.. include:: ../global.rst.inc .. _chunkers: Chunkers ======== Borg splits file contents into chunks and deduplicates them: a chunk that was already stored (same content, same id) is not stored again. The *chunker* decides where chunk boundaries are placed. Most chunkers are *content-defined*: boundaries depend on the data itself, so when a file changes a little (bytes inserted or removed somewhere), boundaries after the change re-align and most chunks are recognized as already stored. This document describes the available chunkers and their trade-offs, twice: first for users who just want a good choice for their situation, then in technical depth for readers with a cryptography background. Choosing a chunker ------------------ What the choice affects +++++++++++++++++++++++ * **Speed** - how fast ``borg create`` can process data that is not already deduplicated. All chunkers are fast (hundreds of MB/s to over 1 GB/s on one CPU core); for many setups, disk or network is the limit, not the chunker. * **Deduplication** - how well changed files deduplicate against their previous versions. All content-defined chunkers here are equally good at this; only the "fixed" chunker is different (see below). * **Privacy of chunk sizes** - what somebody who can *see* your (encrypted) repository can learn from the sizes of the stored chunks. The chunk-size fingerprinting threat ++++++++++++++++++++++++++++++++++++ Borg encrypts chunk contents, but whoever stores your repository (a cloud provider, a rented server, anyone who can read the repository files) can still see the *sizes* of your chunks. The sequence of chunk sizes of a file works like a fingerprint: somebody who has the same file (a leaked document, a known public file, ...) can chunk it the same way and check whether the size pattern occurs in your repository - "does this person have a copy of X?". Chunkers differ in how well they resist this: * ``buzhash`` and ``buzhash64`` / ``fastcdc`` place boundaries using a rolling hash. ``buzhash64`` and ``fastcdc`` mix a secret key into that hash, which helps, but research published in 2025 showed that observing enough chunk boundaries of *known* data allows recovering such keys - so against a capable adversary who can inject or guess file contents, these chunkers do not reliably hide the fingerprints. * ``rabin-aes``, ``goldilocks-aes`` and ``toeplitz-aes`` place boundaries using real cryptography (AES): without the key, chunk boundaries are indistinguishable from random and the observed sizes yield no usable information about the chunking secrets. This protection has a price: they run at roughly half the speed of ``fastcdc`` (still >600 MB/s per core on modern hardware). Note: independently of the chunker, an attacker who *already knows* an exact file and its chunking can always check whether identical chunks exist - deduplication requires that identical content deduplicates. Also, borg can additionally obfuscate stored chunk sizes (see ``--compression obfuscate``), which is complementary to a fingerprinting-resistant chunker. The chunkers at a glance ++++++++++++++++++++++++ ============== ========= ============ =========================== ===================================== name speed dedup fingerprinting resistance notes ============== ========= ============ =========================== ===================================== fixed fastest positional n/a (no content dependency) fixed block size; for disk images fastcdc fastest good improved, but not sound keyed, window-less Gear hash buzhash64 fast good improved, but not sound keyed, normalized chunking buzhash fast good weak (seed only) borg 1.x compatible dedup toeplitz-aes medium good strong (AES-based) best collision bound, secret table rabin-aes medium good strong (AES-based) secret polynomial + AES goldilocks-aes slower good strong (AES-based) reference construction, comparison ============== ========= ============ =========================== ===================================== Recommendations +++++++++++++++ * **You keep deduplicating against a repo created with borg 1.x:** use ``buzhash`` (only it is dedup-compatible with borg 1.x). * **General use, maximum speed:** use ``fastcdc`` (fastest) or ``buzhash64``. * **Your repository is stored somewhere you do not fully trust and you care about the fingerprinting threat above:** use ``toeplitz-aes`` (or ``rabin-aes``; both are strong here, ``toeplitz-aes`` has the edge in analysis and equal speed). * **Raw disk / VM images, fixed-layout data:** use ``fixed`` with a block size matching the image's internal structure; content-defined chunking gains little there and ``fixed`` is the fastest option. * ``goldilocks-aes`` exists mainly as a scientific comparison baseline; it is as secure as the other AES chunkers but slower - prefer ``toeplitz-aes``. All content-defined chunkers accept min/max chunk size exponents and a mask controlling the average chunk size, and (except ``buzhash``) a normalized chunking level that tightens the chunk size distribution; see :ref:`data-structures` for the exact parameter formats. Technical background -------------------- This section assumes familiarity with universal hashing and PRFs. Model and the broken class ++++++++++++++++++++++++++ All content-defined chunkers here are fixed-size-window chunkers (FSWC): at stream position ``i`` a decision function looks at (at most) the last ``w`` bytes and decides "cut here or not" (plus min/max size clamping and, for most, FastCDC-style normalized chunking with a strict/loose mask pair). The adversary observes cut positions - equivalently, chunk sizes - possibly for partially known or chosen plaintext. The classic approach cuts directly on bits of a rolling hash ``H_K(window)``: buzhash (32-bit cyclic polynomial, borg-1.x-compatible seed twist), ``buzhash64`` (CSPRNG-keyed balanced table, 4095-byte window), ``fastcdc`` (CSPRNG-keyed Gear table, window-less: bytes age out of the 64-bit state by left shift, so the effective window is <= 64 bytes with triangular bit influence, which is why its cut mask uses the high bits). All these hashes are GF(2)-linear (or affine) in their key material, so every observed boundary is an algebraic constraint on the key; "Breaking and Fixing Content-Defined Chunking" (Truong, Merz, Scarlata, Günther, Paterson, CCS 2025, eprint 2025/558) and "Chunking Attacks on File Backup Services" (eprint 2025/532) give practical key-recovery attacks against this entire class as deployed in several backup tools. Keying the tables raises the bar but is not sound; no amount of masking fixes the output channel. The UHF-then-PRF construction +++++++++++++++++++++++++++++ ``rabin-aes``, ``goldilocks-aes`` and ``toeplitz-aes`` implement the provably secure construction from eprint 2025/558 ("Chk-PHTE"): a rolling *universal hash* compresses the 64-byte window into a 64-bit digest, then AES-128 with an independent secret key is applied to the digest (as a little-endian u64 in bytes 0..7 of the block, zero padding), and the cut decision looks only at the AES output: cut iff the low ``mask_bits`` bits of the first 8 ciphertext bytes (LE) are zero. Both secrets are derived from the repository's id key with a per-chunker domain. Since AES output bits are pseudorandom, the only property required of the UHF is ε-almost-universality: for fixed ``x != y``, ``Pr_K[H_K(x) = H_K(y)] <= ε``. Colliding windows necessarily receive equal decisions - that is the *only* residual leakage - and the security bound degrades with (number of processed positions)² * ε, so ε directly determines how much data one chunker key can process before the guarantee becomes vacuous. Equal-content chunks still produce equal sizes; that is inherent to deduplication. The window is fixed at 64 bytes for all three (hence ``chunk_min_exp >= 6``: the roll needs 64 bytes of in-chunk history at the first legal cut position). Digest streams do not depend on where cuts happen, so digests can be computed ahead and encrypted in batches: each kernel has three bit-identical code paths (batched OpenSSL EVP AES-128-ECB; arm64 crypto-extension intrinsics; x86-64 AES-NI) with two even/odd rolling lanes and groups of 8 interleaved AES blocks on the hardware paths. The three universal hashes ++++++++++++++++++++++++++ ``toeplitz-aes`` Tabulated LFSR-based Toeplitz hashing (Krawczyk, CRYPTO '94): ``digest = sum_j x^(63-j) * T[b_j]`` over GF(2)[x] mod a *fixed public* irreducible ``P = x^64 + x^4 + x^3 + x + 1``; the secret is the uniform table ``T`` of 256 u64 (2 KiB). For ``x != y`` the difference is ``sum_v c_v * T[v]`` where some ``c_v`` is a nonzero polynomial of degree < 64 - invertible mod the irreducible degree-64 ``P`` - so the sum is uniform: **ε = 2^-64 exactly**, optimal for a 64-bit digest and unconditional (nothing is sampled; the bound does not depend on choosing a good ``P``). The roll is a shift plus a branchless masked XOR with only plaintext-indexed loads. The fixed 64-byte window is essential: the argument needs 64 *distinct* powers of ``x`` of degree < 64. The tempting simplification - plain rotations instead of the LFSR step, i.e. keyed buzhash - fails exactly here: rotations satisfy ``R^64 = I`` (over GF(2), ``x^64 - 1 = (x+1)^64``), coefficient sums can collapse to rank 1, and e.g. two all-same-byte windows collide with probability 1/2. The same algebra is why the classic buzhash window is 4095 and not 4096: with the window a multiple of the word size, a uniform run hashes to a constant independent of its byte value. ``rabin-aes`` Rabin fingerprint over GF(2)[x] mod a *secret, random, irreducible* ``P`` of degree 64 (top bit implicit; table-driven rolling). Two distinct 64-byte windows differ by a polynomial of degree <= 511, which has at most 7 irreducible degree-64 factors out of ~2^58 candidates: ε ≈ 2^-55, probabilistic over the sampled ``P``. Key material: 8 bytes (the polynomial), found by rejection sampling with Rabin's irreducibility test. Note: the reduction table is indexed by digest bits, i.e. there is a secret-dependent memory access in the hot loop. ``goldilocks-aes`` The paper's reference UHF: polynomial evaluation hash over GF(p), p = 2^64 - 2^32 + 1, at a secret uniform point ``K``, byte-wise Horner over the window. The difference of two distinct windows is a nonzero polynomial in ``K`` of degree <= 63: ε <= 63/p ≈ 2^-58. Key material: 8 bytes. All table indices are plaintext bytes (no secret-dependent loads); the rolling multiply keeps every state canonical since the state feeds AES verbatim. Verified bit-equivalent (states and ciphertexts) to the authors' artifact implementation. The field multiply needs the full 128-bit product of two 64-bit values: on 64-bit platforms that is the compiler's ``__uint128_t``, on 32-bit ones (armhf, i386, ...) a portable fallback built from 32x32 -> 64 multiplies. Both compute the same product, so cut points are identical everywhere - 32-bit platforms just run this chunker (and only this one) slower. Constructions considered and rejected: Gear as the UHF (triangular aging gives ε ≈ 1/2 via the oldest byte), hardware CRC (fixed public polynomial, unkeyable), NH/UMAC-style multilinear hashes (position-keyed, not rollable), carryless-multiply Rabin via PMULL (measured slower than the table kernel on Apple Silicon, and it competes with AES for the vector pipes). Performance and limits ++++++++++++++++++++++ Measured on an Apple M-series core (1 GiB random data, parameters 19,23,21, best of 10 runs; "nc2" = normalized chunking level 2; EVP = portable OpenSSL path): ============== ======== ========= ========== chunker hw MB/s nc2 MB/s EVP nc2 ============== ======== ========= ========== fastcdc 1261 1324 n/a buzhash64 974 1036 n/a toeplitz-aes 666 713 450 rabin-aes 661 698 434 goldilocks-aes 364 394 304 ============== ======== ========= ========== Chunk-size distributions, dedup behavior and shift resilience of all content-defined chunkers are statistically identical (the AES output is uniform). Notably, ``toeplitz-aes`` and ``rabin-aes`` tie on the hardware path although the Toeplitz roll is cheaper: at ~700 MB/s the grouped-AES hardware path is AES/transfer-bound, not roll-chain-bound (the cheaper roller shows only on the EVP path). Future speedups must therefore come from the AES side, not the UHF. With a 64-bit digest, the (positions)² * ε proof bound stays meaningful up to roughly tens of GiB per chunker key (best for ``toeplitz-aes``); beyond that no attack is known, but the guarantee is heuristic. The upgrade path is a wider digest (e.g. two independent tables/keys filling the full AES block), at roughly doubled rolling cost. References ++++++++++ * K. T. Truong, S.-P. Merz, M. Scarlata, F. Günther, K. G. Paterson: *Breaking and Fixing Content-Defined Chunking*, ACM CCS 2025, https://eprint.iacr.org/2025/558 * B. Alexeev, C. Percival, Y. X. Zhang: *Chunking Attacks on File Backup Services using Content-Defined Chunking*, https://eprint.iacr.org/2025/532 * H. Krawczyk: *LFSR-based Hashing and Authentication*, CRYPTO '94 * D. Lemire, O. Kaser: *Faster 64-bit universal hashing using carry-less multiplications* (CLHASH; source of the fixed GF(2^64) polynomial) * M. O. Rabin: *Fingerprinting by Random Polynomials*, 1981