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 Data structures and file formats 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.

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

• 8. 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)

• 13. 15. Rabin: Fingerprinting by Random Polynomials, 1981