1 files changed, 78 insertions, 0 deletions
diff --git a/pyecsca/sca/re/epa.py b/pyecsca/sca/re/epa.py
new file mode 100644
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+++ b/pyecsca/sca/re/epa.py
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+"""
+Provides functionality inspired by the Exceptional Procedure Attack [EPA]_.
+"""
+
+from typing import Type, Callable, Literal
+
+from public import public
+
+from pyecsca.ec.context import local
+from pyecsca.ec.formula.fake import FakePoint
+from pyecsca.ec.mult import ScalarMultiplier
+from pyecsca.ec.mult.fake import cached_fake_mult
+from pyecsca.ec.params import DomainParameters
+from pyecsca.sca.re.rpa import MultipleContext
+
+
+@public
+def errors_out(
+    scalar: int,
+    params: DomainParameters,
+    mult_class: Type[ScalarMultiplier],
+    mult_factory: Callable,
+    check_funcs: dict[str, Callable],
+    check_condition: Literal["all"] | Literal["necessary"],
+    precomp_to_affine: bool,
+) -> bool:
+    """
+
+    :param scalar:
+    :param params:
+    :param mult_class:
+    :param mult_factory:
+    :param check_funcs:
+    :param check_condition:
+    :param precomp_to_affine:
+    :return:
+
+    .. note::
+        The scalar multiplier must not short-circuit.
+    """
+    mult = cached_fake_mult(mult_class, mult_factory, params)
+    ctx = MultipleContext(keep_base=True)
+    with local(ctx, copy=False):
+        mult.init(params, FakePoint(params.curve.coordinate_model))
+
+    with local(ctx, copy=False):
+        out = mult.multiply(scalar)
+
+    affine_points = {out, *ctx.precomp.values()} if precomp_to_affine else {out}
+    if check_condition == "all":
+        points = set(ctx.points.keys())
+    elif check_condition == "necessary":
+        points = set(affine_points)
+        queue = set(affine_points)
+        while queue:
+            point = queue.pop()
+            for parent in ctx.parents[point]:
+                points.add(parent)
+                queue.add(parent)
+    else:
+        raise ValueError("check_condition must be 'all' or 'necessary'")
+
+    # Special case the "to affine" transform and checks
+    # This actually passes the multiple itself to the check, not the inputs(parents)
+    for point in affine_points:
+        if "affine" in check_funcs:
+            func = check_funcs["affine"]
+            if func(ctx.points[point]):
+                return True
+    # Now handle the regular checks
+    for point in points:
+        formula = ctx.formulas[point]
+        if formula in check_funcs:
+            func = check_funcs[formula]
+            inputs = list(map(lambda pt: ctx.points[pt], ctx.parents[point]))
+            if func(*inputs):
+                return True
+    return False