Add HASHAB signing support for iPod Nano 6G/7G

- Add hashab.py module using WASM calcHashAB (from dstaley/hashab)
- Bundle calcHashAB.wasm binary
- Implements 57-byte HASHAB signature for Locations.itdb.cbk
- Required by Nano 6G/7G firmware to accept SQLite databases

src/hashab.py

@@ -0,0 +1,123 @@
+"""
+HASHAB implementation for iPod Nano 6G and 7G.
+
+Uses a WebAssembly module (calcHashAB.wasm) from dstaley/hashab — a clean-room
+reimplementation of Apple's white-box AES signing algorithm.
+
+The output is a 57-byte signature used in two places:
+1. iTunesDB binary mhbd header at offset 0xAB
+2. Locations.itdb.cbk header (first 57 bytes)
+
+Source: https://github.com/dstaley/hashab (The Unlicense)
+"""
+
+import hashlib
+import logging
+from pathlib import Path
+
+logger = logging.getLogger(__name__)
+
+HASHAB_SIZE = 57
+
+# Path to the WASM module (shipped alongside this file)
+_WASM_DIR = Path(__file__).parent / "wasm"
+_WASM_PATH = _WASM_DIR / "calcHashAB.wasm"
+
+# Lazy-loaded WASM engine (expensive to create — reuse across calls)
+_wasm_instance = None
+_wasm_store = None
+
+
+def _get_wasm_instance():
+    """Load the WASM module and return (store, instance)."""
+    global _wasm_instance, _wasm_store
+
+    if _wasm_instance is not None:
+        return _wasm_store, _wasm_instance
+
+    try:
+        import wasmtime
+    except ImportError:
+        raise ImportError(
+            "wasmtime is required for HASHAB (iPod Nano 6G/7G). "
+            "Install with: pip install wasmtime"
+        )
+
+    if not _WASM_PATH.exists():
+        raise FileNotFoundError(
+            f"WASM module not found at {_WASM_PATH}. "
+            "Download calcHashAB.wasm from "
+            "https://github.com/dstaley/hashab/releases/tag/2025-01-04"
+        )
+
+    engine = wasmtime.Engine()
+    store = wasmtime.Store(engine)
+    module = wasmtime.Module.from_file(engine, str(_WASM_PATH))
+    instance = wasmtime.Instance(store, module, [])
+
+    _wasm_store = store
+    _wasm_instance = instance
+
+    logger.debug("HASHAB WASM module loaded from %s", _WASM_PATH)
+    return store, instance
+
+
+def compute_hashab(sha1_digest: bytes, uuid: bytes) -> bytes:
+    """
+    Compute 57-byte HASHAB signature using the WASM module.
+
+    Args:
+        sha1_digest: 20-byte SHA1 hash
+        uuid: 8-byte FireWire GUID from SysInfo
+
+    Returns:
+        57-byte signature
+    """
+    if len(sha1_digest) != 20:
+        raise ValueError(f"SHA1 must be 20 bytes, got {len(sha1_digest)}")
+    if len(uuid) < 8:
+        raise ValueError(f"UUID must be at least 8 bytes, got {len(uuid)}")
+
+    store, instance = _get_wasm_instance()
+    exports = instance.exports(store)
+    memory = exports["memory"]
+    get_input_sha1 = exports["getInputSha1"]
+    get_input_uuid = exports["getInputUuid"]
+    get_output = exports["getOutput"]
+    calculate_hash = exports["calculateHash"]
+
+    sha1_ptr = get_input_sha1(store)
+    uuid_ptr = get_input_uuid(store)
+    output_ptr = get_output(store)
+
+    mem_data = memory.data_ptr(store)
+
+    for i in range(20):
+        mem_data[sha1_ptr + i] = sha1_digest[i]
+
+    for i in range(8):
+        mem_data[uuid_ptr + i] = uuid[i]
+
+    calculate_hash(store)
+
+    result = bytes(mem_data[output_ptr + i] for i in range(HASHAB_SIZE))
+    return result
+
+
+def parse_firewire_guid(sysinfo_path: str) -> bytes:
+    """Parse FirewireGuid from SysInfo file.
+    
+    SysInfo contains lines like:
+        FirewireGuid: 0x000A270024A41BB7
+    
+    Returns:
+        8-byte FireWire GUID
+    """
+    with open(sysinfo_path, 'r') as f:
+        for line in f:
+            if line.startswith('FirewireGuid:'):
+                hex_str = line.split(':')[1].strip()
+                if hex_str.startswith('0x'):
+                    hex_str = hex_str[2:]
+                return bytes.fromhex(hex_str)
+    raise ValueError(f"No FirewireGuid found in {sysinfo_path}")