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Signatures in OAuth

Hi again. I already went into the basics of OAuth in a previous post, and alluded to the signatures being the hard part of the implementation. I'm using a python-oauth module for the client, but for the server, I decided to implement my own in Perl and Mojolicious, since I had problems with Net::OAuth that I deemed a protocol violation. Besides, I learn more this way.

There are multiple signature methods supported in OAuth 1.0, specifically three.

  2. HMAC-SHA1
  3. RSA-SHA1

Unsurprisingly, PLAINTEXT is the simplest. In this case, the signature is a simple combination of the consumer secret and the token secret, if there is any yet, with a slight twist. To ensure that the characters are treated properly, they must first be "normalized". This means decoding them as you would any percent-encoded string, and then...

  • convert them to UTF-8

  • escape them with percent-encoding from RFC 3986 like so

    • these characters are not encoded: alphanumerics, "-", ".", "_", "~"
    • all other characters must be encoded
    • any hexidecimal used in encoding must be upper-case

I found that hard to read, but I did find this bit of Perl to solve the problem in Net::OAuth, which I converted to a Mojolicious helper.

helper encode => sub {
    my $self = shift;
    my $str = shift;
    $str = "" unless defined $str;
        if ($str =~ /[\x80-\xFF]/ and !utf8::is_utf8($str)) {
            warn("your OAuth message appears to contain some "
                . "multi-byte characters that need to be decoded "
                . "via or a PerlIO layer first. "
                . "This may result in an incorrect signature.");

Decoding is even simpler.

helper decode => sub {
    my $self = shift;
    my $str = shift;
    return uri_unescape($str);

In Python, the same code looks like so.

import urllib

def escape(s):
    """Escape a URL including any /."""
    return urllib.quote(s, safe='~')

def _utf8_str(s):
    """Convert unicode to utf-8."""
    if isinstance(s, unicode):
        return s.encode("utf-8")
        return str(s)

escaped = escape(_utf8_str(s))

# Unquoting is just
unquoted = urllib.unquote(s)

So, this needs to be done to each of the parameters, in this case, the consumer key and the token secret, if any, and then they are combined with an ampersand. So, if your consumer key is, say, "myconsumerkey", and you don't have a token yet, then the initial PLAINTEXT signature is myconsumerkey&.

Now, this isn't too bad, but once you get into HMAC-SHA1 signatures, it gets a lot worse. The signature from the PLAINTEXT method becomes the key for the signature, and you'll already have the code for that now, but the raw input to the HMAC-SHA1 algorithm is a base string, that is rather difficult to construct. The input is the HTTP method, the request URI, both normalized like above and contatenated with an ampersand. Then, this will be contatenated with an ampersand to all of the input parameters in the request, constructed in a particular way.

  1. Take all input parameter names and values from all sources, and normalize them like above. (but skip the oauth_signature parameter)
  2. Sort all parameters by the normalized parameter name.
  3. Pair the names and values, contatenated with an "=".
  4. Concatenate all pairs with ampersands in the sorted order.
  5. Escape the entire string using the method above.

This is the base string to the HMAC-SHA1 algorithm, along with the key we mentioned. The final signature should match what the client generated. Oh, and if you're running your service on a nonstandard port (80 or 443), then you must include the port in the URI.


A call to http://localhost/initiate on port 80 or 443, a GET request, with the following params:

{'oauth_nonce': '21823552', 'oauth_timestamp': 1356129798,
 'oauth_consumer_key': 'Mitel test', 'oauth_signature_method': 'HMAC-SHA1',
 'oauth_version': '1.0', 'oauth_signature': 'pevzNqSnJ8QtqFUDWVlYhVRp8D0=',
 'oauth_callback': 'oob'}

The base string would look like this:


with a key of:


and a final signature of:


Oddly, if I used the b64encode method in Digest::HMAC_SHA1, a final "=" sign is missing on the final result, so I had to pull in MIME::Base64 and do this instead:

my $sig = encode_base64($hmac->digest);

The equivalent Python in the oauth library does this:

import binascii

# HMAC object.
    import hashlib # 2.5
    hashed =, raw, hashlib.sha1)
    import sha # Deprecated
    hashed =, raw, sha)

# Calculate the digest base 64.
return binascii.b2a_base64(hashed.digest())[:-1]

That just leaves RSA-SHA1, but that requires a pre-existing SSL relationship with the server, using SSL certificates. As such, I'm not worrying about it just yet. I don't think it'll be used much.

I'll need to do some interop testing with a few different clients, I'm hoping that they're not all snowflakes. The point of the rigid nature of the base string construction is that the final product is supposed to be reproducable.

The base string construction is definitely the hardest part, and I've read that the signatures were dropped in OAuth 2.0 because they were too hard to do. I'd rather not drop the added security, and while they're a pain, there are sample implementations to follow. I think that OAuth 1.0 is a better choice. And it's, like, finished.

Comments !