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Codebreaking

The first challenge question for my cryptography course with udacity.com was to decrypt two intercepted messages encoded using a One Time Pad. The messages were encrypted using the same key, which is a big operator error for sending secure messages with a one time pad.

Despite having considerable computer resources available it took me a full afternoon before I obtained the full key. I ended up using a very tedious manual process which pretty much involved assuming common words would present. I ended up trying a few hundred words before finding an invalid assumption I'd made.

Back on the right track I tried words surrounded by spaces starting at all possible locations. By assuming a word appears in a message you get a section of bits that could potentially be the key aligned with that word. Using this key on the other message usually gives garbage - except for the few cases where it output something resembling English, meaning you "might" have guessed the right word in the right place.

After a very manual process of decided what scrambled text could be part of English I decoded the messages. Hopefully I can come up with smarter, more automated solutions to the future challenges!

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