Make certain aspects of the library configurable.

Fixes #5.

5 files changed, 206 insertions, 24 deletions

diff --git a/pyecsca/cfg.py b/pyecsca/cfg.py
new file mode 100644
index 0000000..b1c4007
--- /dev/null
+++ b/pyecsca/cfg.py
@@ -0,0 +1,145 @@
+from copy import deepcopy
+from contextvars import ContextVar, Token
+
+from public import public
+
+
+@public
+class ECConfig(object):
+    """Configuration for the :py:mod:`pyecsca.ec` package."""
+    _no_inverse_action: str = "error"
+    _non_residue_action: str = "error"
+    _unsatisfied_formula_assumption_action: str = "error"
+    _unsatisfied_coordinate_assumption_action: str = "error"
+    _mod_implementation: str = "gmp"
+
+    @property
+    def no_inverse_action(self) -> str:
+        """
+        The action to take when a non-invertible element is to be inverted. One of:
+
+         - `"error"`: Raise :py:class:`pyecsca.ec.error.NonInvertibleError`.
+         - `"warning"`: Raise :py:class:`pyecsca.ec.error.NonInvertibleWarning`.
+         - `"ignore"`: Ignore the event and compute as if nothing happened."""
+        return self._no_inverse_action
+
+    @no_inverse_action.setter
+    def no_inverse_action(self, value: str):
+        if value not in ("error", "warning", "ignore"):
+            raise ValueError("Action has to be one of 'error', 'warning', 'ignore'.")
+        self._no_inverse_action = value
+
+    @property
+    def non_residue_action(self) -> str:
+        """
+        The action to take when a the square-root of a non-residue is to be computed. One of:
+
+         - `"error"`: Raise :py:class:`pyecsca.ec.error.NonResidueError`.
+         - `"warning"`: Raise :py:class:`pyecsca.ec.error.NonResidueWarning`.
+         - `"ignore"`: Ignore the event and compute as if nothing happened."""
+        return self._non_residue_action
+
+    @non_residue_action.setter
+    def non_residue_action(self, value: str):
+        if value not in ("error", "warning", "ignore"):
+            raise ValueError("Action has to be one of 'error', 'warning', 'ignore'.")
+        self._non_residue_action = value
+
+    @property
+    def unsatisfied_formula_assumption_action(self) -> str:
+        """
+        The action to take when a formula assumption is unsatisfied during execution.
+        This works for assumption that can be ignored without a fatal error,
+        which are those that are not used to compute a value of an undefined parameter.
+        For example, things of the form `Z1 = 1`.
+        One of:
+
+         - `"error"`: Raise :py:class:`pyecsca.ec.error.UnsatisfiedAssumptionError`.
+         - `"warning"`: Raise :py:class:`pyecsca.ec.error.UnsatisfiedAssumptionWarning`.
+         - `"ignore"`: Ignore the event and compute as if nothing happened.
+        """
+        return self._unsatisfied_formula_assumption_action
+
+    @unsatisfied_formula_assumption_action.setter
+    def unsatisfied_formula_assumption_action(self, value: str):
+        if value not in ("error", "warning", "ignore"):
+            raise ValueError("Action has to be one of 'error', 'warning', 'ignore'.")
+        self._unsatisfied_formula_assumption_action = value
+
+    @property
+    def unsatisfied_coordinate_assumption_action(self) -> str:
+        """
+        The action to take when a coordinate assumption is unsatisfied during curve creation.
+        This works for assumption that can be ignored without a fatal error,
+        which are those that are not used to compute a value of an undefined parameter.
+        For example, things of the form `a = -1`.
+        One of:
+
+         - `"error"`: Raise :py:class:`pyecsca.ec.error.UnsatisfiedAssumptionError`.
+         - `"warning"`: Raise :py:class:`pyecsca.ec.error.UnsatisfiedAssumptionWarning`.
+         - `"ignore"`: Ignore the event and compute as if nothing happened.
+        """
+        return self._unsatisfied_coordinate_assumption_action
+
+    @unsatisfied_coordinate_assumption_action.setter
+    def unsatisfied_coordinate_assumption_action(self, value: str):
+        if value not in ("error", "warning", "ignore"):
+            raise ValueError("Action has to be one of 'error', 'warning', 'ignore'.")
+        self._unsatisfied_coordinate_assumption_action = value
+
+    @property
+    def mod_implementation(self) -> str:
+        """
+        The selected :py:class:`pyecsca.ec.mod.Mod` implementation. One of:
+
+         - `"gmp"`: Requires the GMP library and `gmpy2` package.
+         - `"python"`: Doesn't require anything.
+        """
+        return self._mod_implementation
+
+    @mod_implementation.setter
+    def mod_implementation(self, value: str):
+        if value not in ("python", "gmp"):
+            raise ValueError(f"Bad Mod implementaiton, can be one of 'python' or 'gmp'.")
+        self._mod_implementation = value
+
+
+@public
+class Config(object):
+    """A runtime configuration for the library."""
+    ec: ECConfig
+    """Configuration for the :py:mod:`pyecsca.ec` package."""
+
+    def __init__(self):
+        self.ec = ECConfig()
+
+
+_config: ContextVar[Config] = ContextVar("config", default=Config())
+
+
+@public
+def getconfig() -> Config:
+    return _config.get()
+
+
+@public
+def setconfig(cfg: Config) -> Token:
+    return _config.set(cfg)
+
+
+@public
+def resetconfig(token: Token):
+    _config.reset(token)
+
+
+@public
+class TemporaryConfig(object):
+    def __init__(self):
+        self.new_config = deepcopy(getconfig())
+
+    def __enter__(self) -> Config:
+        self.token = setconfig(self.new_config)
+        return self.new_config
+
+    def __exit__(self, t, v, tb):
+        resetconfig(self.token)
\ No newline at end of file
diff --git a/pyecsca/ec/error.py b/pyecsca/ec/error.py
index f350d9e..8aabe06 100644
--- a/pyecsca/ec/error.py
+++ b/pyecsca/ec/error.py
@@ -1,5 +1,5 @@
 from public import public
-
+from ..cfg import getconfig
 
 @public
 class NonInvertibleError(ArithmeticError):
@@ -11,6 +11,16 @@ class NonInvertibleWarning(UserWarning):
     pass
 
 
+def raise_non_invertible():
+    """Raise either :py:class:`NonInvertibleError` or :py:class:`NonInvertiblerWarning` or ignore.
+    Depends on the current config value of `no_inverse_action`."""
+    cfg = getconfig()
+    if cfg.ec.no_inverse_action == "error":
+        raise NonInvertibleError("Element not invertible.")
+    elif cfg.ec.no_inverse_action == "warning":
+        raise NonInvertibleWarning("Element not invertible.")
+
+
 @public
 class NonResidueError(ArithmeticError):
     pass
@@ -21,6 +31,16 @@ class NonResidueWarning(UserWarning):
     pass
 
 
+def raise_non_residue():
+    """Raise either :py:class:`NonResidueError` or :py:class:`NonResidueWarning` or ignore.
+    Depends on the current config value of `non_residue_action`."""
+    cfg = getconfig()
+    if cfg.ec.non_residue_action == "error":
+        raise NonResidueError("No square root exists.")
+    elif cfg.ec.non_residue_action == "warning":
+        raise NonResidueWarning("No square root exists.")
+
+
 @public
 class UnsatisfiedAssumptionError(ValueError):
     pass
@@ -29,3 +49,12 @@ class UnsatisfiedAssumptionError(ValueError):
 @public
 class UnsatisfiedAssumptionWarning(UserWarning):
     pass
+
+
+def raise_unsatisified_assumption(action: str, msg: str):
+    """Raise either :py:class:`UnsatisfiedAssumptionError` or :py:class:`UnsatisfiedAssumptionWarning` or ignore.
+    Depends on the value of `action`."""
+    if action == "error":
+        raise UnsatisfiedAssumptionError(msg)
+    elif action == "warning":
+        raise UnsatisfiedAssumptionWarning(msg)
diff --git a/pyecsca/ec/formula.py b/pyecsca/ec/formula.py
index 03681ec..741bb99 100644
--- a/pyecsca/ec/formula.py
+++ b/pyecsca/ec/formula.py
@@ -9,9 +9,10 @@ from public import public
 from sympy import sympify, FF, symbols, Poly
 
 from .context import ResultAction, getcontext, NullContext
-from .error import UnsatisfiedAssumptionError
+from .error import UnsatisfiedAssumptionError, raise_unsatisified_assumption
 from .mod import Mod
 from .op import CodeOp, OpType
+from ..cfg import getconfig
 
 
 @public
@@ -131,7 +132,7 @@ class Formula(ABC):
             for coord, value in point.coords.items():
                 params[coord + str(i + 1)] = value
         # Validate assumptions and compute formula parameters.
-        field = int(params[next(iter(params.keys()))].n)  # This is nasty...
+        field = int(params[next(iter(params.keys()))].n)  # TODO: This is nasty...
         for assumption in self.assumptions:
             assumption_string = unparse(assumption)[1:-2]
             lhs, rhs = assumption_string.split(" == ")
@@ -141,7 +142,9 @@ class Formula(ABC):
                 compiled = compile(assumption, "", mode="eval")
                 holds = eval(compiled, None, alocals)
                 if not holds:
-                    raise UnsatisfiedAssumptionError(f"Unsatisfied assumption in the formula ({assumption_string}).")
+                    # The assumption doesn't hold, see what is the current configured action and do it.
+                    raise_unsatisified_assumption(getconfig().ec.unsatisfied_formula_assumption_action,
+                                                  f"Unsatisfied assumption in the formula ({assumption_string}).")
             else:
                 k = FF(field)
                 expr = sympify(f"{rhs} - {lhs}")
diff --git a/pyecsca/ec/mod.py b/pyecsca/ec/mod.py
index 65018f7..3943b0c 100644
--- a/pyecsca/ec/mod.py
+++ b/pyecsca/ec/mod.py
@@ -4,9 +4,9 @@ from functools import wraps, lru_cache
 from abc import abstractmethod
 from public import public
 
-from .error import NonInvertibleError, NonResidueError
+from .error import raise_non_invertible, raise_non_residue
 from .context import ResultAction
-
+from ..cfg import getconfig
 
 has_gmp = False
 try:
@@ -104,7 +104,7 @@ class RandomModAction(ResultAction):
         return f"{self.__class__.__name__}({self.order:x})"
 
 
-_mod_classes = []
+_mod_classes = {}
 
 
 @public
@@ -114,8 +114,12 @@ class Mod(object):
         if cls != Mod:
             return cls.__new__(cls, *args, **kwargs)
         if not _mod_classes:
-            raise ValueError("Cannot find a working Mod class.")
-        return _mod_classes[-1].__new__(_mod_classes[-1], *args, **kwargs)
+            raise ValueError("Cannot find any working Mod class.")
+        selected_class = getconfig().ec.mod_implementation
+        if selected_class not in _mod_classes:
+            # Fallback to something
+            selected_class = next(iter(_mod_classes.keys()))
+        return _mod_classes[selected_class].__new__(_mod_classes[selected_class], *args, **kwargs)
 
     def __init__(self, x, n):
         self.x = x
@@ -245,10 +249,10 @@ class RawMod(Mod):
 
     def inverse(self):
         if self.x == 0:
-            raise NonInvertibleError("Inverting zero.")
+            raise_non_invertible()
         x, y, d = extgcd(self.x, self.n)
         if d != 1:
-            raise NonInvertibleError("Element not invertible.")
+            raise_non_invertible()
         return RawMod(x, self.n)
 
     def is_residue(self):
@@ -267,7 +271,7 @@ class RawMod(Mod):
         if self.x == 0:
             return RawMod(0, self.n)
         if not self.is_residue():
-            raise NonResidueError("No square root exists.")
+            raise_non_residue()
         if self.n % 4 == 3:
             return self ** int((self.n + 1) // 4)
         q = self.n - 1
@@ -330,7 +334,7 @@ class RawMod(Mod):
         return RawMod(pow(self.x, n, self.n), self.n)
 
 
-_mod_classes.append(RawMod)
+_mod_classes["python"] = RawMod
 
 
 @public
@@ -430,13 +434,14 @@ if has_gmp:
 
         def inverse(self):
             if self.x == 0:
-                raise NonInvertibleError("Inverting zero!")
+                raise_non_invertible()
             if self.x == 1:
                 return GMPMod(1, self.n)
             try:
                 res = gmpy2.invert(self.x, self.n)
             except ZeroDivisionError:
-                raise NonInvertibleError("Element not invertible.")
+                raise_non_invertible()
+                res = 0
             return GMPMod(res, self.n)
 
         def is_residue(self):
@@ -460,7 +465,7 @@ if has_gmp:
             if self.x == 0:
                 return GMPMod(0, self.n)
             if not self.is_residue():
-                raise NonResidueError("No square root exists.")
+                raise_non_residue()
             if self.n % 4 == 3:
                 return self ** int((self.n + 1) // 4)
             q = self.n - 1
@@ -527,4 +532,4 @@ if has_gmp:
             return GMPMod(gmpy2.powmod(self.x, gmpy2.mpz(n), self.n), self.n)
 
 
-    _mod_classes.append(GMPMod)
+    _mod_classes["gmp"] = GMPMod
diff --git a/pyecsca/ec/params.py b/pyecsca/ec/params.py
index 2ec9706..b9495b8 100644
--- a/pyecsca/ec/params.py
+++ b/pyecsca/ec/params.py
@@ -11,11 +11,12 @@ from public import public
 
 from .coordinates import AffineCoordinateModel, CoordinateModel
 from .curve import EllipticCurve
-from .error import UnsatisfiedAssumptionError
+from .error import UnsatisfiedAssumptionError, raise_unsatisified_assumption
 from .mod import Mod
 from .model import (CurveModel, ShortWeierstrassModel, MontgomeryModel, EdwardsModel,
                     TwistedEdwardsModel)
 from .point import Point, InfinityPoint
+from ..cfg import getconfig
 
 
 @public
@@ -127,15 +128,15 @@ def _create_params(curve, coords, infty):
         for assumption in coord_model.assumptions:
             # Try to execute assumption, if it works, check with curve parameters
             # if it doesn't work, move all over to rhs and construct a sympy polynomial of it
-            # then find roots and take first  one for new value for new coordinate parameter.
+            # then find roots and take first one for new value for new coordinate parameter.
             try:
                 alocals: Dict[str, Union[Mod, int]] = {}
                 compiled = compile(assumption, "", mode="exec")
                 exec(compiled, None, alocals)
                 for param, value in alocals.items():
                     if params[param] != value:
-                        raise UnsatisfiedAssumptionError(
-                            f"Coordinate model {coord_model} has an unsatisifed assumption on the {param} parameter (= {value}).")
+                        raise_unsatisified_assumption(getconfig().ec.unsatisfied_coordinate_assumption_action,
+                                                      f"Coordinate model {coord_model} has an unsatisifed assumption on the {param} parameter (= {value}).")
             except NameError:
                 k = FF(field)
                 assumption_string = unparse(assumption)
@@ -148,9 +149,8 @@ def _create_params(curve, coords, infty):
                 poly = Poly(expr, symbols(param), domain=k)
                 roots = poly.ground_roots()
                 for root in roots.keys():
-                    if root >= 0:
-                        params[param] = Mod(int(root), field)
-                        break
+                    params[param] = Mod(int(root), field)
+                    break
                 else:
                     raise UnsatisfiedAssumptionError(f"Coordinate model {coord_model} has an unsatisifed assumption on the {param} parameter (0 = {expr}).")