[zk-token-sdk] Grouped ElGamal ciphertext and validity proof

[samkim-crypto]

Problem

What remains in the zk token proof program is the ciphertext validity (and batched ciphertext validity) proof, which certifies that a variant of ElGamal ciphertext is well-formed.

The issue is that the ciphertext validity proof that is needed for the confidential transfer instruction is not a validity proof on the standard ElGamal ciphertext, which is an encryption of a message under a single ElGamal public key. Instead, the validity proof is on a "grouped" variant of ElGamal ciphertext where the message is encrypted under multiple ElGamal public keys at once.

We can add VerifyCiphertextValidity and VerifyBatchedCiphertextValidityProof in the proof program, but this may be confused as certifying validity on a regular ElGamal ciphertext. A more suitable name for these two proofs would be VerifyGroupedCiphertextValidity and VerifyBatchedGroupedCiphertextValidity.

We can go ahead and rename the implementation in #31816 as VerifyGroupedCiphertextValidity and VerifyBatchedGroupedCiphertextValidity, but a cleaner way would be to actually create a GroupedElGamalCiphertext type in the encryption module and define these proof instructions with respect to the GroupedElGamalCiphertext type.

Defining GroupedElGamalCiphertext would also help in simplifying/cleaning up the VerifyTransfer and VerifyTransferWithFee instruction data as the TransferAmountEncryption and FeeEncryption types can be defined with respect to GroupedElGamalCiphertext type.

Proposed Solution

• Define GroupedElGamalCiphertext type ([zk-token-sdk] Add GroupedElGamalCiphertext type #31849)
• Rename ciphertext validity proof instructions in [zk-token-sdk] Add VerifyGroupedCiphertext2HandlesValidity and VerifyBatchedGroupedCiphertext2HandlesValidity proof instructions #31816 to VerifyGroupedCiphertextValidity and VerifyBatchedGroupedCiphertextValidity instructions
• Update TransferAmountEncryption and FeeEncryption types as simple wrappers around the type GroupedElGamalCiphertext