Signing a Transaction

To prove an input account ownership

Transactions in zkBob are signed by the spending key $\sigma$. To verify a transaction signature the prover should use an intermediate key A.

Transaction hashing

A client application should sign a 'composite' transaction hash instead of full transaction data. The transaction hash is calculated from the input and output hashes:

$H = Hash_{sponge}(Hash_{account}(Acc^\text{in}), Hash_{note}(Note_0^\text{in}), Hash_{note}(Note_1^\text{in}), Hash_{note}(Note_2^\text{in}), TxCommit)$

where

• $Hash$ is a Poseidon multi-hash (sponged) routine in the different modes

• $Acc^\text{in}$is an input account

• $Note_i^\text{in}$is an input notes,

• $TxCommit$ - is a transaction commitment hash (Merkle subtree root). It depends on transaction output account and notes.

Signing

Next, a client uses the account spending key to sign a transaction hash $H$:

$r = Blake2s(\sigma, H)$, where$Blake2s$ is the 256-bit hash function

$R = rG$, $A=\sigma G$ (moving $r$ and $\sigma$ to the JubJub Elliptic curve field)

$S = r + Hash_{eddsa}(R.x, A.x, H)\sigma$

The output signature $(S, R)$ will be sent with a intermediate key $A = \sigma G$

Verifying

To verify a transaction signature a validator should perform the following computations:

$SG == R + Hash_{eddsa}(R.x, A.x, H)A$