Initial commit: Kecalek Android client

Complete Android client for encrypted chat platform.
78+ Kotlin files: crypto (X3DH, Double Ratchet, AES-GCM, Ed25519, X25519,
RSA-PSS), network (TCP/TLS, 50 endpoints), Hilt DI, Room+SQLCipher DB,
Jetpack Compose UI with Catppuccin Mocha theme.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/src/main/java/com/kecalek/chat/crypto/MessagePadding.kt

@@ -0,0 +1,82 @@
+package com.kecalek.chat.crypto
+
+import java.nio.ByteBuffer
+import java.security.SecureRandom
+
+/**
+ * Bucket-based message padding for metadata privacy.
+ * Pads messages to fixed bucket sizes to prevent message-length analysis.
+ *
+ * Format: 0x01 + plaintext + random_padding + pad_length(4 bytes big-endian)
+ * Compatible with Python pad_plaintext/unpad_plaintext.
+ */
+object MessagePadding {
+
+    private const val PAD_MAGIC: Byte = 0x01
+    private val PAD_BUCKETS = intArrayOf(64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536)
+    private val secureRandom = SecureRandom()
+
+    /**
+     * Pad plaintext to nearest bucket size.
+     * @param plaintext raw plaintext bytes
+     * @return padded bytes: 0x01 + plaintext + random_padding + pad_length(4B)
+     */
+    fun pad(plaintext: ByteArray): ByteArray {
+        // content = magic + plaintext
+        val content = ByteArray(1 + plaintext.size)
+        content[0] = PAD_MAGIC
+        System.arraycopy(plaintext, 0, content, 1, plaintext.size)
+
+        // minimum total size = content + 4 bytes for pad_length
+        val minSize = content.size + 4
+
+        // find nearest bucket
+        var targetSize = minSize
+        for (bucket in PAD_BUCKETS) {
+            if (bucket >= minSize) {
+                targetSize = bucket
+                break
+            }
+        }
+
+        // pad_length includes itself (4 bytes) + random padding bytes
+        val padLength = targetSize - content.size
+        val randomPadSize = padLength - 4
+
+        val result = ByteArray(targetSize)
+        System.arraycopy(content, 0, result, 0, content.size)
+
+        // fill random padding
+        if (randomPadSize > 0) {
+            val randomBytes = ByteArray(randomPadSize)
+            secureRandom.nextBytes(randomBytes)
+            System.arraycopy(randomBytes, 0, result, content.size, randomPadSize)
+        }
+
+        // write pad_length as big-endian uint32 at the end
+        val lenBytes = ByteBuffer.allocate(4).putInt(padLength).array()
+        System.arraycopy(lenBytes, 0, result, targetSize - 4, 4)
+
+        return result
+    }
+
+    /**
+     * Remove padding from padded message.
+     * @param data padded message bytes
+     * @return original plaintext
+     */
+    fun unpad(data: ByteArray): ByteArray {
+        // Legacy unpadded messages (JSON starting with '{')
+        if (data.isEmpty() || data[0] != PAD_MAGIC) return data
+        if (data.size < 5) return data
+
+        // Read pad_length from last 4 bytes
+        val padLength = ByteBuffer.wrap(data, data.size - 4, 4).int
+
+        // Validate
+        if (padLength < 4 || padLength > data.size - 1) return data
+
+        // Strip magic prefix (1 byte) and padding (padLength bytes from end)
+        return data.copyOfRange(1, data.size - padLength)
+    }
+}