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I've been redirected here for more, or rather more accurate help from the crypto section of stackexchange.

Long story short, we have this assignment that is bugging me. We are supposed to be employees in a corporate environment that has managed to steal an encrypted message from another company of the field. Now, of course, are asked to decipher the ciphertext. The algorithm is unknown, but we know that we should get an english plaintext, but i cannot seem to find the solution.

The ciphertext is consisted of a long sequence of Fs, 5s and 6s. I have noticed stuff such as:

  • the only "double" sequence is 55. There's no FF or 66.

  • F is always followed by a 5, you cannot find F6 in the ciphertext.

As a hint, we were told that "other than the algorithm, we should also find the alphabet".

I thought of some solutions, but e.g. i don't think that simple substitution can lead in any reasonable result.

For more context, the ciphertext is here:

5656F55656F556F5F55656565656F556F5F556F5F556F55656F556F55656F556F556F5F556F5F556F556F5F55656F556F55656F556F5F55656565656565656F5F5F55656F5565656F5565656F5F556F55656F556F556F5F556F5F556F5F5F5F5F5F5F556F55656565656565656F5F556F556F5F5F5F5F556F5F55656F556565656565656F5565656565656565656F5565656F55656F556565656F556565656F5F556F55656F5F55656F5F5F5F55656F556F5565656F5F55656F556F5F5F5F5F55656F55656F5F5F556F5F5565656F5F5F55656F5F5F556F55656F556F556F5F556F5F556F5F5F5F5F5F5565656F556F5F55656F55656F5F556F5565656565656565656F556F5F5F5F55656F556565656F55656565656565656F5565656F5F55656F5F556F55656F5F556F5F556F556F5F5565656F556565656565656F556F556F556565656F5F5565656F5F55656F5F5F5F55656565656F556F556F5

Any help is highly appreciated.

Thank you.

EDIT: @Gareth McCaughan : Nah not really, it's a university assignment.

EDIT 2: Just because there is a misconception on another thread about homework and what not: I never asked anyone to solve the exercise for me. I am asking for hints, hence the generic "any help is appreciated". I forgot to do it on this post, but on my crypto.se post i explicitly asked people to NOT solve the exercise for me, just give me possible hints that i was possibly not seeing, based on their experience. Heck, that's why i even put the ciphertext in that spoiler thing you got here, just in case anyone could help me already just by the description i gave, without seeing the ciphertext. If you think it's such a biggie, please do remove the post, i don't want to cause any confusion or drama among your users/community, would be a shame.

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    $\begingroup$ Where does this challenge come from? Is it from a competition of some sort? $\endgroup$ – Gareth McCaughan Nov 8 '17 at 17:38
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    $\begingroup$ Preliminarily, it looks like two bits, F5, and 56, decodes to the below, haven't found more yet, 010110101001001000000010111111110010100000100110111111101111111110111011011110111100101100110000110101110011010000011011000100111000110001011010100100100000011011010011111010010010110101101010010010100110101101001111111100011011101110111010011011001011111111101000011011110111111110111001100101100100101001110111111101010111100111001100001111101010 $\endgroup$ – Sconibulus Nov 8 '17 at 17:39
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    $\begingroup$ F5 looks like a seperator. For reference, lengths of 56s after F5s: 1, 2, 1, 1, 0, 1, 0, 1, 0, 0, 0, 0, 0, 0, 1, 8, 0, 1, 1, 0, 0, 0, 0, 1, 0, 2, 7, 9, 3, 2, 4, 4, 0, 1, 2, 0, 2, 0, 0, 0, 2, 1, 3, 0, 2, 1, 0, 0, 0, 0, 2, 2, 0, 0, 1, 0, 3, 0, 0, 2, 0, 0, 1, 2, 1, 1, 0, 1, 0, 1, 0, 0, 0, 0, 0, 2, 2, 1, 0, 5, 1, 0, 1, 0, 1, 2, 1, 2, 1, 1, 0, 1, 0, 1, 1, 0, 2, 1, 2, 1, 0, 8, 0, 0, 2, 3, 3, 3, 1, 0, 2, 2, 0, 1, 9, 1, 0, 0, 0, 2, 4, 8, 3, 0, 2, 0, 1, 2, 0, 1, 0, 1, 1, 0, 3, 7, 1, 1, 4, 0, 3, 0, 2, 0, 0, 0, 5, 1, 1, 0 $\endgroup$ – internet_user Nov 8 '17 at 22:08
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    $\begingroup$ I don't think PSE should be doing your university assignments. The purpose of the assignment is presumably for you to learn something, and you won't learn it by getting the denizens of PSE to do it for you... $\endgroup$ – Gareth McCaughan Nov 9 '17 at 0:41
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    $\begingroup$ 348 is divisible by six. Braille is six bit. Is there a standard Braille to binary transformation? Alternately, there are these: en.wikipedia.org/wiki/Six-bit_character_code. If something in here was covered in class, that might be it. $\endgroup$ – Hugh Meyers Nov 9 '17 at 15:40

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