4chan captcha key:
6Ldp2bsSAAAAAAJ5uyx_lx34lJeEpTLVkP5k04qc

Google supports a noscript version of the new recaptcha, described here https://developers.google.com/recaptcha/docs/faq

https://www.google.com/recaptcha/api/fallback?k=6Ldp2bsSAAAAAAJ5uyx_lx34lJeEpTLVkP5k04qc
Put this in an iframe and when the user has successfully filled in the captcha the user is told to copy paste the key into the real website field (outside the iframe, g-captcha-response)

https://www.google.com/recaptcha/api/challenge?k=6Ldp2bsSAAAAAAJ5uyx_lx34lJeEpTLVkP5k04qc
contains an image and and a field named "c" with the challenge

after the user has solved the image, do a POST to

https://www.google.com/recaptcha/api/fallback?k=6Ldp2bsSAAAAAAJ5uyx_lx34lJeEpTLVkP5k04qc
POST:
c = the challenge given
response = text response from the user

Next if the captcha was solved, get the hash inside the textarea of div.fbc-verification-token
If the textarea isn't present, the captcha wasn't solved correctly you'll reuse the received html.

next send off the reply to 4chan
POST:
g-captcha-response: HASH


Initial HTML:

<!DOCTYPE HTML>
<html dir="ltr">
    <head>
        <meta http-equiv='content-type' content='text/html; charset=UTF-8'>
        <title>reCAPTCHA-uitdaging</title>
        <link rel="stylesheet" href="https://www.gstatic.com/recaptcha/api2/r20150202092117/fallback__ltr.css" type="text/css" charset="utf-8">
    </head>
    <body>
        <div class="fbc">
            <div class="fbc-alert"></div>
            <div class="fbc-header">
                <div class="fbc-logo">
                    <div class="fbc-logo-img"></div>
                    <div class="fbc-logo-text">reCAPTCHA</div>
                </div>
            </div>
            <div>
                <div class="fbc-challenge">
                    <form method="POST">
                        <input type="hidden" name="c" value="03AHJ_VuvGIGhhDoAQAIA8K-vUZHFTAlzF2lAYqmqNIEACc2s6Tu1pZMJScewJo48x0zPzepF1fSgczr9N6TqRoqw-OfTbd4IcAOi7qIQHaTjiftUCI4kpZxRCfmwnJbpeQWl_-SB9Hcg2Jfn-Ri8hnlpHFi5_TRB__Qd5Ni3NoSRP4RBSITkzDB7hQcZN4dZFzGXuB7NJalhTDw5eqwfVre6zIPqxgMW3jkXPTh5C4vAL_NNvNlAXRhMSLBAwxWKV_72fBpr_tWaOOi4z1Jvall4PuYmPi9vj5Ls2A46XKZr85YnJFzaZ4F8ooG5OEv7yM9ZKVByw95rO" />
                        <div class="fbc-message">
                            <label for="response">Voer de tekst in</label>
                        </div>
                        <div class="fbc-input-field">
                            <input dir="ltr" type="text" id="response" name="response" autocomplete="off" autocorrect="off" autocapitalize="off" spellcheck="false" />
                        </div>
                        <div class="fbc-payload">
                            <img src="/recaptcha/api2/payload?c=03AHJ_VuvGIGhhDoAQAIA8K-vUZHFTAlzF2lAYqmqNIEACc2s6Tu1pZMJScewJo48x0zPzepF1fSgczr9N6TqRoqw-OfTbd4IcAOi7qIQHaTjiftUCI4kpZxRCfmwnJbpeQWl_-SB9Hcg2Jfn-Ri8hnlpHFi5_TRB__Qd5Ni3NoSRP4RBSITkzDB7hQcZN4dZFzGXuB7NJalhTDw5eqwfVre6zIPqxgMW3jkXPTh5C4vAL_NNvNlAXRhMSLBAwxWKV_72fBpr_tWaOOi4z1Jvall4PuYmPi9vj5Ls2A46XKZr85YnJFzaZ4F8ooG5OEv7yM9ZKVByw95rO&amp;k=6Ldp2bsSAAAAAAJ5uyx_lx34lJeEpTLVkP5k04qc" alt="reCAPTCHA-uitdagingsafbeelding" />
                        </div>
                        <div class="fbc-button-verify">
                            <input type="submit" value="Verifiëren" />
                        </div>
                    </form>
                </div>
                <div class="fbc-buttons">
                    <div class="fbc-button-holder">
                        <form method="POST" tabindex="-1">
                            <input type="hidden" name="c" value="03AHJ_VuvGIGhhDoAQAIA8K-vUZHFTAlzF2lAYqmqNIEACc2s6Tu1pZMJScewJo48x0zPzepF1fSgczr9N6TqRoqw-OfTbd4IcAOi7qIQHaTjiftUCI4kpZxRCfmwnJbpeQWl_-SB9Hcg2Jfn-Ri8hnlpHFi5_TRB__Qd5Ni3NoSRP4RBSITkzDB7hQcZN4dZFzGXuB7NJalhTDw5eqwfVre6zIPqxgMW3jkXPTh5C4vAL_NNvNlAXRhMSLBAwxWKV_72fBpr_tWaOOi4z1Jvall4PuYmPi9vj5Ls2A46XKZr85YnJFzaZ4F8ooG5OEv7yM9ZKVByw95rO" />
                            <input type="hidden" name="reason" value="r" />
                            <input class="fbc-button-reload fbc-button" type="submit" value="Een nieuwe uitdaging proberen" />
                        </form>
                    </div>
                    <div class="fbc-button-holder">
                        <form method="POST" tabindex="-1">
                            <input type="hidden" name="c" value="03AHJ_VuvGIGhhDoAQAIA8K-vUZHFTAlzF2lAYqmqNIEACc2s6Tu1pZMJScewJo48x0zPzepF1fSgczr9N6TqRoqw-OfTbd4IcAOi7qIQHaTjiftUCI4kpZxRCfmwnJbpeQWl_-SB9Hcg2Jfn-Ri8hnlpHFi5_TRB__Qd5Ni3NoSRP4RBSITkzDB7hQcZN4dZFzGXuB7NJalhTDw5eqwfVre6zIPqxgMW3jkXPTh5C4vAL_NNvNlAXRhMSLBAwxWKV_72fBpr_tWaOOi4z1Jvall4PuYmPi9vj5Ls2A46XKZr85YnJFzaZ4F8ooG5OEv7yM9ZKVByw95rO" />
                            <input type="hidden" name="reason" value="a" />
                            <input class="fbc-button-audio fbc-button" type="submit" value="Een audio-uitdaging proberen" />
                        </form>
                    </div>
                </div>
            </div>
            <div class="fbc-separator"></div>
            <div class="fbc-paste-area">
                <div class="fbc-privacy">
                    <a href="https://www.google.com/intl/nl/policies/privacy/" target="_blank">Privacy</a> - <a href="https://www.google.com/intl/nl/policies/terms/" target="_blank">Voorwaarden</a>
                </div>
            </div>
        </div>
    </body>
</html>

Copy-paste code HTML:
<!DOCTYPE HTML>
<html dir="ltr">
    <head>
        <meta http-equiv='content-type' content='text/html; charset=UTF-8'>
        <title>reCAPTCHA-uitdaging</title>
        <link rel="stylesheet" href="https://www.gstatic.com/recaptcha/api2/r20150202092117/fallback__ltr.css" type="text/css" charset="utf-8">
    </head>
    <body>
        <div class="fbc">
            <div class="fbc-alert"></div>
            <div class="fbc-header">
                <div class="fbc-logo">
                    <div class="fbc-logo-img"></div>
                    <div class="fbc-logo-text">reCAPTCHA</div>
                </div>
            </div>
            <div class="fbc-success">
                <div class="fbc-message">Kopieëer deze code</div>
                <div class="fbc-verification-token">
                    <textarea dir="ltr" readonly onclick="this.select()">
                        03AHJ_Vuvppg-3fh3FZzsT5ArpJD66n9npxpdMArjhFJTH9F_l0-Yp4hQ8LNCDvULQL1hM-WsiMotodqbXN1XDYtmwS18KHmCbLWJEK8UTw4nXKUouTQhKqBK_smQtUPo4nO8vb5VWgFEDjutq3rajk_pwF-TeeXovo3l7prXLHGkZZ6751kGoys3uf9v6QZkANtvVoFtzMvLQ1aOjHB5EftJ7baw6ZFP9GfB6J6r5ANqAvtDrt0ZD7myVIMbYes24QuMOOhl4x8_g
                    </textarea>
                </div>
                <div class="fbc-message">Deze code is 2 minuten geldig</div>
            </div>
            <div class="fbc-separator"></div>
            <div class="fbc-message fbc-paste">Plak de code hier</div>
            <div class="fbc-paste-area">
                <div class="fbc-privacy">
                    <a href="https://www.google.com/intl/nl/policies/privacy/" target="_blank">Privacy</a> - <a href="https://www.google.com/intl/nl/policies/terms/" target="_blank">Voorwaarden</a>
                </div>
            </div>
        </div>
    </body>
</html>


