droidsaw-apk

APK and AAB parser plus security analysis for the droidsaw workspace. Decodes the container and walks every layer inside it — AndroidManifest.xml (binary XML), resources.arsc, ELF shared libraries, APK signing blocks (v1 PKCS#7 + v2 / v3 / v4), v4 sidecars, AAB protobuf, baseline profiles, network security config, raw assets — and emits typed Finding records. Pure Rust, BSD-3-Clause.

Signing analysis

signing_info() returns a SigningInfo carrying every scheme present:

Scheme	Parser	Verifier
v1 (JAR / PKCS#7)	signing::SigningInfo::from_pkcs7	full CMS SignedData validation: cert chain, SHA-1 / SHA-256 / SHA-384 / SHA-512 digests, SignedAttributes canonical sort per RFC 5652 §5.4
v2	signing::detect_signing_block	block ID 0x7109871a, digest verification per AOSP, signer cert chain
v3	same path, block ID 0xf05368c0	rotation lineage; v3 → v2 → v1 cert pinning
v4	signing::SigningInfo::attach_v4_sidecar	.apk.idsig Merkle-tree root + signer cert validation
source stamp	source-stamp parser	pubkey binding cross-checked against META-INF/com.android.stamp.{type,source}

signing/verify.rs carries the algorithm dispatcher: RSA (PKCS#1 v1.5 + PSS) over SHA-2 family, ECDSA on P-256 / P-384, DSA-SHA2-256 (block algo 0x0301). RustCrypto verifiers only — keygen and signing paths are not present.

signing/subject.rs parses subject DNs per RFC 4514 with full \X escape handling (,, =, \, +, <, >, #, ;, leading-#, trailing space).

signer_allowlist.rs carries the known-signer table that gates NO_LAUNCHER_INTENT_FILTER reclassification — a deep-link tool signed by a known vendor that ships without a launcher activity does not generate the same finding as an unknown package missing one.

Cryptographic factoring attacks

signing/factoring.rs runs four independent attacks against RSA moduli extracted from the cert chain via signing::extract_key_material:

• ROCA fingerprint (CVE-2017-15361) — Infineon RSALib weak-key detector via the Howgrave-Graham generator-set check; prime table capped at 271.
• Fermat factoring — close-prime modulus detection, configurable iteration bound.
• Wiener's attack — small-d RSA private-exponent vulnerability (acts on (N, e)).
• Batch-GCD — pairwise (batch_gcd) for small key corpora, product-and-remainder tree (batch_gcd_quasilinear) for large corpora.

The factoring stack is independent of the parse path — caller collects KeyMaterial from any number of APKs, then runs the batch attack across the whole set.

Container analysis

Surface	Module	What lands
binary XML decoder	binary_xml.rs	string pool with lossy-flag preservation, element tree with depth + breadth caps
AndroidManifest.xml	manifest.rs	permissions, components, intent filters, deep links, exported-component risk profile
resources.arsc	resources.rs	resource table, ResTable_config parsing, resource shadowing detection, type-id zero rejection
AAB protobuf	aab.rs	base + feature modules, lossy-attribute preservation, XML node recursion-depth cap
baseline profiles	baseline_profile.rs	.prof / .profm parsing with zlib-bomb cap
v4 sidecar	v4_sidecar.rs	.apk.idsig Merkle root
ELF native libs	elf.rs	hardening flags, JNI exports, .init_array, RWX segments, imported symbols, section-header bounds
META-INF/MANIFEST.MF	manifest_mf.rs	base64 attribute decoding with positional-= validation
network security config	network_security.rs	cleartext, pinning, trust anchors
accessibility XML	accessibility.rs	declared accessibility service config
asset entropy	asset_format.rs + apk/asset_entropy_audit.rs	per-asset Shannon entropy + format classification, encrypted-asset fingerprints
obfuscation metrics	obfuscation.rs	R8 / ProGuard / commercial-shielding signal
SBOM / SDK inventory	sdk_inventory.rs, firebase.rs, backend_vendor.rs	bundled-SDK + backend-vendor inventory

ZIP-structural anomalies

apk/anomaly.rs carries the EOCD-class detectors. APPNOTE.TXT §4.4.21–§4.4.24 are the spec gauge:

• DoubleEocd { count, offsets } — more than one End-of-Central-Directory record present.
• SuspiciousEocdComment — EOCD comment field carries unexpected bytes (packer smuggle vector).
• NonZeroDiskNumber — disk-number / disk-with-CD / disk-entries fields non-zero on a single-disk archive.

The fixtures packer_double_eocd, packer_eocd_comment_false_positive, packer_eocd_disk_number_nonzero, and packer_eocd_magic_flood pin the detector behavior.

YARA scanning

yara_scan.rs is the YARA-X (Rust port) integration. Four bundled rule packs under rules/:

• antianalysis.yar
• credential.yar
• crypto.yar
• packer.yar

Surface:

• bundled_rules() / bundled_credential_rules() — embedded rule sets.
• compile_rules_str_restricted() — directive-policy enforcer for caller-supplied rules.
• load_rules_from_dir() — load and compile a custom rule directory.
• scan_bytes_with_scanner_budgeted() — scan with a YaraMatchBudget cap; emits an overflow Finding on truncation.

yara_verify.rs carries provenance-aware false-positive suppression — a match inside a known-vendor SDK or a known-clean library namespace does not generate a finding. detect_library_namespaces uses Aho-Corasick (single pass over the APK buffer).

Architecture

src/
├── apk/         orchestrates per-entry audits (security, anomaly, classifier, JNI, …)
├── signing/     der_walker · factoring · keyinfo · oids · subject · types · verify
├── manifest.rs  AndroidManifest model
├── binary_xml.rs AXML decoder
├── resources.rs ARSC parser
├── elf.rs       ELF parser
├── aab.rs       AAB protobuf
├── yara_scan.rs · yara_verify.rs
└── …flat src/ layout, ~29 files

Test-only feature flags (test-construct-verified, test-support) gate the Verified<T> constructor and Apk::empty_for_testing() so production callers cannot bypass the verifier or the parser entry point.

Correctness

No panics on adversarial input

The crate root denies the panic lint floor on every non-test module:

#![forbid(unsafe_code)]
#![cfg_attr(not(test), deny(
    clippy::unwrap_used, clippy::expect_used, clippy::panic,
    clippy::unreachable, clippy::todo, clippy::arithmetic_side_effects,
    clippy::indexing_slicing, clippy::string_slice, clippy::as_conversions,
    /* …17 more lints… */
))]

See src/lib.rs lines 72–117. Parser paths return typed Err(ApkError) (see src/error.rs); slice access goes through .get(i).ok_or(…), arithmetic through checked_*. Enforced by cargo clippy and libFuzzer runs.

Fuzz coverage

25 libFuzzer targets under fuzz/fuzz_targets/ cover the parser surface:

• Container & format parsers: fuzz_apk_parse, fuzz_xapk_parse, fuzz_aab_decode_xml_node, fuzz_binary_xml, fuzz_arsc, fuzz_baseline_profile, fuzz_v4_sidecar
• Signing & cryptography: fuzz_signing_block, fuzz_detect_signing_block, fuzz_v1_pkcs7, fuzz_subject_dn
• Native & DEX: fuzz_elf_parse, fuzz_apk_dex_extractor, fuzz_dex_sdk_versions_fallback
• Analysis & scanning: fuzz_obfuscation_analyze, fuzz_yara_prefilter, fuzz_yara_scan_with_skip, scan_tail
• Manifest & auxiliary: fuzz_manifest_heuristics, fuzz_parse_multiple
• Anomaly detection: fuzz_v1_orphan_zip_entry
• Differential & auxiliary: parser_differential_arsc, parser_differential_axml, parser_differential_axml_permissive_recovery_combinatorial
• Additional: fuzz_aab_decode_xml_node (AAB XML), fuzz_v1_signing_garbage, fuzz_v4_signed_data_layout via embedded harnesses

A new parser path lands with its fuzz target; corpus seeds tracked under tests/fixtures/adversarial/ are regenerated by tests/regenerate_fuzz_seeds.rs.

Fixture ratchet

tests/fixtures/apk_matrix/ carries 28 named fixtures: aab_base_module, aab_feature_module, debuggable_true, exported_component_heavy, locale_fallback_manifest, manifest_not_first, many_permissions, minimal_valid, multi_native_libs, multidex, packer_aab_slash_bomb, packer_double_eocd, packer_eocd_comment_false_positive, packer_eocd_disk_number_nonzero, packer_eocd_magic_flood, packer_zip_comment_smuggle, unknown_chunk_arsc, v1_signed, v1_signed_signedattrs_unsorted, v1_signed_tampered, v1_signing_garbage, v4_signed, with_baseline_profile, with_dex, with_native_lib, with_raw_manifest. The ratchet is tests/fixture_ratchet.rs; a regressed fixture is a build break.

Verify locally:

cargo test -p droidsaw-apk fixture_ratchet
cargo test -p droidsaw-apk typed_errors

Kani harnesses

Seven proof files under proofs/ with 21 total harnesses (gated #[cfg(kani)]):

• aab_xml_depth.rs — AAB protobuf XML node recursion depth cap.
• der_set_member_cmp.rs — DER SET member comparison correctness.
• manifest_mf_base64_positional.rs — MANIFEST.MF base64 trailing-= position invariant.
• resources_type_id_nonzero.rs — resources.arsc type-id zero rejection.
• restableconfig_parse_bound.rs — ResTable_config parse bytes-consumed bound.
• subject_dn_escape.rs — RFC 4514 §2.4 escape totality on bounded DN inputs.
• v4_signed_data_layout.rs — v4 signing block layout and data bounds verification.

Inputs

Form	Entry point
.apk	Apk::parse(path)
.xapk	Apk::parse_xapk(path)
.aab (Play bundle)	Apk::parse_aab(path)
split APK set (base + config splits)	Apk::parse_multiple(&paths)

DEX bytecode and Hermes bundles found inside APKs are exposed as DexEntry / AabStringRecord; decompilation of those layers lives in droidsaw-dex and droidsaw-hermes.

Output

Apk::parse returns a typed [Apk] struct with parsed manifest, resource table, signing block, ELF metadata, DEX stats, asset entropy, and AAB string pool. Apk::audit_security() emits [Finding] records carrying severity, kind, layer, and detail. No println!, no panics on adversarial input.

Performance

Vec::with_capacity across the APK parse and audit hot paths replaces Vec::new() on collections that grow to a known upper bound (entry count from the central directory, signer count from the signing block). Pre-allocation is gated behind guard::bound_count from droidsaw-common, which cross-checks the count against the slice's byte length before use.

Workspace

This crate is a member of the droidsaw workspace. See the top-level droidsaw crate for the cross-layer pipeline (DEX → Java, Hermes → JavaScript, taint across the React Native bridge).

Public re-exports from droidsaw-common: Finding, Layer, Severity, EntropyPattern, EntropyProfile, entropy_profile, shannon_entropy.

License

BSD-3-Clause.