routing: add representation of blinded payments

This commit adds a representation of blinded payments, which include a
blinded path and aggregate routing parameters to be used in payment to
the path.

routing/blinding.go

@@ -0,0 +1,79 @@
+package routing
+
+import (
+	"errors"
+	"fmt"
+
+	sphinx "github.com/lightningnetwork/lightning-onion"
+	"github.com/lightningnetwork/lnd/lnwire"
+)
+
+var (
+	// ErrNoBlindedPath is returned when the blinded path in a blinded
+	// payment is missing.
+	ErrNoBlindedPath = errors.New("blinded path required")
+
+	// ErrInsufficientBlindedHops is returned when a blinded path does
+	// not have enough blinded hops.
+	ErrInsufficientBlindedHops = errors.New("blinded path requires " +
+		"at least one hop")
+
+	// ErrHTLCRestrictions is returned when a blinded path has invalid
+	// HTLC maximum and minimum values.
+	ErrHTLCRestrictions = errors.New("invalid htlc minimum and maximum")
+)
+
+// BlindedPayment provides the path and payment parameters required to send a
+// payment along a blinded path.
+type BlindedPayment struct {
+	// BlindedPath contains the unblinded introduction point and blinded
+	// hops for the blinded section of the payment.
+	BlindedPath *sphinx.BlindedPath
+
+	// BaseFee is the total base fee to be paid for payments made over the
+	// blinded path.
+	BaseFee uint32
+
+	// ProportionalFee is the aggregated proportional fee for payments
+	// made over the blinded path.
+	ProportionalFee uint32
+
+	// CltvExpiryDelta is the total expiry delta for the blinded path. Note
+	// this does not include the final cltv delta for the receiving node
+	// (which should be provided in an invoice).
+	CltvExpiryDelta uint16
+
+	// HtlcMinimum is the highest HLTC minimum supported along the blinded
+	// path (while some hops may have lower values, we're effectively
+	// bounded by the highest minimum).
+	HtlcMinimum uint64
+
+	// HtlcMaximum is the lowest HTLC maximum supported along the blinded
+	// path (while some hops may have higher values, we're effectively
+	// bounded by the lowest maximum).
+	HtlcMaximum uint64
+
+	// Features is the set of relay features available for the payment.
+	Features *lnwire.FeatureVector
+}
+
+// Validate performs validation on a blinded payment.
+func (b *BlindedPayment) Validate() error {
+	if b.BlindedPath == nil {
+		return ErrNoBlindedPath
+	}
+
+	// The sphinx library inserts the introduction node as the first hop,
+	// so we expect at least one hop.
+	if len(b.BlindedPath.BlindedHops) < 1 {
+		return fmt.Errorf("%w got: %v", ErrInsufficientBlindedHops,
+			len(b.BlindedPath.BlindedHops))
+	}
+
+	if b.HtlcMaximum < b.HtlcMinimum {
+		return fmt.Errorf("%w: %v < %v", ErrHTLCRestrictions,
+			b.HtlcMaximum, b.HtlcMinimum)
+	}
+
+	return nil
+}