[Python] Why does my Collapsing Matrix Cipher decryption produce wrong plaintext?

I implemented the Collapsing Matrix Cipher decryption in Python.

Cipher Specification

Grid Initialization

• Create a 5×5 grid.
• Populate it with the unique letters of the keyword (ignore non-alphabetic characters), followed by the remaining letters of the alphabet in order.
• Treat i and j as the same letter; skip j during grid creation. When decrypting, i and j are interchangeable.

Character Decryption

• For each ciphertext character C, find its position (row, col) in the current grid.
• The original plaintext character P is at (row+1, col) (wrap around so row 4 becomes row 0).

Collapse Step

• After finding P, remove it from the grid.
• Shift all characters after P in row-major order (left-to-right, top-to-bottom) forward by one to fill the gap.
• Use the updated grid for the next character decryption.

Here is the complete code I’m using:

import sys

def preprocess_keyword(keyword):
seen = set()
keyword = keyword.lower().replace('j', 'i')
grid_letters = []

for c in keyword:
if c.isalpha() and c not in seen:
seen.add(c)
grid_letters.append(c)

for char_code in range(ord('a'), ord('z') + 1):
c = chr(char_code)
if c == 'j':
continue
if c not in seen:
seen.add(c)
grid_letters.append(c)

return [grid_letters[i*5i+1)*5] for i in range(5)]

def decrypt_char(grid, cipher_char):
cipher_char = 'i' if cipher_char == 'j' else cipher_char
trow, tcol = -1, -1
for r in range(5):
try:
c = grid[r].index(cipher_char)
trow, tcol = r, c
break
except ValueError:
continue
if trow == -1:
return None
p_row = (trow + 1) % 5
p_col = tcol
return grid[p_row][p_col]

def collapse_grid(grid, removed_char):
flat = [char for row in grid for char in row]
try:
idx = flat.index(removed_char)
except ValueError:
return grid
for i in range(idx, 24):
flat = flat[i + 1]
flat[24] = None
return [flat[i*5i+1)*5] for i in range(5)]

def process_test_case(keyword, ciphertext):
grid = preprocess_keyword(keyword)
plaintext = ''

for c in ciphertext:
p = decrypt_char(grid, c)
if p is not None:
plaintext += p
grid = collapse_grid(grid, p)

return plaintext

def main():
lines = sys.stdin.read().splitlines()
line_idx = 0
while line_idx < len(lines) and not lines[line_idx].strip():
line_idx += 1
if line_idx >= len(lines): return
num_test_cases = int(lines[line_idx])
line_idx += 1

outputs = []
for _ in range(num_test_cases):
while line_idx < len(lines) and not lines[line_idx].strip():
line_idx += 1
if line_idx >= len(lines): break
keyword = lines[line_idx]
line_idx += 1
if line_idx >= len(lines): break
ciphertext = lines[line_idx]
line_idx += 1
outputs.append(process_test_case(keyword, ciphertext))

print('\n\n'.join(outputs))

if __name__ == "__main__":
main()


Test case:

2

crypto
kcnarfm


Expected output:

hackathon


Actual output:

soxgbm


I expected the decryption to match the example provided in the problem statement, but instead I’m getting incorrect plaintext. I’ve verified my grid initialization, so I suspect the issue is with my collapse_grid logic or the way I determine the plaintext character in decrypt_char.

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