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Let me start by saying that I have a mathematical background, and have very little experience with cryptography. I only know the very basics.

With that said, I recently read about Homomorphic Encryption (HE), and thought that it was a very interesting concept. What one really cares about, though, is Fully Homomorphic Encryption (FHE), which allows you to (essentially) perform an unlimited amount of additions and multiplications (and possibly other operations) on ciphertexts. HE schemes are sometimes transformed into FHE schemes using the so-called concept of bootstrapping.

How I understand bootstrapping is that one uses limited knowledge about the secret key in order to reduce the existing noise in the ciphertext, allowing for more operations to be performed on it. The extra knowledge that is gained from the secret key is assumed not to endanger security.

While trying to read more about bootstrapping, I felt overwhelmed by the technical details. My current goal is to understand the mere essence of bootstrapping. I thus wonder if there is a very basic example which is still able to capture the very basic idea behind bootstrapping?

The example need not even be of “cryptographic nature”, it might as well just describe a related concept showing how such noise reduction might be realised.

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Let me try a basic high-level explanation.

The modern approach is to encrypt messages $m$ as ciphertexts $c$ under a secret key $s$. To achieve security, we slightly alter $c$ with something called a noise/error $e$, which becomes part of $c$. If we perform some (homomorphic) operations on $c$, $e$ increases. There now is a limit on the number of operations: $e$ must not grow too much, since then we cannot decrypt correctly anymore. (At this point, you may visualize the noise as noise in a radio signal, whose music becomes indistinguishable if there's too much of the background noise) Furthermore, we can consider decryption as an operation that removes the noise $e$ and gives us back the original message $m$. Needless to say, decryption requires the secret key $s$.

Now, to achieve FHE, i.e. unlimited operations on $c$, we need a way to reduce the noise $e$ for future operations, but without decrypting $c$ at all. Currently, we only know one way to achieve this: We need to homomorphically evaluate the decryption procedure. Because we know that the decryption procedure removes the noise in general, we can apply it in such a way that it reduces the large noise accumulated in a ciphertext.

Now the details are hard to grab, but for applying the decryption procedure homomorphically, you need to create a ciphertext encrypting (something like) the message $s$, the secret key itself. Without this, you wouldn't be able to evaluate decryption homomorphically, since $s$ is part of the decryption formula. As you noticed, this demands circular security, i.e. it is secure to encrypt $s$ under $s$ itself.

Now, informally speaking, if your evaluation of the decryption as a sequence of homomorphic operations creates less noise than it actually removes, you end up with less noise than before. Thus you can continue performing operations on $c$ and repeat this bootstrapping (= enabling something larger in terms of more operations) process to achieve FHE.

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