diff --git a/src/holt59/aoc/2015/day25.py b/src/holt59/aoc/2015/day25.py
new file mode 100644
index 0000000..0389521
--- /dev/null
+++ b/src/holt59/aoc/2015/day25.py
@@ -0,0 +1,16 @@
+import re
+from typing import Any, Iterator
+
+from ..base import BaseSolver
+from ..tools.math import pow_mod
+
+
+class Solver(BaseSolver):
+    def solve(self, input: str) -> Iterator[Any]:
+        m = re.search(r"row\s*([0-9]+)\s*,\s*column\s*([0-9]+)", input)
+        assert m is not None
+
+        row, col = int(m.group(1)), int(m.group(2))
+        n = (row * (row - 1)) // 2 + col * (col + 1) // 2 + (row - 1) * (col - 1)
+
+        yield (20151125 * pow_mod(252533, n - 1, 33554393)) % 33554393
diff --git a/src/holt59/aoc/inputs/holt59/2015/day25.txt b/src/holt59/aoc/inputs/holt59/2015/day25.txt
new file mode 100644
index 0000000..9dcb923
--- /dev/null
+++ b/src/holt59/aoc/inputs/holt59/2015/day25.txt
@@ -0,0 +1 @@
+To continue, please consult the code grid in the manual.  Enter the code at row 3010, column 3019.
diff --git a/src/holt59/aoc/inputs/tests/2015/day25.txt b/src/holt59/aoc/inputs/tests/2015/day25.txt
new file mode 100644
index 0000000..31e0d72
--- /dev/null
+++ b/src/holt59/aoc/inputs/tests/2015/day25.txt
@@ -0,0 +1 @@
+To continue, please consult the code grid in the manual.  Enter the code at row 6, column 5.
diff --git a/src/holt59/aoc/tools/__init__.py b/src/holt59/aoc/tools/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/src/holt59/aoc/tools/math.py b/src/holt59/aoc/tools/math.py
new file mode 100644
index 0000000..c02a802
--- /dev/null
+++ b/src/holt59/aoc/tools/math.py
@@ -0,0 +1,21 @@
+def pow_mod(b: int, e: int, m: int):
+    """
+    Compute (b ** e) % m using right-to-left binary method.
+
+    See https://en.wikipedia.org/wiki/Modular_exponentiation.
+
+    Args:
+        b: Base to exponentiate.
+        e: Exponent.
+        m: Modulus.
+
+    Returns:
+        (b ** e) % m.
+    """
+    r = 1
+    while e > 0:
+        if e % 2 == 1:
+            r = (r * b) % m
+        e >>= 1
+        b = (b * b) % m
+    return r