diff --git a/digid_eherkenning/mock/idp/forms.py b/digid_eherkenning/mock/idp/forms.py
index bd29efb..7de7f18 100644
--- a/digid_eherkenning/mock/idp/forms.py
+++ b/digid_eherkenning/mock/idp/forms.py
@@ -1,10 +1,15 @@
 from django import forms
 from django.utils.translation import gettext_lazy as _
 
+from ...validators import BSNValidator
+
 
 class PasswordLoginForm(forms.Form):
     auth_name = forms.CharField(
-        max_length=255, required=True, label=_("DigiD gebruikersnaam")
+        max_length=255,
+        required=True,
+        label=_("DigiD gebruikersnaam"),
+        validators=[BSNValidator()],
     )
     auth_pass = forms.CharField(
         max_length=255, required=True, label=_("Wachtwoord"), widget=forms.PasswordInput
diff --git a/digid_eherkenning/validators.py b/digid_eherkenning/validators.py
index 8fc3eb5..82be08a 100644
--- a/digid_eherkenning/validators.py
+++ b/digid_eherkenning/validators.py
@@ -1,4 +1,6 @@
+from django.core.exceptions import ValidationError
 from django.core.validators import RegexValidator
+from django.utils.deconstruct import deconstructible
 from django.utils.translation import gettext_lazy as _
 
 # See `OINType` in eherkenning-dc.xml XSD
@@ -6,3 +8,55 @@
     regex=r"[0-9]{20}",
     message=_("A valid OIN consists of 20 digits."),
 )
+
+
+validate_digits = RegexValidator(
+    regex="^[0-9]+$", message=_("Expected a numerical value.")
+)
+
+
+class Proef11ValidatorBase:
+    value_size = NotImplemented
+    error_messages = {
+        "too_short": NotImplemented,
+        "wrong": NotImplemented,
+    }
+
+    def __call__(self, value):
+        """
+        Validates that a string value is a valid 11-proef number (BSN, RSIN etc) by applying the
+        '11-proef' checking.
+        :param value: String object representing a presumably good 11-proef number.
+        """
+        # Initial sanity checks.
+        validate_digits(value)
+        if len(value) != self.value_size:
+            raise ValidationError(
+                self.error_messages["too_short"],
+                params={"size": self.value_size},
+                code="invalid",
+            )
+
+        # 11-proef check.
+        total = 0
+        for multiplier, char in enumerate(reversed(value), start=1):
+            if multiplier == 1:
+                total += -multiplier * int(char)
+            else:
+                total += multiplier * int(char)
+
+        if total % 11 != 0:
+            raise ValidationError(self.error_messages["wrong"])
+
+
+@deconstructible
+class BSNValidator(Proef11ValidatorBase):
+    """
+    Validate a BSN value by applying the "11-proef".
+    """
+
+    value_size = 9
+    error_messages = {
+        "too_short": _("BSN should have %(size)i characters."),
+        "wrong": _("Invalid BSN."),
+    }
diff --git a/tests/test_mock_forms.py b/tests/test_mock_forms.py
new file mode 100644
index 0000000..454539e
--- /dev/null
+++ b/tests/test_mock_forms.py
@@ -0,0 +1,24 @@
+import pytest
+
+from digid_eherkenning.mock.idp.forms import PasswordLoginForm
+
+
+@pytest.mark.parametrize(
+    "auth_name, auth_pass, name_has_error, pass_has_error",
+    [
+        ("296648875", "password", False, False),  # OK
+        ("abcdefghe", "password", True, False),  # bsn wrong type
+        ("2966488759", "password", True, False),  # bsn too long
+        ("29664887", "password", True, False),  # bsn too short
+        ("123456789", "password", True, False),  # bsn wrong checksum
+        ("296648875", "", False, True),  # missing password
+    ],
+)
+def test_password_login_form_validate(
+    auth_name, auth_pass, name_has_error, pass_has_error
+):
+    form = PasswordLoginForm(data={"auth_name": auth_name, "auth_pass": auth_pass})
+
+    assert form.has_error("auth_name") is name_has_error
+    assert form.has_error("auth_pass") is pass_has_error
+    assert form.is_valid() is not (name_has_error or pass_has_error)