secret connection: check for low order points (#3040)

> Implement a check for the blacklisted low order points, ala the X25519 has_small_order() function in libsodium

(#3010 (comment))
resolves first half of #3010

p2p/conn/secret_connection.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	crand "crypto/rand"
 	"crypto/sha256"
+	"crypto/subtle"
 	"encoding/binary"
 	"errors"
 	"io"
@@ -28,6 +29,8 @@ const aeadSizeOverhead = 16 // overhead of poly 1305 authentication tag
 const aeadKeySize = chacha20poly1305.KeySize
 const aeadNonceSize = chacha20poly1305.NonceSize
 
+var ErrSmallOrderRemotePubKey = errors.New("detected low order point from remote peer")
+
 // SecretConnection implements net.Conn.
 // It is an implementation of the STS protocol.
 // See https://github.com/tendermint/tendermint/blob/0.1/docs/sts-final.pdf for
@@ -251,6 +254,9 @@ func shareEphPubKey(conn io.ReadWriteCloser, locEphPub *[32]byte) (remEphPub *[3
 			if err2 != nil {
 				return nil, err2, true // abort
 			}
+			if hasSmallOrder(_remEphPub) {
+				return nil, ErrSmallOrderRemotePubKey, true
+			}
 			return _remEphPub, nil, false
 		},
 	)
@@ -266,6 +272,52 @@ func shareEphPubKey(conn io.ReadWriteCloser, locEphPub *[32]byte) (remEphPub *[3
 	return &_remEphPub, nil
 }
 
+// use the samne blacklist as lib sodium (see https://eprint.iacr.org/2017/806.pdf for reference):
+// https://github.com/jedisct1/libsodium/blob/536ed00d2c5e0c65ac01e29141d69a30455f2038/src/libsodium/crypto_scalarmult/curve25519/ref10/x25519_ref10.c#L11-L17
+var blacklist = [][32]byte{
+	// 0 (order 4)
+	{0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00},
+	// 1 (order 1)
+	{0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00},
+	// 325606250916557431795983626356110631294008115727848805560023387167927233504
+	//   (order 8)
+	{0xe0, 0xeb, 0x7a, 0x7c, 0x3b, 0x41, 0xb8, 0xae, 0x16, 0x56, 0xe3,
+		0xfa, 0xf1, 0x9f, 0xc4, 0x6a, 0xda, 0x09, 0x8d, 0xeb, 0x9c, 0x32,
+		0xb1, 0xfd, 0x86, 0x62, 0x05, 0x16, 0x5f, 0x49, 0xb8, 0x00},
+	// 39382357235489614581723060781553021112529911719440698176882885853963445705823
+	//    (order 8)
+	{0x5f, 0x9c, 0x95, 0xbc, 0xa3, 0x50, 0x8c, 0x24, 0xb1, 0xd0, 0xb1,
+		0x55, 0x9c, 0x83, 0xef, 0x5b, 0x04, 0x44, 0x5c, 0xc4, 0x58, 0x1c,
+		0x8e, 0x86, 0xd8, 0x22, 0x4e, 0xdd, 0xd0, 0x9f, 0x11, 0x57},
+	// p-1 (order 2)
+	{0xec, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+		0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+		0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x7f},
+	// p (=0, order 4)
+	{0xed, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+		0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+		0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x7f},
+	// p+1 (=1, order 1)
+	{0xee, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+		0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+		0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x7f},
+}
+
+func hasSmallOrder(pubKey [32]byte) bool {
+	isSmallOrderPoint := false
+	for _, bl := range blacklist {
+		if subtle.ConstantTimeCompare(pubKey[:], bl[:]) == 1 {
+			isSmallOrderPoint = true
+			break
+		}
+	}
+	return isSmallOrderPoint
+}
+
 func deriveSecretAndChallenge(dhSecret *[32]byte, locIsLeast bool) (recvSecret, sendSecret *[aeadKeySize]byte, challenge *[32]byte) {
 	hash := sha256.New
 	hkdf := hkdf.New(hash, dhSecret[:], nil, []byte("TENDERMINT_SECRET_CONNECTION_KEY_AND_CHALLENGE_GEN"))

p2p/conn/secret_connection_test.go

@@ -100,6 +100,32 @@ func TestSecretConnectionHandshake(t *testing.T) {
 	}
 }
 
+func TestShareLowOrderPubkey(t *testing.T) {
+	var fooConn, barConn = makeKVStoreConnPair()
+	locEphPub, _ := genEphKeys()
+
+	// all blacklisted low order points:
+	for _, remLowOrderPubKey := range blacklist {
+		_, _ = cmn.Parallel(
+			func(_ int) (val interface{}, err error, abort bool) {
+				_, err = shareEphPubKey(fooConn, locEphPub)
+
+				require.Error(t, err)
+				require.Equal(t, err, ErrSmallOrderRemotePubKey)
+
+				return nil, nil, false
+			},
+			func(_ int) (val interface{}, err error, abort bool) {
+				readRemKey, err := shareEphPubKey(barConn, &remLowOrderPubKey)
+
+				require.NoError(t, err)
+				require.Equal(t, locEphPub, readRemKey)
+
+				return nil, nil, false
+			})
+	}
+}
+
 func TestConcurrentWrite(t *testing.T) {
 	fooSecConn, barSecConn := makeSecretConnPair(t)
 	fooWriteText := cmn.RandStr(dataMaxSize)