storage_xts/

lib.rs

// Copyright 2026 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

use cipher::inout::InOut;
use cipher::typenum::consts::U16;
use cipher::{
    Array, BlockCipherDecBackend, BlockCipherDecClosure, BlockCipherEncBackend,
    BlockCipherEncClosure, BlockSizeUser,
};
use static_assertions::assert_cfg;
use storage_ptr_slice::{MutPtrByteSlice, PtrByteSlice};
use zerocopy::{FromBytes, Immutable, IntoBytes, KnownLayout};

// This assumes little-endianness which is likely to always be the case.
assert_cfg!(target_endian = "little");

#[derive(IntoBytes, KnownLayout, FromBytes, Immutable, Debug, Clone, Copy, PartialEq, Eq)]
#[repr(C)]
pub struct Tweak(pub u128);

impl Tweak {
    pub fn new(val: u128) -> Self {
        Self(val)
    }

    fn update(&mut self) {
        self.0 = (self.0 << 1) ^ ((self.0 as i128 >> 127) as u128 & 0x87);
    }
}

/// To be used with encrypt|decrypt_with_backend.
pub struct XtsProcessor<'a, 'b> {
    tweak: Tweak,
    src: PtrByteSlice<'a>,
    dst: MutPtrByteSlice<'b>,
}

fn xts_encrypt_chunk<B: BlockCipherEncBackend<BlockSize = U16>>(
    backend: &B,
    mut val: u128,
    tweak: &Tweak,
) -> u128 {
    // XOR plaintext with tweak.
    val ^= tweak.0;

    let arr: &mut Array<u8, U16> = val.as_mut_bytes().try_into().unwrap();
    backend.encrypt_block(InOut::from(arr));

    // XOR ciphertext with tweak.
    val ^ tweak.0
}

/// Internal only. Assumes all fields have been validated.
fn xts_encrypt_buffer<B: BlockCipherEncBackend<BlockSize = U16>>(
    backend: &B,
    src: PtrByteSlice<'_>,
    mut dst: MutPtrByteSlice<'_>,
    tweak: &mut Tweak,
) {
    debug_assert!(src.as_ptr().cast::<u128>().is_aligned());
    debug_assert!(dst.as_ptr().cast::<u128>().is_aligned());
    let src_chunks = src.iter_as::<u128>();
    let dst_chunks = dst.iter_as_mut::<u128>();

    for (src_chunk, dst_chunk) in src_chunks.zip(dst_chunks) {
        let val = xts_encrypt_chunk(backend, src_chunk.read(), &tweak);
        dst_chunk.write(val);
        tweak.update();
    }
}

fn xts_decrypt_chunk<B: BlockCipherDecBackend<BlockSize = U16>>(
    backend: &B,
    mut val: u128,
    tweak: &Tweak,
) -> u128 {
    // XOR ciphertext with tweak.
    val ^= tweak.0;

    let arr: &mut Array<u8, U16> = val.as_mut_bytes().try_into().unwrap();
    backend.decrypt_block(InOut::from(arr));

    // XOR plaintext with tweak.
    val ^ tweak.0
}

/// Internal only. Assumes all fields have been validated.
fn xts_decrypt_buffer<B: BlockCipherDecBackend<BlockSize = U16>>(
    backend: &B,
    src: PtrByteSlice<'_>,
    mut dst: MutPtrByteSlice<'_>,
    tweak: &mut Tweak,
) {
    debug_assert!(src.as_ptr().cast::<u128>().is_aligned());
    debug_assert!(dst.as_ptr().cast::<u128>().is_aligned());
    let src_chunks = src.iter_as::<u128>();
    let dst_chunks = dst.iter_as_mut::<u128>();

    for (src_chunk, dst_chunk) in src_chunks.zip(dst_chunks) {
        let val = xts_decrypt_chunk(backend, src_chunk.read(), &tweak);
        dst_chunk.write(val);
        tweak.update();
    }
}

impl<'a, 'b> XtsProcessor<'a, 'b> {
    // `tweak` should be encrypted. `src` and `dst` must have the same length and be 16 byte
    // aligned.
    pub fn new(tweak: Tweak, src: PtrByteSlice<'a>, mut dst: MutPtrByteSlice<'b>) -> Self {
        assert_eq!(src.len(), dst.len(), "Source and destination lengths must match");
        assert!(src.as_ptr().cast::<u128>().is_aligned(), "src must be 16 byte aligned");
        assert!(dst.as_ptr().cast::<u128>().is_aligned(), "dst must be 16 byte aligned");
        dst.zero_no_rfo();
        Self { tweak, src, dst }
    }

    /// Creates an XtsProcessor for in-place operation on a single buffer.
    pub fn new_in_place(tweak: Tweak, buf: MutPtrByteSlice<'a>) -> XtsProcessor<'a, 'a> {
        assert!(buf.as_ptr().cast::<u128>().is_aligned(), "buf must be 16 byte aligned");
        let len = buf.len();
        let ptr = buf.as_ptr_slice().as_ptr();
        // SAFETY: We are creating a PtrByteSlice that aliases with the MutPtrByteSlice.
        // This is safe because PtrByteSlice only allows read access, and we control the
        // execution in `call` to ensure we don't violate safety (we read a block, then write it,
        // so we don't have concurrent read/write on the same sub-block).
        let src = unsafe { PtrByteSlice::new(std::ptr::slice_from_raw_parts(ptr, len)) };
        XtsProcessor { tweak, src, dst: buf }
    }
}

impl BlockSizeUser for XtsProcessor<'_, '_> {
    type BlockSize = U16;
}

impl BlockCipherEncClosure for XtsProcessor<'_, '_> {
    fn call<B: BlockCipherEncBackend<BlockSize = Self::BlockSize>>(self, backend: &B) {
        let Self { mut tweak, src, dst } = self;
        xts_encrypt_buffer(backend, src, dst, &mut tweak);
    }
}

impl BlockCipherDecClosure for XtsProcessor<'_, '_> {
    fn call<B: BlockCipherDecBackend<BlockSize = Self::BlockSize>>(self, backend: &B) {
        let Self { mut tweak, src, dst } = self;
        xts_decrypt_buffer(backend, src, dst, &mut tweak);
    }
}

/// Handles ciphertext stealing in order to allow non-BlockSize lengths to be used as long as they
/// are >= BlockSize. To be used with encrypt|decrypt_with_backend. Conforms to IEEE 1619-2007.
pub struct XtsCtsProcessor<'a, 'b> {
    tweak: Tweak,
    src: PtrByteSlice<'a>,
    dst: MutPtrByteSlice<'b>,
}

impl BlockSizeUser for XtsCtsProcessor<'_, '_> {
    type BlockSize = U16;
}

impl<'a, 'b> XtsCtsProcessor<'a, 'b> {
    /// `tweak` should be encrypted. `src` and `dst` must have the same length and be 16 byte
    /// aligned.
    pub fn new(tweak: Tweak, src: PtrByteSlice<'a>, mut dst: MutPtrByteSlice<'b>) -> Self {
        assert_eq!(src.len(), dst.len(), "Source and destination lengths must match");
        assert!(src.len() >= size_of::<u128>());
        assert!(src.as_ptr().cast::<u128>().is_aligned(), "src must be 16 byte aligned");
        assert!(dst.as_ptr().cast::<u128>().is_aligned(), "dst must be 16 byte aligned");
        dst.zero_no_rfo();
        Self { tweak, src, dst }
    }

    /// Creates an XtsCtsProcessor for in-place operation on a single buffer.
    pub fn new_in_place(tweak: Tweak, buf: MutPtrByteSlice<'a>) -> XtsCtsProcessor<'a, 'a> {
        assert!(buf.len() >= size_of::<u128>());
        assert!(buf.as_ptr().cast::<u128>().is_aligned(), "buf must be 16 byte aligned");
        let len = buf.len();
        let ptr = buf.as_ptr_slice().as_ptr();
        // SAFETY: We are creating a PtrByteSlice that aliases with the MutPtrByteSlice.
        // This is safe because PtrByteSlice only allows read access, and we control the
        // execution in `call` to ensure we don't violate safety.
        let src = unsafe { PtrByteSlice::new(std::ptr::slice_from_raw_parts(ptr, len)) };
        XtsCtsProcessor { tweak, src, dst: buf }
    }
}

impl BlockCipherEncClosure for XtsCtsProcessor<'_, '_> {
    fn call<B: BlockCipherEncBackend<BlockSize = Self::BlockSize>>(self, backend: &B) {
        let Self { mut tweak, src, mut dst } = self;
        let len = src.len();
        let base_size = len & (!0x0F);

        // Fast path for non-CTS (len is an exact multiple of 16):
        if len == base_size {
            xts_encrypt_buffer(backend, src, dst, &mut tweak);
            return;
        }

        // All but the last two blocks are normal.
        if base_size > 16 {
            let src_base = src.subslice(0..(base_size - 16));
            let dst_base = dst.subslice_mut(0..(base_size - 16));
            xts_encrypt_buffer(backend, src_base, dst_base, &mut tweak);
        }

        let last_full_block_range = (base_size - 16)..base_size;
        let last_full_block =
            src.subslice(last_full_block_range.clone()).read().expect("Size validated above");
        let intermediate_ciphertext = xts_encrypt_chunk(backend, last_full_block, &tweak);

        let extra = len & 0x0F;
        let intermediate_cipher_bytes = intermediate_ciphertext.to_le_bytes();

        // Construct combined block: start with intermediate_cipher_bytes at the end and put the
        // remaining plaintext at the beginning. Must read from second-to-last before writing to
        // last block to support in-place operation.
        let mut combined_block_bytes = intermediate_cipher_bytes;
        src.subslice(base_size..len).copy_to_slice(&mut combined_block_bytes[0..extra]);

        // Write the first ciphertext bytes into the partial block slot.
        dst.subslice_mut(base_size..len).copy_from_slice(&intermediate_cipher_bytes[0..extra]);

        tweak.update();

        let combined_block = u128::from_le_bytes(combined_block_bytes);
        let full_ciphertext = xts_encrypt_chunk(backend, combined_block, &tweak);

        // Write the encrypted full ciphertext block into the second-to-last block slot.
        dst.subslice_mut(last_full_block_range)
            .write(full_ciphertext)
            .expect("Size validated above");
    }
}

impl BlockCipherDecClosure for XtsCtsProcessor<'_, '_> {
    fn call<B: BlockCipherDecBackend<BlockSize = Self::BlockSize>>(self, backend: &B) {
        let Self { mut tweak, src, mut dst } = self;
        let len = src.len();
        let base_size = len & (!0x0F);

        // Fast path for non-CTS (len is an exact multiple of 16):
        if len == base_size {
            xts_decrypt_buffer(backend, src, dst, &mut tweak);
            return;
        }

        // All but the last two blocks are normal.
        if base_size > 16 {
            let src_base = src.subslice(0..(base_size - 16));
            let dst_base = dst.subslice_mut(0..(base_size - 16));
            xts_decrypt_buffer(backend, src_base, dst_base, &mut tweak);
        }

        let last_full_block_tweak = tweak;
        let last_full_block_range = (base_size - 16)..base_size;

        tweak.update();
        let extra = len & 0x0F;

        // Decrypt full ciphertext block at last_full_block_range using updated tweak:
        let full_ciphertext =
            src.subslice(last_full_block_range.clone()).read().expect("Size validated above");
        let decrypted_combined_block = xts_decrypt_chunk(backend, full_ciphertext, &tweak);
        let combined_bytes = decrypted_combined_block.to_le_bytes();

        // Reconstruct intermediate ciphertext for second-to-last block. Start with combined_bytes
        // at the end and put the input ciphertext at the beginning. Must read from second-to-last
        // before writing to last block to support in-place operation.
        let mut intermediate_bytes = combined_bytes;
        src.subslice(base_size..len).copy_to_slice(&mut intermediate_bytes[0..extra]);

        // Write recovered partial plaintext.
        dst.subslice_mut(base_size..len).copy_from_slice(&combined_bytes[0..extra]);

        let last_block_ciphertext = u128::from_le_bytes(intermediate_bytes);
        let val = xts_decrypt_chunk(backend, last_block_ciphertext, &last_full_block_tweak);
        dst.subslice_mut(last_full_block_range).write(val).expect("Size validated above");
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use cipher::inout::InOut;
    use cipher::typenum::consts::U1;
    use cipher::{Block, ParBlocksSizeUser};
    use std::cell::RefCell;
    use test_case::test_case;

    struct MockCipher {
        recorded_blocks: RefCell<Vec<u128>>,
        key: u128,
    }

    impl MockCipher {
        fn new(key: u128) -> Self {
            Self { recorded_blocks: RefCell::new(Vec::new()), key }
        }
    }

    impl BlockSizeUser for MockCipher {
        type BlockSize = U16;
    }

    impl ParBlocksSizeUser for MockCipher {
        type ParBlocksSize = U1;
    }

    impl BlockCipherEncBackend for MockCipher {
        fn encrypt_block(&self, mut block: InOut<'_, '_, Block<Self>>) {
            // SAFETY: Block<Self> is Array<u8, U16>, which is 16 bytes.
            let mut val =
                unsafe { std::ptr::read_unaligned(block.get_in().as_ptr() as *const u128) };
            self.recorded_blocks.borrow_mut().push(val);
            val ^= self.key;
            // SAFETY: Block<Self> is Array<u8, U16>, which is 16 bytes.
            unsafe {
                std::ptr::write_unaligned(block.get_out().as_mut_ptr() as *mut u128, val);
            }
        }
    }

    impl BlockCipherDecBackend for MockCipher {
        fn decrypt_block(&self, mut block: InOut<'_, '_, Block<Self>>) {
            // SAFETY: Block<Self> is Array<u8, U16>, which is 16 bytes.
            let mut val =
                unsafe { std::ptr::read_unaligned(block.get_in().as_ptr() as *const u128) };
            self.recorded_blocks.borrow_mut().push(val);
            // Lazy scramble. See above for the encryption method.
            val = val.rotate_right(3);
            val ^= self.key;
            // SAFETY: Block<Self> is Array<u8, U16>, which is 16 bytes.
            unsafe {
                std::ptr::write_unaligned(block.get_out().as_mut_ptr() as *mut u128, val);
            }
        }
    }

    /// A simple fake scrambler that includes a rotate left to keep the XOR from canceling itself
    /// out.
    struct MockNonLinearCipher {
        key: u128,
    }

    impl MockNonLinearCipher {
        fn new(key: u128) -> Self {
            Self { key }
        }
    }

    impl BlockSizeUser for MockNonLinearCipher {
        type BlockSize = U16;
    }

    impl ParBlocksSizeUser for MockNonLinearCipher {
        type ParBlocksSize = U1;
    }

    impl BlockCipherEncBackend for MockNonLinearCipher {
        fn encrypt_block(&self, mut block: InOut<'_, '_, Block<Self>>) {
            let mut val =
                unsafe { std::ptr::read_unaligned(block.get_in().as_ptr() as *const u128) };
            val = val.rotate_left(11) ^ self.key;
            unsafe {
                std::ptr::write_unaligned(block.get_out().as_mut_ptr() as *mut u128, val);
            }
        }
    }

    impl BlockCipherDecBackend for MockNonLinearCipher {
        fn decrypt_block(&self, mut block: InOut<'_, '_, Block<Self>>) {
            let mut val =
                unsafe { std::ptr::read_unaligned(block.get_in().as_ptr() as *const u128) };
            val = (val ^ self.key).rotate_right(11);
            unsafe {
                std::ptr::write_unaligned(block.get_out().as_mut_ptr() as *mut u128, val);
            }
        }
    }

    #[repr(C)]
    #[derive(FromBytes, IntoBytes, Immutable)]
    struct Blocks<const N: usize>([u128; N]);

    static_assertions::const_assert!(std::mem::align_of::<Blocks<1>>() == 16);
    static_assertions::const_assert!(std::mem::align_of::<Blocks<2>>() == 16);

    impl<const N: usize> Default for Blocks<N> {
        fn default() -> Self {
            Self([0u128; N])
        }
    }

    #[test]
    fn test_xts_out_of_place() {
        let mut plaintext: Blocks<2> = Default::default();
        for (i, x) in plaintext.as_mut_bytes().iter_mut().enumerate() {
            *x = i as u8;
        }
        let mut ciphertext: Blocks<2> = Default::default();

        let src = PtrByteSlice::from(plaintext.as_bytes());
        let dst = MutPtrByteSlice::from(ciphertext.as_mut_bytes());

        let tweak_val = 0x123456789abcdef0123456789abcdef0u128;
        let tweak = Tweak::new(tweak_val);
        let key = 0xffeeddccbbaa99887766554433221100u128;

        let processor = XtsProcessor::new(tweak, src, dst);
        let cipher = MockCipher::new(key);

        BlockCipherEncClosure::call(processor, &cipher);

        // Verify ciphertext.
        // Since our mock cipher is just XOR with key, the tweak should cancel out.
        // C = P ^ K.
        let expected_c0 =
            u128::from_le_bytes(plaintext.as_bytes()[0..16].try_into().unwrap()) ^ key;
        let expected_c1 =
            u128::from_le_bytes(plaintext.as_bytes()[16..32].try_into().unwrap()) ^ key;

        let actual_c0 = u128::from_le_bytes(ciphertext.as_bytes()[0..16].try_into().unwrap());
        let actual_c1 = u128::from_le_bytes(ciphertext.as_bytes()[16..32].try_into().unwrap());

        assert_eq!(actual_c0, expected_c0);
        assert_eq!(actual_c1, expected_c1);

        // Verify recorded blocks (should be P ^ T).
        assert_eq!(cipher.recorded_blocks.borrow().len(), 2);

        let p0 = u128::from_le_bytes(plaintext.as_bytes()[0..16].try_into().unwrap());
        let p1 = u128::from_le_bytes(plaintext.as_bytes()[16..32].try_into().unwrap());

        let mut t0 = tweak;
        assert_eq!(cipher.recorded_blocks.borrow()[0], p0 ^ t0.0);
        t0.update();
        assert_eq!(cipher.recorded_blocks.borrow()[1], p1 ^ t0.0);
    }

    #[test]
    fn test_xts_in_place() {
        let mut buf: Blocks<2> = Default::default();
        for (i, x) in buf.as_mut_bytes().iter_mut().enumerate() {
            *x = i as u8;
        }

        let tweak_val = 0x123456789abcdef0123456789abcdef0u128;
        let tweak = Tweak::new(tweak_val);
        let key = 0xffeeddccbbaa99887766554433221100u128;

        // Save original plaintext for verification.
        let p0 = u128::from_le_bytes(buf.as_bytes()[0..16].try_into().unwrap());
        let p1 = u128::from_le_bytes(buf.as_bytes()[16..32].try_into().unwrap());

        let slice = MutPtrByteSlice::from(buf.as_mut_bytes());
        let processor = XtsProcessor::new_in_place(tweak, slice);
        let cipher = MockCipher::new(key);

        BlockCipherEncClosure::call(processor, &cipher);

        // Verify in-place ciphertext.
        let expected_c0 = p0 ^ key;
        let expected_c1 = p1 ^ key;

        let actual_c0 = u128::from_le_bytes(buf.as_bytes()[0..16].try_into().unwrap());
        let actual_c1 = u128::from_le_bytes(buf.as_bytes()[16..32].try_into().unwrap());

        assert_eq!(actual_c0, expected_c0);
        assert_eq!(actual_c1, expected_c1);

        // Verify recorded blocks.
        assert_eq!(cipher.recorded_blocks.borrow().len(), 2);
        let mut t0 = tweak;
        assert_eq!(cipher.recorded_blocks.borrow()[0], p0 ^ t0.0);
        t0.update();
        assert_eq!(cipher.recorded_blocks.borrow()[1], p1 ^ t0.0);
    }

    #[test_case(16; "exact_block")]
    #[test_case(17; "one_byte_more_than_block")]
    #[test_case(31; "one_byte_less_than_two_blocks")]
    #[test_case(32; "exact_two_blocks")]
    #[test_case(80; "bunch_of_blocks_exact")]
    #[test_case(87; "bunch_of_blocks_cts")]
    fn test_cts_encrypt_decrypt(len: usize) {
        let tweak = Tweak::new(0x123456789abcdef0123456789abcdef0);
        let key = 0xffeeddccbbaa99887766554433221100;

        let mut plaintext_vec = vec![0u128; (len + 15) / 16];
        let plaintext_bytes = plaintext_vec.as_mut_bytes();
        for (i, b) in plaintext_bytes[..len].iter_mut().enumerate() {
            *b = ((i % 255) + 1) as u8;
        }
        let plaintext = &plaintext_bytes[..len];

        let mut ciphertext_vec = vec![0u128; (len + 15) / 16];
        let ciphertext_bytes = ciphertext_vec.as_mut_bytes();

        let mut decrypted_vec = vec![0u128; (len + 15) / 16];
        let decrypted_bytes = decrypted_vec.as_mut_bytes();

        let cipher = MockNonLinearCipher::new(key);

        // Encrypt out-of-place
        {
            let src = PtrByteSlice::from(plaintext);
            let dst = MutPtrByteSlice::from(&mut ciphertext_bytes[..len]);
            let processor = XtsCtsProcessor::new(tweak, src, dst);
            BlockCipherEncClosure::call(processor, &cipher);
        }

        let ciphertext = &ciphertext_bytes[..len];

        // Verify none of the original blocks/content are intact
        assert_ne!(ciphertext, plaintext, "Ciphertext should not match plaintext");
        for chunk_start in (0..len).step_by(16) {
            let chunk_end = (chunk_start + 16).min(len);
            assert_ne!(
                &ciphertext[chunk_start..chunk_end],
                &plaintext[chunk_start..chunk_end],
                "Chunk at {chunk_start}..{chunk_end} should not match plaintext"
            );
        }

        // Decrypt out-of-place
        {
            let src = PtrByteSlice::from(ciphertext);
            let dst = MutPtrByteSlice::from(&mut decrypted_bytes[..len]);
            let processor = XtsCtsProcessor::new(tweak, src, dst);
            BlockCipherDecClosure::call(processor, &cipher);
        }

        let decrypted = &decrypted_bytes[..len];

        // Verify decrypted matches original plaintext
        assert_eq!(decrypted, plaintext, "Decrypted text should match original plaintext");
    }

    #[test]
    fn test_cts_encrypt_decrypt_in_place() {
        const LEN: usize = 87;

        let tweak = Tweak::new(0x123456789abcdef0123456789abcdef0);
        let key = 0xffeeddccbbaa99887766554433221100;

        let mut buf_vec = vec![0u128; (LEN + 15) / 16];
        let buf_bytes = buf_vec.as_mut_bytes();
        for (i, b) in buf_bytes[..LEN].iter_mut().enumerate() {
            *b = ((i % 255) + 1) as u8;
        }
        let original_plaintext = buf_bytes[..LEN].to_vec();

        let cipher = MockNonLinearCipher::new(key);

        // Encrypt in-place
        {
            let slice = MutPtrByteSlice::from(&mut buf_bytes[..LEN]);
            let processor = XtsCtsProcessor::new_in_place(tweak, slice);
            BlockCipherEncClosure::call(processor, &cipher);
        }

        // Verify in-place ciphertext differs from original plaintext
        assert_ne!(
            &buf_bytes[..LEN],
            &original_plaintext[..],
            "In-place ciphertext should not match plaintext"
        );
        for chunk_start in (0..LEN).step_by(16) {
            let chunk_end = (chunk_start + 16).min(LEN);
            assert_ne!(
                &buf_bytes[chunk_start..chunk_end],
                &original_plaintext[chunk_start..chunk_end],
                "Chunk at {chunk_start}..{chunk_end} should not match plaintext"
            );
        }

        // Decrypt in-place
        {
            let slice = MutPtrByteSlice::from(&mut buf_bytes[..LEN]);
            let processor = XtsCtsProcessor::new_in_place(tweak, slice);
            BlockCipherDecClosure::call(processor, &cipher);
        }

        // Verify decrypted matches original plaintext
        assert_eq!(
            &buf_bytes[..LEN],
            &original_plaintext[..],
            "Decrypted in-place text should match original plaintext"
        );
    }

    #[test_case(1; "one_block")]
    #[test_case(2; "two_blocks")]
    #[test_case(5; "five_blocks")]
    fn test_cts_matches_normal_xts_on_exact_blocks(num_blocks: usize) {
        let tweak = Tweak::new(0x9876543210abcdef9876543210abcdef);
        let key = 0x0123456789abcdef0123456789abcdef;

        let len = num_blocks * 16;
        let mut plaintext_vec = vec![0u128; num_blocks];
        let plaintext_bytes = plaintext_vec.as_mut_bytes();
        for (i, b) in plaintext_bytes[..len].iter_mut().enumerate() {
            *b = (i as u8).wrapping_mul(17).wrapping_add(3);
        }
        let plaintext = &plaintext_bytes[..len];

        let mut normal_cts_vec = vec![0u128; num_blocks];
        let mut cts_vec = vec![0u128; num_blocks];

        let cipher = MockNonLinearCipher::new(key);

        // Normal XTS encryption
        {
            let src = PtrByteSlice::from(plaintext);
            let dst = MutPtrByteSlice::from(&mut normal_cts_vec.as_mut_bytes()[..len]);
            let processor = XtsProcessor::new(tweak, src, dst);
            BlockCipherEncClosure::call(processor, &cipher);
        }

        // CTS XTS encryption
        {
            let src = PtrByteSlice::from(plaintext);
            let dst = MutPtrByteSlice::from(&mut cts_vec.as_mut_bytes()[..len]);
            let processor = XtsCtsProcessor::new(tweak, src, dst);
            BlockCipherEncClosure::call(processor, &cipher);
        }

        assert_eq!(
            normal_cts_vec, cts_vec,
            "CTS XTS should match normal XTS for {num_blocks} blocks"
        );
    }

    #[test]
    fn test_cts_different_tweaks_different_ciphertext() {
        const LEN: usize = 87;

        let tweak1 = Tweak::new(0x123456789abcdef0123456789abcdef0);
        let tweak2 = Tweak::new(0xfeef876543210fedcba9876543210fed);
        let key = 0xffeeddccbbaa99887766554433221100;

        let mut plaintext_vec = vec![0u128; (LEN + 15) / 16];
        let plaintext_bytes = plaintext_vec.as_mut_bytes();
        for (i, b) in plaintext_bytes[..LEN].iter_mut().enumerate() {
            *b = ((i % 255) + 1) as u8;
        }
        let plaintext = &plaintext_bytes[..LEN];

        let mut ciphertext1_vec = vec![0u128; (LEN + 15) / 16];
        let ciphertext1_bytes = ciphertext1_vec.as_mut_bytes();

        let mut ciphertext2_vec = vec![0u128; (LEN + 15) / 16];
        let ciphertext2_bytes = ciphertext2_vec.as_mut_bytes();

        let cipher = MockNonLinearCipher::new(key);

        // Encrypt with tweak1
        {
            let src = PtrByteSlice::from(plaintext);
            let dst = MutPtrByteSlice::from(&mut ciphertext1_bytes[..LEN]);
            let processor = XtsCtsProcessor::new(tweak1, src, dst);
            BlockCipherEncClosure::call(processor, &cipher);
        }

        // Encrypt with tweak2
        {
            let src = PtrByteSlice::from(plaintext);
            let dst = MutPtrByteSlice::from(&mut ciphertext2_bytes[..LEN]);
            let processor = XtsCtsProcessor::new(tweak2, src, dst);
            BlockCipherEncClosure::call(processor, &cipher);
        }

        assert_ne!(
            &ciphertext1_bytes[..LEN],
            &ciphertext2_bytes[..LEN],
            "Different tweaks should produce different ciphertexts"
        );
    }

    #[test]
    #[should_panic(expected = "Source and destination lengths must match")]
    fn test_cts_panic_length_mismatch() {
        let tweak = Tweak::new(0);
        let src_buf = vec![0u128; 2];
        let mut dst_buf = vec![0u128; 3];
        let src = PtrByteSlice::from(src_buf.as_bytes());
        let dst = MutPtrByteSlice::from(dst_buf.as_mut_bytes());
        let _ = XtsCtsProcessor::new(tweak, src, dst);
    }

    #[test]
    #[should_panic]
    fn test_cts_panic_too_short() {
        let tweak = Tweak::new(0);
        let src_buf = vec![0u128; 2];
        let mut dst_buf = vec![0u128; 2];
        let src = PtrByteSlice::from(&src_buf.as_bytes()[..15]);
        let dst = MutPtrByteSlice::from(&mut dst_buf.as_mut_bytes()[..15]);
        let _ = XtsCtsProcessor::new(tweak, src, dst);
    }

    #[test]
    #[should_panic(expected = "src must be 16 byte aligned")]
    fn test_cts_panic_unaligned_src() {
        let tweak = Tweak::new(0);
        let src_buf = vec![0u128; 3];
        let mut dst_buf = vec![0u128; 2];
        let src = PtrByteSlice::from(&src_buf.as_bytes()[1..17]);
        let dst = MutPtrByteSlice::from(&mut dst_buf.as_mut_bytes()[..16]);
        let _ = XtsCtsProcessor::new(tweak, src, dst);
    }

    #[test]
    #[should_panic(expected = "dst must be 16 byte aligned")]
    fn test_cts_panic_unaligned_dst() {
        let tweak = Tweak::new(0);
        let src_buf = vec![0u128; 2];
        let mut dst_buf = vec![0u128; 3];
        let src = PtrByteSlice::from(&src_buf.as_bytes()[..16]);
        let dst = MutPtrByteSlice::from(&mut dst_buf.as_mut_bytes()[1..17]);
        let _ = XtsCtsProcessor::new(tweak, src, dst);
    }
}