diff --git a/client/log_verifier_test.go b/client/log_verifier_test.go
index 610f0cf7e7..dc5464aa4d 100644
--- a/client/log_verifier_test.go
+++ b/client/log_verifier_test.go
@@ -15,6 +15,7 @@
 package client
 
 import (
+	"crypto"
 	"testing"
 
 	"github.com/google/trillian"
@@ -32,7 +33,7 @@ func TestVerifyRootErrors(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Failed to open test key, err=%v", err)
 	}
-	signer := tcrypto.NewSHA256Signer(key)
+	signer := tcrypto.NewSigner(0, key, crypto.SHA256)
 	pk, err := pem.UnmarshalPublicKey(testonly.DemoPublicKey)
 	if err != nil {
 		t.Fatalf("Failed to load public key, err=%v", err)
diff --git a/crypto/signer.go b/crypto/signer.go
index b5860360dc..f8303808dd 100644
--- a/crypto/signer.go
+++ b/crypto/signer.go
@@ -35,11 +35,22 @@ var sigpbHashLookup = map[crypto.Hash]sigpb.DigitallySigned_HashAlgorithm{
 // Signer is responsible for signing log-related data and producing the appropriate
 // application specific signature objects.
 type Signer struct {
-	Hash   crypto.Hash
-	Signer crypto.Signer
+	KeyHint []byte
+	Hash    crypto.Hash
+	Signer  crypto.Signer
 }
 
-// NewSHA256Signer creates a new SHA256 based Signer.
+// NewSigner returns a new signer. The signer will set the KeyHint field, when available, with KeyID.
+func NewSigner(keyID int64, signer crypto.Signer, hash crypto.Hash) *Signer {
+	return &Signer{
+		KeyHint: types.SerializeKeyHint(keyID),
+		Hash:    hash,
+		Signer:  signer,
+	}
+}
+
+// NewSHA256Signer creates a new SHA256 based Signer and a KeyID of 0.
+// TODO(gbelvin): remove
 func NewSHA256Signer(signer crypto.Signer) *Signer {
 	return &Signer{
 		Hash:   crypto.SHA256,
@@ -98,7 +109,7 @@ func (s *Signer) SignLogRoot(r *types.LogRootV1) (*trillian.SignedLogRoot, error
 	}
 	signature, err := s.Sign(hash)
 	if err != nil {
-		glog.Warningf("%v: signer failed to sign log root: %v", root.LogId, err)
+		glog.Warningf("%v: signer failed to sign log root: %v", s.KeyHint, err)
 		return nil, err
 	}
 
@@ -111,7 +122,7 @@ func (s *Signer) SignLogRoot(r *types.LogRootV1) (*trillian.SignedLogRoot, error
 func (s *Signer) SignMapRoot(root *trillian.SignedMapRoot) (*sigpb.DigitallySigned, error) {
 	signature, err := s.SignObject(root)
 	if err != nil {
-		glog.Warningf("%v: signer failed to sign map root: %v", root.MapId, err)
+		glog.Warningf("%v: signer failed to sign map root: %v", s.KeyHint, err)
 		return nil, err
 	}
 
diff --git a/crypto/signer_test.go b/crypto/signer_test.go
index 8f85821800..b5cd361cb8 100644
--- a/crypto/signer_test.go
+++ b/crypto/signer_test.go
@@ -15,6 +15,7 @@
 package crypto
 
 import (
+	"crypto"
 	"encoding/json"
 	"errors"
 	"testing"
@@ -35,7 +36,7 @@ func TestSign(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Failed to open test key, err=%v", err)
 	}
-	signer := NewSHA256Signer(key)
+	signer := NewSigner(0, key, crypto.SHA256)
 
 	for _, test := range []struct {
 		message []byte
@@ -72,7 +73,7 @@ func TestSign_SignerFails(t *testing.T) {
 		t.Fatalf("Failed to load private key: %v", err)
 	}
 
-	_, err = NewSHA256Signer(testonly.NewSignerWithErr(key, errors.New("sign"))).Sign([]byte(message))
+	_, err = NewSigner(0, testonly.NewSignerWithErr(key, errors.New("sign")), crypto.SHA256).Sign([]byte(message))
 	if err == nil {
 		t.Fatalf("Ignored a signing error: %v", err)
 	}
@@ -95,7 +96,7 @@ func TestSignWithSignedLogRoot_SignerFails(t *testing.T) {
 	if err != nil {
 		t.Fatalf("HashLogRoot(): %v", err)
 	}
-	_, err = NewSHA256Signer(s).Sign(hash)
+	_, err = NewSigner(0, s, crypto.SHA256).Sign(hash)
 	testonly.EnsureErrorContains(t, err, "signfail")
 }
 
@@ -104,7 +105,7 @@ func TestSignLogRoot(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Failed to open test key, err=%v", err)
 	}
-	signer := NewSHA256Signer(key)
+	signer := NewSigner(0, key, crypto.SHA256)
 
 	for _, test := range []struct {
 		root *types.LogRootV1
@@ -137,7 +138,7 @@ func TestSignMapRoot(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Failed to open test key, err=%v", err)
 	}
-	signer := NewSHA256Signer(key)
+	signer := NewSigner(0, key, crypto.SHA256)
 
 	for _, test := range []struct {
 		root trillian.SignedMapRoot
diff --git a/crypto/verifier_test.go b/crypto/verifier_test.go
index 1876c36c77..cfb1aa68b1 100644
--- a/crypto/verifier_test.go
+++ b/crypto/verifier_test.go
@@ -15,6 +15,7 @@
 package crypto
 
 import (
+	"crypto"
 	"testing"
 
 	"github.com/google/trillian"
@@ -70,7 +71,7 @@ func TestSignVerify(t *testing.T) {
 		msg := []byte("foo")
 		var signature *sigpb.DigitallySigned
 		if !test.skipSigning {
-			signature, err = NewSHA256Signer(key).Sign(msg)
+			signature, err = NewSigner(0, key, crypto.SHA256).Sign(msg)
 			if err != nil {
 				t.Errorf("%s: Sign()=(_,%v), want (_,nil)", test.name, err)
 				continue
@@ -89,7 +90,7 @@ func TestSignVerifyObject(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Failed to open test key, err=%v", err)
 	}
-	signer := NewSHA256Signer(key)
+	signer := NewSigner(0, key, crypto.SHA256)
 
 	type subfield struct {
 		c int
diff --git a/log/sequencer.go b/log/sequencer.go
index 149f9e2726..a515e4450c 100644
--- a/log/sequencer.go
+++ b/log/sequencer.go
@@ -138,19 +138,19 @@ func (s Sequencer) buildMerkleTreeFromStorageAtRoot(ctx context.Context, root tr
 	mt, err := merkle.NewCompactMerkleTreeWithState(s.hasher, root.TreeSize, func(depth int, index int64) ([]byte, error) {
 		nodeID, err := storage.NewNodeIDForTreeCoords(int64(depth), index, maxTreeDepth)
 		if err != nil {
-			glog.Warningf("%v: Failed to create nodeID: %v", root.LogId, err)
+			glog.Warningf("%x: Failed to create nodeID: %v", s.signer.KeyHint, err)
 			return nil, err
 		}
 		nodes, err := tx.GetMerkleNodes(ctx, root.TreeRevision, []storage.NodeID{nodeID})
 
 		if err != nil {
-			glog.Warningf("%v: Failed to get Merkle nodes: %v", root.LogId, err)
+			glog.Warningf("%x: Failed to get Merkle nodes: %v", s.signer.KeyHint, err)
 			return nil, err
 		}
 
 		// We expect to get exactly one node here
 		if nodes == nil || len(nodes) != 1 {
-			return nil, fmt.Errorf("%v: Did not retrieve one node while loading CompactMerkleTree, got %#v for ID %v@%v", root.LogId, nodes, nodeID.String(), root.TreeRevision)
+			return nil, fmt.Errorf("%x: Did not retrieve one node while loading CompactMerkleTree, got %#v for ID %v@%v", s.signer.KeyHint, nodes, nodeID.String(), root.TreeRevision)
 		}
 
 		return nodes[0].Hash, nil
diff --git a/log/sequencer_test.go b/log/sequencer_test.go
index 9f2c0d0e11..2adff402ef 100644
--- a/log/sequencer_test.go
+++ b/log/sequencer_test.go
@@ -31,9 +31,10 @@ import (
 	"github.com/google/trillian/merkle/rfc6962"
 	"github.com/google/trillian/quota"
 	"github.com/google/trillian/storage"
-	stestonly "github.com/google/trillian/storage/testonly"
 	"github.com/google/trillian/testonly"
 	"github.com/google/trillian/util"
+
+	stestonly "github.com/google/trillian/storage/testonly"
 )
 
 var (
@@ -73,7 +74,6 @@ var expectedSignedRoot = trillian.SignedLogRoot{
 	TimestampNanos: fakeTimeForTest.UnixNano(),
 	TreeRevision:   6,
 	TreeSize:       17,
-	LogId:          0,
 	Signature: &sigpb.DigitallySigned{
 		SignatureAlgorithm: sigpb.DigitallySigned_ECDSA,
 		HashAlgorithm:      sigpb.DigitallySigned_SHA256,
@@ -87,7 +87,6 @@ var expectedSignedRoot16 = trillian.SignedLogRoot{
 	TreeRevision:   6,
 	TreeSize:       16,
 	RootHash:       testRoot16.RootHash,
-	LogId:          0,
 	Signature: &sigpb.DigitallySigned{
 		SignatureAlgorithm: sigpb.DigitallySigned_ECDSA,
 		HashAlgorithm:      sigpb.DigitallySigned_SHA256,
@@ -101,7 +100,6 @@ var expectedSignedRoot0 = trillian.SignedLogRoot{
 	TimestampNanos: fakeTimeForTest.UnixNano(),
 	TreeRevision:   1,
 	TreeSize:       0,
-	LogId:          0,
 	Signature: &sigpb.DigitallySigned{
 		SignatureAlgorithm: sigpb.DigitallySigned_ECDSA,
 		HashAlgorithm:      sigpb.DigitallySigned_SHA256,
@@ -251,7 +249,7 @@ func createTestContext(ctrl *gomock.Controller, params testParameters) (testCont
 		}
 	}
 
-	signer := crypto.NewSHA256Signer(params.signer)
+	signer := crypto.NewSigner(0, params.signer, gocrypto.SHA256)
 	qm := params.qm
 	if qm == nil {
 		qm = quota.Noop()
@@ -550,7 +548,7 @@ func TestIntegrateBatch_PutTokens(t *testing.T) {
 	// Needed to create a signer
 	hasher := rfc6962.DefaultHasher
 	ts := util.NewFakeTimeSource(fakeTimeForTest)
-	signer := crypto.NewSHA256Signer(cryptoSigner)
+	signer := crypto.NewSigner(0, cryptoSigner, gocrypto.SHA256)
 
 	// Needed for IntegrateBatch calls
 	const treeID int64 = 1234
diff --git a/quota/mysqlqm/mysql_quota_test.go b/quota/mysqlqm/mysql_quota_test.go
index 25f66bb9d7..464621bd50 100644
--- a/quota/mysqlqm/mysql_quota_test.go
+++ b/quota/mysqlqm/mysql_quota_test.go
@@ -16,21 +16,25 @@ package mysqlqm_test
 
 import (
 	"context"
+	"crypto"
 	"database/sql"
 	"fmt"
 	"testing"
 	"time"
 
 	"github.com/google/trillian"
-	"github.com/google/trillian/crypto/sigpb"
 	"github.com/google/trillian/quota"
 	"github.com/google/trillian/quota/mysqlqm"
 	"github.com/google/trillian/storage"
 	"github.com/google/trillian/storage/mysql"
 	"github.com/google/trillian/storage/testdb"
-	"github.com/google/trillian/storage/testonly"
+	"github.com/google/trillian/testonly"
 	"github.com/google/trillian/trees"
+	"github.com/google/trillian/types"
 	"github.com/kylelemons/godebug/pretty"
+
+	tcrypto "github.com/google/trillian/crypto"
+	stestonly "github.com/google/trillian/storage/testonly"
 )
 
 func TestQuotaManager_GetTokens(t *testing.T) {
@@ -293,7 +297,7 @@ func createTree(ctx context.Context, db *sql.DB) (*trillian.Tree, error) {
 		as := mysql.NewAdminStorage(db)
 		err := as.ReadWriteTransaction(ctx, func(ctx context.Context, tx storage.AdminTX) error {
 			var err error
-			tree, err = tx.CreateTree(ctx, testonly.LogTree)
+			tree, err = tx.CreateTree(ctx, stestonly.LogTree)
 			return err
 		})
 		if err != nil {
@@ -304,7 +308,13 @@ func createTree(ctx context.Context, db *sql.DB) (*trillian.Tree, error) {
 	{
 		ls := mysql.NewLogStorage(db, nil)
 		err := ls.ReadWriteTransaction(ctx, tree, func(ctx context.Context, tx storage.LogTreeTX) error {
-			return tx.StoreSignedLogRoot(ctx, trillian.SignedLogRoot{LogId: tree.TreeId, RootHash: []byte{0}, Signature: &sigpb.DigitallySigned{}})
+			signer := tcrypto.NewSigner(0, testonly.NewSignerWithFixedSig(nil, nil), crypto.SHA256)
+			slr, err := signer.SignLogRoot(&types.LogRootV1{RootHash: []byte{0}})
+			if err != nil {
+				return err
+			}
+			return tx.StoreSignedLogRoot(ctx, *slr)
+
 		})
 		if err != nil {
 			return nil, err
diff --git a/server/sequencer_manager_test.go b/server/sequencer_manager_test.go
index aaa2b99051..7fc0375d9b 100644
--- a/server/sequencer_manager_test.go
+++ b/server/sequencer_manager_test.go
@@ -36,6 +36,7 @@ import (
 	"github.com/google/trillian/storage"
 	stestonly "github.com/google/trillian/storage/testonly"
 	"github.com/google/trillian/testonly"
+	"github.com/google/trillian/types"
 	"github.com/google/trillian/util"
 )
 
@@ -63,7 +64,7 @@ var testLeaf0Updated = &trillian.LogLeaf{
 var testRoot0 = trillian.SignedLogRoot{
 	TreeSize:     0,
 	TreeRevision: 0,
-	LogId:        testLogID1,
+	KeyHint:      types.SerializeKeyHint(testLogID1),
 	RootHash:     []byte{},
 	Signature: &sigpb.DigitallySigned{
 		HashAlgorithm:      sigpb.DigitallySigned_SHA256,
diff --git a/storage/cloudspanner/log_storage.go b/storage/cloudspanner/log_storage.go
index 8aca9719a1..58dd899e1b 100644
--- a/storage/cloudspanner/log_storage.go
+++ b/storage/cloudspanner/log_storage.go
@@ -340,7 +340,6 @@ func (tx *logTX) LatestSignedLogRoot(ctx context.Context) (trillian.SignedLogRoo
 		TimestampNanos: currentSTH.TsNanos,
 		RootHash:       currentSTH.RootHash,
 		TreeSize:       currentSTH.TreeSize,
-		LogId:          currentSTH.TreeId,
 		TreeRevision:   currentSTH.TreeRevision,
 		Signature:      apiSig,
 	}, nil
diff --git a/storage/mysql/log_storage.go b/storage/mysql/log_storage.go
index 96220a36d6..511511d5fe 100644
--- a/storage/mysql/log_storage.go
+++ b/storage/mysql/log_storage.go
@@ -34,6 +34,7 @@ import (
 	"github.com/google/trillian/monitoring"
 	"github.com/google/trillian/storage"
 	"github.com/google/trillian/storage/cache"
+	"github.com/google/trillian/types"
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/status"
 
@@ -668,8 +669,8 @@ func (t *logTreeTX) fetchLatestRoot(ctx context.Context) (trillian.SignedLogRoot
 		TimestampNanos: timestamp,
 		TreeRevision:   treeRevision,
 		Signature:      &rootSignature,
-		LogId:          t.treeID,
 		TreeSize:       treeSize,
+		KeyHint:        types.SerializeKeyHint(t.treeID),
 	}, nil
 }
 
diff --git a/storage/mysql/log_storage_test.go b/storage/mysql/log_storage_test.go
index 2500f162e3..a22ee12a25 100644
--- a/storage/mysql/log_storage_test.go
+++ b/storage/mysql/log_storage_test.go
@@ -30,6 +30,7 @@ import (
 	"github.com/google/trillian"
 	"github.com/google/trillian/storage"
 	"github.com/google/trillian/storage/testonly"
+	"github.com/google/trillian/types"
 	"github.com/kylelemons/godebug/pretty"
 
 	spb "github.com/google/trillian/crypto/sigpb"
@@ -865,7 +866,7 @@ func TestLatestSignedLogRoot(t *testing.T) {
 	tree := logTree(logID)
 
 	root := trillian.SignedLogRoot{
-		LogId:          logID,
+		KeyHint:        types.SerializeKeyHint(logID),
 		TimestampNanos: 98765,
 		TreeSize:       16,
 		TreeRevision:   5,
@@ -902,7 +903,7 @@ func TestDuplicateSignedLogRoot(t *testing.T) {
 
 	runLogTX(s, tree, t, func(ctx context.Context, tx storage.LogTreeTX) error {
 		root := trillian.SignedLogRoot{
-			LogId:          logID,
+			KeyHint:        types.SerializeKeyHint(logID),
 			TimestampNanos: 98765,
 			TreeSize:       16,
 			TreeRevision:   5,
@@ -928,7 +929,7 @@ func TestLogRootUpdate(t *testing.T) {
 	tree := logTree(logID)
 
 	root := trillian.SignedLogRoot{
-		LogId:          logID,
+		KeyHint:        types.SerializeKeyHint(logID),
 		TimestampNanos: 98765,
 		TreeSize:       16,
 		TreeRevision:   5,
@@ -936,7 +937,7 @@ func TestLogRootUpdate(t *testing.T) {
 		Signature:      &spb.DigitallySigned{Signature: []byte("notempty")},
 	}
 	root2 := trillian.SignedLogRoot{
-		LogId:          logID,
+		KeyHint:        types.SerializeKeyHint(logID),
 		TimestampNanos: 98766,
 		TreeSize:       16,
 		TreeRevision:   6,
diff --git a/storage/mysql/storage_test.go b/storage/mysql/storage_test.go
index 3101e435bb..2e3333a96c 100644
--- a/storage/mysql/storage_test.go
+++ b/storage/mysql/storage_test.go
@@ -27,12 +27,14 @@ import (
 
 	"github.com/golang/glog"
 	"github.com/google/trillian"
-	"github.com/google/trillian/crypto/sigpb"
 	"github.com/google/trillian/merkle"
 	"github.com/google/trillian/merkle/rfc6962"
 	"github.com/google/trillian/storage"
 	"github.com/google/trillian/storage/testdb"
+	"github.com/google/trillian/testonly"
+	"github.com/google/trillian/types"
 
+	tcrypto "github.com/google/trillian/crypto"
 	storageto "github.com/google/trillian/storage/testonly"
 )
 
@@ -256,13 +258,16 @@ func createLogForTests(db *sql.DB) int64 {
 		panic(fmt.Sprintf("Error creating log: %v", err))
 	}
 
+	signer := tcrypto.NewSigner(0, testonly.NewSignerWithFixedSig(nil, nil), crypto.SHA256)
+
 	ctx := context.Background()
 	l := NewLogStorage(db, nil)
 	err = l.ReadWriteTransaction(ctx, tree, func(ctx context.Context, tx storage.LogTreeTX) error {
-		if err := tx.StoreSignedLogRoot(ctx, trillian.SignedLogRoot{
-			LogId:     tree.TreeId,
-			RootHash:  []byte{0},
-			Signature: &sigpb.DigitallySigned{Signature: []byte("asignature")}}); err != nil {
+		root, err := signer.SignLogRoot(&types.LogRootV1{RootHash: []byte{0}})
+		if err != nil {
+			return fmt.Errorf("Error creating new SignedLogRoot: %v", err)
+		}
+		if err := tx.StoreSignedLogRoot(ctx, *root); err != nil {
 			return fmt.Errorf("Error storing new SignedLogRoot: %v", err)
 		}
 		return nil
diff --git a/trillian.pb.go b/trillian.pb.go
index 2c4c01798d..acd9ae586f 100644
--- a/trillian.pb.go
+++ b/trillian.pb.go
@@ -409,8 +409,16 @@ type SignedLogRoot struct {
 	// TreeSize is the number of entries in the tree.
 	TreeSize     int64                  `protobuf:"varint,3,opt,name=tree_size,json=treeSize" json:"tree_size,omitempty"`
 	Signature    *sigpb.DigitallySigned `protobuf:"bytes,4,opt,name=signature" json:"signature,omitempty"`
-	LogId        int64                  `protobuf:"varint,5,opt,name=log_id,json=logId" json:"log_id,omitempty"`
 	TreeRevision int64                  `protobuf:"varint,6,opt,name=tree_revision,json=treeRevision" json:"tree_revision,omitempty"`
+	// key_hint is a hint to identify the public key for signature verification.
+	// key_hint is not authenticated and may be incorrect or missing, in which
+	// case all known public keys may be used to verify the signature.
+	// When directly communicating with a Trillian gRPC server, the key_hint will
+	// typically contain the LogID encoded as a big-endian 64-bit integer;
+	// however, in other contexts the key_hint is likely to have different
+	// contents (e.g. it could be a GUID, a URL + TreeID, or it could be
+	// derived from the public key itself).
+	KeyHint []byte `protobuf:"bytes,7,opt,name=key_hint,json=keyHint,proto3" json:"key_hint,omitempty"`
 }
 
 func (m *SignedLogRoot) Reset()                    { *m = SignedLogRoot{} }
@@ -446,18 +454,18 @@ func (m *SignedLogRoot) GetSignature() *sigpb.DigitallySigned {
 	return nil
 }
 
-func (m *SignedLogRoot) GetLogId() int64 {
+func (m *SignedLogRoot) GetTreeRevision() int64 {
 	if m != nil {
-		return m.LogId
+		return m.TreeRevision
 	}
 	return 0
 }
 
-func (m *SignedLogRoot) GetTreeRevision() int64 {
+func (m *SignedLogRoot) GetKeyHint() []byte {
 	if m != nil {
-		return m.TreeRevision
+		return m.KeyHint
 	}
-	return 0
+	return nil
 }
 
 // SignedMapRoot represents a commitment by a Map to a particular tree.
@@ -536,72 +544,73 @@ func init() {
 func init() { proto.RegisterFile("trillian.proto", fileDescriptor3) }
 
 var fileDescriptor3 = []byte{
-	// 1057 bytes of a gzipped FileDescriptorProto
-	0x1f, 0x8b, 0x08, 0x00, 0x00, 0x00, 0x00, 0x00, 0x02, 0xff, 0xac, 0x56, 0x5b, 0x6f, 0xe2, 0xc6,
-	0x17, 0x8f, 0x81, 0x80, 0x39, 0x5c, 0xe2, 0x4c, 0x6e, 0x0e, 0x2b, 0xfd, 0x97, 0x7f, 0x5a, 0xa9,
-	0x69, 0x1e, 0xc8, 0x2e, 0x6d, 0x22, 0x55, 0xfb, 0x50, 0x39, 0xc1, 0x09, 0x90, 0x04, 0xd0, 0xd8,
-	0xdd, 0x6a, 0xf3, 0x62, 0x0d, 0x78, 0x6a, 0xac, 0xc5, 0xd8, 0xb2, 0x87, 0xd5, 0x7a, 0x9f, 0xfb,
-	0x52, 0xb5, 0x1f, 0xb3, 0x5f, 0xa3, 0x52, 0x35, 0x63, 0x9b, 0x10, 0xd2, 0xbd, 0xa8, 0xea, 0x4b,
-	0x32, 0xe7, 0xfc, 0x2e, 0x73, 0x8e, 0xe7, 0x8c, 0x31, 0xd4, 0x59, 0xe8, 0xce, 0x66, 0x2e, 0x99,
-	0xb7, 0x82, 0xd0, 0x67, 0x3e, 0x92, 0xb3, 0xb8, 0xd1, 0x98, 0x84, 0x71, 0xc0, 0xfc, 0xd3, 0xb7,
-	0x34, 0x8e, 0x82, 0x71, 0xfa, 0x2f, 0x61, 0x35, 0xd4, 0x14, 0x8b, 0x5c, 0x27, 0x18, 0x27, 0x7f,
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+	0x41, 0x07, 0x9f, 0x9d, 0x3e, 0xfb, 0xe1, 0xd9, 0xc0, 0x36, 0x47, 0xfa, 0xe0, 0xe9, 0x33, 0x55,
+	0x42, 0xfb, 0xb0, 0x6b, 0x19, 0xa6, 0x65, 0x73, 0x71, 0x8e, 0x37, 0xb0, 0x5a, 0xe1, 0x1a, 0xb3,
+	0x17, 0x13, 0xe3, 0xd4, 0xb2, 0xb7, 0xf0, 0x55, 0x74, 0x00, 0x7b, 0xa7, 0xb3, 0xe9, 0xf8, 0xc2,
+	0xe4, 0xae, 0xa7, 0x4f, 0x06, 0x36, 0x77, 0xcb, 0x8f, 0x7e, 0x97, 0xa0, 0x51, 0xbe, 0xe1, 0x78,
+	0xb2, 0x45, 0x0e, 0x16, 0x36, 0x0c, 0xdb, 0xb4, 0x74, 0xcb, 0x50, 0xef, 0x20, 0x80, 0x9a, 0x7e,
+	0x6a, 0x8d, 0x5f, 0x1a, 0xaa, 0xc4, 0xd7, 0x67, 0x78, 0xf6, 0xda, 0x98, 0xaa, 0x15, 0xf4, 0x10,
+	0xee, 0x0f, 0x8d, 0x39, 0x36, 0x4e, 0x75, 0xcb, 0x18, 0xda, 0xe6, 0xec, 0xcc, 0xb2, 0x87, 0xc6,
+	0xa5, 0x61, 0x19, 0x43, 0xb5, 0xda, 0xad, 0x28, 0xd2, 0x16, 0x60, 0xa4, 0xe3, 0x61, 0x09, 0x90,
+	0x05, 0xa0, 0x05, 0xca, 0x10, 0xeb, 0xe3, 0xe9, 0x78, 0x7a, 0xae, 0xee, 0x3c, 0x3a, 0x07, 0xa5,
+	0x78, 0x77, 0xf2, 0x84, 0x6f, 0xe5, 0x62, 0xbd, 0x9a, 0xf3, 0x54, 0xea, 0x50, 0xbd, 0x9c, 0x9d,
+	0xab, 0x12, 0x5f, 0x5c, 0xe9, 0x73, 0xb5, 0xc2, 0x9f, 0xce, 0x1c, 0x1b, 0x33, 0x3c, 0x34, 0xb0,
+	0x31, 0xb4, 0x79, 0xb0, 0xfa, 0x62, 0x04, 0x87, 0xcb, 0xd0, 0x2f, 0x86, 0xe2, 0xf6, 0xe7, 0xca,
+	0x8b, 0xb6, 0x95, 0xdb, 0x73, 0x6e, 0xce, 0xa5, 0xd7, 0x5d, 0xd7, 0x63, 0xab, 0xcd, 0xa2, 0xbf,
+	0x0c, 0xfd, 0x93, 0xfc, 0x7b, 0xa2, 0xa0, 0x2c, 0x6a, 0x82, 0xf3, 0xdd, 0xdf, 0x01, 0x00, 0x00,
+	0xff, 0xff, 0x00, 0xc9, 0xa2, 0x26, 0xf4, 0x08, 0x00, 0x00,
 }
diff --git a/trillian.proto b/trillian.proto
index c897cf423a..4f72b557e3 100644
--- a/trillian.proto
+++ b/trillian.proto
@@ -217,8 +217,17 @@ message SignedLogRoot {
   int64 tree_size = 3;
   sigpb.DigitallySigned signature = 4;
 
-  int64 log_id = 5;
+  reserved 5; // log_id is associated with the public key that validates signature.
   int64 tree_revision = 6;
+  // key_hint is a hint to identify the public key for signature verification.
+  // key_hint is not authenticated and may be incorrect or missing, in which
+  // case all known public keys may be used to verify the signature.
+  // When directly communicating with a Trillian gRPC server, the key_hint will
+  // typically contain the LogID encoded as a big-endian 64-bit integer;
+  // however, in other contexts the key_hint is likely to have different
+  // contents (e.g. it could be a GUID, a URL + TreeID, or it could be
+  // derived from the public key itself).
+  bytes key_hint = 7;
 }
 
 // SignedMapRoot represents a commitment by a Map to a particular tree.
diff --git a/types/logroot.go b/types/logroot.go
index f92112e7c3..c10e881e97 100644
--- a/types/logroot.go
+++ b/types/logroot.go
@@ -85,3 +85,22 @@ func (l *LogRootV1) MarshalBinary() ([]byte, error) {
 		V1:      l,
 	})
 }
+
+// SerializeKeyHint returns a byte slice with logID serialized as a big endian uint64.
+func SerializeKeyHint(logID int64) []byte {
+	hint := make([]byte, 8)
+	binary.BigEndian.PutUint64(hint, uint64(logID))
+	return hint
+}
+
+// ParseKeyHint converts a keyhint into a keyID.
+func ParseKeyHint(hint []byte) (int64, error) {
+	if len(hint) != 8 {
+		return 0, fmt.Errorf("hint is %v bytes, want %v", len(hint), 4)
+	}
+	keyID := int64(binary.BigEndian.Uint64(hint))
+	if keyID < 0 {
+		return 0, fmt.Errorf("hint %x is negative", keyID)
+	}
+	return keyID, nil
+}
diff --git a/types/logroot_test.go b/types/logroot_test.go
index 1d9d9e0960..1c4ff02805 100644
--- a/types/logroot_test.go
+++ b/types/logroot_test.go
@@ -84,3 +84,25 @@ func MustMarshalLogRoot(root *LogRootV1) []byte {
 	}
 	return b
 }
+
+func TestKeyHint(t *testing.T) {
+	for _, tc := range []struct {
+		hint    []byte
+		want    int64
+		wantErr bool
+	}{
+		{hint: SerializeKeyHint(4), want: 4},
+		{hint: SerializeKeyHint(3561657513447883733), want: 3561657513447883733},
+		{hint: []byte{0, 0, 0, 0, 0, 0, 0, 4}, want: 4},
+		{hint: []byte{0xff, 0, 0, 0, 0, 0, 4}, want: 0, wantErr: true},    // Integer overflow
+		{hint: []byte{0, 0, 0, 0, 0, 0, 0, 4, 0}, want: 0, wantErr: true}, // Wrong byte len
+	} {
+		logID, err := ParseKeyHint(tc.hint)
+		if got, want := err != nil, tc.wantErr; got != want {
+			t.Errorf("ParseKeyHint(%v): %v, wantErr: %v", tc.hint, err, want)
+		}
+		if got, want := logID, tc.want; got != want {
+			t.Errorf("ParseKeyHint(%v): %v, want: %v", tc.hint, got, want)
+		}
+	}
+}