amp/child: add Child deriviation

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Conner Fromknecht 2021-01-15 14:51:51 -08:00
parent 39f51b5568
commit 0144bdb241
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85
amp/child.go Normal file

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package amp
import (
"crypto/sha256"
"encoding/binary"
"fmt"
"github.com/lightningnetwork/lnd/lntypes"
)
// Share represents an n-of-n sharing of a secret 32-byte value. The secret can
// be recovered by XORing all n shares together.
type Share [32]byte
// ChildDesc contains the information necessary to derive a child hash/preimage
// pair that is attached to a particular HTLC. This information will be known by
// both the sender and receiver in the process of fulfilling an AMP payment.
type ChildDesc struct {
// Share is one of n shares of the root seed. Once all n shares are
// known to the receiver, the Share will also provide entropy to the
// derivation of child hash and preimage.
Share Share
// Index is 32-bit value that can be used to derive up to 2^32 child
// hashes and preimages from a single Share. This allows the payment
// hashes sent over the network to be refreshed without needing to
// modify the Share.
Index uint32
}
// Child is a payment hash and preimage pair derived from the root seed. In
// addition to the derived values, a Child carries all information required in
// the derivation apart from the root seed (unless n=1).
type Child struct {
// ChildDesc contains the data required to derive the child hash and
// preimage below.
ChildDesc
// Preimage is the child payment preimage that can be used to settle the
// HTLC carrying Hash.
Preimage lntypes.Preimage
// Hash is the child payment hash that to be carried by the HTLC.
Hash lntypes.Hash
}
// String returns a human-readable description of a Child.
func (c *Child) String() string {
return fmt.Sprintf("share=%x, index=%d -> preimage=%v, hash=%v",
c.Share, c.Index, c.Preimage, c.Hash)
}
// DeriveChild computes the child preimage and child hash for a given (root,
// share, index) tuple. The derivation is defined as:
//
// child_preimage = SHA256(root || share || be32(index)),
// child_hash = SHA256(child_preimage).
func DeriveChild(root Share, desc ChildDesc) *Child {
var (
indexBytes [4]byte
preimage lntypes.Preimage
hash lntypes.Hash
)
// Serialize the child index in big-endian order.
binary.BigEndian.PutUint32(indexBytes[:], desc.Index)
// Compute child_preimage as SHA256(root || share || child_index).
h := sha256.New()
_, _ = h.Write(root[:])
_, _ = h.Write(desc.Share[:])
_, _ = h.Write(indexBytes[:])
copy(preimage[:], h.Sum(nil))
// Compute child_hash as SHA256(child_preimage).
h = sha256.New()
_, _ = h.Write(preimage[:])
copy(hash[:], h.Sum(nil))
return &Child{
ChildDesc: desc,
Preimage: preimage,
Hash: hash,
}
}