Private transactions

Private transactions have extra attributes to public Ethereum transactions:

• privateFrom - The Tessera public key of the transaction sender
• privateFor - The Tessera public keys of the transaction recipients, or
• privacyGroupId - The privacy group to receive the transaction

• restriction - Whether the private transaction is restricted or unrestricted:

  • restricted private transactions, only the nodes participating in the transaction receive and store the payload of the private transaction.
  • unrestricted private transactions, all nodes in the network receive the payload of the private transaction, but only the nodes participating in the transaction can read the transaction.

  Important

  Besu implements restricted private transactions only.

The gas and gasPrice are used by the privacy marker transaction not the private transaction itself.

For more information about creating and sending private transactions, see the How To documentation.

Besu and Tessera keys

Besu and Tessera nodes both have public/private key pairs identifying them. A Besu node sending a private transaction to a Tessera node signs the transaction with the Besu node private key. The privateFrom and privateFor attributes specified in the RLP-encoded transaction string for eea_sendRawTransaction are the public keys of the Tessera nodes sending and receiving the transaction.

Nonces

Private transaction processing involves two transactions, the private transaction distributed to involved participants and the privacy marker transaction included on the public blockchain. Each each of these transactions has its own nonce.

Private transaction nonce

Besu maintains separate private states for each privacy group. The private transaction nonce for an account is specific to the privacy group. That is, the nonce for account A for privacy group ABC is different to the nonce for account A for privacy group AB.

A nonce is the number of previous transactions made by the sender.

Privacy marker transaction nonce

The nonce for the privacy marker transaction is the public nonce for the account.

Nonce validation

Unlike public transactions, private transactions are not submitted to the transaction pool. The private transaction is distributed directly to the participants in the transaction and the privacy marker transaction is submitted to the transaction pool.

Unlike public transaction nonces, private transaction nonces are not validated when the private transaction is submitted. If a private transaction has an incorrect nonce, the privacy marker transaction is still valid and is added to a block. The private transaction execution fails when processing the privacy marker transaction for the private transaction with the incorrect nonce.

The following private transaction flow illustrates when nonce validation occurs:

1. Submit a private transaction with a nonce value.
2. The private transaction is distributed to all participants in the privacy group.
3. The privacy marker transaction is created and submitted to the transaction pool with a nonce of 0 if using one time accounts. If using a specific account with --privacy-marker-transaction-signing-key-file, the nonce for that account is obtained and used for the privacy marker transaction.
4. The privacy marker transaction is mined and included in the block.

5. After the block containing the privacy marker transaction is imported, and the privacy marker transaction is processed, the private transaction is retrieved from the private transaction manager and executed.

   If the private transaction was submitted with a correct nonce in step 1, the nonce is validated as correct, if an incorrect nonce was submitted, the private transaction execution fails.