Added documents

docs/Impl-Crypto.md

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+# Before start
+
+`AES-128-CFB8`, `RSA1024` and `SHA1` has been implemented in `libmc-protocol-context` module, it uses
+`cryptography-kotlin`(and its platform based provider)
+
+> All impl example below are based on Java's built-in security & cryptography API
+
+# AES-128-CFB8
+
+```kotlin
+class JavaAesCipher(sharedKey: ByteArray) : NetworkCipher {
+    private val encryptCipher = Cipher.getInstance("AES/CFB8/NoPadding").apply {
+        init(Cipher.ENCRYPT_MODE, SecretKeySpec(sharedKey, "AES"), IvParameterSpec(sharedKey))
+    }
+
+    private val decryptCipher = Cipher.getInstance("AES/CFB8/NoPadding").apply {
+        init(Cipher.DECRYPT_MODE, SecretKeySpec(sharedKey, "AES"), IvParameterSpec(sharedKey))
+    }
+
+    override fun encrypt(buffer: ByteArray, offset: Int, length: Int) {
+        encryptCipher.update(buffer, offset, length, buffer, offset)
+    }
+
+    override fun decrypt(buffer: ByteArray, offset: Int, length: Int) {
+        decryptCipher.update(buffer, offset, length, buffer, offset)
+    }
+}
+```
+
+# RSA1024
+
+```kotlin
+private fun encrypt(publicKeyBytes: ByteArray, data: ByteArray): ByteArray {
+    val keySpec = X509EncodedKeySpec(publicKeyBytes)
+    val keyFactory = KeyFactory.getInstance("RSA")
+    val publicKey = keyFactory.generatePublic(keySpec)
+    val cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding")
+    cipher.init(Cipher.ENCRYPT_MODE, publicKey)
+    return cipher.doFinal(data)
+}
+
+public val CustomProtocolContext: ProtocolContextBuilder.() -> Unit = {
+    rsaEncryptor = RSA1024Encryptor { key: ByteArray, data: ByteArray -> encrypt(key, data) }
+}
+```
+
+# SHA1
+
+```kotlin
+private fun minecraftServerIdHash(serverId: String, secretKey: ByteArray, publicKey: ByteArray): String {
+    val serverIdBytes: ByteArray = serverId.encodeToByteArray()
+    for (b in serverIdBytes) require((b.toInt() and 0xFF) <= 0x7F) { "serverId contains non-US-ASCII character" }
+    val data = ByteArray(serverIdBytes.size + secretKey.size + publicKey.size)
+    System.arraycopy(serverIdBytes, 0, data, 0, serverIdBytes.size)
+    System.arraycopy(secretKey, 0, data, serverIdBytes.size, secretKey.size)
+    System.arraycopy(publicKey, 0, data, serverIdBytes.size + secretKey.size, publicKey.size)
+    val digest = MessageDigest.getInstance("SHA-1")
+    val hash = digest.digest(data)
+    return BigInteger(hash).toString(16)
+}
+
+public val CustomProtocolContext: ProtocolContextBuilder.() -> Unit = {
+    sha1Hasher = Sha1Hasher { serverId: String, secretKey: ByteArray, publicKey: ByteArray ->
+        minecraftServerIdHash(serverId, secretKey, publicKey)
+    }
+}
+```