# zkBob Keys

zkBob is based on complex cryptography. There are several keys needed for private transactions. The relationship between these keys is presented in the scheme below.

![](/files/a5ZN0UwEWJ5Rm4toksTc)

* **Spending key (**$$\sigma$$**)** is the top secret key. It is used to derive other keys and to sign transactions. At a high level it is just a random 256-bit number which should be stored securely on the client side. The simplest way to get a spending key is to produce a random number.  But in a production client software implementation more complex approaches should be used (e.g. [hierarchical deterministic wallets](https://github.com/bitcoin/bips/blob/master/bip-0044.mediawiki)). Concrete approaches for spending key derivation by a client are not discussed here.
* **Transaction verifier key (**$$A$$**)** is used for transaction signature verification. It's derived from the spending key and multiplied by the generator point in the JubJub elliptic curve field: $$A = \sigma G$$
* **Intermediate key (**$$\eta$$**)** is derived from the $$A$$ key by the[ $$Poseidon$$ hash function](/implementation/untitled/the-poseidon-hash.md):$$\eta = Hash(A.x)$$. It is used in several cases:
  * to calculate the account[ nullifier](/implementation/transaction-overview/the-nullifiers.md)
  * to derive the outgoing viewing key ($$\kappa$$)
  * to decrypt incoming notes
* **Receiving key** is used to decrypt incoming notes in the memo block. It is a combination of the intermediate key and ephemeral key generated for each note.
* **Outgoing viewing key (**$$\kappa$$**)** is used to decrypt the whole memo block in the transaction which is initiated by itself. The key is derived from the intermediate key by the keccak hash function: $$\kappa = keccak\_{256}(\eta, \text{"this is the suffix for the symmetric encryption key"})$$
* **Private payment address** $$(d, P\_d)$$ - is a set of random diversifiers $$d$$ and point $$P\_d = \eta G\_p = \eta \text{ToSubGroupHash}\_{E(F\_r)}(d)$$


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