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I am new to cryptography and have started looking at some basic encryption. I have the following question to answer:

Decrypt the the following encrypted quotation using key = 9, mod 26:

fqjcb rwjwj vnjax bnkhj whxcq nawjv nfxdu mbvnu ujbbf nnc

So far I have decrypted the message to the following:

Whats in a name arose by any other name would smell as sweet

However, this doesn't seem to make sense as a complete decrypted message - have I done something wrong in my decryption or is my answer correct?

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    $\begingroup$ Well, it reads as an english phrase (more or less) and the chances for this to happen with a bad decryption are rather low... That said I did not actually try to decrypt this myself. Note for everybody who wants to VTC this as "request to analyze data", please note that the OP doesn't ask for analysis but rather for verification of his decryption technique which he obviously already applied (and he does know the key) $\endgroup$ – SEJPM Apr 4 '16 at 20:05
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    $\begingroup$ I'm kinda shocked there are people, and British people at that, who wouldn't recognise the quotation. I really don't mean any offence, it's just... strange and puzzling to me. $\endgroup$ – SáT Apr 5 '16 at 1:06
  • $\begingroup$ @SáT Perhaps they're the same people who think that "wherefore" means "where" at the beginning of the same speech. :P $\endgroup$ – IMSoP Apr 5 '16 at 16:33
  • $\begingroup$ @SáT, shakespear just isn't a part of the educational plans in many countries or is not done as intense as in Great Britain. People usually learn a lot about their local great authors and less about foreign ones and those are mainly reserved for the language course where you probably only read one shakespear book (at most) - more often than not, this will be "MacBeth", unless you're studying english at an university (to become a teacher) where you also have to read more of this sort of literature. (at least that's how it is in Germany) $\endgroup$ – SEJPM Apr 5 '16 at 16:41
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Your punctuation is slightly off:

What's in a name? A rose by any other name would smell as sweet.

It's a reference to Romeo and Juliet

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A quick search turns up a quote of Shakespeare from Romeo & Julia. It is often a good idea to simply search the internet, even for (smallish) hexadecimal values or values encoded as base 64.

In general it can be expected that decryption has succeeded if the attacker gets English text without invalid words. Words are usually not encrypted separately, so if they are correct the likelyhood that the decryption succeeded is very high.

To check that the plaintext is correct it is required that the ciphertext size is at least somewhat larger than the key size. If it is not then it is impossible to distinguish between the correct plaintext and other candidates. OTP, where the key size is the same size as the plaintext/ciphertext, is a good example of the latter; each possible sentence has the same probability.

In your case the key is very small (the number 9), so the key must be correct to get this plaintext. The only way that another key would also be valid is when the creator of the key / ciphertext deliberately allowed two different outcomes. This is probably not possible for a Caesar cipher over a sufficiently sized sentence though.


Note that it's common for these kind of puzzles to have a plaintext that is just slightly wrong. I'm presuming some professor or assistent is silently sniggling somewhere...

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    $\begingroup$ "google the plaintext" is just as unexpected of a solution for many people as "google the hash", good catch (+1), I'm sure I wouldn't have tried this (and in fact I didn't before commenting). $\endgroup$ – SEJPM Apr 4 '16 at 20:13
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    $\begingroup$ Had my fun with Shakespeare, significantly added to the answer. $\endgroup$ – Maarten Bodewes Apr 5 '16 at 12:03

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