---
title: "AES GCM Stream Spec"
---
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# AES GCM Stream file format extension

## Background and Motivation

Iceberg supports a number of data file formats. Two of these formats (Parquet and ORC) have built-in encryption capabilities, that allow to protect sensitive information in the data files. However, besides the data files, Iceberg tables also have metadata files, that keep sensitive information too (e.g., min/max values in manifest files, or bloom filter bitsets in puffin files). Metadata file formats (AVRO, JSON, Puffin) don't have encryption support.

Moreover, with the exception of Parquet, no Iceberg data or metadata file format supports integrity verification, required for end-to-end tamper proofing of Iceberg tables.

This document specifies details of a simple file format extension that adds encryption and tamper-proofing to any existing file format.

## Goals

* Metadata encryption: enable encryption of manifests, manifest lists, snapshots and stats.
* Avro data encryption: enable encryption of data files in tables that use the Avro format.
* Support read splitting: enable seekable decrypting streams that can be used with splittable formats like Avro.
* Tamper proofing of Iceberg data and metadata files.

## Overview

The output stream, produced by a metadata or data writer, is split into equal-size blocks (plus last block that can be shorter). Each block is enciphered (encrypted/signed) with a given encryption key, and stored in a file in the AES GCM Stream format. Upon reading, the stored cipherblocks are verified for integrity; then decrypted and passed to metadata or data readers.

## Encryption algorithm

AES GCM Stream uses the standard AEG GCM cipher, and supports all AES key sizes: 128, 192 and 256 bits.

AES GCM is an authenticated encryption. Besides data confidentiality (encryption), it supports two levels of integrity verification (authentication): of the data (default), and of the data combined with an optional AAD (“additional authenticated data”). An AAD is a free text to be authenticated, together with the data. The structure of AES GCM Stream AADs is described below.

AES GCM requires a unique vector to be provided for each encrypted block. In this document, the unique input to GCM encryption is called nonce (“number used once”). AES GCM Stream encryption uses the RBG-based (random bit generator) nonce construction as defined in the section 8.2.2 of the NIST SP 800-38D document. For each encrypted block, AES GCM Stream generates a unique nonce with a length of 12 bytes (96 bits).

## Format specification

### File structure

The AES GCM Stream files have the following structure

```
Magic BlockLength CipherBlock₁ CipherBlock₂ ... CipherBlockₙ
```

where

- `Magic` is four bytes 0x41, 0x47, 0x53, 0x31 ("AGS1", short for: AES GCM Stream, version 1)
- `BlockLength` is four bytes (little endian) integer keeping the length of the equal-size split blocks before encryption. The length is specified in bytes.
- `CipherBlockᵢ` is the i-th enciphered block in the file, with the structure defined below.

### Cipher Block structure

Cipher blocks have the following structure

| nonce | ciphertext | tag |
|-------|------------|-----|

where

- `nonce` is the AES GCM nonce, with a length of 12 bytes.
- `ciphertext` is the encrypted block. Its length is identical to the length of the block before encryption ("plaintext"). The length of all plaintext blocks, except the last, is `BlockLength` bytes. The last block has a non-zero length <= `BlockLength`.
- `tag` is the AES GCM tag, with a length of 16 bytes.

AES GCM Stream encrypts all blocks by the GCM cipher, without padding. The AES GCM cipher must be implemented by a cryptographic provider according to the NIST SP 800-38D specification. In AES GCM Stream, an input to the GCM cipher is an AES encryption key, a nonce, a plaintext and an AAD (described below). The output is a ciphertext with the length equal to that of plaintext, and a 16-byte authentication tag used to verify the ciphertext and AAD integrity.

### Additional Authenticated Data

The AES GCM cipher protects against byte replacement inside a ciphertext block - but, without an AAD, it can't prevent replacement of one ciphertext block with another (encrypted with the same key). AES GCM Stream leverages AADs to protect against swapping ciphertext blocks inside a file or between files. AES GCM Stream can also protect against swapping full files - for example, replacement of a metadata file with an old version. AADs are built to reflects the identity of a file and of the blocks inside the file.

AES GCM Stream constructs a block AAD from two components: an AAD prefix - a string provided by Iceberg for the file (with the file ID), and an AAD suffix - the block sequence number in the file, as an int in a 4-byte little-endian form. The block AAD is a direct concatenation of the prefix and suffix parts.

### File length

An attacker can delete a few last blocks in an encrypted file. To detect the attack, the reader implementations of the AES GCM Stream must use the file length value taken from a trusted source (such as a signed file metadata), and not from the file system.
