| payment_request_hash || SHA256 Hash of Non-Encrypted, Serialized PaymentRequest (used for validation)
|}
==InvoiceRequest / ReturnPaymentRequest Process==
# NOTE: The sender is the entity wishing to send value to the receiver.
===Overview===
1. Sender creates InvoiceRequest message
2. Sender sends InvocieRequest to Receiver
3. Receiver validates InvoiceRequest
4. Receiver creates return PaymentRequest message
5. Receiver encrypts the PaymentRequest message
6. Receiver creates ReturnPaymentRequest
7. Receiver returns ReturnPaymentRequest message to Sender
8. Sender validates ReturnPaymentRequest
9. Sender decrypts and validates encrypted PaymentRequest
===InvoiceRequest Message Creation===
* Create an InvoiceRequest message
* REQUIRED: Set sender_public_key. This is the public key of an EC keypair using secp256k1.
* Set amount if desired
* Set notification_url to URL that will accept ReturnPaymentRequest from Receiver
* If NOT including certificate, set pki_type to "none"
* If including certificate:
** Set pki_type to "x509+sha256"
** Set pki_data as it would be set in BIP-0070 (see [Certificates](https://github.com/bitcoin/bips/blob/master/bip-0070.mediawiki#Certificates) section)
** Sign InvoiceRequest with signature == "" using the X509 Certificate's private key
===ReturnPaymentRequest Message Creation and PaymentRequest Encryption===
* Generate EC secret point using [ECDH](https://en.wikipedia.org/wiki/Elliptic_curve_Diffie–Hellman) with the Sender's EC public key and the Receiver's EC private key.
* Generate Symmetric Encryption Key and Initialization vector using [HMAC_DRBG](http://csrc.nist.gov/publications/nistpubs/800-90A/SP800-90A.pdf) also referenced in [RFC6979](https://tools.ietf.org/html/rfc6979) in the following way:
** HMAC_DRBG Initialization Entropy is set to the EC secret point's X value
** HMAC_DRBG Initialization Nonce is set to the InvoiceRequest's sender_public_key
* Encrypt the serialized PaymentRequest using AES-256-CBC using the Encryption Key and IV previously generated
* Create ReturnPaymentRequest message
* Set encrypted_payment_request to be the encrypted value of the PaymentRequest
* Set receiver_public_key to the Receiver's EC public key (of which the private key was previously used in ECDH secret point calculation)
* Set ephemeral_public_key to the public key of an EC keypair created using the secret point's X value.
* Set payment_request_hash to generated SHA256 hash of the serialized PaymentRequest (without encryption)
===ReturnPaymentRequest Validation and Decryption===
* Generate EC secret point using [ECDH](https://en.wikipedia.org/wiki/Elliptic_curve_Diffie–Hellman) with the Sender's EC private key and the Receiver's EC public key.
* Generate Symmetric Decryption Key and Initialization vector using [HMAC_DRBG](http://csrc.nist.gov/publications/nistpubs/800-90A/SP800-90A.pdf) also referenced in [RFC6979](https://tools.ietf.org/html/rfc6979) in the following way:
** HMAC_DRBG Initialization Entropy is set to the EC secret point's X value
** HMAC_DRBG Initialization Nonce is set to the InvoiceRequest's sender_public_key
