gh-36020: Fixes #35804 by computing the DDT of non-square sboxes properly.
    
Fixes #35804.

After some investigation, it was found that many non-square sboxes did
not work.
In cases where the output size is bigger than the input size (e.g.
`SBox(16,16,16,16,16,16,16,16)`), an incorrect DDT is produced.
In cases where where input size is bigger, it leads to an IndexError
such as in #35804.

`di` spans from [0, `nrows`)
When multiplied by `nrows`, a size of `nrows * (nrows - 1)` can be
achieved which can be greater than the maximum size of the table when
`nrows > ncols` (leading to the behaviour seen earlier).

Changing the multiplication of nrows to ncols fixes the IndexError as
well as produce the correct SBox for `SBox(16,16,16,16,16,16,16,16)`.

Tested with doctests along with the following script:
```sage
import random
from sage.crypto.sbox import SBox

random.seed(b"sbox")

test = lambda x: print(SBox(x).difference_distribution_table())

# Square sbox
test([7,6,0,4,2,5,1,3])
for _ in range(10):
        sbox = random.sample(range(16), 16)
        print(sbox)
        test(sbox)

# Non-square sboxes
for _ in range(10): # input > output
        sbox = random.sample(list(range(4)) * 4, 16)
        print(sbox)
        test(sbox)

for _ in range(10): # input < output
        sbox = random.sample(range(16), 4)
        print(sbox)
        test(sbox)

print("Passed all cases!")
```

<!-- ^^^^^
Please provide a concise, informative and self-explanatory title.
Don't put issue numbers in there, do this in the PR body below.
For example, instead of "Fixes #1234" use "Introduce new method to
calculate 1+1"
-->
<!-- Describe your changes here in detail -->

<!-- Why is this change required? What problem does it solve? -->
<!-- If this PR resolves an open issue, please link to it here. For
example "Fixes #12345". -->
<!-- If your change requires a documentation PR, please link it
appropriately. -->

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->
<!-- If your change requires a documentation PR, please link it
appropriately -->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
<!-- Feel free to remove irrelevant items. -->

- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [x] I have updated the documentation accordingly.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on
- #12345: short description why this is a dependency
- #34567: ...
-->

<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
    
URL: #36020
Reported by: Potato K
Reviewer(s): Kwankyu Lee

Author: Release Manager

src/sage/crypto/sbox.pyx

@@ -639,6 +639,54 @@ cdef class SBox(SageObject):
             [0 0 2 2 2 2 0 0]
             [0 2 2 0 0 2 2 0]
             [0 0 0 0 2 2 2 2]
+            sage: S = SBox(7,4,8,6)
+            sage: S.difference_distribution_table()
+            [4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]
+            [0 0 0 2 0 0 0 0 0 0 0 0 0 0 2 0]
+            [0 0 2 0 0 0 0 0 0 0 0 0 0 0 0 2]
+            [0 2 0 0 0 0 0 0 0 0 0 0 2 0 0 0]
+
+        TESTS::
+
+        Testing square SBoxes::
+        
+            sage: from sage.crypto.sbox import SBox
+            sage: S = SBox(7,6,0,4,2,5,1,3)
+            sage: S.difference_distribution_table()
+            [8 0 0 0 0 0 0 0]
+            [0 2 2 0 2 0 0 2]
+            [0 0 2 2 0 0 2 2]
+            [0 2 0 2 2 0 2 0]
+            [0 2 0 2 0 2 0 2]
+            [0 0 2 2 2 2 0 0]
+            [0 2 2 0 0 2 2 0]
+            [0 0 0 0 2 2 2 2]
+
+        Testing non-square SBoxes::
+
+            sage: from sage.crypto.sbox import SBox
+            sage: S = SBox(8,8,8,8)
+            sage: S.difference_distribution_table()
+            [4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]
+            [4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]
+            [4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]
+            [4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]
+            sage: S = SBox(7,4,8,6)
+            sage: S.difference_distribution_table()
+            [4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]
+            [0 0 0 2 0 0 0 0 0 0 0 0 0 0 2 0]
+            [0 0 2 0 0 0 0 0 0 0 0 0 0 0 0 2]
+            [0 2 0 0 0 0 0 0 0 0 0 0 2 0 0 0]
+            sage: S = SBox(0,0,0,1,0,0,1,3)
+            sage: S.difference_distribution_table()
+            [8 0 0 0]
+            [4 2 2 0]
+            [2 4 0 2]
+            [2 4 0 2]
+            [4 2 2 0]
+            [6 0 0 2]
+            [2 4 0 2]
+            [2 4 0 2]
         """
         cdef Py_ssize_t nrows = 1 << self.m
         cdef Py_ssize_t ncols = 1 << self.n
@@ -649,7 +697,7 @@ cdef class SBox(SageObject):
         for i in range(nrows):
             si = self._S_list[i]
             for di in range(nrows):
-                L[di*nrows + si ^ self._S_list[i ^ di]] += 1
+                L[di*ncols + si ^ self._S_list[i ^ di]] += 1
 
         A = matrix(ZZ, nrows, ncols, L)
         A.set_immutable()