From f94816908c7e8d7805129127fe188cca6bdba6bd Mon Sep 17 00:00:00 2001
From: Taksh <takshkothari09@gmail.com>
Date: Tue, 6 Oct 2026 10:35:37 +0530
Subject: [PATCH] Reject noncanonical Bech32 padding and invalid prefix lengths

---
 .../java/com/bitchat/android/nostr/Bech32.kt  |  6 ++-
 .../android/nostr/Bech32PaddingTest.kt        | 47 +++++++++++++++++++
 docs/client-rewrite-contracts.md              |  6 +++
 3 files changed, 58 insertions(+), 1 deletion(-)
 create mode 100644 app/src/test/java/com/bitchat/android/nostr/Bech32PaddingTest.kt

diff --git a/app/src/main/java/com/bitchat/android/nostr/Bech32.kt b/app/src/main/java/com/bitchat/android/nostr/Bech32.kt
index 3e55ab67c..6daa22df0 100644
--- a/app/src/main/java/com/bitchat/android/nostr/Bech32.kt
+++ b/app/src/main/java/com/bitchat/android/nostr/Bech32.kt
@@ -12,6 +12,7 @@ object Bech32 {
      * Encode data with HRP (Human Readable Part)
      */
     fun encode(hrp: String, data: ByteArray): String {
+        require(hrp.length in 1..83) { "Invalid HRP length" }
         val values = convertBits(data, 8, 5, true).toList()
         val checksum = createChecksum(hrp, values)
         val combined = values + checksum
@@ -25,7 +26,7 @@ object Bech32 {
      */
     fun decode(bech32String: String): Pair<String, ByteArray> {
         val separatorIndex = bech32String.lastIndexOf('1')
-        require(separatorIndex >= 0) { "No separator found" }
+        require(separatorIndex in 1..83) { "Invalid HRP length or missing separator" }
         
         val hrp = bech32String.substring(0, separatorIndex)
         val dataString = bech32String.substring(separatorIndex + 1)
@@ -76,6 +77,9 @@ object Bech32 {
         
         if (pad && bits > 0) {
             result.add((acc shl (toBits - bits)) and maxv)
+        } else if (!pad) {
+            require(bits < fromBits) { "Excess padding" }
+            require(((acc shl (toBits - bits)) and maxv) == 0) { "Nonzero padding" }
         }
         
         return result.toIntArray()
diff --git a/app/src/test/java/com/bitchat/android/nostr/Bech32PaddingTest.kt b/app/src/test/java/com/bitchat/android/nostr/Bech32PaddingTest.kt
new file mode 100644
index 000000000..0600279db
--- /dev/null
+++ b/app/src/test/java/com/bitchat/android/nostr/Bech32PaddingTest.kt
@@ -0,0 +1,47 @@
+package com.bitchat.android.nostr
+
+import org.junit.Assert.assertArrayEquals
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertThrows
+import org.junit.Test
+
+class Bech32PaddingTest {
+    @Test
+    fun `a literal canonical npub decodes to its key`() {
+        val npub = "npub1qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqzqujme"
+        val (hrp, key) = Bech32.decode(npub)
+        assertEquals("npub", hrp)
+        assertArrayEquals(ByteArray(32), key)
+        assertEquals(npub, Bech32.encode(hrp, key))
+    }
+
+    @Test
+    fun `valid checksum does not permit nonzero discarded padding`() {
+        // The last payload symbol differs by one bit from the canonical zero key.
+        val npub = "npub1qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqplkg8xt"
+        assertThrows(IllegalArgumentException::class.java) { Bech32.decode(npub) }
+    }
+
+    @Test
+    fun `a whole extra padding symbol cannot alias an empty payload`() {
+        assertThrows(IllegalArgumentException::class.java) { Bech32.decode("test1q6j8x73") }
+    }
+
+    @Test
+    fun `empty and oversized human readable prefixes are rejected`() {
+        for (value in listOf("10a06t8", "x".repeat(84) + "1e5y3yu")) {
+            assertThrows(IllegalArgumentException::class.java) { Bech32.decode(value) }
+        }
+        for (hrp in listOf("", "x".repeat(84))) {
+            assertThrows(IllegalArgumentException::class.java) { Bech32.encode(hrp, byteArrayOf()) }
+        }
+    }
+
+    @Test
+    fun `all byte lengths preserve their canonical padding`() {
+        for (length in 0..64) {
+            val bytes = ByteArray(length) { (it * 37).toByte() }
+            assertArrayEquals(bytes, Bech32.decode(Bech32.encode("test", bytes)).second)
+        }
+    }
+}
diff --git a/docs/client-rewrite-contracts.md b/docs/client-rewrite-contracts.md
index 05d5e1ad7..f93cd1025 100644
--- a/docs/client-rewrite-contracts.md
+++ b/docs/client-rewrite-contracts.md
@@ -36,6 +36,12 @@ randomization used by senders. Android caps outbound seal and gift-wrap
 randomization at 22h, leaving 2 hours of slack inside iOS's 24-hour
 subscription window, while retaining its 48-hour receive lookback.
 
+Bech32 byte decoding must reject nonzero discarded bits and a residual whole
+five-bit symbol. A valid checksum alone does not make byte padding canonical.
+Human-readable prefixes contain 1–83 characters; Nostr's extended data payloads
+are not subject to Bitcoin address length limits. `Bech32PaddingTest` pins these
+rules with literal checksum-valid positive and negative vectors.
+
 ## Rewrite acceptance gate
 
 From a configured Android development environment, run: