fairplay/README.md (198bf39) - apple-crates · Git

README.md

# fairplay

Clean Rust port of https://github.com/EthanArbuckle/fairplay-decrypt.

A Rust library for the recovered FairPlay page-key algorithm and AES-128-CBC
decryption of protected executable regions. It has no IPA, bundle, filesystem,
Mach-O or device dependencies. The caller supplies the region and metadata.

```rust
use fairplay::{Decryptor, PageRecords, PAGE_SIZE};

fn decrypt_selected_region(
    credential: &[u8; 16],
    sinf: &[u8],
    supf: &[u8],
    original_file_offset: u64,
    encrypted_region: &mut [u8],
) -> Result<usize, Box<dyn std::error::Error>> {
    let decoder = Decryptor::from_sinf(credential, sinf)?;
    let records = PageRecords::from_supf(supf, 0x0100000c, 0)?;
    let first_page = u32::try_from(original_file_offset / PAGE_SIZE as u64)?;
    Ok(decoder.decrypt_region_in_place(encrypted_region, first_page, records)?)
}
```

Cache a `Decryptor` for binaries sharing a SINF. `from_sinf_fields` accepts an
already parsed `SinfFields { name, iviv, private }`; `from_work_key` accepts the
recovered 16-byte masked working key directly. A working key differs from the
saved credential and from an individual page AES key.

`PageRecords::from_bytes` borrows flat 32-byte records from an existing SUPF
parser. `from_supf` provides optional metadata parsing and selects an exact CPU
type/subtype; it supports SUPF 3/5 and tags 507/510/612. SINF decoding handles
both 440-byte and 448-byte private records. No code parses the executable.

## Caller responsibilities

- Pass only the protected region, containing a nonzero whole number of
  4096-byte pages. Decryption resets CBC with each record's IV and removes no
  padding. Extra SUPF records are allowed.
- Compute the first absolute page index from the original complete executable
  offset. For a FAT binary this includes the architecture slice's file base:
  `(slice_base + cryptoff) / 4096`. Do not reset it to zero for a section or slice.
- Match the SUPF architecture and record range to the selected region. Records
  start at the beginning of the protected region. For a later subregion, advance
  both `first_page_index` and the record range via `records.slice(start..end)`.
- Clear the executable's encryption flag only after decryption succeeds, and
  handle bundle metadata, signing and output publication in the consuming code.

For page-at-a-time or parallel processing, `PageRecords::get` provides a
`PageRecord` for `decrypt_page_in_place`. `derive_page_key` is available when
another component owns CBC decryption. The decoder can be shared across threads.

Region decryption allocates no buffers. Size, record count and page-index
validation happens before mutation. Those checks do not authenticate page
ciphertext: the recovered scheme uses CBC without an authentication tag. The
SINF private record's markers detect credentials that fail its decoding check.
Secrets are omitted from Debug and temporary SINF keys/plaintext and AES key
schedules use zeroization.

apple-crates

Add FairPlay crate reimplementing fairplay-decrypt and fix APS