Digital signature. It is formed as sha256( account + "{up}" + currency + "{up}" + desc + "{up}" + sum + "{up}" + secretKey), where sha256 is the hashing method; "{up}" is the parameter separator in the hash function; account, sum, currency, desc are the payment initialization parameters described above; secretKey is the secret key of the project (available in your personal account); Important Note. If you do not transmit currency to the payment form, this parameter should not be used in signature generation.