Bolt11 invoices: do not re-encode amounts (#771)

* Bolt11 invoices: do not re-encode amounts

Bolt11 amounts may not be minimally encoded (the specs says they SHOULD be but it's not a hard requirement).
We should preserve the original amount encoding: if it was not minimally encoded and we re-encode it using its
minimal representation (for example "5" instead of "5000m") then read() will fail to verify the invoice checksum and return an "invoice isn't canonically encoded" error.   
---------

Co-authored-by: Pierre-Marie Padiou <[email protected]>

Author: docker-lordvf6ik

modules/core/src/commonMain/kotlin/fr/acinq/lightning/payment/Bolt11Invoice.kt

@@ -13,14 +13,22 @@ import fr.acinq.lightning.utils.*
 import fr.acinq.lightning.wire.LightningCodecs
 import kotlin.experimental.and
 
+/**
+ * @param encodedAmount We counterintuitively store the string-serialized amount instead of the numeric value. The goal is to preserve the
+ *                      validity of the signature for invoices generated by 3rd party software when the amount is not canonically encoded.
+ */
 data class Bolt11Invoice(
     val prefix: String,
-    override val amount: MilliSatoshi?,
+    val encodedAmount: String,
     val timestampSeconds: Long,
     override val nodeId: PublicKey,
     val tags: List<TaggedField>,
     val signature: ByteVector
 ) : PaymentRequest() {
+    constructor(prefix: String, amount: MilliSatoshi?, timestampSeconds: Long, nodeId: PublicKey, tags: List<TaggedField>, signature: ByteVector): this(prefix, encodeAmount(amount), timestampSeconds, nodeId, tags, signature)
+
+    override val amount: MilliSatoshi? = decodeAmount(encodedAmount)
+
     val chain: Chain? get() = prefixes.entries.firstOrNull { it.value == prefix }?.key
 
     override val paymentHash: ByteVector32 get() = tags.find { it is TaggedField.PaymentHash }!!.run { (this as TaggedField.PaymentHash).hash }
@@ -60,7 +68,7 @@ data class Bolt11Invoice(
         else -> timestampSeconds + expirySeconds <= currentTimestampSeconds
     }
 
-    private fun hrp() = prefix + encodeAmount(amount)
+    private fun hrp() = prefix + encodedAmount
 
     private fun rawData(): List<Int5> {
         val data5 = ArrayList<Int5>()
@@ -147,7 +155,7 @@ data class Bolt11Invoice(
 
             return Bolt11Invoice(
                 prefix = prefix,
-                amount = amount,
+                encodedAmount = encodeAmount(amount),
                 timestampSeconds = timestampSeconds,
                 nodeId = privateKey.publicKey(),
                 tags = tags,
@@ -165,7 +173,7 @@ data class Bolt11Invoice(
         fun read(input: String): Try<Bolt11Invoice> = runTrying {
             val (hrp, data) = Bech32.decode(input)
             val prefix = prefixes.values.find { hrp.startsWith(it) } ?: throw IllegalArgumentException("unknown prefix $hrp")
-            val amount = decodeAmount(hrp.drop(prefix.length))
+            val encodedAmount = hrp.drop(prefix.length)
             val timestamp = decodeTimestamp(data.toList())
             // signature and recovery id, encoded on 65 bytes = 5 * 13 bytes = 5 * 13 * 8 bits =  8 * 13 "5-bits integers"
             val sigandrecid = toByteArray(data.copyOfRange(data.size - 8 * 13, data.size).toList())
@@ -203,7 +211,7 @@ data class Bolt11Invoice(
             }
 
             loop(data.drop(7).dropLast(104))
-            val pr = Bolt11Invoice(prefix, amount, timestamp, nodeId, tags, sigandrecid.toByteVector())
+            val pr = Bolt11Invoice(prefix, encodedAmount, timestamp, nodeId, tags, sigandrecid.toByteVector())
             require(pr.signedPreimage().contentEquals(tohash)) { "invoice isn't canonically encoded" }
             pr
         }

modules/core/src/commonTest/kotlin/fr/acinq/lightning/payment/Bolt11InvoiceTestsCommon.kt

@@ -540,6 +540,16 @@ class Bolt11InvoiceTestsCommon : LightningTestSuite() {
         assertNull(pr1.description)
     }
 
+    @Test
+    fun `decode and re-encode non-canonically encoded invoice`() {
+        // invoice encoding is not canonical: 5000m should be 5
+        val encodedInvoice = createInvoiceUnsafe(5.btc.toMilliSatoshi()).copy(encodedAmount = "5000m").write()
+        assertTrue { encodedInvoice.startsWith("lnbcrt5000m") }
+        val invoice = Bolt11Invoice.read(encodedInvoice).get()
+        val reencodedInvoice = invoice.write()
+        assertEquals(encodedInvoice, reencodedInvoice)
+    }
+
     companion object {
         fun createInvoiceUnsafe(
             amount: MilliSatoshi? = null,
@@ -576,5 +586,4 @@ class Bolt11InvoiceTestsCommon : LightningTestSuite() {
             ).sign(privateKey)
         }
     }
-
 }