15.3: ElGamal Encryption
- Page ID
- 86474
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ElGamal encryption is a public-key encryption scheme that is based on DHKA.
The public parameters are a choice of cyclic group \(\mathbb{G}\) with \(n\) elements and generator \(g\).
The scheme satisfies correctness, since for all \(M\):
\[\begin{aligned} \operatorname{Dec}(s k, \operatorname{Enc}(p k, M)) &=\operatorname{Dec}\left(s k,\left(g^{b}, M \cdot A^{b}\right)\right) \\ &=\left(M \cdot A^{b}\right)\left(\left(g^{b}\right)^{a}\right)^{-1} \\ &=M \cdot\left(g^{a b}\right)\left(g^{a b}\right)^{-1}=M \end{aligned}\]
Security
Imagine an adversary who is interested in attacking an ElGamal scheme. This adversary sees the public key \(A=g^{a}\) and a ciphertext \(\left(g^{b}, M g^{a b}\right)\) go by. Intuitively, the Decisional Diffie-Hellman assumption says that the value \(g^{a b}\) looks random, even to someone who has seen \(g^{a}\) and \(g^{b}\). Thus, the message \(M\) is masked with a pseudorandom group element - as we’ve seen before, this is a lot like masking the message with a random pad as in one-time pad. The only change here is that instead of the xor operation, we are using the group operation in \(\mathbb{G}\).
More formally, we can prove the security of ElGamal under the DDH assumption:
If the DDH assumption in group \(\mathbb{G}\) is true, then ElGamal in group \(\mathbb{G}\) is CPA$-secure.
Proof
It suffices to show that ElGamal has pseudorandom ciphertexts when the calling program sees only a single ciphertext. In other words, we will show that \(\mathcal{L}_{\text {pk-ots\$-real }} \approx \mathcal{L}_{\text {pk-ots\$-rand, }}\), where these libraries are the \(\mathcal{L}_{\text {pk-cpa\$-}\star}\) libraries from Definition \(15.2\) but with the single-ciphertext restriction used in Definition 15.4. It is left as an exercise to show that \(\mathcal{L}_{\text {pk-ots\$-real }} \approx \mathcal{L}_{\text {pk-ots\$-rand }}\) implies CPA$ security (which in turn implies CPA security); the proof is very similar to that of Claim \(15.5.\)
The sequence of hybrid libraries is given below:
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The starting point is the \(\mathcal{L}_{\text {pk-ots } \$ \text {-real library, shown here }}\) with the details of ElGamal filled in. |
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The main body of QUERY computes some intermediate values \(B\) and \(A^{b}\). But since those lines are only reachable one time, it does not change anything to precompute them at initialization time. |
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We can factor out the generation of \(A, B, C\) in terms of the \(\mathcal{L}_{\text {dh-real}}\) library from the Decisional Diffie-Hellman security definition (Definition 14.5). |
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Applying the security of \(\mathrm{DDH}\), we can replace \(\mathcal{L}_{\text {dh-real }}\) with \(\mathcal{L}_{\text {dh-rand }}\). |
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The call to DHQUERY has been inlined. |
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As before, since the main body of QUERY is only reachable once, we can move the choice of \(B\) and \(C\) into that subroutine instead of at initialization time. |
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When \(b\) is sampled uniformly from \(\mathbb{Z}_{n}\), the expression \(B=g^{b}\) is a uniformly distributed element of \(\mathbb{G}\). Also recall that when \(C\) is a uniformly distributed element of \(\mathbb{G}\), then \(M \cdot C\) is uniformly distributed \(-\) this is analogous to the one-time pad property (see Exercise 2.5). Applying this change gives the library to the left. |
In the final hybrid, the response to QUERY is a pair of uniformly distributed group elements \((B, X)\). Hence that library is exactly \(\mathcal{L}_{\mathrm{pk} \text {-ots\$-rand}}\), as desired.