lnd.xprv/elkrem/elkrem.go
2016-01-14 23:58:05 -08:00

158 lines
5.2 KiB
Go

package elkrem
import (
"fmt"
"github.com/btcsuite/btcd/wire"
)
/* elkrem is a simpler alternative to the 64 dimensional sha-chain.
it's basically a reverse merkle tree. If we want to provide 2**64 possible
hashes, this requires a worst case computation of 63 hashes for the
sender, and worst-case storage of 64 hashes for the receiver.
The operations are left hash L() and right hash R(), which are
hash(parent) and hash(parent, 1) respectively. (concatenate one byte)
Here is a shorter example of a tree with 8 leaves and 15 total nodes.
The sender first computes the bottom left leaf 0b0000. This is
L(L(L(L(root)))). The receiver stores leaf 0.
Next the sender computes 0b0001. R(L(L(L(root)))). Receiver stores.
Next sender computes 0b1000 (8). L(L(L(root))). Receiver stores this, and
discards leaves 0b0000 and 0b0001, as they have the parent node 8.
For total hashes (2**h)-1 requires a tree of height h.
Sender:
as state, must store 1 hash (root) and current index (h bits)
to move to the next index, compute at most h hashes.
Receiver:
as state, must store at most h+1 hashes and the index of each hash (h*(h+1)) bits
to compute a previous index, compute at most h hashes.
*/
// You can calculate h from i but I can't figure out how without taking
// O(i) ops. Feels like there should be a clever O(h) way. 1 byte, whatever.
type ElkremNode struct {
i uint64 // index (ith node)
h uint8 // height of this node
sha *wire.ShaHash // hash
}
type ElkremSender struct {
current uint64 // last sent hash index
treeHeight uint8 // height of tree (size is 2**height -1 )
maxIndex uint64 // top of the tree
root *wire.ShaHash // root hash of the tree
}
type ElkremReceiver struct {
current uint64 // last received index (actually don't need it?)
treeHeight uint8 // height of tree (size is 2**height -1 )
s []ElkremNode // store of received hashes, max size = height
}
func LeftSha(in wire.ShaHash) wire.ShaHash {
return wire.DoubleSha256SH(in.Bytes()) // left is sha(sha(in))
}
func RightSha(in wire.ShaHash) wire.ShaHash {
return wire.DoubleSha256SH(append(in.Bytes(), 0x01)) // sha(sha(in, 1))
}
// iterative descent of sub-tree. w = hash number you want. i = input index
// h = height of input index. sha = input hash
func descend(w, i uint64, h uint8, sha wire.ShaHash) (wire.ShaHash, error) {
for w < i {
if w <= i-(1<<h) { // left
sha = LeftSha(sha)
i = i - (1 << h) // left descent reduces index by 2**h
} else { // right
sha = RightSha(sha)
i-- // right descent reduces index by 1
}
if h == 0 { // avoid underflowing h
break
}
h-- // either descent reduces height by 1
}
if w != i { // somehow couldn't / didn't end up where we wanted to go
return sha, fmt.Errorf("can't generate index %d from %d", w, i)
}
return sha, nil
}
// Creates an Elkrem Sender from a root hash and tree height
func NewElkremSender(th uint8, r wire.ShaHash) ElkremSender {
var e ElkremSender
e.root = &r
e.treeHeight = th
// set max index based on tree height
for j := uint8(0); j <= e.treeHeight; j++ {
e.maxIndex = e.maxIndex<<1 | 1
}
e.maxIndex-- // 1 less than 2**h
return e
}
// Creates an Elkrem Receiver from a tree height
func NewElkremReceiver(th uint8) ElkremReceiver {
var e ElkremReceiver
e.treeHeight = th
return e
}
// Next() increments the index to the next hash and outputs it
func (e *ElkremSender) Next() (*wire.ShaHash, error) {
// increment index
e.current++
return e.AtIndex(e.current)
}
// w is the wanted index, i is the root index
func (e *ElkremSender) AtIndex(w uint64) (*wire.ShaHash, error) {
out, err := descend(w, e.maxIndex, e.treeHeight, *e.root)
return &out, err
}
func (e *ElkremReceiver) AddNext(sha *wire.ShaHash) error {
// note: careful about atomicity / disk writes here
var n ElkremNode
n.sha = sha
t := len(e.s) - 1 // top of stack
if t > 0 && e.s[t-1].h == e.s[t].h { // top 2 elements are equal height
// next node must be parent; verify and remove children
n.h = e.s[t].h + 1 // assign height
l := LeftSha(*sha) // calc l child
r := RightSha(*sha) // calc r child
if !e.s[t-1].sha.IsEqual(&l) { // test l child
return fmt.Errorf("left child doesn't match, expect %s got %s",
e.s[t-1].sha.String(), l.String())
}
if !e.s[t].sha.IsEqual(&r) { // test r child
return fmt.Errorf("right child doesn't match, expect %s got %s",
e.s[t].sha.String(), r.String())
}
e.s = e.s[:len(e.s)-2] // l and r children OK, remove them
} // if that didn't happen, height defaults to 0
e.current++ // increment current index
n.i = e.current // set new node to that incremented index
e.s = append(e.s, n) // append new node to stack
return nil
}
func (e *ElkremReceiver) AtIndex(w uint64) (*wire.ShaHash, error) {
var out ElkremNode // node we will eventually return
for _, n := range e.s { // go through stack
if w <= n.i { // found one bigger than or equal to what we want
out = n
break
}
}
if out.sha == nil { // didn't find anything
return nil, fmt.Errorf("receiver has max %d, less than requested %d",
e.s[len(e.s)-1].i, w)
}
sha, err := descend(w, out.i, out.h, *out.sha)
return &sha, err
}