Ciphertext persistence

pjhnocegood · March 6, 2024, 1:31am

hello.

Looking at fhevm-go, if you put the ciphertext in verifiedCiphertexts map[common.Hash]*verifiedCiphertext and return the key value for that value, the contract stores only the key value in storage.

verifiedCiphertexts map[common.Hash]*verifiedCiphertext
Since is a value stored in memory, wouldn’t it be lost when geth is restarted?

Is there separate logic for storing the ciphertext in storage?

clement · March 6, 2024, 2:03am

Hello!
We store ciphertext in what we call the “protected storage”. For this, we catch the EVM opcode SSTORE and if the content is a verified ciphertext, we save it in the protected storage.

You can see the function which stores the value here:

github.com

zama-ai/fhevm-go/blob/main/fhevm/protected_storage.go#L103


      
          		left -= toAppend
          		ctBytes = append(ctBytes, bytes[0:toAppend]...)
          		protectedSlotIdx.AddUint64(protectedSlotIdx, 1)
          	}
          	return &ciphertextData{metadata: metadata, bytes: ctBytes}
          }
          
          // If a verified ciphertext:
          // * if the ciphertext does not exist in protected storage, persist it with a refCount = 1
          // * if the ciphertexts exists in protected, bump its refCount by 1
          func persistIfVerifiedCiphertext(flagHandleLocation common.Hash, handle common.Hash, protectedStorage common.Address, env EVMEnvironment) {
          	verifiedCiphertext := getVerifiedCiphertextFromEVM(env, handle)
          	if verifiedCiphertext == nil {
          		return
          	}
          	logger := env.GetLogger()
          
          	// Try to read ciphertext metadata from protected storage.
          	metadataKey := crypto.Keccak256Hash(handle.Bytes())
          	metadataInt := newInt(env.GetState(protectedStorage, metadataKey).Bytes())
          	metadata := ciphertextMetadata{}

The function called during Opsstore here fhevm-go/fhevm/instructions.go at main · zama-ai/fhevm-go · GitHub

And where we hook the native opsstore in go-ethereum https://github.com/zama-ai/zbc-go-ethereum/blob/master/core/vm/instructions.go#L520

You can get more details about protected storage, also called “privileged storage”, in section 4.4 of our whitepaper fhevm/fhevm-whitepaper.pdf at main · zama-ai/fhevm · GitHub

pjhnocegood · March 6, 2024, 2:22am

Thank you for the good explanation!!