decrypt numbers multiplied by a certain floating point number 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.
There is a line between encryption and obfuscation, I would say this is on the latter side.

If someone knows the method, and is able to correctly guess even 1 of the original values, simple math will reveal all the original values. Additionally, depending on the type of data in the field, guessing the floating point number may only take seconds even with no 'plaintext' guess, and you have thousands of values to test it on.

Most encryption is based on some kind of permutation and is reversible, floating point values have a maximum level of precision, and can cause the original value to be lost in some cases. Rounding may be able to recover them, but if the original values also have a certain level of precision, there may be no way to recover them, assuming you are not recovering from a backup, but rather trying to decode in Excel.

Examples
Here is an example of an information disclosure that recovers the key.

Say 2 of the encoded values are $388874698.219672$ and $293536403.581274$. If a strong assumption can be made by the table format or header names that these are relatively small integers, say dollar values, you can multiply the ratio by successive values until you find another value with the same relative rounding error. Lets assume they are prices under $1000:

$R = 388874698.219672/ 293536403.581274$

$R = 1.32479206488609$

Multiply $R$ by an incrementing integer counter and round to 6 decimal places till we get an integer. We can even implement the attack in Excel and visually compare, say on the victim's laptop. Extend the following down 100000 rows:

A1 = ROUND(ROW(A1)*1.32479206488609,6)

Then page down till you see an integer. We run into one at row 19357, with a value of 25644. It can be reasonably assumed these translate to 193.57 and 256.44. A little bit of math recovers the 'key' used hide them, which will probably have rounding errors. Testing this against more values and averaging out some more guesses will narrow down which digits of the key are significant and verify if it is correct. An automated attack will take a fraction of a second, even with much larger values.

Since the ratio of values is preserved when using multiplication, guesses can be made about the nature of the data, and information can be exposed even without knowing the real value. mikeazo's example regarding salary information will expose if someone makes more or less, and by what percentage that difference is.