Creating a signature

Overview

This page explains how to generate an OAuth 1.0a HMAC-SHA1 signature for a HTTP request. This signature will be suitable for passing to the Twitter API as part of an authorized request, as described in Authorizing a request.

The request used to demonstrate signing is a POST to https://api.twitter.com/1/statuses/update.json. The raw request looks like this:

POST /1/statuses/update.json?include_entities=true HTTP/1.1
Accept: */*
Connection: close
User-Agent: OAuth gem v0.4.4
Content-Type: application/x-www-form-urlencoded
Content-Length: 76
Host: api.twitter.com

status=Hello%20Ladies%20%2b%20Gentlemen%2c%20a%20signed%20OAuth%20request%21

Collecting the request method and URL

To produce a signature, start by determining the HTTP method and URL of the request. These two are known when creating the request, so they are easy to obtain.

The request method will almost always be GET or POST for Twitter API requests.

The base URL is the URL to which the request is directed, minus any query string or hash parameters. It is important to use the correct protocol here, so make sure that the “https://” or “http://” portion of the URL matches the actual request sent to the API. As a best practice, you should always be using “https://” with the Twitter API.

Collecting parameters

Next, gather all of the parameters included in the request. There are two such locations for these additional parameters - the URL (as part of the querystring) and the request body. The sample request includes a single parameter in both locations (shown in bold):

POST /1/statuses/update.json?include_entities=true HTTP/1.1
Accept: */*
Connection: close
User-Agent: OAuth gem v0.4.4
Content-Type: application/x-www-form-urlencoded
Content-Length: 76
Host: api.twitter.com

status=Hello%20Ladies%20%2b%20Gentlemen%2c%20a%20signed%20OAuth%20request%21

In the HTTP request the parameters are URL encoded, but you should collect the raw values. In addition to the request parameters, every oauth_* parameter needs to be included in the signature, so collect those too. Here are the parameters from Authorizing a request:

These values need to be encoded into a single string which will be used later on. The process to build the string is very specific:

1. Percent encode every key and value that will be signed.
2. Sort the list of parameters alphabetically[1] by encoded key[2].
3. For each key/value pair:
4. Append the encoded key to the output string.
5. Append the ‘=’ character to the output string.
6. Append the encoded value to the output string.
7. If there are more key/value pairs remaining, append a ‘&’ character to the output string.

[1] Note: The OAuth spec says to sort lexigraphically, which is the default alphabetical sort for many libraries.

[2] Note: In case of two parameters with the same encoded key, the OAuth spec says to continue sorting based on value. However, Twitter does not accept duplicate keys in API requests.

The following string will be produced by repeating these steps with the parameters collected above:

include_entities=true&oauth_consumer_key=xvz1evFS4wEEPTGEFPHBog&oauth_nonce=kYjzVBB8Y0ZFabxSWbWovY3uYSQ2pTgmZeNu2VS4cg&oauth_signature_method=HMAC-SHA1&oauth_timestamp=1318622958&oauth_token=370773112-GmHxMAgYyLbNEtIKZeRNFsMKPR9EyMZeS9weJAEb&oauth_version=1.0&status=Hello%20Ladies%20%2B%20Gentlemen%2C%20a%20signed%20OAuth%20request%21

Creating the signature base string

The three values collected so far must be joined to make a single string, from which the signature will be generated. This is called the signature base string by the OAuth specification.

To encode the HTTP method, base URL, and parameter string into a single string:

1. Convert the HTTP Method to uppercase and set the output string equal to this value.
2. Append the ‘&’ character to the output string.
3. Percent encode the URL and append it to the output string.
4. Append the ‘&’ character to the output string.
5. Percent encode the parameter string and append it to the output string.

This will produce the following:

POST&https%3A%2F%2Fapi.twitter.com%2F1%2Fstatuses%2Fupdate.json&include_entities%3Dtrue%26oauth_consumer_key%3Dxvz1evFS4wEEPTGEFPHBog%26oauth_nonce%3DkYjzVBB8Y0ZFabxSWbWovY3uYSQ2pTgmZeNu2VS4cg%26oauth_signature_method%3DHMAC-SHA1%26oauth_timestamp%3D1318622958%26oauth_token%3D370773112-GmHxMAgYyLbNEtIKZeRNFsMKPR9EyMZeS9weJAEb%26oauth_version%3D1.0%26status%3DHello%2520Ladies%2520%252B%2520Gentlemen%252C%2520a%2520signed%2520OAuth%2520request%2521

Make sure to percent encode the parameter string! The signature base string should contain exactly 2 ampersand ‘&’ characters. The percent ‘%’ characters in the parameter string should be encoded as %25 in the signature base string.

Getting a signing key

The last pieces of data to collect are secrets which identify the application making the request, and the user the request is on behalf of. It is very important to note that these values are incredibly sensitive and should never be shared with anyone.

The value which identifies your application to Twitter is called the consumer secret and can be found by going to dev.twitter.com/apps and viewing the settings page for your application. This will be the same for every request your application sends.

The value which identifies the account your application is acting on behalf of is called the oauth token secret. This value can be obtained in several ways, all of which are described at Obtaining access tokens.

Once again, it is very important to keep these values private to your application. If you feel that your values have been compromised, regenerate your tokens (the tokens on this page have been marked as invalid for real requests).

Both of these values need to be combined to form a signing key which will be used to generate the signature. The signing key is simply the percent encoded consumer secret, followed by an ampersand character ‘&’, followed by the percent encoded token secret:

Note that there are some flows, such as when obtaining a request token, where the token secret is not yet known. In this case, the signing key should consist of the percent encodedconsumer secret followed by an ampersand character ‘&’.

kAcSOqF21Fu85e7zjz7ZN2U4ZRhfV3WpwPAoE3Z7kBw&LswwdoUaIvS8ltyTt5jkRh4J50vUPVVHtR2YPi5kE

Calculating the signature

Finally, the signature is calculated by passing the signature base string and signing key to the HMAC-SHA1 hashing algorithm. The details of the algorithm are explained in depth here, but thankfully there are implementations of HMAC-SHA1 available for every popular language. For example, PHP has the hash_hmac function.

The output of the HMAC signing function is a binary string. This needs to be base64 encoded to produce the signature string. For example, the output given the base string and signing key given on this page is B6 79 C0 AF 18 F4 E9 C5 87 AB 8E 20 0A CD 4E 48 A9 3F 8C B6. That value, when converted to base64, is the OAuth signature for this request: