pre-request script for Postman, to perform HttpSignature calculation. Also SHA-256 message digest.

httpsig-in-postman-pre-request-script.js

function computeHttpSignature(config, headerHash) {
  var template = 'keyId="${keyId}",algorithm="${algorithm}",headers="${headers}",signature="${signature}"',
      sig = template;

  // compute sig here
  var signingBase = '';
  config.headers.forEach(function(h){
    if (signingBase !== '') { signingBase += '\n'; }
    signingBase += h.toLowerCase() + ": " + headerHash[h];
  });

  var hashf = (function() {
      switch (config.algorithm) {
        case 'hmac-sha1': return CryptoJS.HmacSHA1;
        case 'hmac-sha256': return CryptoJS.HmacSHA256;
        case 'hmac-sha512': return CryptoJS.HmacSHA512;
        default : return null;
      }
    }());

  var hash = hashf(signingBase, config.secretkey);
  var signatureOptions = {
        keyId : config.keyId,
        algorithm: config.algorithm,
        headers: config.headers,
        signature : CryptoJS.enc.Base64.stringify(hash)
      };

  // build sig string here
  Object.keys(signatureOptions).forEach(function(key) {
    var pattern = "${" + key + "}",
        value = (typeof signatureOptions[key] != 'string') ? signatureOptions[key].join(' ') : signatureOptions[key];
    sig = sig.replace(pattern, value);
  });

  return sig;
}


var curDate = new Date().toGMTString();
var targetUrl = request.url.trim(); // there may be surrounding ws
targetUrl = targetUrl.replace(new RegExp('^https?://[^/]+/'),'/'); // strip hostname
var method = request.method.toLowerCase();
var sha256digest = CryptoJS.SHA256(request.data);
var base64sha256 = CryptoJS.enc.Base64.stringify(sha256digest);
var computedDigest = 'sha-256=' + base64sha256;

var headerHash = {
      date : curDate,
      digest : computedDigest,
      '(request-target)' : method + ' ' + targetUrl
    };

var config = {
      algorithm : 'hmac-sha256',
      keyId : environment['key-id'],
      secretkey : environment['shared-secret'],
      headers : [ '(request-target)', 'date', 'digest' ]
    };

var sig = computeHttpSignature(config, headerHash);

postman.setEnvironmentVariable('httpsig', sig);
postman.setEnvironmentVariable('computed-digest', computedDigest);
postman.setEnvironmentVariable("current-date", curDate);
postman.setEnvironmentVariable("target-url", targetUrl);

Thanks for the head start on this; I was able to adapt to our use.

Some suggestions:

* Base64 decode the `secretKey` before use

* change `headerHash` to also pull from the request:

var headerHash = Object.assign(Object.assign({},pm.request.headers.toObject(true)),  {
      date : curDate,
      digest : computedDigest,
      '(request-target)' : method + ' ' + targetUrl
    });

* Parameterize or at least call out the hash algorithm name might need adjusting (it did in my case)

Specifically you need to Base64 decode with

CryptoJS.enc.Base64.parse('your-secret-key')