Breaking keyword ciphers

From 6aedd8f4118deada8b750569fb9443fe786c6cfa Mon Sep 17 00:00:00 2001 From: Neil Smith Date: Wed, 2 Jul 2014 22:14:55 +0100 Subject: [PATCH] Added transposition cipher slides --- cipher.py | 28 ++-- slides/further-work.html | 92 ++++++++++ slides/keyword-encipher.html | 4 - slides/transposition-encipher.html | 258 +++++++++++++++++++++++++++++ 4 files changed, 367 insertions(+), 15 deletions(-) create mode 100644 slides/further-work.html create mode 100644 slides/transposition-encipher.html diff --git a/cipher.py b/cipher.py index e39abce..a2df81b 100644 --- a/cipher.py +++ b/cipher.py @@ -27,7 +27,7 @@ def every_nth(text, n, fillvalue=''): >>> every_nth(string.ascii_lowercase, 5, fillvalue='!') ['afkpuz', 'bglqv!', 'chmrw!', 'dinsx!', 'ejoty!'] """ - split_text = [text[i:i+n] for i in range(0, len(text), n)] + split_text = chunks(text, n, fillvalue) return [''.join(l) for l in zip_longest(*split_text, fillvalue=fillvalue)] def combine_every_nth(split_text): @@ -471,15 +471,18 @@ def scytale_encipher(message, rows, fillvalue=' '): >>> scytale_encipher('thequickbrownfox', 4) 'tubnhirfecooqkwx' >>> scytale_encipher('thequickbrownfox', 5) - 'tubnhirfecooqkwx' + 'tubn hirf ecoo qkwx ' >>> scytale_encipher('thequickbrownfox', 6) 'tqcrnxhukof eibwo ' >>> scytale_encipher('thequickbrownfox', 7) - 'tqcrnxhukof eibwo ' + 'tqcrnx hukof eibwo ' """ - transpositions = [i for i in range(math.ceil(len(message) / rows))] + # transpositions = [i for i in range(math.ceil(len(message) / rows))] + # return column_transposition_encipher(message, transpositions, + # fillvalue=fillvalue, fillcolumnwise=False, emptycolumnwise=True) + transpositions = [i for i in range(rows)] return column_transposition_encipher(message, transpositions, - fillcolumnwise=False, emptycolumnwise=True) + fillvalue=fillvalue, fillcolumnwise=True, emptycolumnwise=False) def scytale_decipher(message, rows): """Deciphers using the scytale transposition cipher. @@ -489,16 +492,19 @@ def scytale_decipher(message, rows): 'thequickbrownfox ' >>> scytale_decipher('tubnhirfecooqkwx', 4) 'thequickbrownfox' - >>> scytale_decipher('tubnhirfecooqkwx', 5) - 'thequickbrownfox' + >>> scytale_decipher('tubn hirf ecoo qkwx ', 5) + 'thequickbrownfox ' >>> scytale_decipher('tqcrnxhukof eibwo ', 6) 'thequickbrownfox ' - >>> scytale_decipher('tqcrnxhukof eibwo ', 7) - 'thequickbrownfox ' + >>> scytale_decipher('tqcrnx hukof eibwo ', 7) + 'thequickbrownfox ' """ - transpositions = [i for i in range(math.ceil(len(message) / rows))] + # transpositions = [i for i in range(math.ceil(len(message) / rows))] + # return column_transposition_decipher(message, transpositions, + # fillcolumnwise=False, emptycolumnwise=True) + transpositions = [i for i in range(rows)] return column_transposition_decipher(message, transpositions, - fillcolumnwise=False, emptycolumnwise=True) + fillcolumnwise=True, emptycolumnwise=False) if __name__ == "__main__": diff --git a/slides/further-work.html b/slides/further-work.html new file mode 100644 index 0000000..64a9729 --- /dev/null +++ b/slides/further-work.html @@ -0,0 +1,92 @@ + + + + Breaking keyword ciphers + + + + + + + + + + + + + diff --git a/slides/keyword-encipher.html b/slides/keyword-encipher.html index bb9e679..168bb5f 100644 --- a/slides/keyword-encipher.html +++ b/slides/keyword-encipher.html @@ -161,10 +161,6 @@ Write out the rest of the alphabet... 2. and 3. How to find the appropriate letter of the keyword. -Indexing pulls out letters. `'keyword'[0]` = 'k' ; `'keyword'[3]` = 'w' ; `'keyword'[-1]` = 'd' - -Slices pulls out substrings. `'keyword'[1:4]` = 'eyw' ; `'keyword'[:3]` = 'key' ; `'keyword'[5:]` = 'rd' - `deduplicate(keyword + string_ascii_lowercase[from:] + string.ascii_lowercase)` Question: why not take a slice of the second alphabet copy? diff --git a/slides/transposition-encipher.html b/slides/transposition-encipher.html new file mode 100644 index 0000000..0c09a4b --- /dev/null +++ b/slides/transposition-encipher.html @@ -0,0 +1,258 @@ + + + + Keyword ciphers + + + + +

+
+# Transposition ciphers
+
+    attack the fort at dawn
+
+    a t t a c
+    k t h e f
+    o r t a t
+    d a w n
+
+    akod ttra aean cft
+
+---
+
+# Transposition ciphers
+
+Rather than changing symbols (substitution ciphers),
+
+Rearrange them.
+
+Still disguises the message.
+
+(Good ciphers do both, and more.)
+
+---
+
+# Scytale
+
+Even older than Caesar cipher.
+
+* Wrap a strip round a pole
+* Write the message along it
+* Unwind the strip
+* "Unreadable" unless reader has pole of same diameter
+
+    attack the fort at dawn
+
+    a t t a c
+    k t h e f
+    o r t a t
+    d a w n
+
+    akod ttra aean cft
+
+---
+
+# Generalising: column transposition ciphers
+
+Scytale essentially fills a grid by rows, then reads it by columns
+
+* (Deciphering is the reverse)
+
+Column transposition ciphers:
+
+* Fill a grid
+* Reorder columns based on keyword
+* Read the grid (perhaps different direction)
+
+Scytale is just a special case of column transposition.
+
+---
+
+# Grids and data structures
+
+How to represent a grid?
+
+What operations do we need to do on it?
+
+---
+
+# Grids and data structures
+
+How to represent a grid?
+
+* List of strings
+* Each row is a string
+* Rows in order in the list
+
+What operations do we need to do on it?
+
+* Fill, by rows or columns
+* Empty, by rows or columns
+* Rearrange columns
+* Calculate the size of the grid
+* Pad message to fit a rectangle of the required size
+
+---
+
+# Finding sizes
+
+Know number of columns
+
+Number of rows = ceiling(message length / columns)
+
+Paddding is (rows * columns) - message length
+
+* What to use as default padding? 
+* Keyword parameter!
+
+## Fit 'thequickbrownfox' (16 letters) into grid of 
+
+* 4 columns
+* 5 columns
+
+---
+
+# Fill and empty grid by rows
+
+Split message into row-sized chunks
+
+* slices and ranges
+
+Append all the rows together
+
+* `<string>.join()`
+
+Keep thinking about test cases!
+
+---
+
+# Fill and empty grid by columns
+
+Idea: fill and empty by rows, with a transposition.
+
+`zip(*rows)` and `itertools.zip_longest(*rows)`
+
+---
+
+# Swapping columns
+
+How to represent a transposition (_permutation_, to mathematicians)?
+
+How to create it from a keyword?
+
+---
+
+# Idea of a transposition
+
+Says, for each element, where it should go
+
+```
+0 1 2 3 4 5 6
+t r e a s o n
+
+a e n o r s t 
+3 2 6 5 1 4 0
+```
+
+The transposition `(3, 2, 6, 5, 1, 4, 0)` says that what was in position 3 moves to position 0, what was in position 2 moves to position 1, what was in position 6 moves to position 2, ...
+
+`enumerate(_iterable_)` yields an iterator that walks over the iterator, including the element indexes.
+
+```python
+>>> [i for i in enumerate('treason')]
+[(0, 't'), (1, 'r'), (2, 'e'), (3, 'a'), (4, 's'), (5, 'o'), (6, 'n')]
+>>> [i for i in enumerate((3, 2, 6, 5, 1, 4, 0))]
+[(0, 3), (1, 2), (2, 6), (3, 5), (4, 1), (5, 4), (6, 0)]
+```
+
+Write the `transpose` and `untranspose` functions.
+
+---
+
+# Transposition from a keyword
+
+Deduplicate the keyword
+
+Sort it
+
+Use `<iterable>.index()` to find the positions of the letters in the sorted keyword
+
+---
+
+# Transposition ciphers
+
+Put it all together 
+
+---
+
+# Masking the fill characters
+
+Padding characters can be distinctive.
+
+Make a function that generates a random letter, based on the `normalised_english_counts`
+
+Use `callable()` to check if the `fillvalue` should be called or just inserted
+
+

+ + + + + + + -- 2.34.1