sphinx: update encrypter for attr errors

We enhance the existing onion error encrypter to now include the
attribution data. This will be needed by upstream peers in order to
help verify the error source and hold times.

Author: GeorgeTsagk

crypto.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"crypto/hmac"
 	"crypto/sha256"
+	"encoding/binary"
 	"errors"
 	"fmt"
 
@@ -18,6 +19,14 @@ const (
 	// the onion. Any value lower than 32 will truncate the HMAC both
 	// during onion creation as well as during the verification.
 	HMACSize = 32
+
+	// AMMAG is the string representation for the ammag key type. Used in
+	// cypher stream generation.
+	AMMAG = "ammag"
+
+	// AMMAG_EXT is the string representation for the extended ammag key
+	// type. Used in cypher stream generation.
+	AMMAG_EXT = "ammagext"
 )
 
 // chaChaPolyZeroNonce is a slice of zero bytes used in the chacha20poly1305
@@ -301,10 +310,10 @@ func sharedSecret(priv SingleKeyECDH, pub *btcec.PublicKey) (Hash256, error) {
 // onionEncrypt obfuscates the data with compliance with BOLT#4. As we use a
 // stream cipher, calling onionEncrypt on an already encrypted piece of data
 // will decrypt it.
-func onionEncrypt(sharedSecret *Hash256, data []byte) []byte {
+func onionEncrypt(keyType string, sharedSecret *Hash256, data []byte) []byte {
 	p := make([]byte, len(data))
 
-	ammagKey := generateKey("ammag", sharedSecret)
+	ammagKey := generateKey(keyType, sharedSecret)
 	streamBytes := generateCipherStream(ammagKey, uint(len(data)))
 	xor(p, data, streamBytes)
 
@@ -370,7 +379,9 @@ func (o *OnionErrorDecrypter) DecryptError(encryptedData []byte) (
 
 		// With the shared secret, we'll now strip off a layer of
 		// encryption from the encrypted error payload.
-		encryptedData = onionEncrypt(&sharedSecret, encryptedData)
+		encryptedData = onionEncrypt(
+			AMMAG, &sharedSecret, encryptedData,
+		)
 
 		// Next, we'll need to separate the data, from the MAC itself
 		// so we can reconstruct and verify it.
@@ -413,17 +424,141 @@ func (o *OnionErrorDecrypter) DecryptError(encryptedData []byte) (
 // for backward failure obfuscation of the onion failure blob. By obfuscating
 // the onion failure on every node in the path we are adding additional step of
 // the security and barrier for malware nodes to retrieve valuable information.
-// The reason for using onion obfuscation is to not give
-// away to the nodes in the payment path the information about the exact
-// failure and its origin.
-func (o *OnionErrorEncrypter) EncryptError(initial bool, data []byte) []byte {
+// The reason for using onion obfuscation is to not give away to the nodes in
+// the payment path the information about the exact failure and its origin.
+// Every node down the error path reports the recorded hold times for the HTLC,
+// so this is also passed as an argument to this function in order for this node
+// to append its own value. The attribution data is a structure which helps with
+// identifying malicious intermediate hops that may have modified the failure
+// data.
+func (o *OnionErrorEncrypter) EncryptError(initial bool, legacyData []byte,
+	attrData []byte, holdTime uint32) ([]byte, []byte, error) {
+
+	if initial && attrData != nil {
+		return nil, nil, fmt.Errorf("unable to encrypt, cannot " +
+			"initialize error with existing attribution data")
+	}
+
+	if attrData == nil {
+		attrData = o.initializePayload(holdTime)
+	}
+
 	if initial {
+		if len(legacyData) < minPaddedOnionErrorLength {
+			return nil, nil, fmt.Errorf("initial data size less "+
+				"than %v", minPaddedOnionErrorLength)
+		}
+
 		umKey := generateKey("um", &o.sharedSecret)
 		hash := hmac.New(sha256.New, umKey[:])
-		hash.Write(data)
+		hash.Write(legacyData)
 		h := hash.Sum(nil)
-		data = append(h, data...)
+		legacyData = append(h, legacyData...)
+	} else {
+		if len(attrData) < o.hmacsAndPayloadsLen() {
+			return nil, nil, ErrInvalidAttrStructure
+		}
+
+		// Add our hold time.
+		o.addIntermediatePayload(attrData, holdTime)
+
+		// Shift hmacs to create space for the new hmacs.
+		o.shiftHmacsRight(o.hmacs(attrData))
+	}
+
+	// Update hmac block.
+	o.addHmacs(attrData, legacyData)
+
+	legacy := onionEncrypt(AMMAG, &o.sharedSecret, legacyData)
+	attrError := onionEncrypt(AMMAG_EXT, &o.sharedSecret, attrData)
+
+	return legacy, attrError, nil
+}
+
+func (o *OnionErrorEncrypter) shiftHmacsRight(hmacs []byte) {
+	totalHmacs := (o.hopCount * (o.hopCount + 1)) / 2
+
+	// Work from right to left to avoid overwriting data that is still
+	// needed.
+	srcIdx := totalHmacs - 2
+	destIdx := totalHmacs - 1
+
+	// The variable copyLen contains the number of hmacs to copy for the
+	// current hop.
+	copyLen := 1
+	for i := 0; i < o.hopCount-1; i++ {
+		// Shift the hmacs to the right for the current hop. The hmac
+		// corresponding to the assumed position that is farthest away
+		// from the error source is discarded.
+		copy(
+			hmacs[destIdx*o.hmacSize:],
+			hmacs[srcIdx*o.hmacSize:(srcIdx+copyLen)*o.hmacSize],
+		)
+
+		// The number of hmacs to copy increases by one for each
+		// iteration. The further away from the error source, the more
+		// downstream hmacs exist that are relevant.
+		copyLen++
+
+		// Update indices backwards for the next iteration.
+		srcIdx -= copyLen + 1
+		destIdx -= copyLen
+	}
+
+	// Zero out the hmac slots corresponding to every possible position
+	// relative to the error source for the current hop. This is not
+	// strictly necessary as these slots are overwritten anyway, but we
+	// clear them for cleanliness.
+	for i := 0; i < o.hopCount; i++ {
+		copy(hmacs[i*o.hmacSize:], o.zeroHmac)
 	}
+}
+
+// addHmacs updates the failure data with a series of hmacs corresponding to all
+// possible positions in the path for the current node.
+func (o *OnionErrorEncrypter) addHmacs(data []byte, message []byte) {
+	payloads := o.payloads(data)
+	hmacs := o.hmacs(data)
+
+	for i := 0; i < o.hopCount; i++ {
+		position := o.hopCount - i - 1
+		hmac := o.calculateHmac(
+			o.sharedSecret, position, message, payloads, hmacs,
+		)
+
+		copy(hmacs[i*o.hmacSize:], hmac)
+	}
+}
+
+func (o *OnionErrorEncrypter) initializePayload(holdTime uint32) []byte {
+
+	// Add space for payloads and hmacs.
+	data := make([]byte, o.hmacsAndPayloadsLen())
+
+	payloads := o.payloads(data)
+
+	// Signal final hops in the payload.
+	addPayload(payloads, holdTime)
+
+	return data
+}
+
+func (o *OnionErrorEncrypter) addIntermediatePayload(data []byte,
+	holdTime uint32) {
+
+	payloads := o.payloads(data)
+
+	// Shift payloads to create space for the new payload.
+	o.shiftPayloadsRight(payloads)
+
+	// Signal intermediate hop in the payload.
+	addPayload(payloads, holdTime)
+}
+
+func (o *OnionErrorEncrypter) shiftPayloadsRight(payloads []byte) {
+	copy(payloads[o.payloadLen():], payloads)
+}
 
-	return onionEncrypt(&o.sharedSecret, data)
+func addPayload(payloads []byte, holdTime uint32) {
+	binary.BigEndian.PutUint32(payloads, holdTime)
 }

error.go

@@ -1,6 +1,8 @@
 package sphinx
 
-import "fmt"
+import (
+	"fmt"
+)
 
 var (
 	// ErrReplayedPacket is an error returned when a packet is rejected
@@ -24,4 +26,8 @@ var (
 	// ErrLogEntryNotFound is an error returned when a packet lookup in a replay
 	// log fails because it is missing.
 	ErrLogEntryNotFound = fmt.Errorf("sphinx packet is not in log")
+
+	// ErrInvalidAttrStructure is an error that signals that the provided
+	// attribution data have an invalid length.
+	ErrInvalidAttrStructure = fmt.Errorf("invalid attribution data length")
 )

obfuscation.go

@@ -9,12 +9,18 @@ import (
 // OnionErrorEncrypter is a struct that's used to implement onion error
 // encryption as defined within BOLT0004.
 type OnionErrorEncrypter struct {
+	*AttrErrorStructure
 	sharedSecret Hash256
 }
 
-func NewOnionErrorEncrypter(sharedSecret Hash256) *OnionErrorEncrypter {
+// NewOnionErrorEncrypter creates a new encrypter with the provided shared
+// secret and attributable error structure.
+func NewOnionErrorEncrypter(sharedSecret Hash256,
+	structure *AttrErrorStructure) *OnionErrorEncrypter {
+
 	return &OnionErrorEncrypter{
-		sharedSecret: sharedSecret,
+		sharedSecret:       sharedSecret,
+		AttrErrorStructure: structure,
 	}
 }