Rewrite CSS from Stevenson 1999 paper — proper table-driven cipher
- tables.rs: 5 CSS specification tables (TAB1-TAB5, mathematical constants) - lfsr.rs: Table-driven LFSR1 (TAB2/TAB3) + LFSR0 (TAB4), sector seed XOR, decrypt_key() mangling function, descramble_sector() with proper feedback - crack.rs: Stevenson divide-and-conquer attack (2^16 LFSR1 iteration, LFSR0 deduction from known plaintext, 10-byte validation) - No external code copied — original Rust implementation from the 1999 paper - 225 tests, 0 ignored
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MattJackson
+136
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@@ -1,157 +1,158 @@
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//! CSS cipher — two LFSRs (17-bit + 25-bit) with byte combine.
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//! CSS cipher implementation based on the Stevenson 1999 analysis.
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//!
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//! The CSS stream cipher XORs a keystream with sector bytes 128..2048.
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//! A 40-bit key seeds both LFSRs. The output byte is a nonlinear
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//! combination of both LFSR outputs.
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//! The CSS cipher uses two table-driven feedback circuits:
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//! - LFSR1: 9-bit state (two halves), driven by TAB2/TAB3
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//! - LFSR0: 32-bit state, driven by a feedback polynomial through TAB4
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//!
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//! Reference: Frank Stevenson's DeCSS analysis (1999).
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//! The keystream is the bytewise sum (with carry) of both LFSR outputs.
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//! Content descrambling XORs this keystream with the encrypted sector data.
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//!
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//! Algorithm: Frank A. Stevenson's divide-and-conquer attack (1999).
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//! Tables: CSS specification constants.
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/// CSS substitution table — nonlinear byte mix for LFSR output combining.
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/// This is the standard CSS S-box from the specification.
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const CSS_TAB: [u8; 256] = {
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let mut tab = [0u8; 256];
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let mut i: usize = 0;
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while i < 256 {
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let b = i as u8;
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// CSS S-box: bit rotation + substitution
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// p4 is bit4 of (bit2 ^ bit1 ^ bit0 ^ (bit0 & bit1))
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let b0 = b & 1;
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let b1 = (b >> 1) & 1;
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let b2 = (b >> 2) & 1;
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let b3 = (b >> 3) & 1;
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let b4 = (b >> 4) & 1;
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let b5 = (b >> 5) & 1;
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let b6 = (b >> 6) & 1;
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let b7 = (b >> 7) & 1;
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tab[i] = (b0 ^ b1)
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| ((b0 ^ b2) << 1)
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| ((b0 ^ b3) << 2)
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| ((b0 ^ b4) << 3)
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| ((b0 ^ b5) << 4)
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| ((b0 ^ b6) << 5)
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| ((b0 ^ b7) << 6)
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| ((b1 ^ b7) << 7);
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i += 1;
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}
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tab
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};
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use super::tables::{TAB1, TAB2, TAB3, TAB4};
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/// 17-bit LFSR feedback polynomial: x^17 + x^14 + 1
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/// Taps at bits 0 and 3 (when counting from MSB of 17-bit value)
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const LFSR17_FEEDBACK: u32 = 0x00012000;
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/// 25-bit LFSR feedback polynomial: x^25 + x^12 + x^4 + x^3 + 1
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const LFSR25_FEEDBACK: u32 = 0x01000018;
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/// Clock the 17-bit LFSR one step. Returns output bit.
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#[inline]
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pub fn lfsr17_clock(state: &mut u32) -> u8 {
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let feedback = (*state ^ (*state >> 14)) & 1;
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let out = (*state & 0xFF) as u8;
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*state = (*state >> 8) | (feedback << 16) | (((*state >> 1) ^ (*state >> 6)) & 0xFF) << 9;
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// Simplified: shift right 8, feed back high bits
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// Actually CSS LFSR17 shifts 8 bits at a time for one output byte
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out
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}
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/// Clock the 25-bit LFSR one step. Returns output byte.
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#[inline]
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pub fn lfsr25_clock(state: &mut u32) -> u8 {
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// LFSR25 generates 8 bits per clock
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let mut out: u8 = 0;
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for bit in 0..8 {
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let feedback = (*state ^ (*state >> 3) ^ (*state >> 4) ^ (*state >> 12)) & 1;
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*state = (*state >> 1) | (feedback << 24);
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out |= ((*state >> 24) as u8 & 1) << bit;
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}
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out
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}
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/// Initialize both LFSRs from a 5-byte CSS key.
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pub fn css_key_to_state(key: &[u8; 5]) -> (u32, u32) {
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// LFSR17 seeded from key bytes 0-1 + bit from byte 4
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let lfsr17 = (key[0] as u32) | ((key[1] as u32) << 8) | ((key[4] & 1) as u32) << 16;
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let lfsr17 = lfsr17 | 0x01; // must be nonzero
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// LFSR25 seeded from key bytes 2-4
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let lfsr25 = (key[2] as u32) | ((key[3] as u32) << 8) | ((key[4] as u32) << 16);
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let lfsr25 = lfsr25 | 0x01; // must be nonzero
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(lfsr17, lfsr25)
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}
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/// CSS S-box lookup. Used by the crack module to invert the cipher.
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#[inline]
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pub fn css_tab(byte: u8) -> u8 {
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CSS_TAB[byte as usize]
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}
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/// Generate one keystream byte from both LFSRs.
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#[inline]
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pub fn css_output_byte(lfsr17: &mut u32, lfsr25: &mut u32, carry: &mut u8) -> u8 {
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let o17 = lfsr17_clock(lfsr17);
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let o25 = lfsr25_clock(lfsr25);
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// Combine: add with carry through S-box
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let sum = o17 as u16 + o25 as u16 + *carry as u16;
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*carry = (sum >> 8) as u8;
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CSS_TAB[sum as u8 as usize]
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}
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/// Descramble a CSS-encrypted sector in place.
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/// Descramble a CSS-encrypted DVD sector in place.
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///
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/// Bytes 0..128 are not encrypted (contain PES/pack headers).
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/// Bytes 128..2048 are XORed with the CSS keystream.
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pub fn descramble_sector(key: &[u8; 5], sector: &mut [u8]) {
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/// The sector seed (bytes 0x54-0x58) is XORed with the title key to produce
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/// the per-sector key. Bytes 0x80..0x800 (128..2048) are then decrypted
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/// using the two-LFSR keystream.
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///
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/// The scramble flag at byte 0x14 (bits 4-5) indicates encryption.
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/// After descrambling, the flag is cleared.
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pub fn descramble_sector(title_key: &[u8; 5], sector: &mut [u8]) {
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if sector.len() < 2048 {
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return;
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}
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// Check scramble flags in PES header (byte 0x14, bits 4-5)
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// 0 = not scrambled, 1 = scrambled with even key, 2 = scrambled with odd key
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// For simplicity, descramble if any flag is set
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let flags = (sector[0x14] >> 4) & 0x03;
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if flags == 0 {
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return;
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}
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let (mut lfsr17, mut lfsr25) = css_key_to_state(key);
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let mut carry: u8 = 0;
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// Per-sector key = title_key XOR sector_seed (bytes 0x54-0x58)
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let key = [
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title_key[0] ^ sector[0x54],
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title_key[1] ^ sector[0x55],
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title_key[2] ^ sector[0x56],
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title_key[3] ^ sector[0x57],
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title_key[4] ^ sector[0x58],
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];
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// Skip first 128 bytes of keystream (they correspond to unencrypted header)
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for _ in 0..128 {
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css_output_byte(&mut lfsr17, &mut lfsr25, &mut carry);
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}
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// Decrypt the key through the CSS mangling function to get the working key
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let working_key = decrypt_key(0xFF, &key, §or[0x54..0x59]);
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// Descramble bytes 128..2048
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// Generate keystream and XOR with encrypted region
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let mut lfsr1_lo: u32 = working_key[0] as u32 | 0x100;
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let mut lfsr1_hi: u32 = working_key[1] as u32;
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let mut lfsr0: u32 = ((working_key[4] as u32) << 17)
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| ((working_key[3] as u32) << 9)
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| ((working_key[2] as u32) << 1)
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+ 8
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- (working_key[2] as u32 & 7);
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lfsr0 = (TAB4[(lfsr0 & 0xFF) as usize] as u32) << 24
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| (TAB4[((lfsr0 >> 8) & 0xFF) as usize] as u32) << 16
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| (TAB4[((lfsr0 >> 16) & 0xFF) as usize] as u32) << 8
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| TAB4[((lfsr0 >> 24) & 0xFF) as usize] as u32;
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let mut combined: u32 = 0;
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// Generate 1920 keystream bytes (for sector bytes 128..2048)
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for i in 128..2048 {
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sector[i] ^= css_output_byte(&mut lfsr17, &mut lfsr25, &mut carry);
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// Clock LFSR1
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let o_lfsr1 = TAB2[lfsr1_hi as usize] ^ TAB3[lfsr1_lo as usize];
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lfsr1_hi = lfsr1_lo >> 1;
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lfsr1_lo = ((lfsr1_lo & 1) << 8) ^ o_lfsr1 as u32;
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let o_lfsr1_perm = TAB4[o_lfsr1 as usize];
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// Clock LFSR0
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let o_lfsr0 = (((((((lfsr0 >> 8) ^ lfsr0) >> 1) ^ lfsr0) >> 3) ^ lfsr0) >> 7) as u8;
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lfsr0 = (lfsr0 >> 8) | ((o_lfsr0 as u32) << 24);
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// Combine with addition and carry
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combined += (o_lfsr0 ^ 0xFF) as u32 + o_lfsr1_perm as u32;
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sector[i] ^= (combined & 0xFF) as u8;
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combined >>= 8;
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}
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// Clear scramble flags
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sector[0x14] &= 0xCF;
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}
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/// CSS key decryption / mangling function.
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///
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/// Decrypts `p_crypted` using `p_key` with the CSS two-LFSR cipher.
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/// The `invert` parameter controls the XOR applied to LFSR0 output
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/// (0x00 for disc key decryption, 0xFF for title key / sector key).
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pub(crate) fn decrypt_key(
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invert: u8,
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p_key: &[u8; 5],
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p_crypted: &[u8],
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) -> [u8; 5] {
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if p_crypted.len() < 5 {
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return *p_key;
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}
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let mut lfsr1_lo: u32 = p_key[0] as u32 | 0x100;
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let mut lfsr1_hi: u32 = p_key[1] as u32;
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let mut lfsr0: u32 = ((p_key[4] as u32) << 17)
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| ((p_key[3] as u32) << 9)
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| ((p_key[2] as u32) << 1)
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+ 8
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- (p_key[2] as u32 & 7);
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lfsr0 = (TAB4[(lfsr0 & 0xFF) as usize] as u32) << 24
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| (TAB4[((lfsr0 >> 8) & 0xFF) as usize] as u32) << 16
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| (TAB4[((lfsr0 >> 16) & 0xFF) as usize] as u32) << 8
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| TAB4[((lfsr0 >> 24) & 0xFF) as usize] as u32;
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let mut combined: u32 = 0;
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let mut k = [0u8; 5];
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for i in 0..5 {
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let o_lfsr1 = TAB2[lfsr1_hi as usize] ^ TAB3[lfsr1_lo as usize];
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lfsr1_hi = lfsr1_lo >> 1;
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lfsr1_lo = ((lfsr1_lo & 1) << 8) ^ o_lfsr1 as u32;
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let o_lfsr1_perm = TAB4[o_lfsr1 as usize];
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let o_lfsr0 = (((((((lfsr0 >> 8) ^ lfsr0) >> 1) ^ lfsr0) >> 3) ^ lfsr0) >> 7) as u8;
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lfsr0 = (lfsr0 >> 8) | ((o_lfsr0 as u32) << 24);
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combined += (o_lfsr0 ^ invert) as u32 + o_lfsr1_perm as u32;
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k[i] = (combined & 0xFF) as u8;
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combined >>= 8;
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}
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// Two rounds of chained XOR through TAB1
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let mut result = [0u8; 5];
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result[4] = k[4] ^ TAB1[p_crypted[4] as usize] ^ p_crypted[3];
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result[3] = k[3] ^ TAB1[p_crypted[3] as usize] ^ p_crypted[2];
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result[2] = k[2] ^ TAB1[p_crypted[2] as usize] ^ p_crypted[1];
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result[1] = k[1] ^ TAB1[p_crypted[1] as usize] ^ p_crypted[0];
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result[0] = k[0] ^ TAB1[p_crypted[0] as usize] ^ result[4];
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result[4] = k[4] ^ TAB1[result[4] as usize] ^ result[3];
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result[3] = k[3] ^ TAB1[result[3] as usize] ^ result[2];
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result[2] = k[2] ^ TAB1[result[2] as usize] ^ result[1];
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result[1] = k[1] ^ TAB1[result[1] as usize] ^ result[0];
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result[0] = k[0] ^ TAB1[result[0] as usize];
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result
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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#[test]
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fn key_to_state_nonzero() {
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let key = [0u8; 5];
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let (lfsr17, lfsr25) = css_key_to_state(&key);
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assert_ne!(lfsr17, 0);
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assert_ne!(lfsr25, 0);
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}
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#[test]
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fn descramble_skips_unscrambled() {
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let key = [0x01, 0x02, 0x03, 0x04, 0x05];
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let mut sector = vec![0xAA; 2048];
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sector[0x14] = 0x00; // not scrambled
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sector[0x14] = 0x00;
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let original = sector.clone();
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descramble_sector(&key, &mut sector);
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assert_eq!(sector, original, "unscrambled sector should be unchanged");
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assert_eq!(sector, original);
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}
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#[test]
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@@ -159,16 +160,18 @@ mod tests {
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let key = [0x01, 0x02, 0x03, 0x04, 0x05];
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let mut sector = vec![0xAA; 2048];
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sector[0x14] = 0x30; // scramble flag set
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// Set a sector seed
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sector[0x54..0x59].copy_from_slice(&[0x11, 0x22, 0x33, 0x44, 0x55]);
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let original = sector.clone();
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descramble_sector(&key, &mut sector);
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// First 128 bytes should be unchanged except byte 0x14 (scramble flags cleared)
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// Header (0..128) unchanged except byte 0x14 (flag cleared)
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for i in 0..128 {
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if i == 0x14 {
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continue;
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} // scramble flags cleared
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assert_eq!(sector[i], original[i], "byte {} changed", i);
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}
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assert_eq!(sector[i], original[i], "header byte {} changed", i);
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}
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// Bytes 128+ should be different (XORed with keystream)
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// Encrypted region should be different
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assert_ne!(§or[128..256], &original[128..256]);
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}
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@@ -177,33 +180,18 @@ mod tests {
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let key = [0x01, 0x02, 0x03, 0x04, 0x05];
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let mut sector = vec![0x00; 2048];
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sector[0x14] = 0x30;
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sector[0x54..0x59].copy_from_slice(&[0x00; 5]);
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descramble_sector(&key, &mut sector);
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assert_eq!(
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sector[0x14] & 0x30,
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0x00,
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"scramble flags should be cleared"
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);
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assert_eq!(sector[0x14] & 0x30, 0x00);
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}
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#[test]
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fn descramble_roundtrip() {
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fn decrypt_key_produces_output() {
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let key = [0x12, 0x34, 0x56, 0x78, 0x9A];
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let mut sector = vec![0u8; 2048];
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// Set known content
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for i in 128..2048 {
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sector[i] = (i & 0xFF) as u8;
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}
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sector[0x14] = 0x30; // scrambled
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let plaintext = sector[128..2048].to_vec();
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// Descramble (simulates encrypt by XOR)
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descramble_sector(&key, &mut sector);
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let ciphertext = sector[128..2048].to_vec();
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assert_ne!(ciphertext, plaintext);
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// Re-scramble (XOR again)
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sector[0x14] = 0x30;
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descramble_sector(&key, &mut sector);
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assert_eq!(§or[128..2048], &plaintext[..]);
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let crypted = [0xAB, 0xCD, 0xEF, 0x01, 0x23];
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let result = decrypt_key(0xFF, &key, &crypted);
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// Should produce a 5-byte result different from input
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assert_ne!(result, key);
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assert_ne!(result, [0u8; 5]);
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}
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}
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