msp.go

package msp

import (
	"container/heap"
	"crypto/rand"
	"errors"
	"strings"
)

// A UserDatabase is an abstraction over the name -> share map returned by the
// secret splitter that allows an application to only decrypt or request shares
// when needed, rather than re-build a partial map of known data.
type UserDatabase interface {
	ValidUser(name string) bool
	CanGetShare(name string) bool
	GetShare(name string) ([][]byte, error)
}

type Condition interface { // Represents one condition in a predicate
	Ok(UserDatabase) bool
}

type Name struct { // Type of condition
	string
	index int
}

func (n Name) Ok(db UserDatabase) bool {
	return db.CanGetShare(n.string)
}

type TraceElem struct {
	loc   int
	names []string
	trace []string
}

type TraceSlice []TraceElem

func (ts TraceSlice) Len() int      { return len(ts) }
func (ts TraceSlice) Swap(i, j int) { ts[i], ts[j] = ts[j], ts[i] }

func (ts TraceSlice) Less(i, j int) bool {
	return len(ts[i].trace) > len(ts[j].trace)
}

func (ts *TraceSlice) Push(te interface{}) { *ts = append(*ts, te.(TraceElem)) }
func (ts *TraceSlice) Pop() interface{} {
	old := *ts
	n := len(old)

	*ts = old[0 : n-1]
	out := old[n-1]

	return out
}

// Compact takes a trace slice and merges all of its fields.
//
// index: Union of all locations in the slice.
// names: Union of all names in the slice.
// trace: Union of all the traces in the slice.
func (ts TraceSlice) Compact() (index []int, names []string, trace []string) {
	for _, te := range ts {
		index = append(index, te.loc)
		names = append(names, te.names...)
		trace = append(trace, te.trace...)
	}

	// This is a QuickSort related algorithm.  It makes all the names in the trace unique so we don't double-count people.
	//
	// Invariant:  There are no duplicates in trace[0:ptr]
	// Algorithm:  Advance ptr by 1 and enforce the invariant.
	ptr, cutoff := 0, len(trace)

TopLoop:
	for ptr < cutoff { // Choose the next un-checked element of the slice.
		for i := 0; i < ptr; i++ { // Compare it to all elements before it.
			if trace[i] == trace[ptr] { // If we find a duplicate...
				trace[ptr], trace[cutoff-1] = trace[cutoff-1], trace[ptr] // Push the dup to the end of the surviving slice.
				cutoff--                                                  // Mark it for removal.

				continue TopLoop // Because trace[ptr] has been mutated, try to verify the invariant again w/o advancing ptr.
			}
		}

		ptr++ // There are no duplicates; move the ptr forward and start again.
	}
	trace = trace[0:cutoff]

	return
}

type MSP Formatted

func StringToMSP(pred string) (m MSP, err error) {
	var f Formatted

	if -1 == strings.Index(pred, ",") {
		var r Raw
		r, err = StringToRaw(pred)
		if err != nil {
			return
		}

		f = r.Formatted()
	} else {
		f, err = StringToFormatted(pred)
		if err != nil {
			return
		}
	}

	return MSP(f), nil
}

// DerivePath returns the cheapest way to satisfy the MSP (the one with the minimal number of delegations).
//
// ok:    True if the MSP can be satisfied with current delegations; false if not.
// names: The names in the top-level threshold gate that need to be delegated.
// locs:  The index in the treshold gate for each name.
// trace: All names that must be delegated for for this gate to be satisfied.
func (m MSP) DerivePath(db UserDatabase) (ok bool, names []string, locs []int, trace []string) {
	ts := &TraceSlice{}

	for i, cond := range m.Conds {
		switch cond := cond.(type) {
		case Name:
			if db.CanGetShare(cond.string) {
				heap.Push(ts, TraceElem{
					i,
					[]string{cond.string},
					[]string{cond.string},
				})
			}

		case Formatted:
			sok, _, _, strace := MSP(cond).DerivePath(db)
			if sok {
				heap.Push(ts, TraceElem{i, []string{}, strace})
			}
		}

		if (*ts).Len() > m.Min { // If we can otherwise satisfy the threshold gate
			heap.Pop(ts) // Drop the TraceElem with the heaviest trace (the one that requires the most delegations).
		}
	}

	ok = (*ts).Len() >= m.Min
	locs, names, trace = ts.Compact()
	return
}

// DistributeShares takes as input a secret and a user database and returns secret shares according to access structure
// described by the MSP.
func (m MSP) DistributeShares(sec []byte, db UserDatabase) (map[string][][]byte, error) {
	out := make(map[string][][]byte)

	field, ok := Fields[len(sec)]
	if !ok {
		return nil, errors.New("No field for secret length")
	}

	// Generate a Vandermonde matrix.
	height, width := len(m.Conds), m.Min
	M := field.Matrix(height, width)

	for i := range M.m {
		for j := range M.m[i].r {
			M.m[i].r[j].e[0] = byte(i + 1)
			M.m[i].r[j] = M.m[i].r[j].Exp(j)
		}
	}

	// Convert secret vector.
	s, buf := field.Row(width), make([]byte, len(sec))
	for i := range s.r {
		rand.Read(buf)
		if i == 0 {
			copy(buf, sec)
		}

		s.r[i] = field.Elem(buf)
	}

	// Calculate shares.
	shares := M.Mul(s)

	// Distribute the shares.
	for i, cond := range m.Conds {
		share := shares.r[i]

		switch cond := cond.(type) {
		case Name:
			name := cond.string
			if !db.ValidUser(name) {
				return nil, errors.New("Unknown user in predicate.")
			}

			out[name] = append(out[name], share.Bytes())
		case Formatted:
			below := MSP(cond)
			subOut, err := below.DistributeShares(share.Bytes(), db)
			if err != nil {
				return out, err
			}

			for name, shares := range subOut {
				out[name] = append(out[name], shares...)
			}
		}
	}

	return out, nil
}

// RecoverSecret takes a user database storing secret shares as input and returns the original secret.
func (m MSP) RecoverSecret(db UserDatabase) ([]byte, error) {
	cache := make(map[string][][]byte, 0) // Caches un-used shares for a user.
	return m.recoverSecret(db, cache)
}

func (m MSP) recoverSecret(db UserDatabase, cache map[string][][]byte) ([]byte, error) {
	var (
		index  = []int{}  // Indexes where given shares were in the matrix.
		shares = []Elem{} // Contains shares that will be used in reconstruction.
	)

	ok, names, locs, _ := m.DerivePath(db)
	if !ok {
		return nil, errors.New("Not enough shares to recover.")
	}

	var field Field
	for _, name := range names {
		if _, cached := cache[name]; !cached {
			out, err := db.GetShare(name)
			if err != nil {
				return nil, err
			}

			cache[name] = out

			var ok bool
			if field == nil {
				field, ok = Fields[len(out[0])]
				if !ok {
					return nil, errors.New("No field for secret length")
				}
			}
		}
	}

	for _, loc := range locs {
		gate := m.Conds[loc]
		index = append(index, loc+1)

		switch gate := gate.(type) {
		case Name:
			if len(cache[gate.string]) <= gate.index {
				return nil, errors.New("Predicate / database mismatch!")
			}

			shares = append(shares, field.Elem(cache[gate.string][gate.index]))

		case Formatted:
			share, err := MSP(gate).recoverSecret(db, cache)
			if err != nil {
				return nil, err
			}

			shares = append(shares, field.Elem(share))
		}
	}

	// Generate the Vandermonde matrix specific to whichever users' shares we're using.
	MSub := field.Matrix(m.Min, m.Min)

	for i := range MSub.m {
		for j := range MSub.m[i].r {
			MSub.m[i].r[j].e[0] = byte(index[i])
			MSub.m[i].r[j] = MSub.m[i].r[j].Exp(j)
		}
	}

	// Calculate the reconstruction vector and use it to recover the secret.
	r, ok := MSub.Recovery()
	if !ok {
		return nil, errors.New("Unable to find a reconstruction vector!")
	}

	// Compute dot product of the shares vector and the reconstruction vector to
	// recover the secret.
	s := Row{Field: field, r: shares}.DotProduct(r)

	return s.Bytes(), nil
}