nearby/presence/ldt/src/lib.rs - beto-core - Git at Google

 // Copyright 2022 Google LLC
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //      http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.

 //! Provides an implementation of [LDT](https://eprint.iacr.org/2017/841.pdf).

 #![no_std]

 #[cfg(feature = "std")]
 extern crate std;

 use core::{fmt, marker::PhantomData, ops};
 use crypto_provider::CryptoProvider;
 use ldt_tbc::{ConcatenatedKeyArray, TweakableBlockCipher, TweakableBlockCipherKey};
 use ldt_tbc::{TweakableBlockCipherDecrypter, TweakableBlockCipherEncrypter};

 /// Common functionality for [LdtEncryptCipher] and [LdtDecryptCipher]
 pub trait LdtCipher<const B: usize, T: TweakableBlockCipher<B>> {
     /// The range of input lengths the cipher can operate on
     const VALID_INPUT_LEN: ops::Range<usize>;

     /// Create a new cipher with the provided [TweakableBlockCipher] and [Mix] function
     fn new(key: &LdtKey<T::Key>) -> Self;
 }

 /// Implementation of the [LDT](https://eprint.iacr.org/2017/841.pdf) length doubler encryption cipher.
 ///
 /// `B` is the block size.
 /// `T` is the provided implementation of a Tweakable Block Cipher
 /// `M` is the implementation of a [pure mix function](https://eprint.iacr.org/2017/841.pdf)
 pub struct LdtEncryptCipher<const B: usize, T: TweakableBlockCipher<B>, M: Mix> {
     cipher_1: T::EncryptionCipher,
     cipher_2: T::EncryptionCipher,
     // marker to use `M`
     mix_phantom: PhantomData<M>,
 }

 impl<const B: usize, T: TweakableBlockCipher<B>, M: Mix> LdtCipher<B, T>
     for LdtEncryptCipher<B, T, M>
 {
     const VALID_INPUT_LEN: ops::Range<usize> = input_len_range::<B>();

     fn new(key: &LdtKey<T::Key>) -> Self {
         LdtEncryptCipher {
             cipher_1: T::EncryptionCipher::new(&key.key_1),
             cipher_2: T::EncryptionCipher::new(&key.key_2),
             mix_phantom: PhantomData,
         }
     }
 }

 impl<const B: usize, T: TweakableBlockCipher<B>, M: Mix> LdtEncryptCipher<B, T, M> {
     /// Encrypt `data` in place, performing the pad operation with `padder`.
     ///
     /// Unless you have particular padding needs, use [DefaultPadder].
     ///
     /// # Errors
     /// - if `data` has a length outside of `[B, B * 2)`.
     pub fn encrypt<P: Padder<B, T>>(&self, data: &mut [u8], padder: &P) -> Result<(), LdtError> {
         do_ldt::<B, T, _, _, _, P>(
             data,
             |cipher, tweak, block| cipher.encrypt(tweak, block),
             padder,
             M::forwards,
             &self.cipher_1,
             &self.cipher_2,
         )
     }
 }

 /// Implementation of the [LDT](https://eprint.iacr.org/2017/841.pdf) length doubler decryption cipher.
 ///
 /// `B` is the block size.
 /// `T` is the provided implementation of a Tweakable Block Cipher
 /// `M` is the implementation of a [pure mix function](https://eprint.iacr.org/2017/841.pdf)
 #[repr(C)]
 pub struct LdtDecryptCipher<const B: usize, T: TweakableBlockCipher<B>, M: Mix> {
     cipher_1: T::DecryptionCipher,
     cipher_2: T::DecryptionCipher,
     // marker to use `M`
     mix_phantom: PhantomData<M>,
 }

 impl<const B: usize, T: TweakableBlockCipher<B>, M: Mix> LdtDecryptCipher<B, T, M> {
     /// Create an [LdtDecryptCipher] with the provided Tweakable block cipher and Mix function

     /// Decrypt `data` in place, performing the pad operation with `padder`.
     ///
     /// Unless you have particular padding needs, use [DefaultPadder].
     ///
     /// # Errors
     /// - if `data` has a length outside of `[B, B * 2)`.
     pub fn decrypt<P: Padder<B, T>>(&self, data: &mut [u8], padder: &P) -> Result<(), LdtError> {
         do_ldt::<B, T, _, _, _, P>(
             data,
             |cipher, tweak, block| cipher.decrypt(tweak, block),
             padder,
             M::backwards,
             // cipher order swapped for decryption
             &self.cipher_2,
             &self.cipher_1,
         )
     }
 }

 impl<const B: usize, T: TweakableBlockCipher<B>, M: Mix> LdtCipher<B, T>
     for LdtDecryptCipher<B, T, M>
 {
     const VALID_INPUT_LEN: ops::Range<usize> = input_len_range::<B>();

     fn new(key: &LdtKey<T::Key>) -> Self {
         LdtDecryptCipher {
             cipher_1: T::DecryptionCipher::new(&key.key_1),
             cipher_2: T::DecryptionCipher::new(&key.key_2),
             mix_phantom: PhantomData,
         }
     }
 }

 /// Returns the range of valid input lengths to encrypt or decrypt with LDT for a given tweakable
 /// block cipher block size `B`, namely `[B, B * 2)`.
 const fn input_len_range<const B: usize>() -> ops::Range<usize> {
     B..B * 2
 }

 // internal implementation of ldt cipher operations, re-used by encryption and decryption, by providing
 // the corresponding cipher_op and mix operation
 fn do_ldt<const B: usize, T, O, C, X, P>(
     data: &mut [u8],
     cipher_op: O,
     padder: &P,
     mix: X,
     first_cipher: &C,
     second_cipher: &C,
 ) -> Result<(), LdtError>
 where
     T: TweakableBlockCipher<B>,
     // Encrypt or decrypt in place with a tweak
     O: Fn(&C, T::Tweak, &mut [u8; B]),
     // Mix a/b into block-sized chunks
     X: for<'a, 'b> Fn(&'a [u8], &'b [u8]) -> (&'b [u8], &'a [u8]),
     P: Padder<B, T>,
 {
     if !input_len_range::<B>().contains(&data.len()) {
         return Err(LdtError::InvalidLength(data.len()));
     }
     let s = data.len() - B;
     debug_assert!(s < B);

     // m1 length B, m2 length s (s < B)
     let (m1, m2) = data.split_at(B);
     debug_assert_eq!(s, m2.len());
     let m1_ciphertext = {
         let mut m1_plaintext = [0_u8; B];
         // m1 is of length B, so no panic
         m1_plaintext[..].copy_from_slice(m1);
         let tweak = padder.pad_tweak(m2);
         cipher_op(first_cipher, tweak, &mut m1_plaintext);
         m1_plaintext
     };
     // |z| = B - s, |m3| = s
     let (z, m3) = m1_ciphertext.split_at(B - s);
     debug_assert_eq!(s, m3.len());

     // c3 and c2 are the last s bytes of their size-B arrays, respectively
     let (c3, c2) = mix(m3, m2);

     let c1 = {
         let mut z_c3 = [0; B];
         z_c3[(B - s)..].copy_from_slice(c3);
         z_c3[0..(B - s)].copy_from_slice(z);

         let tweak = padder.pad_tweak(c2);
         cipher_op(second_cipher, tweak, &mut z_c3);
         z_c3
     };

     let (left, right) = data.split_at_mut(B);
     left.copy_from_slice(&c1);
     right.copy_from_slice(c2);
     Ok(())
 }

 /// Errors produced by LDT encryption/decryption.
 #[derive(Debug, PartialEq, Eq)]
 pub enum LdtError {
     /// Data to encrypt/decrypt is the wrong length -- must be in `[B, 2 * B)` for block size `B`
     /// of the underlying [TweakableBlockCipher].
     InvalidLength(usize),
 }

 impl fmt::Display for LdtError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
             LdtError::InvalidLength(len) => {
                 write!(f, "Invalid length ({len}), must be in [block size, 2 * block size)")
             }
         }
     }
 }

 /// A key for an LDT cipher.
 ///
 /// `T` is the key type used for the underlying tweakable block cipher.
 pub struct LdtKey<T: TweakableBlockCipherKey> {
     key_1: T,
     key_2: T,
 }

 impl<T: TweakableBlockCipherKey> LdtKey<T> {
     /// Build a key from two separate tweakable block cipher keys.
     pub fn from_separate(key_1: T, key_2: T) -> Self {
         Self { key_1, key_2 }
     }

     /// Build a key from an array representing two concatenated tweakable block cipher keys.
     pub fn from_concatenated(key: &T::ConcatenatedKeyArray) -> Self {
         let (key_1, key_2) = T::split_from_concatenated(key);
         Self::from_separate(key_1, key_2)
     }

     /// Build a random key from a secure RNG.
     pub fn from_random<C: CryptoProvider>(rng: &mut C::CryptoRng) -> Self {
         Self::from_concatenated(&ConcatenatedKeyArray::from_random::<C>(rng))
     }

     /// Returns the key material as a concatenated array with the contents of the two tweakable
     /// block cipher keys.
     pub fn as_concatenated(&self) -> T::ConcatenatedKeyArray {
         self.key_1.concatenate_with(&self.key_2)
     }
 }

 /// A [pure mix function](https://eprint.iacr.org/2017/841.pdf).
 pub trait Mix {
     /// Mix `a` and `b`, writing into the last `s` bytes of the output arrays.
     /// `a` and `b` must be the same length `s`, and no longer than the block size `B`.
     /// Must be the inverse of [Mix::backwards].
     fn forwards<'a, 'b>(a: &'a [u8], b: &'b [u8]) -> (&'b [u8], &'a [u8]);

     /// Mix `a` and `b`, writing into the last `s` bytes of the output arrays.
     /// `a` and `b` must be the same length, and no longer than the block size `B`.
     /// Must be the inverse of [Mix::forwards].
     fn backwards<'a, 'b>(a: &'a [u8], b: &'b [u8]) -> (&'b [u8], &'a [u8]);
 }

 /// Per section 2.4, swapping `a` and `b` is a valid mix function
 pub struct Swap {}

 impl Mix for Swap {
     fn forwards<'a, 'b>(a: &'a [u8], b: &'b [u8]) -> (&'b [u8], &'a [u8]) {
         debug_assert_eq!(a.len(), b.len());
         (b, a)
     }

     fn backwards<'a, 'b>(a: &'a [u8], b: &'b [u8]) -> (&'b [u8], &'a [u8]) {
         // backwards is the same as forwards.
         Self::forwards(a, b)
     }
 }

 /// Pads partial-block input into tweaks.
 ///
 /// This is exposed as a separate trait to allow for padding methods beyond the default.
 pub trait Padder<const B: usize, T: TweakableBlockCipher<B>> {
     /// Build a tweak for `T` out of `data`.
     /// `data` must be shorter than the tweak size so that some padding can take place.
     ///
     /// # Panics
     ///
     /// Panics if the length of `data` >= the tweak size.
     fn pad_tweak(&self, data: &[u8]) -> T::Tweak;
 }

 /// The default padding algorithm per section 2 of [LDT paper](https://eprint.iacr.org/2017/841.pdf)
 #[derive(Default)]
 pub struct DefaultPadder;

 impl<const B: usize, T: TweakableBlockCipher<B>> Padder<B, T> for DefaultPadder {
     fn pad_tweak(&self, data: &[u8]) -> T::Tweak {
         Self::default_padding(data).into()
     }
 }

 impl DefaultPadder {
     /// Expand `data` to an array of the appropriate size, and append a 1 bit after the original data.
     ///
     /// `T` is the tweak size to pad to in bytes.
     ///
     /// # Panics
     ///
     /// Panics if the length of the data to pad is >= the tweak size.
     // allow index_slicing, since we are ensuring panic is impossible in assert
     #[allow(clippy::indexing_slicing)]
     fn default_padding<const N: usize>(data: &[u8]) -> [u8; N] {
         // If this assert fails, our LDT impl is broken - always pads data < block size
         assert!(data.len() < N);
         let mut out = [0; N];
         out[0..data.len()].copy_from_slice(data);
         // 0b1 followed by zeros for all remaining bits.
         // Since the array was initialized with 0, nothing left to do.
         out[data.len()] = 128;
         out
     }
 }

 /// Pads with the default algorithm to the tweak size, then XORs that with the provided bytes.
 ///
 /// This is useful as a means to perturb the cipher output without having to alter the input
 /// directly.
 ///
 /// `T` is the tweak size to pad to in bytes
 #[derive(Debug, PartialEq, Eq)]
 pub struct XorPadder<const T: usize> {
     xor_bytes: [u8; T],
 }

 impl<const T: usize> From<[u8; T]> for XorPadder<T> {
     fn from(bytes: [u8; T]) -> Self {
         XorPadder { xor_bytes: bytes }
     }
 }

 impl<const B: usize, T: TweakableBlockCipher<B>> Padder<B, T> for XorPadder<B> {
     fn pad_tweak(&self, data: &[u8]) -> T::Tweak {
         let mut out = DefaultPadder::default_padding::<B>(data);
         debug_assert_eq!(self.xor_bytes.len(), out.len());

         // xor into the padded data
         out.iter_mut()
             .zip(self.xor_bytes.iter())
             .for_each(|(out_byte, xor_byte): (&mut u8, &u8)| *out_byte ^= xor_byte);

         out.into()
     }
 }
	// Copyright 2022 Google LLC
	//
	// Licensed under the Apache License, Version 2.0 (the "License");
	// you may not use this file except in compliance with the License.
	// You may obtain a copy of the License at
	//
	// http://www.apache.org/licenses/LICENSE-2.0
	//
	// Unless required by applicable law or agreed to in writing, software
	// distributed under the License is distributed on an "AS IS" BASIS,
	// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	// See the License for the specific language governing permissions and
	// limitations under the License.

	//! Provides an implementation of [LDT](https://eprint.iacr.org/2017/841.pdf).

	#![no_std]

	#[cfg(feature = "std")]
	extern crate std;

	use core::{fmt, marker::PhantomData, ops};
	use crypto_provider::CryptoProvider;
	use ldt_tbc::{ConcatenatedKeyArray, TweakableBlockCipher, TweakableBlockCipherKey};
	use ldt_tbc::{TweakableBlockCipherDecrypter, TweakableBlockCipherEncrypter};

	/// Common functionality for [LdtEncryptCipher] and [LdtDecryptCipher]
	pub trait LdtCipher<const B: usize, T: TweakableBlockCipher<B>> {
	/// The range of input lengths the cipher can operate on
	const VALID_INPUT_LEN: ops::Range<usize>;

	/// Create a new cipher with the provided [TweakableBlockCipher] and [Mix] function
	fn new(key: &LdtKey<T::Key>) -> Self;
	}

	/// Implementation of the [LDT](https://eprint.iacr.org/2017/841.pdf) length doubler encryption cipher.
	///
	/// `B` is the block size.
	/// `T` is the provided implementation of a Tweakable Block Cipher
	/// `M` is the implementation of a [pure mix function](https://eprint.iacr.org/2017/841.pdf)
	pub struct LdtEncryptCipher<const B: usize, T: TweakableBlockCipher<B>, M: Mix> {
	cipher_1: T::EncryptionCipher,
	cipher_2: T::EncryptionCipher,
	// marker to use `M`
	mix_phantom: PhantomData<M>,
	}

	impl<const B: usize, T: TweakableBlockCipher<B>, M: Mix> LdtCipher<B, T>
	for LdtEncryptCipher<B, T, M>
	{
	const VALID_INPUT_LEN: ops::Range<usize> = input_len_range::<B>();

	fn new(key: &LdtKey<T::Key>) -> Self {
	LdtEncryptCipher {
	cipher_1: T::EncryptionCipher::new(&key.key_1),
	cipher_2: T::EncryptionCipher::new(&key.key_2),
	mix_phantom: PhantomData,
	}
	}
	}

	impl<const B: usize, T: TweakableBlockCipher<B>, M: Mix> LdtEncryptCipher<B, T, M> {
	/// Encrypt `data` in place, performing the pad operation with `padder`.
	///
	/// Unless you have particular padding needs, use [DefaultPadder].
	///
	/// # Errors
	/// - if `data` has a length outside of `[B, B * 2)`.
	pub fn encrypt<P: Padder<B, T>>(&self, data: &mut [u8], padder: &P) -> Result<(), LdtError> {
	do_ldt::<B, T, _, _, _, P>(
	data,
	\|cipher, tweak, block\| cipher.encrypt(tweak, block),
	padder,
	M::forwards,
	&self.cipher_1,
	&self.cipher_2,
	)
	}
	}

	/// Implementation of the [LDT](https://eprint.iacr.org/2017/841.pdf) length doubler decryption cipher.
	///
	/// `B` is the block size.
	/// `T` is the provided implementation of a Tweakable Block Cipher
	/// `M` is the implementation of a [pure mix function](https://eprint.iacr.org/2017/841.pdf)
	#[repr(C)]
	pub struct LdtDecryptCipher<const B: usize, T: TweakableBlockCipher<B>, M: Mix> {
	cipher_1: T::DecryptionCipher,
	cipher_2: T::DecryptionCipher,
	// marker to use `M`
	mix_phantom: PhantomData<M>,
	}

	impl<const B: usize, T: TweakableBlockCipher<B>, M: Mix> LdtDecryptCipher<B, T, M> {
	/// Create an [LdtDecryptCipher] with the provided Tweakable block cipher and Mix function

	/// Decrypt `data` in place, performing the pad operation with `padder`.
	///
	/// Unless you have particular padding needs, use [DefaultPadder].
	///
	/// # Errors
	/// - if `data` has a length outside of `[B, B * 2)`.
	pub fn decrypt<P: Padder<B, T>>(&self, data: &mut [u8], padder: &P) -> Result<(), LdtError> {
	do_ldt::<B, T, _, _, _, P>(
	data,
	\|cipher, tweak, block\| cipher.decrypt(tweak, block),
	padder,
	M::backwards,
	// cipher order swapped for decryption
	&self.cipher_2,
	&self.cipher_1,
	)
	}
	}

	impl<const B: usize, T: TweakableBlockCipher<B>, M: Mix> LdtCipher<B, T>
	for LdtDecryptCipher<B, T, M>
	{
	const VALID_INPUT_LEN: ops::Range<usize> = input_len_range::<B>();

	fn new(key: &LdtKey<T::Key>) -> Self {
	LdtDecryptCipher {
	cipher_1: T::DecryptionCipher::new(&key.key_1),
	cipher_2: T::DecryptionCipher::new(&key.key_2),
	mix_phantom: PhantomData,
	}
	}
	}

	/// Returns the range of valid input lengths to encrypt or decrypt with LDT for a given tweakable
	/// block cipher block size `B`, namely `[B, B * 2)`.
	const fn input_len_range<const B: usize>() -> ops::Range<usize> {
	B..B * 2
	}

	// internal implementation of ldt cipher operations, re-used by encryption and decryption, by providing
	// the corresponding cipher_op and mix operation
	fn do_ldt<const B: usize, T, O, C, X, P>(
	data: &mut [u8],
	cipher_op: O,
	padder: &P,
	mix: X,
	first_cipher: &C,
	second_cipher: &C,
	) -> Result<(), LdtError>
	where
	T: TweakableBlockCipher<B>,
	// Encrypt or decrypt in place with a tweak
	O: Fn(&C, T::Tweak, &mut [u8; B]),
	// Mix a/b into block-sized chunks
	X: for<'a, 'b> Fn(&'a [u8], &'b [u8]) -> (&'b [u8], &'a [u8]),
	P: Padder<B, T>,
	{
	if !input_len_range::<B>().contains(&data.len()) {
	return Err(LdtError::InvalidLength(data.len()));
	}
	let s = data.len() - B;
	debug_assert!(s < B);

	// m1 length B, m2 length s (s < B)
	let (m1, m2) = data.split_at(B);
	debug_assert_eq!(s, m2.len());
	let m1_ciphertext = {
	let mut m1_plaintext = [0_u8; B];
	// m1 is of length B, so no panic
	m1_plaintext[..].copy_from_slice(m1);
	let tweak = padder.pad_tweak(m2);
	cipher_op(first_cipher, tweak, &mut m1_plaintext);
	m1_plaintext
	};
	// \|z\| = B - s, \|m3\| = s
	let (z, m3) = m1_ciphertext.split_at(B - s);
	debug_assert_eq!(s, m3.len());

	// c3 and c2 are the last s bytes of their size-B arrays, respectively
	let (c3, c2) = mix(m3, m2);

	let c1 = {
	let mut z_c3 = [0; B];
	z_c3[(B - s)..].copy_from_slice(c3);
	z_c3[0..(B - s)].copy_from_slice(z);

	let tweak = padder.pad_tweak(c2);
	cipher_op(second_cipher, tweak, &mut z_c3);
	z_c3
	};

	let (left, right) = data.split_at_mut(B);
	left.copy_from_slice(&c1);
	right.copy_from_slice(c2);
	Ok(())
	}

	/// Errors produced by LDT encryption/decryption.
	#[derive(Debug, PartialEq, Eq)]
	pub enum LdtError {
	/// Data to encrypt/decrypt is the wrong length -- must be in `[B, 2 * B)` for block size `B`
	/// of the underlying [TweakableBlockCipher].
	InvalidLength(usize),
	}

	impl fmt::Display for LdtError {
	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
	match self {
	LdtError::InvalidLength(len) => {
	write!(f, "Invalid length ({len}), must be in [block size, 2 * block size)")
	}
	}
	}
	}

	/// A key for an LDT cipher.
	///
	/// `T` is the key type used for the underlying tweakable block cipher.
	pub struct LdtKey<T: TweakableBlockCipherKey> {
	key_1: T,
	key_2: T,
	}

	impl<T: TweakableBlockCipherKey> LdtKey<T> {
	/// Build a key from two separate tweakable block cipher keys.
	pub fn from_separate(key_1: T, key_2: T) -> Self {
	Self { key_1, key_2 }
	}

	/// Build a key from an array representing two concatenated tweakable block cipher keys.
	pub fn from_concatenated(key: &T::ConcatenatedKeyArray) -> Self {
	let (key_1, key_2) = T::split_from_concatenated(key);
	Self::from_separate(key_1, key_2)
	}

	/// Build a random key from a secure RNG.
	pub fn from_random<C: CryptoProvider>(rng: &mut C::CryptoRng) -> Self {
	Self::from_concatenated(&ConcatenatedKeyArray::from_random::<C>(rng))
	}

	/// Returns the key material as a concatenated array with the contents of the two tweakable
	/// block cipher keys.
	pub fn as_concatenated(&self) -> T::ConcatenatedKeyArray {
	self.key_1.concatenate_with(&self.key_2)
	}
	}

	/// A [pure mix function](https://eprint.iacr.org/2017/841.pdf).
	pub trait Mix {
	/// Mix `a` and `b`, writing into the last `s` bytes of the output arrays.
	/// `a` and `b` must be the same length `s`, and no longer than the block size `B`.
	/// Must be the inverse of [Mix::backwards].
	fn forwards<'a, 'b>(a: &'a [u8], b: &'b [u8]) -> (&'b [u8], &'a [u8]);

	/// Mix `a` and `b`, writing into the last `s` bytes of the output arrays.
	/// `a` and `b` must be the same length, and no longer than the block size `B`.
	/// Must be the inverse of [Mix::forwards].
	fn backwards<'a, 'b>(a: &'a [u8], b: &'b [u8]) -> (&'b [u8], &'a [u8]);
	}

	/// Per section 2.4, swapping `a` and `b` is a valid mix function
	pub struct Swap {}

	impl Mix for Swap {
	fn forwards<'a, 'b>(a: &'a [u8], b: &'b [u8]) -> (&'b [u8], &'a [u8]) {
	debug_assert_eq!(a.len(), b.len());
	(b, a)
	}

	fn backwards<'a, 'b>(a: &'a [u8], b: &'b [u8]) -> (&'b [u8], &'a [u8]) {
	// backwards is the same as forwards.
	Self::forwards(a, b)
	}
	}

	/// Pads partial-block input into tweaks.
	///
	/// This is exposed as a separate trait to allow for padding methods beyond the default.
	pub trait Padder<const B: usize, T: TweakableBlockCipher<B>> {
	/// Build a tweak for `T` out of `data`.
	/// `data` must be shorter than the tweak size so that some padding can take place.
	///
	/// # Panics
	///
	/// Panics if the length of `data` >= the tweak size.
	fn pad_tweak(&self, data: &[u8]) -> T::Tweak;
	}

	/// The default padding algorithm per section 2 of [LDT paper](https://eprint.iacr.org/2017/841.pdf)
	#[derive(Default)]
	pub struct DefaultPadder;

	impl<const B: usize, T: TweakableBlockCipher<B>> Padder<B, T> for DefaultPadder {
	fn pad_tweak(&self, data: &[u8]) -> T::Tweak {
	Self::default_padding(data).into()
	}
	}

	impl DefaultPadder {
	/// Expand `data` to an array of the appropriate size, and append a 1 bit after the original data.
	///
	/// `T` is the tweak size to pad to in bytes.
	///
	/// # Panics
	///
	/// Panics if the length of the data to pad is >= the tweak size.
	// allow index_slicing, since we are ensuring panic is impossible in assert
	#[allow(clippy::indexing_slicing)]
	fn default_padding<const N: usize>(data: &[u8]) -> [u8; N] {
	// If this assert fails, our LDT impl is broken - always pads data < block size
	assert!(data.len() < N);
	let mut out = [0; N];
	out[0..data.len()].copy_from_slice(data);
	// 0b1 followed by zeros for all remaining bits.
	// Since the array was initialized with 0, nothing left to do.
	out[data.len()] = 128;
	out
	}
	}

	/// Pads with the default algorithm to the tweak size, then XORs that with the provided bytes.
	///
	/// This is useful as a means to perturb the cipher output without having to alter the input
	/// directly.
	///
	/// `T` is the tweak size to pad to in bytes
	#[derive(Debug, PartialEq, Eq)]
	pub struct XorPadder<const T: usize> {
	xor_bytes: [u8; T],
	}

	impl<const T: usize> From<[u8; T]> for XorPadder<T> {
	fn from(bytes: [u8; T]) -> Self {
	XorPadder { xor_bytes: bytes }
	}
	}

	impl<const B: usize, T: TweakableBlockCipher<B>> Padder<B, T> for XorPadder<B> {
	fn pad_tweak(&self, data: &[u8]) -> T::Tweak {
	let mut out = DefaultPadder::default_padding::<B>(data);
	debug_assert_eq!(self.xor_bytes.len(), out.len());

	// xor into the padded data
	out.iter_mut()
	.zip(self.xor_bytes.iter())
	.for_each(\|(out_byte, xor_byte): (&mut u8, &u8)\| *out_byte ^= xor_byte);

	out.into()
	}
	}