## Monday, March 29, 2010

### Fully Homomorphic Encryption Scheme

The paper this week is about how to construct "fully homomorphic" encryption scheme.

Homomorphic encryption is a form of encryption where a specific algebraic operation performed on the ciphertext has a regular algebraic effect on the plain text. Let us take a look at the RSA system. Its public key consists a pair of integers $(n,e)$. The encryption function of a message $m$ is by calculating $m^e (mod n)$. Suppose that for message $m_1$ and $m_2$, we have their encryption $c_1,c_2$. We know that $(m_1*m_2)^e = (m_1)^e * (m_2)^e = c1*c2 (mod n)$. Therefore, the product of two ciphertexts is corresponding to the encryption of the product of the plaintexts. RSA is therefore known to be homomorphic with respect to the multiplication operation. However, the homomorphism is not preserved for addition: adding $c_1$ and $c_2$ is not the encryption of $m_1+m_2$ (or other algebraic operations on $m_1$ and $m_2$).

Since basically every computation on the integer ring can be done by a circuit with addition and multiplication gate, people would like to see an encryption scheme that is homomorphic for both addition and multiplication (on the plain text). This allows us to compute the encryption of arbitrary function on the plain texts by only looking at the ciphertexts. A encryption scheme that is homomorphic with respect to both addition and multiplication is called "fully homomorphic". Prior to this work, it is only known how to construct the encryption scheme that is homomorphic for a single operation (either addition or multiplication) or for "simple" circuits of both addition and multiplication gates. One of the famous early work is due to Boneh-Goh-Nissim which supports function of unlimited addition but only a single multiplication.

In his breakthrough paper in STOC 2010, Gentry constructed a encryption scheme that is fully homomorphic. Its security assumption is based on solving certain lattice problem under worst case. There are mostly two key ideas for constructing such a homomorphic encryption scheme. The first idea is that if the encryption scheme can ensure that it is homomorphic with respect to its own decryption circuit, then we can somehow bootstrap the encryption scheme to make it fully homomorphic. Given the first idea, it remains to construct such a encryption that is bootstrapable. This will require the decryption function to be simple enough. Gentry has some smart ideas of "squashing" the circuit complexity of the decryption function.

As the encryption scheme of the original paper is based on ideal lattice which is quite complex algebraic object, we will instead present a followup work (by Gentry along with Marten van Dijk, Shai Halevi, and Vinod Vaikuntanathan) which simplify the first paper. The link of the new paper is
http://eprint.iacr.org/2009/616.
In this new paper, all the encryption and decryption is implemented by addition and multiplication over integers (instead of using the ideal of the lattice). The two key ideas (bootstrapping and squashing) are still preserved in the new paper.

There are many applications for homomorphic encryption schemes. For example, we can use such an encryption scheme for voting without releasing the privacy of each voter's preference. Basically, every voter encrypts their preference and the voting system executes the voting rules (for e.g., evaluate the majority function) only on the ciphertext. Homomorphic encryption scheme is also believed to have wide applications in the context of cloud computing.

However, the current scheme is still quite impractical. To encrypt one bit, it requires $\eta^8$ bits (I think) where $\eta$ is the security parameter. If we take $\eta$ to be 1000, then it means that to encrypt one bit, we need $10^{24}$ bits. Also, there is a huge loss of efficiency for converting a general algorithm into a circuit of the multiplication and addition gates.

As for the class, I suggest you have a look at the paper at:
http://eprint.iacr.org/2009/616
No other crypto knowledge is needed for this talk.

#### 1 comment:

1. Is this sort of 'one secret hides the next' technique commonly used in cryptography / coding? (I've seen this at least
once before
.)