My solution

python/solution.py

@@ -0,0 +1,76 @@
+from typing import List
+
+def create_square(key: str) -> List[List[str]]:
+    """
+    Create a 5x5 Polybius Square
+
+    @param key: The key used to encode the Polybius square
+    @return: 5x5 Matrix representation of the generated square
+    """
+    square = [['']*5 for i in range(5)]
+
+    i, j = 0, 0
+    inserted = set()
+    #                       J = I
+    for c in key + "ABCDEFGHIKLMNOPQRSTUVWXYZ":
+        if c in inserted:
+            continue
+        square[i][j] = c
+        inserted.add(c)
+        j += 1
+        if j % 5 == 0:
+            j = 0
+            i += 1
+
+    return square
+
+def find_char(char: str, square: List[List[str]]) -> tuple[int, int]:
+    """
+    Finds the location of a character inside the Polybius Square
+
+    @param char: The character to search for inside the square
+    @param square: The Polybius square generated from the `create_square()` function
+    @return: (row, col) location of the character within the square
+    """
+    for i in range(5):
+        for j in range(5):
+            if square[i][j] == char:
+                return (i,j)
+
+def decrypt(ciphertext: str, square: List[List[str]]) -> str:
+    """
+    Decrypts the input ciphertext https://en.wikipedia.org/wiki/Playfair_cipher
+
+    @param ciphertext: The input ciphertext
+    @param square: The Polybius square generated from the `create_square()` function
+    @return: Plaintext
+    """
+    bigraphs = [ciphertext[i:i+2] for i in range(0, len(ciphertext), 2)]
+    result = []
+
+    for b in bigraphs:
+        r1, c1 = find_char(b[0], square)
+        r2, c2 = find_char(b[1], square)
+
+        if r1 == r2:
+            result.append(square[r1][(c1 - 1) % 5])
+            result.append(square[r2][(c2 - 1) % 5])
+
+        elif c1 == c2:
+            result.append(square[(r1 - 1) % 5][c1])
+            result.append(square[(r2 - 1) % 5][c2])
+
+        else:
+            result.append(square[r1][c2])
+            result.append(square[r2][c1])
+
+    result = [c for c in result if c != 'X']
+    return "".join(result)
+
+if __name__ == "__main__":
+    key = "SUPERSPY"
+    ciphertext = "IKEWENENXLNQLPZSLERUMRHEERYBOFNEINCHCV"
+
+    square = create_square(key)
+
+    print(decrypt(ciphertext, square))