Refactor common STR verification functionality

protocol/auditlog.go

@@ -11,10 +11,21 @@ import (
 )
 
 type directoryHistory struct {
+	*AudState
 	addr      string
-	signKey   sign.PublicKey
-	snapshots map[uint64]*DirSTR
-	latestSTR *DirSTR
+	snapshots []*DirSTR
+}
+
+// caller validates that initSTR is for epoch 0
+func newDirectoryHistory(addr string, signKey sign.PublicKey, initSTR *DirSTR) *directoryHistory {
+	a := NewAuditor(signKey, initSTR)
+	h := &directoryHistory{
+		a,
+		addr,
+		make([]*DirSTR, 0),
+	}
+	h.updateVerifiedSTR([]*DirSTR{initSTR})
+	return h
 }
 
 // A ConiksAuditLog maintains the histories
@@ -27,21 +38,25 @@ type directoryHistory struct {
 // chronological order.
 type ConiksAuditLog map[[crypto.HashSizeByte]byte]*directoryHistory
 
-// updateLatestSTR inserts a new STR into a directory history;
-// assumes the STR has been validated by the caller
-func (h *directoryHistory) updateLatestSTR(newLatest *DirSTR) {
-	h.snapshots[newLatest.Epoch] = newLatest
-	h.latestSTR = newLatest
+// updateVerifiedSTR inserts a new range of STRs into a directory history;
+// assumes the STRs have been validated by the caller
+func (h *directoryHistory) updateVerifiedSTR(newVerified []*DirSTR) {
+	h.Update(LatestSTRInRange(newVerified))
+	h.snapshots = append(h.snapshots, newVerified...)
 }
 
-// caller validates that initSTR is for epoch 0
-func newDirectoryHistory(addr string, signKey sign.PublicKey, initSTR *DirSTR) *directoryHistory {
-	h := new(directoryHistory)
-	h.addr = addr
-	h.signKey = signKey
-	h.snapshots = make(map[uint64]*DirSTR)
-	h.updateLatestSTR(initSTR)
-	return h
+// Audit checks that a directory's STR history
+// is linear and updates the audtor's state
+// if the checks pass.
+// Audit() first checks the oldest STR in the
+// STR range received in message against the h.verfiedSTR,
+// and then verifies the remaining STRs in msg, and
+// finally updates the snapshots if the checks pass.
+// Audit() is called when an auditor receives new STRs
+// from a directory.
+func (h *directoryHistory) Audit(msg *Response) error {
+	// TODO: Implement as part of the auditor-server protocol
+	return CheckPassed
 }
 
 // NewAuditLog constructs a new ConiksAuditLog. It creates an empty
@@ -72,10 +87,11 @@ func (l ConiksAuditLog) get(dirInitHash [crypto.HashSizeByte]byte) (*directoryHi
 // signing key signKey, and a list of snapshots snaps representing the
 // directory's STR history so far, in chronological order.
 // Insert() returns an ErrAuditLog if the auditor attempts to create
-// a new history for a known directory, an ErrMalformedDirectoryMessage
-// if oldSTRs is malformed, and nil otherwise.
+// a new history for a known directory, and nil otherwise.
 // Insert() only creates the initial entry in the log for addr. Use Update()
 // to insert newly observed STRs for addr in subsequent epochs.
+// Insert() assumes that the caller
+// has called Audit() on snaps before calling Insert().
 // FIXME: pass Response message as param
 // masomel: will probably want to write a more generic function
 // for "catching up" on a history in case an auditor misses epochs
@@ -100,6 +116,7 @@ func (l ConiksAuditLog) Insert(addr string, signKey sign.PublicKey,
 	// create the new directory history
 	h = newDirectoryHistory(addr, signKey, snaps[0])
 
+	// FIXME: remove this check --> caller calls Audit() before this function
 	// add each STR into the history
 	// start at 1 since we've inserted the initial STR above
 	// This loop automatically catches if snaps is malformed
@@ -112,26 +129,28 @@ func (l ConiksAuditLog) Insert(addr string, signKey sign.PublicKey,
 
 		// verify the consistency of each new STR before inserting
 		// into the audit log
-		if err := verifySTRConsistency(signKey, h.latestSTR, str); err != nil {
+		if err := h.verifySTRConsistency(snaps[i-1], str); err != nil {
 			return err
 		}
-
-		h.updateLatestSTR(snaps[i])
 	}
 
 	// Finally, add the new history to the log
+	if len(snaps) > 1 {
+		h.updateVerifiedSTR(snaps[1:])
+	}
 	l.set(dirInitHash, h)
 
 	return nil
 }
 
-// Update verifies the consistency of a newly observed STR newSTR for
-// the directory addr, and inserts the newSTR into addr's directory history
-// if the checks (i.e. STR signature and hash chain verifications) pass.
-// Update() returns nil if the checks pass, and the appropriate consistency
-// check error otherwise. Update() assumes that Insert() has been called for
-// addr prior to its first call and thereby expects that an entry for addr
-// exists in the audit log l.
+// Update inserts a newly observed STR newSTR into the log entry for the
+// directory history given by dirInitHash (hash of direcotry's initial STR).
+// Update() assumes that Insert() has been called for
+// dirInitHash prior to its first call and thereby expects that an
+// entry for addr exists in the audit log l, and that the caller
+// has called Audit() on newSTR before calling Update().
+// Update() returns ErrAuditLog if the audit log doesn't contain an
+// entry for dirInitHash
 // FIXME: pass Response message as param
 func (l ConiksAuditLog) Update(dirInitHash [crypto.HashSizeByte]byte, newSTR *DirSTR) error {
 
@@ -141,12 +160,14 @@ func (l ConiksAuditLog) Update(dirInitHash [crypto.HashSizeByte]byte, newSTR *Di
 		return ErrAuditLog
 	}
 
-	if err := verifySTRConsistency(h.signKey, h.latestSTR, newSTR); err != nil {
+	// FIXME: remove this check --> caller calls Audit() before this function
+	if err := h.verifySTRConsistency(h.VerifiedSTR(), newSTR); err != nil {
 		return err
 	}
 
 	// update the latest STR
-	h.updateLatestSTR(newSTR)
+	// FIXME: use STR slice from Response msg
+	h.updateVerifiedSTR([]*DirSTR{newSTR})
 	return nil
 }
 
@@ -178,7 +199,7 @@ func (l ConiksAuditLog) GetObservedSTRs(req *AuditingRequest) (*Response,
 	}
 
 	// make sure the request is well-formed
-	if req.EndEpoch > h.latestSTR.Epoch || req.StartEpoch > req.EndEpoch {
+	if req.EndEpoch > h.VerifiedSTR().Epoch || req.StartEpoch > req.EndEpoch {
 		return NewErrorResponse(ErrMalformedClientMessage),
 			ErrMalformedClientMessage
 	}

protocol/auditor.go

@@ -0,0 +1,174 @@
+// This module implements a generic CONIKS auditor, i.e. the
+// functionality that clients and auditors need to verify
+// a server's STR history.
+
+package protocol
+
+import (
+	"fmt"
+	"reflect"
+
+	"github.com/coniks-sys/coniks-go/crypto"
+	"github.com/coniks-sys/coniks-go/crypto/sign"
+)
+
+// Auditor provides a generic interface allowing different
+// auditor types to implement specific auditing functionality.
+type Auditor interface {
+	AuditDirectory([]*DirSTR) error
+}
+
+// AudState verifies the hash chain of a specific directory.
+type AudState struct {
+	signKey     sign.PublicKey
+	verifiedSTR *DirSTR
+}
+
+var _ Auditor = (*AudState)(nil)
+
+// New instantiates a new auditor state from a persistance storage.
+func NewAuditor(signKey sign.PublicKey, verified *DirSTR) *AudState {
+	a := &AudState{
+		signKey:     signKey,
+		verifiedSTR: verified,
+	}
+	return a
+}
+
+// VerifiedSTR returns the newly verified STR.
+func (a *AudState) VerifiedSTR() *DirSTR {
+	return a.verifiedSTR
+}
+
+// Update updates the auditor's verifiedSTR to newSTR
+func (a *AudState) Update(newSTR *DirSTR) {
+	a.verifiedSTR = newSTR
+}
+
+// CompareWithVerified checks whether the received STR is the same as
+// the verified STR in the AudState using reflect.DeepEqual().
+func (a *AudState) compareWithVerified(str *DirSTR) error {
+	if reflect.DeepEqual(a.verifiedSTR, str) {
+		return nil
+	}
+	return CheckBadSTR
+}
+
+// CheckSTRAgainstVerified checks an STR str against the a.verifiedSTR.
+// If str's Epoch is the same as the verified, CheckSTRAgainstVerified()
+// compares the two STRs directly. If str is one epoch ahead of the
+// a.verifiedSTR, CheckSTRAgainstVerified() checks the consistency between
+// the two STRs.
+// CheckSTRAgainstVerified() returns nil if the check passes,
+// or the appropriate consistency check error if any of the checks fail,
+// or str's epoch is anything other than the same or one ahead of
+// a.verifiedSTR.
+func (a *AudState) checkSTRAgainstVerified(str *DirSTR) error {
+	// FIXME: check whether the STR was issued on time and whatnot.
+	// Maybe it has something to do w/ #81 and client transitioning between epochs.
+	// Try to verify w/ what's been saved
+
+	switch {
+	case str.Epoch == a.verifiedSTR.Epoch:
+		// Checking an STR in the same epoch
+		if err := a.compareWithVerified(str); err != nil {
+			return err
+		}
+	case str.Epoch == a.verifiedSTR.Epoch+1:
+		// Otherwise, expect that we've entered a new epoch
+		if err := a.verifySTRConsistency(a.verifiedSTR, str); err != nil {
+			return err
+		}
+	default:
+		return CheckBadSTR
+	}
+
+	return nil
+}
+
+// VerifySTRConsistency checks the consistency between 2 snapshots.
+// It uses the signing key signKey to verify the STR's signature.
+// The signKey param either comes from a client's
+// pinned signing key in its consistency state,
+// or an auditor's pinned signing key in its history.
+func (a *AudState) verifySTRConsistency(prevSTR, str *DirSTR) error {
+	// verify STR's signature
+	if !a.signKey.Verify(str.Serialize(), str.Signature) {
+		return CheckBadSignature
+	}
+	if str.VerifyHashChain(prevSTR) {
+		return nil
+	}
+
+	// TODO: verify the directory's policies as well. See #115
+	return CheckBadSTR
+}
+
+// VerifySTRHistory checks the consistency of a range
+// of a directory's STRs. It begins by verifying the STR consistency between
+// the given prevSTR and the first STR in the given range, and
+// then verifies the consistency between each subsequent STR pair.
+func (a *AudState) verifySTRRange(prevSTR *DirSTR, strs []*DirSTR) error {
+
+	prev := prevSTR
+	for i := 0; i < len(strs); i++ {
+		str := strs[i]
+		if str == nil {
+			// FIXME: if this comes from the auditor, this
+			// should really be an ErrMalformedAuditorMessage
+			return ErrMalformedDirectoryMessage
+		}
+
+		// verify the consistency of each STR in the range
+		if err := a.verifySTRConsistency(prev, str); err != nil {
+			return err
+		}
+
+		prev = str
+	}
+
+	return nil
+}
+
+// AuditDirectory validates a range of STRs received from a CONIKS directory.
+// AuditDirectory() checks the consistency of the oldest STR in the range
+// against the verifiedSTR, and verifies the remaining
+// range if the message contains more than one STR.
+// AuditDirectory() returns the appropriate consistency check error
+// if any of the checks fail, or nil if the checks pass.
+func (a *AudState) AuditDirectory(strs []*DirSTR) error {
+
+	// check STR against the latest verified STR
+	if err := a.checkSTRAgainstVerified(strs[0]); err != nil {
+		return err
+	}
+
+	// verify the entire range if we have received more than one STR
+	if len(strs) > 1 {
+		if err := a.verifySTRRange(strs[0], strs[1:]); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+// ComputeDirectoryIdentity returns the hash of
+// the directory's initial STR as a byte array.
+// It panics if the STR isn't an initial STR (i.e. str.Epoch != 0).
+func ComputeDirectoryIdentity(str *DirSTR) [crypto.HashSizeByte]byte {
+	if str.Epoch != 0 {
+		panic(fmt.Sprintf("[coniks] Expect epoch 0, got %x", str.Epoch))
+	}
+
+	var initSTRHash [crypto.HashSizeByte]byte
+	copy(initSTRHash[:], crypto.Digest(str.Signature))
+	return initSTRHash
+}
+
+// LatestSTRInRange returns the STR in the last element of a
+// range of STRs. The range is assumed to be sorted
+// by increasing order of STR epoch.
+func LatestSTRInRange(strs []*DirSTR) *DirSTR {
+	return strs[len(strs)-1]
+}