feat(06-01): add XOR header obfuscation and buffer-based serialization to format.rs

- Add XOR_KEY constant (FORMAT.md Section 9.1)
- Add xor_header_buf() for cyclic 8-byte XOR encode/decode
- Add write_header_to_buf() for buffer-based header serialization
- Add read_header_auto() with XOR bootstrapping detection
- Add serialize_toc() and read_toc_from_buf() helpers for TOC encryption
- Add parse_header_from_buf() internal helper
- Add 6 new unit tests (XOR round-trip, magic change, auto-detect, buf helpers)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/format.rs

@@ -1,5 +1,4 @@
-use std::io::Read;
-use std::io::Write;
+use std::io::{Cursor, Read, Write};
 
 /// Custom magic bytes: leading 0x00 signals binary, remaining bytes are unrecognized.
 pub const MAGIC: [u8; 4] = [0x00, 0xEA, 0x72, 0x63];
@@ -10,6 +9,9 @@ pub const VERSION: u8 = 1;
 /// Fixed header size in bytes.
 pub const HEADER_SIZE: u32 = 40;
 
+/// Fixed 8-byte XOR obfuscation key (FORMAT.md Section 9.1).
+pub const XOR_KEY: [u8; 8] = [0xA5, 0x3C, 0x96, 0x0F, 0xE1, 0x7B, 0x4D, 0xC8];
+
 /// Archive header (40 bytes fixed at offset 0x00).
 #[derive(Debug, Clone)]
 pub struct Header {
@@ -76,6 +78,112 @@ pub fn write_toc_entry(writer: &mut impl Write, entry: &TocEntry) -> anyhow::Res
     Ok(())
 }
 
+/// XOR-obfuscate or de-obfuscate a header buffer in-place.
+///
+/// XOR is its own inverse, so the same function encodes and decodes.
+/// Applies the 8-byte XOR_KEY cyclically across the first 40 bytes of the buffer.
+pub fn xor_header_buf(buf: &mut [u8]) {
+    assert!(buf.len() >= 40, "buffer must be at least 40 bytes");
+    for i in 0..40 {
+        buf[i] ^= XOR_KEY[i % 8];
+    }
+}
+
+/// Serialize the 40-byte archive header into a fixed buffer.
+///
+/// Returns a `[u8; 40]` buffer that can be XOR-obfuscated before writing.
+pub fn write_header_to_buf(header: &Header) -> [u8; 40] {
+    let mut buf = [0u8; 40];
+    buf[0..4].copy_from_slice(&MAGIC);
+    buf[4] = header.version;
+    buf[5] = header.flags;
+    buf[6..8].copy_from_slice(&header.file_count.to_le_bytes());
+    buf[8..12].copy_from_slice(&header.toc_offset.to_le_bytes());
+    buf[12..16].copy_from_slice(&header.toc_size.to_le_bytes());
+    buf[16..32].copy_from_slice(&header.toc_iv);
+    buf[32..40].copy_from_slice(&header.reserved);
+    buf
+}
+
+/// Parse a header from a 40-byte buffer (already validated for magic).
+///
+/// Verifies: version == 1, reserved flags bits 4-7 are zero.
+fn parse_header_from_buf(buf: &[u8; 40]) -> anyhow::Result<Header> {
+    let version = buf[4];
+    anyhow::ensure!(version == VERSION, "Unsupported version: {}", version);
+
+    let flags = buf[5];
+    anyhow::ensure!(
+        flags & 0xF0 == 0,
+        "Unknown flags set: 0x{:02X} (bits 4-7 must be zero)",
+        flags
+    );
+
+    let file_count = u16::from_le_bytes([buf[6], buf[7]]);
+    let toc_offset = u32::from_le_bytes([buf[8], buf[9], buf[10], buf[11]]);
+    let toc_size = u32::from_le_bytes([buf[12], buf[13], buf[14], buf[15]]);
+
+    let mut toc_iv = [0u8; 16];
+    toc_iv.copy_from_slice(&buf[16..32]);
+
+    let mut reserved = [0u8; 8];
+    reserved.copy_from_slice(&buf[32..40]);
+
+    Ok(Header {
+        version,
+        flags,
+        file_count,
+        toc_offset,
+        toc_size,
+        toc_iv,
+        reserved,
+    })
+}
+
+/// Read 40 raw bytes and parse the header, with XOR bootstrapping.
+///
+/// Implements FORMAT.md Section 10 steps 1-3:
+/// 1. Read 40 bytes.
+/// 2. Check magic: if match, parse normally; if no match, XOR and re-check.
+/// 3. Parse header fields from the (possibly de-XORed) buffer.
+pub fn read_header_auto(reader: &mut impl Read) -> anyhow::Result<Header> {
+    let mut buf = [0u8; 40];
+    reader.read_exact(&mut buf)?;
+
+    // Check magic bytes
+    if buf[0..4] != MAGIC {
+        // Attempt XOR de-obfuscation
+        xor_header_buf(&mut buf);
+        anyhow::ensure!(
+            buf[0..4] == MAGIC,
+            "Invalid magic bytes: expected {:02X?}, got {:02X?} (tried XOR de-obfuscation)",
+            MAGIC,
+            &buf[0..4]
+        );
+    }
+
+    parse_header_from_buf(&buf)
+}
+
+/// Serialize all TOC entries to a Vec<u8> buffer.
+///
+/// The buffer can be encrypted before writing to the archive.
+pub fn serialize_toc(entries: &[TocEntry]) -> anyhow::Result<Vec<u8>> {
+    let mut buf = Vec::new();
+    for entry in entries {
+        write_toc_entry(&mut buf, entry)?;
+    }
+    Ok(buf)
+}
+
+/// Parse TOC entries from a byte slice (using a Cursor).
+///
+/// Used for reading TOC from a decrypted buffer.
+pub fn read_toc_from_buf(buf: &[u8], file_count: u16) -> anyhow::Result<Vec<TocEntry>> {
+    let mut cursor = Cursor::new(buf);
+    read_toc(&mut cursor, file_count)
+}
+
 /// Read and parse the 40-byte archive header.
 ///
 /// Verifies: magic bytes, version == 1, reserved flags bits 4-7 are zero.
@@ -396,4 +504,159 @@ mod tests {
         // FORMAT.md worked example: 110 + 109 = 219
         assert_eq!(compute_toc_size(&[entry_hello, entry_data]), 219);
     }
+
+    #[test]
+    fn test_xor_roundtrip() {
+        let header = Header {
+            version: 1,
+            flags: 0x0F,
+            file_count: 2,
+            toc_offset: HEADER_SIZE,
+            toc_size: 256,
+            toc_iv: [0x42; 16],
+            reserved: [0u8; 8],
+        };
+
+        let original_buf = write_header_to_buf(&header);
+        let mut buf = original_buf;
+
+        // XOR once (encode)
+        xor_header_buf(&mut buf);
+        // XOR again (decode) -- must restore original
+        xor_header_buf(&mut buf);
+
+        assert_eq!(buf, original_buf);
+    }
+
+    #[test]
+    fn test_xor_changes_magic() {
+        let header = Header {
+            version: 1,
+            flags: 0x0F,
+            file_count: 2,
+            toc_offset: HEADER_SIZE,
+            toc_size: 256,
+            toc_iv: [0x42; 16],
+            reserved: [0u8; 8],
+        };
+
+        let mut buf = write_header_to_buf(&header);
+
+        // Before XOR, magic is present
+        assert_eq!(&buf[0..4], &MAGIC);
+
+        // After XOR, magic bytes must NOT be recognizable
+        xor_header_buf(&mut buf);
+        assert_ne!(&buf[0..4], &MAGIC);
+    }
+
+    #[test]
+    fn test_read_header_auto_plain() {
+        // Plain (non-XOR'd) header should be parsed correctly
+        let header = Header {
+            version: 1,
+            flags: 0x01,
+            file_count: 3,
+            toc_offset: HEADER_SIZE,
+            toc_size: 330,
+            toc_iv: [0u8; 16],
+            reserved: [0u8; 8],
+        };
+
+        let buf = write_header_to_buf(&header);
+        let mut cursor = Cursor::new(buf.as_slice());
+        let read_back = read_header_auto(&mut cursor).unwrap();
+
+        assert_eq!(read_back.version, 1);
+        assert_eq!(read_back.flags, 0x01);
+        assert_eq!(read_back.file_count, 3);
+    }
+
+    #[test]
+    fn test_read_header_auto_xored() {
+        // XOR'd header should be de-obfuscated and parsed correctly
+        let header = Header {
+            version: 1,
+            flags: 0x0F,
+            file_count: 5,
+            toc_offset: HEADER_SIZE,
+            toc_size: 512,
+            toc_iv: [0xBB; 16],
+            reserved: [0u8; 8],
+        };
+
+        let mut buf = write_header_to_buf(&header);
+        xor_header_buf(&mut buf);
+
+        let mut cursor = Cursor::new(buf.as_slice());
+        let read_back = read_header_auto(&mut cursor).unwrap();
+
+        assert_eq!(read_back.version, 1);
+        assert_eq!(read_back.flags, 0x0F);
+        assert_eq!(read_back.file_count, 5);
+        assert_eq!(read_back.toc_size, 512);
+        assert_eq!(read_back.toc_iv, [0xBB; 16]);
+    }
+
+    #[test]
+    fn test_write_header_to_buf_matches_write_header() {
+        let header = Header {
+            version: 1,
+            flags: 0x01,
+            file_count: 2,
+            toc_offset: HEADER_SIZE,
+            toc_size: 219,
+            toc_iv: [0xAA; 16],
+            reserved: [0u8; 8],
+        };
+
+        // write_header to a Vec
+        let mut vec_buf = Vec::new();
+        write_header(&mut vec_buf, &header).unwrap();
+
+        // write_header_to_buf to a fixed array
+        let arr_buf = write_header_to_buf(&header);
+
+        assert_eq!(vec_buf.as_slice(), &arr_buf[..]);
+    }
+
+    #[test]
+    fn test_serialize_toc_and_read_toc_from_buf() {
+        let entries = vec![
+            TocEntry {
+                name: "file1.txt".to_string(),
+                original_size: 100,
+                compressed_size: 80,
+                encrypted_size: 96,
+                data_offset: 300,
+                iv: [0x11; 16],
+                hmac: [0x22; 32],
+                sha256: [0x33; 32],
+                compression_flag: 1,
+                padding_after: 128,
+            },
+            TocEntry {
+                name: "file2.bin".to_string(),
+                original_size: 200,
+                compressed_size: 180,
+                encrypted_size: 192,
+                data_offset: 524,
+                iv: [0x44; 16],
+                hmac: [0x55; 32],
+                sha256: [0x66; 32],
+                compression_flag: 0,
+                padding_after: 256,
+            },
+        ];
+
+        let buf = serialize_toc(&entries).unwrap();
+        let parsed = read_toc_from_buf(&buf, 2).unwrap();
+
+        assert_eq!(parsed.len(), 2);
+        assert_eq!(parsed[0].name, "file1.txt");
+        assert_eq!(parsed[0].padding_after, 128);
+        assert_eq!(parsed[1].name, "file2.bin");
+        assert_eq!(parsed[1].data_offset, 524);
+        assert_eq!(parsed[1].padding_after, 256);
+    }
 }