aws-sdkでs3にファイルをアップロードする時、マルチパートアップロードだとEtagがmd5にならない。
なので、Transform経由で横取りして算出する。

digeststream.js

'use strict';

var util = require('util');
var Transform = require('stream').Transform;
var createHash = require('crypto').createHash;

function DigestStream(algorithm, opts) {
  if (!(this instanceof DigestStream)) {
    return new DigestStream(algorithm, opts);
  }

  opts = opts || {};
  opts.objectMode = false;

  Transform.call(this, opts);

  this._hash = createHash(algorithm);
}
util.inherits(DigestStream, Transform);

DigestStream.prototype._transform = function(chunk, encoding, callback) {
  this.push(chunk);
  this._hash.update(chunk);

  callback();
};

DigestStream.prototype._flush = function(callback) {
  this.emit('digest', this._hash.digest('hex'));

  callback();
};

module.exports = DigestStream;

#使用例

app.js

var fs = require('fs');
var zlib = require('zlib');
var AWS = require('aws-sdk');
var config = require('./config.json');
var s3 = new AWS.S3(config.s3);
var DigestStream = require('./digeststream.js');

var digestStream = new DigestStream('md5');
digestStream.on('digest', function(value) {
  console.log(value);
});

var stream = fs.createReadStream('./bigdata').pipe(zlib.createGzip()).pipe(digestStream);

var params = {
  Bucket: 'bucket',
  Key: 'key',
  Body: stream
};

s3.upload(params, function(err, data) {
  if (err) console.log(err, err.stack); // an error occurred
  else     console.log(data);           // successful response
});