The third comment (on the article) is interesting. In theory, a 'XOR' among sources of data is 'secure' if any one of them is truly random. Let's look at this

The third comment (on the article) is interesting. In theory, a 'XOR' among sources of data is 'secure' if any one of them is truly random. Let's look at this further. Imagine you have a stream of entropy, and then you xor it with a second stream. The first stream is secure (truly random) but the second stream is defective: it's all 1's. Question: Do you now have a random stream, or is it biased towards 0's or 1's or in any other way? Answer: If they're independent it's secure. Let's see why. In fact, you can consider the correct (random) stream as being a OTP on the plaintext which is all the rest. In this case the 'rest' is all 1's, but it could be something partially random. So, any random-enough source acts like an OTP on the final output - Except you throw away the key. Naturally, as OTP's are secure, the resulting 'ciphertext' (looking at it as though it were an OTP operation) does not exhibit any evidence of the 'all 1's plaintext that you "applied" it to. It doesn't matter what order you do the XOR's, or how many are secure (random enough) and how many are insecure.[1] So, if you have 27 sources of entropy you XOR by, then if any one of them are 'random enough' then the output is 'random enough'. There's just one problem: your output could be LESS random if the 'random enough' "key" could somehow be recovered. For example, if instead of all 1's, for our second stream we XOR'd by a mirror of the first (otherwise secure) stream, then our results will be all 0: a horribly biased RNG. So, if any of the 27 (or however many) sources 'know' about the most random source, they can 'undo' the entropy that is being added, and add hteir own. so you might be XORing with a copy of the 'random enough' source, netting all 0's, and also xoring by the output of an encrypted output of an iterated counter, encrypted with, e.g. the NSA's private key: then to anyone that didn't have the NSA's private key it would look like the stream was still random enough. But in fact, the entropy added by one source is removed in another. The correct way around this is for hte sources to not have enough information about each other to be able to 'leak' keys this way (undo entropy). So the third comment on the article is very interesting in this light. [1] XOR is commutative so you can rearrange the expression this way for any number of sources of entropy; it doesn't matter which one is secure: e.g. if D is secure in A XOR B XOR C XOR D XOR E XOR F, the output is still the same as (A XOR B XOR C XOR E XOR F) XOR D (by the commutative property) and therefore secure.