A friend has about 10K numbers in a an excel sheet. He wanted to encrypt these n

Question

A friend has about 10K numbers in a an excel sheet. He wanted to encrypt these numbers in order to share the file with a 3rd party that will help him reorganize the tables. What he did, to hide the numbers, is to multiply all the numbers by a floating point number.

Based on my limited knowledge of encryption I argued that this method seems fairly weak (compared to other methods of encryption). My question is - Am I right in my argument or is it really difficult to decipher this sort of encryption.

Explanation / Answer

From the information we can describe the encryption algorithm as follows. Given a floating point number (m) and there is some number (nx) where m * nx = kx.

kx is the encrypted number where x is some position in the list of 10,000 numbers.

So know that nx is going to be some realistic data. In this case you pointed out it was a money value. Money values are probably very common in different data today. Then we can assume nx is some number with two decimal places from .00 to 999999999.99, or bigger if we need.

Let's just consider the first two encrypted numbers for now, n1 and n2.

If n1 is .01, then m = k1,/.01

Then if in n2 we would know n2 = (k2)/(k1,/.01)

If n2 is an expected value like 100.05, because it only has at most two decimals, we may have found they key m. If n2 is an obvious bogus number like 1515.14041, we know that is not an expected value and throw it out and .01 was not n1. If we get a nice number such as 100.05, we can calculate m and plug it in and solve for other values of nx and based of the results come to a conclusion that we have found the original values.

If .01 didn't work, the number would increase to .02 and then for as long as you would want.

Then number of times needed to do this is probably going to be small and may be done in a small or reasonable amount of time.

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