lnwire: document start of single funder workflow

This commit adds some additional documentation in the form of comments
to the start of the revised single funder workflow.

A primary change lies in the introduction of the exchange of Channel
Derivation Points (CDP’s) for both sides. Using CDP’s we can derive
channel authentication proofs which are both unforgettable and binding.

lnwire/single_funding_request.go

@@ -1,71 +1,104 @@
 package lnwire
 
 import (
+	"bytes"
 	"fmt"
-	"github.com/roasbeef/btcd/btcec"
-	"github.com/roasbeef/btcd/wire"
-	"github.com/roasbeef/btcutil"
 	"io"
+
+	"github.com/roasbeef/btcd/btcec"
+	"github.com/roasbeef/btcutil"
 )
 
+// SingleFundingRequest is the message Alice sends to Bob if we should like
+// to create a channel with Bob where she's the sole provider of funds to the
+// channel. Single funder channels simplify the initial funding workflow, are
+// supported by nodes backed by SPV Bitcoin clients, and have a simpler
+// security models than dual funded channels.
+//
+// NOTE: In order to avoid a slow loris like resource exhaustion attack, the
+// responder of a single funding channel workflow *should not* watch the
+// blockchain for the funding transaction. Instead, it is the initiator's job
+// to provide the responder with an SPV proof of funding transaction inclusion
+// after a sufficient number of confirmations.
 type SingleFundingRequest struct {
-	//ChannelID
+	// ChannelID serves to uniquely identify the future channel created by
+	// the initiated single funder workflow.
 	ChannelID uint64
 
-	//Default 0
+	// ChannelType represents the type of channel this request would like
+	// to open. At this point, the only supported channels are type 0
+	// channels, which are channels with regular commitment transactions
+	// utilizing HTLC's for payments.
 	ChannelType uint8
 
-	//Bitcoin: 0.
+	// CoinType represents which blockchain the channel will be opened
+	// using. By default, this field should be set to 0, indicating usage
+	// of the Bitcoin blockchain.
 	CoinType uint64
 
-	//Amount of fees per kb
+	// FeePerKb is the required number of satoshis per KB that the
-	//Assumes requester pays
+	// requester will pay at all timers, for both the funding transaction
+	// and commitment transaction. This value can later be updated once the
+	// channel is open.
 	FeePerKb btcutil.Amount
 
-	// The funding requester can request payment
+	// FundingAmount is the number of satoshis the the initiator would like
-	// This wallet only allows positive values,
+	// to commit to the channel.
-	// which is a payment to the responder
+	FundingAmount btcutil.Amount
-	// (This can be used to fund the Reserve)
-	// If the responder disagrees, then the funding request fails
-	// THIS VALUE GOES INTO THE RESPONDER'S FUNDING AMOUNT
-	// total requester input value = RequesterFundingAmount + PaymentAmount + "Total Change" + Fees(?)
-	// RequesterFundingAmount = "Available Balance" + RequesterReserveAmount
-	// Payment SHOULD NOT be acknowledged until the minimum confirmation has elapsed
-	// (Due to double-spend risks the recipient will not want to acknolwedge confirmation until later)
-	// This is to make a payment as part of opening the channel
-	PaymentAmount btcutil.Amount
 
-	// CLTV/CSV lock-time to use
+	// CsvDelay is the number of blocks to use for the relative time lock
-	LockTime uint32
+	// in the pay-to-self output of both commitment transactions.
+	// TODO(roasbeef): bool for seconds or blocks?
+	CsvDelay uint32
 
-	RevocationHash   [20]byte
+	// ChannelDerivationPoint is an secp256k1 point which will be used to
-	Pubkey           *btcec.PublicKey
+	// derive the public key the initiator will use for the half of the
-	DeliveryPkScript PkScript // *MUST* be either P2PKH or P2SH
+	// 2-of-2 multi-sig. Using the channel derivation point (CDP), and the
-	ChangePkScript   PkScript // *MUST* be either P2PKH or P2SH
+	// initiators identity public key (A), the channel public key is
+	// computed as: C = A + CDP. In order to be valid all CDP's MUST have
+	// an odd y-coordinate.
+	ChannelDerivationPoint *btcec.PublicKey
+
+	// RevocationHash is the initial revocation hash to be used for the
+	// initiator's commitment transaction to derive their revocation public
+	// key as: P + G*revocationHash, where P is the initiator's channel
+	// public key.
+	RevocationHash [20]byte
+
+	// DeliveryPkScript defines the public key script that the initiator
+	// would like to use to receive their balance in the case of a
+	// cooperative close. Only the following script templates are
+	// supported: P2PKH, P2WKH, P2SH, and P2WSH.
+	DeliveryPkScript PkScript
+
+	// TODO(roasbeef): confirmation depth
 }
 
+// Decode deserializes the serialized SingleFundingRequest stored in the passed
+// io.Reader into the target SingleFundingRequest using the deserialization
+// rules defined by the passed protocol version.
+//
+// This is part of the lnwire.Message interface.
 func (c *SingleFundingRequest) Decode(r io.Reader, pver uint32) error {
 	// ChannelID (8)
 	// ChannelType (1)
 	// CoinType	(8)
 	// FeePerKb (8)
-	// PaymentAmount (8)
+	// FundingAmount (8)
-	// LockTime (4)
+	// CsvDelay (4)
+	// Channel Derivation Point (32)
 	// Revocation Hash (20)
-	// Pubkey (32)
 	// DeliveryPkScript (final delivery)
-	// ChangePkScript (change for extra from inputs)
 	err := readElements(r,
 		&c.ChannelID,
 		&c.ChannelType,
 		&c.CoinType,
 		&c.FeePerKb,
-		&c.PaymentAmount,
+		&c.FundingAmount,
-		&c.LockTime,
+		&c.CsvDelay,
+		&c.ChannelDerivationPoint,
 		&c.RevocationHash,
-		&c.Pubkey,
+		&c.DeliveryPkScript)
-		&c.DeliveryPkScript,
-		&c.ChangePkScript)
 	if err != nil {
 		return err
 	}
@@ -73,13 +106,16 @@ func (c *SingleFundingRequest) Decode(r io.Reader, pver uint32) error {
 	return nil
 }
 
-// Creates a new SingleFundingRequest
+// NewSingleFundingRequest creates, and returns a new empty SingleFundingRequest.
 func NewSingleFundingRequest() *SingleFundingRequest {
 	return &SingleFundingRequest{}
 }
 
-// Serializes the item from the SingleFundingRequest struct
+// Encode serializes the target SingleFundingRequest into the passed io.Writer
-// Writes the data to w
+// implementation. Serialization will observe the rules defined by the passed
+// protocol version.
+//
+// This is part of the lnwire.Message interface.
 func (c *SingleFundingRequest) Encode(w io.Writer, pver uint32) error {
 	// ChannelID (8)
 	// ChannelType (1)
@@ -90,18 +126,16 @@ func (c *SingleFundingRequest) Encode(w io.Writer, pver uint32) error {
 	// Revocation Hash (20)
 	// Pubkey (32)
 	// DeliveryPkScript (final delivery)
-	// ChangePkScript (change for extra from inputs)
 	err := writeElements(w,
 		c.ChannelID,
 		c.ChannelType,
 		c.CoinType,
 		c.FeePerKb,
-		c.PaymentAmount,
+		c.FundingAmount,
-		c.LockTime,
+		c.CsvDelay,
+		c.ChannelDerivationPoint,
 		c.RevocationHash,
-		c.Pubkey,
+		c.DeliveryPkScript)
-		c.DeliveryPkScript,
-		c.ChangePkScript)
 	if err != nil {
 		return err
 	}
@@ -109,72 +143,80 @@ func (c *SingleFundingRequest) Encode(w io.Writer, pver uint32) error {
 	return nil
 }
 
+// Command returns the uint32 code which uniquely identifies this message as a
+// SingleFundingRequest on the wire.
+//
+// This is part of the lnwire.Message interface.
 func (c *SingleFundingRequest) Command() uint32 {
 	return CmdSingleFundingRequest
 }
 
+// MaxPayloadLength returns the maximum allowed payload length for a
+// SingleFundingRequest. This is calculated by summing the max length of all
+// the fields within a SingleFundingRequest. To enforce a maximum
+// DeliveryPkScript size, the size of a P2PKH public key script is used.
+// Therefore, the final breakdown is: 8 + 1 + 8 + 8 + 8 + 4 + 32 + 20 + 25 = 114.
+//
+// This is part of the lnwire.Message interface.
 func (c *SingleFundingRequest) MaxPayloadLength(uint32) uint32 {
-	return 141
+	return 114
 }
 
-// Makes sure the struct data is valid (e.g. no negatives or invalid pkscripts)
+// Validate examines each populated field within the SingleFundingRequest for
+// field sanity. For example, all fields MUST NOT be negative, and all pkScripts
+// must belong to the allowed set of public key scripts.
+//
+// This is part of the lnwire.Message interface.
 func (c *SingleFundingRequest) Validate() error {
-	var err error
+	// Negative values is are allowed.
-
+	if c.FeePerKb < 0 {
-	// No negative values
-	if c.RequesterFundingAmount < 0 {
-		return fmt.Errorf("RequesterFundingAmount cannot be negative")
-	}
-
-	if c.RequesterReserveAmount < 0 {
-		return fmt.Errorf("RequesterReserveAmount cannot be negative")
-	}
-
-	if c.MinFeePerKb < 0 {
 		return fmt.Errorf("MinFeePerKb cannot be negative")
 	}
-	if c.MinTotalFundingAmount < 0 {
+	if c.FundingAmount < 0 {
-		return fmt.Errorf("MinTotalFundingAmount cannot be negative")
+		return fmt.Errorf("FundingAmount cannot be negative")
 	}
 
-	// Validation of what makes sense...
+	// The CSV delay MUST be non-zero.
-	if c.MinTotalFundingAmount < c.RequesterFundingAmount {
+	if c.CsvDelay == 0 {
-		return fmt.Errorf("Requester's minimum too low.")
+		return fmt.Errorf("Commitment transaction must have non-zero " +
-	}
+			"CSV delay")
-	if c.RequesterFundingAmount < c.RequesterReserveAmount {
-		return fmt.Errorf("Reserve must be below Funding Amount")
 	}
 
-	// This wallet only allows payment from the requester to responder
+	// The channel derivation point must be non-nil, and have an odd
-	if c.PaymentAmount < 0 {
+	// y-coordinate.
-		return fmt.Errorf("This wallet requieres payment to be greater than zero.")
+	if c.ChannelDerivationPoint == nil {
+		return fmt.Errorf("The channel derivation point must be non-nil")
+	}
+	if c.ChannelDerivationPoint.Y.Bit(0) != 1 {
+		return fmt.Errorf("The channel derivation point must have an odd " +
+			"y-coordinate")
 	}
 
-	// Make sure there's not more than 127 inputs
+	// The revocation hash MUST be non-zero.
-	if len(c.Inputs) > 127 {
+	var zeroHash [20]byte
-		return fmt.Errorf("Too many inputs")
+	if bytes.Equal(c.RevocationHash[:], zeroHash[:]) {
+		return fmt.Errorf("Initial revocation hash must be non-zero")
 	}
 
-	// DeliveryPkScript is either P2SH or P2PKH
+	// The delivery pkScript must be amongst the supported script
-	err = ValidatePkScript(c.DeliveryPkScript)
+	// templates.
-	if err != nil {
+	if !isValidPkScript(c.DeliveryPkScript) {
-		return err
+		// TODO(roasbeef): move into actual error
-	}
+		return fmt.Errorf("Valid delivery public key scripts MUST be: " +
-
+			"P2PKH, P2WKH, P2SH, or P2WSH.")
-	// ChangePkScript is either P2SH or P2PKH
-	err = ValidatePkScript(c.ChangePkScript)
-	if err != nil {
-		return err
 	}
 
 	// We're good!
 	return nil
 }
 
+// String returns the string representation of the SingleFundingRequest.
+//
+// This is part of the lnwire.Message interface.
 func (c *SingleFundingRequest) String() string {
 	var serializedPubkey []byte
-	if &c.Pubkey != nil && c.Pubkey.X != nil {
+	if &c.ChannelDerivationPoint != nil && c.ChannelDerivationPoint.X != nil {
-		serializedPubkey = c.Pubkey.SerializeCompressed()
+		serializedPubkey = c.ChannelDerivationPoint.SerializeCompressed()
 	}
 
 	return fmt.Sprintf("\n--- Begin SingleFundingRequest ---\n") +
@@ -182,11 +224,10 @@ func (c *SingleFundingRequest) String() string {
 		fmt.Sprintf("ChannelType:\t\t\t%x\n", c.ChannelType) +
 		fmt.Sprintf("CoinType:\t\t\t%d\n", c.CoinType) +
 		fmt.Sprintf("FeePerKb:\t\t\t%s\n", c.FeePerKb.String()) +
-		fmt.Sprintf("PaymentAmount:\t\t\t%s\n", c.PaymentAmount.String()) +
+		fmt.Sprintf("FundingAmount:\t\t\t%s\n", c.FundingAmount.String()) +
-		fmt.Sprintf("LockTime\t\t\t%d\n", c.LockTime) +
+		fmt.Sprintf("CsvDelay\t\t\t%d\n", c.CsvDelay) +
+		fmt.Sprintf("ChannelDerivationPoint\t\t\t\t%x\n", serializedPubkey) +
 		fmt.Sprintf("RevocationHash\t\t\t%x\n", c.RevocationHash) +
-		fmt.Sprintf("Pubkey\t\t\t\t%x\n", serializedPubkey) +
 		fmt.Sprintf("DeliveryPkScript\t\t%x\n", c.DeliveryPkScript) +
-		fmt.Sprintf("ChangePkScript\t\t%x\n", c.ChangePkScript) +
 		fmt.Sprintf("--- End SingleFundingRequest ---\n")
 }