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Can anyone tell me the name of this cipher please?
I know it's a simple substitution cipher, I just don't know the name of it.

Cipher Key:

help

Cipher alphabet:

a|b|c|d|e|f|g|h|i|j|k|l|m|n|o|p|q|r|s|t|u|v|w|x|y|z
h|e|l|p|a|b|c|d|f|g|i|j|k|m|n|o|q|r|s|t|u|v|w|x|y|z

Text:

this is a test

Cipher text:

tdfs fs h tast

I think I have found the answer: a Caesar variant, a "Mixed Alphabet" substitution cipher.
Does it have a specific name?

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What are you asking? You provide one question near the top of your message, then continue to discuss a different construction of classical cipher. –  figlesquidge Dec 13 '13 at 12:09
    
@figlesquidge The question is still the question at the top. Is this cipher named or have I got it right and it's just called a variant of the 2 I talked about. –  Chris Dec 13 '13 at 12:10
    
Fair enough. In that case I think you could improve your question by removing the section below the horizontal line - if you are only asking about the first section then everything below it is superfluous. –  figlesquidge Dec 13 '13 at 14:04
    
You are asking "is it A or B", and the answer is "it's neither, A and B are both better (and more efficient) than this". –  tylo Dec 13 '13 at 14:05
1  
@figlesquidge Point taken and question edited. –  Chris Dec 13 '13 at 15:20
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2 Answers

up vote 7 down vote accepted

It's called a keyword cipher. See this question for some ways to break it.

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It took me a while, to actually understand the question.

To your first question: No, this kind of scheme has no name, because it is actually worse than any of the classical ciphers. Well, if you consider any substitution table (over 26 letters) a variant of the Caesar cipher, then... you have such a variant. But it is much weaker than the random substitution, and probably even weaker than the original Caesar cipher. As far as I know, this has no name, and it is surely not a classical cipher. The weakness is really obvious: The later part of the alphabet is just the identity, and this is known even before looking at frequency analysis.

Considering your "improvement", this is not a Vigenere variant, because you actually have a static shift of the substitution by 1 letter each step. So your period is static (26), and you don't gain much of an advantage.

Considering your last "variants"... I have no idea what you actually try to achieve. Leaving out spaces or "ordering them in blocks with fixed size" is not gonna matter. This is no security measure. You are not adding "complexity", you make it more complex to use the cipher for no reason.

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Ok, Thanks for the answer. –  Chris Dec 13 '13 at 14:47
    
In the Internet, there is actually a single site, which calls such a substitution a "keyed Caesar cipher", but I couldn't find any other reference with this name. But they use this substitution and then do the normal Caesar shift. Just the substitution is not much better than not doing anything. I guess the 2nd part of my answer is void, since you removed half of your question –  tylo Dec 13 '13 at 16:03
    
Thanks. And yea, I removed it as suggested by figlesquidge to make the question clearer. –  Chris Dec 13 '13 at 16:07
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