diff --git a/api/forms.py b/api/forms.py
new file mode 100644
index 00000000..1f6d7de0
--- /dev/null
+++ b/api/forms.py
@@ -0,0 +1,35 @@
+from django import forms
+from django.forms import HiddenInput
+from django.utils.translation import gettext_lazy as _
+
+
+class ThirdPartyAuthForm(forms.Form):
+    """Form to complete to authenticate on the sith from a third-party app.
+
+    For the form to be valid, the user approve the EULA (french: CGU)
+    and give its username from the third-party app.
+    """
+
+    cgu_accepted = forms.BooleanField(
+        required=True,
+        label=_("I have read and I accept the terms and conditions of use"),
+        error_messages={
+            "required": _("You must approve the terms and conditions of use.")
+        },
+    )
+    is_username_valid = forms.BooleanField(
+        required=True,
+        error_messages={"required": _("You must confirm that this is your username.")},
+    )
+    client_id = forms.IntegerField(widget=HiddenInput())
+    third_party_app = forms.CharField(widget=HiddenInput())
+    cgu_link = forms.URLField(widget=HiddenInput())
+    username = forms.CharField(widget=HiddenInput())
+    callback_url = forms.URLField(widget=HiddenInput())
+    signature = forms.CharField(widget=HiddenInput())
+
+    def __init__(self, *args, label_suffix: str = "", initial, **kwargs):
+        super().__init__(*args, label_suffix=label_suffix, initial=initial, **kwargs)
+        self.fields["is_username_valid"].label = _(
+            "I confirm that %(username)s is my username on %(app)s"
+        ) % {"username": initial.get("username"), "app": initial.get("third_party_app")}
diff --git a/api/urls.py b/api/urls.py
index 2c3f12ff..63ad7052 100644
--- a/api/urls.py
+++ b/api/urls.py
@@ -1,5 +1,9 @@
+from django.urls import path, register_converter
 from ninja_extra import NinjaExtraAPI
 
+from api.views import ThirdPartyAuthResultView, ThirdPartyAuthView
+from core.converters import ResultConverter
+
 api = NinjaExtraAPI(
     title="PICON",
     description="Portail Interactif de Communication avec les Outils Numériques",
@@ -8,3 +12,14 @@ api = NinjaExtraAPI(
     csrf=True,
 )
 api.auto_discover_controllers()
+
+register_converter(ResultConverter, "res")
+
+urlpatterns = [
+    path("auth/", ThirdPartyAuthView.as_view(), name="third-party-auth"),
+    path(
+        "auth/<res:result>/",
+        ThirdPartyAuthResultView.as_view(),
+        name="third-party-auth-result",
+    ),
+]
diff --git a/api/views.py b/api/views.py
new file mode 100644
index 00000000..34e52732
--- /dev/null
+++ b/api/views.py
@@ -0,0 +1,129 @@
+import hmac
+from urllib.parse import unquote
+
+import pydantic
+import requests
+from django.conf import settings
+from django.contrib import messages
+from django.contrib.auth.mixins import LoginRequiredMixin
+from django.core.exceptions import PermissionDenied
+from django.urls import reverse, reverse_lazy
+from django.utils.translation import gettext as _
+from django.views.generic import FormView, TemplateView
+from ninja import Schema
+from ninja_extra.shortcuts import get_object_or_none
+from pydantic import HttpUrl
+
+from api.forms import ThirdPartyAuthForm
+from api.models import ApiClient
+from core.models import SithFile
+from core.schemas import UserProfileSchema
+from core.utils import hmac_hexdigest
+
+
+class ThirdPartyAuthParamsSchema(Schema):
+    client_id: int
+    third_party_app: str
+    cgu_link: HttpUrl
+    username: str
+    callback_url: HttpUrl
+    signature: str
+
+
+class ThirdPartyAuthView(LoginRequiredMixin, FormView):
+    form_class = ThirdPartyAuthForm
+    template_name = "api/third_party/auth.jinja"
+    success_url = reverse_lazy("core:index")
+
+    def parse_params(self) -> ThirdPartyAuthParamsSchema:
+        """Parse and check the authentication parameters.
+
+        Raises:
+            PermissionDenied: if the verification failed.
+        """
+        # This is here rather than in ThirdPartyAuthForm because
+        # the given parameters and their signature are checked during both
+        # POST (for obvious reasons) and GET (in order not to make
+        # the user fill a form just to get an error he won't understand)
+        params = self.request.GET or self.request.POST
+        params = {key: unquote(val) for key, val in params.items()}
+        try:
+            params = ThirdPartyAuthParamsSchema(**params)
+        except pydantic.ValidationError as e:
+            raise PermissionDenied("Wrong data format") from e
+        client: ApiClient = get_object_or_none(ApiClient, id=params.client_id)
+        if not client:
+            raise PermissionDenied
+        if not hmac.compare_digest(
+            hmac_hexdigest(client.hmac_key, params.model_dump(exclude={"signature"})),
+            params.signature,
+        ):
+            raise PermissionDenied("Bad signature")
+        return params
+
+    def dispatch(self, request, *args, **kwargs):
+        self.params = self.parse_params()
+        return super().dispatch(request, *args, **kwargs)
+
+    def get(self, *args, **kwargs):
+        messages.warning(
+            self.request,
+            _(
+                "You are going to link your AE account and your %(app)s account. "
+                "Continue only if this page was opened from %(app)s."
+            )
+            % {"app": self.params.third_party_app},
+        )
+        return super().get(*args, **kwargs)
+
+    def get_initial(self):
+        return self.params.model_dump()
+
+    def form_valid(self, form):
+        client = ApiClient.objects.get(id=form.cleaned_data["client_id"])
+        user = UserProfileSchema.from_orm(self.request.user).model_dump()
+        data = {"user": user, "signature": hmac_hexdigest(client.hmac_key, user)}
+        response = requests.post(form.cleaned_data["callback_url"], data=data)
+        self.success_url = reverse(
+            "api-link:third-party-auth-result",
+            kwargs={"result": "success" if response.ok else "failure"},
+        )
+        return super().form_valid(form)
+
+    def get_context_data(self, **kwargs):
+        return super().get_context_data(**kwargs) | {
+            "third_party_app": self.params.third_party_app,
+            "third_party_cgu": self.params.cgu_link,
+            "sith_cgu": SithFile.objects.get(id=settings.SITH_CGU_FILE_ID),
+        }
+
+
+class ThirdPartyAuthResultView(LoginRequiredMixin, TemplateView):
+    """View that the user will see if its authentication on sith was successful.
+
+    This can show either a success or a failure message :
+    - success : everything is good, the user is successfully authenticated
+      and can close the page
+    - failure : the authentication has been processed on the sith side,
+      but the request to the callback url received an error.
+      In such a case, there is nothing much we can do but to advice
+      the user to contact the developers of the third-party app.
+    """
+
+    template_name = "core/base.jinja"
+    success_message = _(
+        "You have been successfully authenticated. You can now close this page."
+    )
+    error_message = _(
+        "Your authentication on the AE website was successful, "
+        "but an error happened during the interaction "
+        "with the third-party application. "
+        "Please contact the managers of the latter."
+    )
+
+    def get(self, request, *args, **kwargs):
+        if self.kwargs.get("result") == "success":
+            messages.success(request, self.success_message)
+        else:
+            messages.error(request, self.error_message)
+        return super().get(request, *args, **kwargs)
diff --git a/core/static/core/forms.scss b/core/static/core/forms.scss
index d761331c..d2e6e032 100644
--- a/core/static/core/forms.scss
+++ b/core/static/core/forms.scss
@@ -141,7 +141,6 @@ form {
   display: block;
   margin: calc(var(--nf-input-size) * 1.5) auto 10px;
   line-height: 1;
-  white-space: nowrap;
 
   .helptext {
     margin-top: .25rem;
diff --git a/sith/urls.py b/sith/urls.py
index e6629373..af3203b3 100644
--- a/sith/urls.py
+++ b/sith/urls.py
@@ -34,6 +34,7 @@ urlpatterns = [
     path("", include(("core.urls", "core"), namespace="core")),
     path("sitemap.xml", cache_page(86400)(sitemap), {"sitemaps": sitemaps}),
     path("api/", api.urls),
+    path("api-link/", include(("api.urls", "api-link"), namespace="api-link")),
     path("rootplace/", include(("rootplace.urls", "rootplace"), namespace="rootplace")),
     path(
         "subscription/",