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If the decrypting key is 448, then what is the key size? Suppose I am encrypting a plain text with a key by choosing some random numbers between 0 to 1000 then how I can calculate the key size?

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closed as unclear what you're asking by DrLecter, Ricky Demer, e-sushi, archie, Gilles May 12 at 10:05

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"If the decrypting key" for what "is 448"? $\;$ –  Ricky Demer May 12 at 6:51
    
I find the question answerable, even if there are several possible answers, starting with ≈9.967 bit (key size as entropy), 10 bit (bit size of fixed-size bitstring coding all possible keys), and marginally 9 bit (number of bits in actual key), or anything above that (name your definition of key size tied to a particular encoding). –  fgrieu May 12 at 11:43
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1 Answer 1

If the keys of a cryptosystem are obtained

by choosing some random numbers between 0 to 1000

(where numbers is understood as meaning integers), then there are $1001$ equaly probable keys. Hence the key size (or key entropy) is $\log_2(1001)=\log(1001)/\log(2)\approx9.967\text{ bit}$, and the representation of the key as a bitstring must be $10\text{ bit}$ at least (obtained by rounding up).

The actual value ($448$) of the key is only relevant to check that $0\le448\le1000$.

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Can you round up the result of log_2? Please validate my answer :) –  TruthSerum May 12 at 10:04
    
@TruthSerum: when keys are non-negative integers coded in binary, the size of the bitstring coding the key is $\lceil\log_2(m)\rceil$ where $m$ is the number of possible keys (with $m=n+1$ if $n$ is the maximum key); that matches your $1+\lfloor\log_2(n)\rfloor$. We should plug $m=1001$ into that (giving 10 bit), not $m=448$ (giving 9 bit) which would yield the bit size of the number coding a particular value of the key, which I think is less relevant. –  fgrieu May 12 at 11:13
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