till now there is not solution to match normal file ETag and Multipart file ETag and MD5 of local file.