First stab at shachain. Untested. Still doin't really get it...

revocation/shachain.go

@@ -1,15 +1,24 @@
 package revocation
 
 import (
+	"bytes"
+	"crypto/rand"
+	"crypto/sha256"
+	"fmt"
+	"math"
 	"sync"
 
 	"github.com/btcsuite/btcd/wire"
 )
 
+const (
+	maxIndex = 1<<64 - 1
+)
+
 // chainFragment...
-type chainFragment struct {
+type chainBranch struct {
 	index uint64
-	hash  wire.ShaHash
+	hash  [32]byte
 }
 
 // HyperShaChain...
@@ -18,20 +27,120 @@ type HyperShaChain struct {
 	sync.RWMutex
 
 	lastChainIndex uint64
+	numValid       uint64
 
-	chainFragments []chainFragment
+	chainBranches [64]chainBranch
+
+	lastHash wire.ShaHash
 }
 
 // NewHyperShaChain...
-func NewHyperShaChain(seed wire.ShaHash) *HyperShaChain {
+func NewHyperShaChain(seed *[32]byte, deriveTo uint64) (*HyperShaChain, error) {
+	var shaSeed *[32]byte
+
+	// If no seed is specified, generate a new one.
+	if seed == nil {
+		_, err := rand.Read(shaSeed[:])
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	// The last possible value in the chain is our starting index.
+	start := uint64(maxIndex)
+	stop := deriveTo
+
+	curHash := derive(start, stop, *shaSeed)
+
 	// TODO(roasbeef): from/to or static size?
+	return &HyperShaChain{lastChainIndex: deriveTo, lastHash: curHash}, nil
+}
+
+// derive...
+func derive(from, to uint64, startingHash [32]byte) [32]byte {
+	nextHash := startingHash
+
+	numBranches := from ^ to
+	toDerive := uint64(math.Log2(float64(numBranches))) // uh.....
+	for i := toDerive - 1; i >= 0; i-- {
+		if (numBranches>>i)&1 == 1 {
+			// Flip the ith bit, then hash the current state to
+			// advance down the tree.
+			nextHash[i/8] ^= (1 << (i % 8))
+			nextHash = sha256.Sum256(nextHash[:])
+		}
+	}
+
+	return nextHash
+}
+
+// getHash...
+// index should be commitment #
+func (h *HyperShaChain) GetHash(index uint64) (*[32]byte, error) {
+	for i := uint64(0); i < h.numValid; i++ {
+		/* If we can get from key to index only by resetting bits,
+		 * we can derive from it => index has no bits key doesn't. */
+		if !canDerive(h.chainBranches[i].index, index) {
+			continue
+		}
+
+		nextHash := derive(h.chainBranches[i].index, index,
+			h.chainBranches[i].hash)
+
+		return &nextHash, nil
+	}
+
+	return nil, fmt.Errorf("unable to derive hash # %v", index)
+}
+
+// canDerive...
+func canDerive(from, to uint64) bool {
+	return ^from&to == 1
+}
+
+// addHash
+func (h *HyperShaChain) AddNextHash(hash [32]byte) error {
+	nextIdx := h.lastChainIndex + 1
+	// Hashes for a remote chain must be added in order.
+	if nextIdx != h.lastChainIndex+1 || nextIdx == 0 && h.numValid != 0 {
+		return fmt.Errorf("shachain values must be added in order, attempted"+
+			"to add index %v, chain is at %v", nextIdx, h.lastChainIndex)
+	}
+
+	i := uint64(0)
+	for ; i < h.numValid; i++ {
+		if canDerive(nextIdx, h.chainBranches[i].index) {
+			// Ensure we can actually derive this value.
+			derivation := derive(nextIdx, h.chainBranches[i].index, hash)
+			if !bytes.Equal(derivation[:], h.chainBranches[i].hash[:]) {
+				// TODO(roasbeef): better err message
+				return fmt.Errorf("chain corruption")
+			}
+			break
+		}
+	}
+
+	h.chainBranches[i].index = nextIdx
+	copy(h.chainBranches[i].hash[:], hash[:])
+	copy(h.lastHash[:], hash[:])
+	h.numValid = i + 1
+	h.lastChainIndex = nextIdx
 	return nil
 }
 
-// NextHash...
-func (s *HyperShaChain) NextHash() {
+// CurrentHash...
+func (h *HyperShaChain) CurrentHash() (*wire.ShaHash, error) {
+	h.Lock()
+	defer h.Unlock()
+	return &h.lastHash, nil
 }
 
-// GetHash...
-func (s *HyperShaChain) GetHash(index uint64) {
+// MarshallBinary...
+func (h *HyperShaChain) Encode(b bytes.Buffer) error {
+	return nil
+}
+
+// UnmarshallBinary...
+func (h *HyperShaChain) Decode(b bytes.Buffer) error {
+	return nil
 }