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 7ad59f57..21d0dbfb 100644
--- a/app/src/main/java/com/bitchat/android/nostr/Bech32.kt
+++ b/app/src/main/java/com/bitchat/android/nostr/Bech32.kt
@@ -28,6 +28,13 @@ object Bech32 {
      * alphanumeric mode. Mixed case is invalid and is rejected.
      */
     fun decode(bech32String: String): Pair<String, ByteArray> {
+        // BIP-173 restricts every character to printable US-ASCII, 33..126.
+        // This has to run before the case test and before lowercase(): a
+        // non-ASCII uppercase homoglyph such as U+212A KELVIN SIGN reads as
+        // uppercase, so an otherwise all-uppercase string carrying one shows no
+        // mixed case, and lowercase() then folds it into an ASCII 'k'.
+        require(bech32String.all { it.code in 33..126 }) { "Invalid character" }
+
         val hasLower = bech32String.any { it.isLowerCase() }
         val hasUpper = bech32String.any { it.isUpperCase() }
         require(!(hasLower && hasUpper)) { "Mixed case" }
diff --git a/app/src/test/java/com/bitchat/android/nostr/Bech32CaseTest.kt b/app/src/test/java/com/bitchat/android/nostr/Bech32CaseTest.kt
index 1f66c593..ffad6987 100644
--- a/app/src/test/java/com/bitchat/android/nostr/Bech32CaseTest.kt
+++ b/app/src/test/java/com/bitchat/android/nostr/Bech32CaseTest.kt
@@ -3,6 +3,7 @@ package com.bitchat.android.nostr
 import org.junit.Assert.assertArrayEquals
 import org.junit.Assert.assertEquals
 import org.junit.Assert.assertThrows
+import org.junit.Assume.assumeTrue
 import org.junit.Test
 
 /**
@@ -43,6 +44,18 @@ class Bech32CaseTest {
         assertThrows(IllegalArgumentException::class.java) { Bech32.decode(flipped) }
     }
 
+    @Test
+    fun `a non-ascii homoglyph is rejected rather than folded to ascii`() {
+        val npub = Bech32.encode("npub", pubkey).uppercase()
+        // U+212A KELVIN SIGN is uppercase and lowercases to an ASCII 'k', so an
+        // all-uppercase string carrying one passes a naive mixed-case test and
+        // then folds into a perfectly valid npub.
+        val kelvin = npub.replace("K", "\u212A")
+        assumeTrue(kelvin != npub)
+
+        assertThrows(IllegalArgumentException::class.java) { Bech32.decode(kelvin) }
+    }
+
     @Test
     fun `round trip through encode and decode is stable`() {
         val npub = Bech32.encode("npub", pubkey)