I Modular mathematics

Fiona Rozario · Mar 27, 2017

I have some data which I am encrypting using modulus - (data + 203) mod 200. If the data is 150, I get the result as 153. Is there a way to decrypt and get my data (150) back? I don't know how to calculate inverse of this operation...

Oedipus · Mar 27, 2017

Your encrypting operation is the function ƒ(x)=(x+3) mod 200. For a function ƒ: X→Y to be invertible, for every y∈Y there must be one and only one x∈X so that ƒ(x)=y. In your case, the result 153 can be obtained from multiple data values: 150, 350, 550 and so on. There are multiple x∈X that satisfy ƒ(x)=153, so your operation is not an invertible function. In other words, your decryptor cannot possibly know whether the data was 150,350,550...etc. when it receives the encrypted data 153.

mfb · Mar 27, 2017

data might be limited to a range like [0,199], that would allow decrypting it (simply by taking encrpytedvalue-3 mod 200).

Fiona Rozario · Mar 27, 2017

Yes, my data has a range from 1 to 200...

Fiona Rozario · Mar 27, 2017

Thanks for the help...i am adding 203 to the data (not 3). I tried doing what mfb suggested. But (153-203) is not possible and (203-153) gives 50. 50 mod 200 doesn't give 150...am I going wrong somewhere?

mfb · Mar 27, 2017

Fiona Rozario said: ↑

i am adding 203 to the data (not 3)

That is the same mod 200.

Fiona Rozario said: ↑

But (153-203) is not possible

Of course it is possible.

Fiona Rozario · Mar 27, 2017

mfb said: ↑

That is the same mod 200.Of course it is possible.

Yes...got it. Thanks...

Log in or Sign up

I Modular mathematics

Fiona Rozario

Oedipus

mfb

Staff: Mentor

Fiona Rozario

Fiona Rozario

mfb

Staff: Mentor

Fiona Rozario

Modular Multiplication (Replies: 0)

Modular Inverses? (Replies: 8)

Modular arithmetic (Replies: 5)

Modular Mathematics (Replies: 4)

Modular arithmetic (Replies: 7)