const std = @import("../../std.zig");
const math = std.math;
const mem = std.mem;

const side_channels_mitigations = std.options.side_channels_mitigations;

/// A single AES block.
pub const Block = struct {
    const Repr = [4]u32;

    pub const block_length: usize = 16;

    /// Internal representation of a block.
    repr: Repr align(16),

    /// Convert a byte sequence into an internal representation.
    pub fn fromBytes(bytes: *const [16]u8) Block {
        const s0 = mem.readInt(u32, bytes[0..4], .little);
        const s1 = mem.readInt(u32, bytes[4..8], .little);
        const s2 = mem.readInt(u32, bytes[8..12], .little);
        const s3 = mem.readInt(u32, bytes[12..16], .little);
        return Block{ .repr = Repr{ s0, s1, s2, s3 } };
    }

    /// Convert the internal representation of a block into a byte sequence.
    pub fn toBytes(block: Block) [16]u8 {
        var bytes: [16]u8 = undefined;
        mem.writeInt(u32, bytes[0..4], block.repr[0], .little);
        mem.writeInt(u32, bytes[4..8], block.repr[1], .little);
        mem.writeInt(u32, bytes[8..12], block.repr[2], .little);
        mem.writeInt(u32, bytes[12..16], block.repr[3], .little);
        return bytes;
    }

    /// XOR the block with a byte sequence.
    pub fn xorBytes(block: Block, bytes: *const [16]u8) [16]u8 {
        const block_bytes = block.toBytes();
        var x: [16]u8 = undefined;
        comptime var i: usize = 0;
        inline while (i < 16) : (i += 1) {
            x[i] = block_bytes[i] ^ bytes[i];
        }
        return x;
    }

    /// Encrypt a block with a round key.
    pub fn encrypt(block: Block, round_key: Block) Block {
        const s0 = block.repr[0];
        const s1 = block.repr[1];
        const s2 = block.repr[2];
        const s3 = block.repr[3];

        var x: [4]u32 = undefined;
        x = table_lookup(&table_encrypt, @as(u8, @truncate(s0)), @as(u8, @truncate(s1 >> 8)), @as(u8, @truncate(s2 >> 16)), @as(u8, @truncate(s3 >> 24)));
        var t0 = x[0] ^ x[1] ^ x[2] ^ x[3];
        x = table_lookup(&table_encrypt, @as(u8, @truncate(s1)), @as(u8, @truncate(s2 >> 8)), @as(u8, @truncate(s3 >> 16)), @as(u8, @truncate(s0 >> 24)));
        var t1 = x[0] ^ x[1] ^ x[2] ^ x[3];
        x = table_lookup(&table_encrypt, @as(u8, @truncate(s2)), @as(u8, @truncate(s3 >> 8)), @as(u8, @truncate(s0 >> 16)), @as(u8, @truncate(s1 >> 24)));
        var t2 = x[0] ^ x[1] ^ x[2] ^ x[3];
        x = table_lookup(&table_encrypt, @as(u8, @truncate(s3)), @as(u8, @truncate(s0 >> 8)), @as(u8, @truncate(s1 >> 16)), @as(u8, @truncate(s2 >> 24)));
        var t3 = x[0] ^ x[1] ^ x[2] ^ x[3];

        t0 ^= round_key.repr[0];
        t1 ^= round_key.repr[1];
        t2 ^= round_key.repr[2];
        t3 ^= round_key.repr[3];

        return Block{ .repr = Repr{ t0, t1, t2, t3 } };
    }

    /// Encrypt a block with a round key *WITHOUT ANY PROTECTION AGAINST SIDE CHANNELS*
    pub fn encryptUnprotected(block: Block, round_key: Block) Block {
        const s0 = block.repr[0];
        const s1 = block.repr[1];
        const s2 = block.repr[2];
        const s3 = block.repr[3];

        var x: [4]u32 = undefined;
        x = .{
            table_encrypt[0][@as(u8, @truncate(s0))],
            table_encrypt[1][@as(u8, @truncate(s1 >> 8))],
            table_encrypt[2][@as(u8, @truncate(s2 >> 16))],
            table_encrypt[3][@as(u8, @truncate(s3 >> 24))],
        };
        var t0 = x[0] ^ x[1] ^ x[2] ^ x[3];
        x = .{
            table_encrypt[0][@as(u8, @truncate(s1))],
            table_encrypt[1][@as(u8, @truncate(s2 >> 8))],
            table_encrypt[2][@as(u8, @truncate(s3 >> 16))],
            table_encrypt[3][@as(u8, @truncate(s0 >> 24))],
        };
        var t1 = x[0] ^ x[1] ^ x[2] ^ x[3];
        x = .{
            table_encrypt[0][@as(u8, @truncate(s2))],
            table_encrypt[1][@as(u8, @truncate(s3 >> 8))],
            table_encrypt[2][@as(u8, @truncate(s0 >> 16))],
            table_encrypt[3][@as(u8, @truncate(s1 >> 24))],
        };
        var t2 = x[0] ^ x[1] ^ x[2] ^ x[3];
        x = .{
            table_encrypt[0][@as(u8, @truncate(s3))],
            table_encrypt[1][@as(u8, @truncate(s0 >> 8))],
            table_encrypt[2][@as(u8, @truncate(s1 >> 16))],
            table_encrypt[3][@as(u8, @truncate(s2 >> 24))],
        };
        var t3 = x[0] ^ x[1] ^ x[2] ^ x[3];

        t0 ^= round_key.repr[0];
        t1 ^= round_key.repr[1];
        t2 ^= round_key.repr[2];
        t3 ^= round_key.repr[3];

        return Block{ .repr = Repr{ t0, t1, t2, t3 } };
    }

    /// Encrypt a block with the last round key.
    pub fn encryptLast(block: Block, round_key: Block) Block {
        const s0 = block.repr[0];
        const s1 = block.repr[1];
        const s2 = block.repr[2];
        const s3 = block.repr[3];

        // Last round uses s-box directly and XORs to produce output.
        var x: [4]u8 = undefined;
        x = sbox_lookup(&sbox_encrypt, @as(u8, @truncate(s0)), @as(u8, @truncate(s1 >> 8)), @as(u8, @truncate(s2 >> 16)), @as(u8, @truncate(s3 >> 24)));
        var t0 = mem.readInt(u32, &x, .little);
        x = sbox_lookup(&sbox_encrypt, @as(u8, @truncate(s1)), @as(u8, @truncate(s2 >> 8)), @as(u8, @truncate(s3 >> 16)), @as(u8, @truncate(s0 >> 24)));
        var t1 = mem.readInt(u32, &x, .little);
        x = sbox_lookup(&sbox_encrypt, @as(u8, @truncate(s2)), @as(u8, @truncate(s3 >> 8)), @as(u8, @truncate(s0 >> 16)), @as(u8, @truncate(s1 >> 24)));
        var t2 = mem.readInt(u32, &x, .little);
        x = sbox_lookup(&sbox_encrypt, @as(u8, @truncate(s3)), @as(u8, @truncate(s0 >> 8)), @as(u8, @truncate(s1 >> 16)), @as(u8, @truncate(s2 >> 24)));
        var t3 = mem.readInt(u32, &x, .little);

        t0 ^= round_key.repr[0];
        t1 ^= round_key.repr[1];
        t2 ^= round_key.repr[2];
        t3 ^= round_key.repr[3];

        return Block{ .repr = Repr{ t0, t1, t2, t3 } };
    }

    /// Decrypt a block with a round key.
    pub fn decrypt(block: Block, round_key: Block) Block {
        const s0 = block.repr[0];
        const s1 = block.repr[1];
        const s2 = block.repr[2];
        const s3 = block.repr[3];

        var x: [4]u32 = undefined;
        x = table_lookup(&table_decrypt, @as(u8, @truncate(s0)), @as(u8, @truncate(s3 >> 8)), @as(u8, @truncate(s2 >> 16)), @as(u8, @truncate(s1 >> 24)));
        var t0 = x[0] ^ x[1] ^ x[2] ^ x[3];
        x = table_lookup(&table_decrypt, @as(u8, @truncate(s1)), @as(u8, @truncate(s0 >> 8)), @as(u8, @truncate(s3 >> 16)), @as(u8, @truncate(s2 >> 24)));
        var t1 = x[0] ^ x[1] ^ x[2] ^ x[3];
        x = table_lookup(&table_decrypt, @as(u8, @truncate(s2)), @as(u8, @truncate(s1 >> 8)), @as(u8, @truncate(s0 >> 16)), @as(u8, @truncate(s3 >> 24)));
        var t2 = x[0] ^ x[1] ^ x[2] ^ x[3];
        x = table_lookup(&table_decrypt, @as(u8, @truncate(s3)), @as(u8, @truncate(s2 >> 8)), @as(u8, @truncate(s1 >> 16)), @as(u8, @truncate(s0 >> 24)));
        var t3 = x[0] ^ x[1] ^ x[2] ^ x[3];

        t0 ^= round_key.repr[0];
        t1 ^= round_key.repr[1];
        t2 ^= round_key.repr[2];
        t3 ^= round_key.repr[3];

        return Block{ .repr = Repr{ t0, t1, t2, t3 } };
    }

    /// Decrypt a block with a round key *WITHOUT ANY PROTECTION AGAINST SIDE CHANNELS*
    pub fn decryptUnprotected(block: Block, round_key: Block) Block {
        const s0 = block.repr[0];
        const s1 = block.repr[1];
        const s2 = block.repr[2];
        const s3 = block.repr[3];

        var x: [4]u32 = undefined;
        x = .{
            table_decrypt[0][@as(u8, @truncate(s0))],
            table_decrypt[1][@as(u8, @truncate(s3 >> 8))],
            table_decrypt[2][@as(u8, @truncate(s2 >> 16))],
            table_decrypt[3][@as(u8, @truncate(s1 >> 24))],
        };
        var t0 = x[0] ^ x[1] ^ x[2] ^ x[3];
        x = .{
            table_decrypt[0][@as(u8, @truncate(s1))],
            table_decrypt[1][@as(u8, @truncate(s0 >> 8))],
            table_decrypt[2][@as(u8, @truncate(s3 >> 16))],
            table_decrypt[3][@as(u8, @truncate(s2 >> 24))],
        };
        var t1 = x[0] ^ x[1] ^ x[2] ^ x[3];
        x = .{
            table_decrypt[0][@as(u8, @truncate(s2))],
            table_decrypt[1][@as(u8, @truncate(s1 >> 8))],
            table_decrypt[2][@as(u8, @truncate(s0 >> 16))],
            table_decrypt[3][@as(u8, @truncate(s3 >> 24))],
        };
        var t2 = x[0] ^ x[1] ^ x[2] ^ x[3];
        x = .{
            table_decrypt[0][@as(u8, @truncate(s3))],
            table_decrypt[1][@as(u8, @truncate(s2 >> 8))],
            table_decrypt[2][@as(u8, @truncate(s1 >> 16))],
            table_decrypt[3][@as(u8, @truncate(s0 >> 24))],
        };
        var t3 = x[0] ^ x[1] ^ x[2] ^ x[3];

        t0 ^= round_key.repr[0];
        t1 ^= round_key.repr[1];
        t2 ^= round_key.repr[2];
        t3 ^= round_key.repr[3];

        return Block{ .repr = Repr{ t0, t1, t2, t3 } };
    }

    /// Decrypt a block with the last round key.
    pub fn decryptLast(block: Block, round_key: Block) Block {
        const s0 = block.repr[0];
        const s1 = block.repr[1];
        const s2 = block.repr[2];
        const s3 = block.repr[3];

        // Last round uses s-box directly and XORs to produce output.
        var x: [4]u8 = undefined;
        x = sbox_lookup(&sbox_decrypt, @as(u8, @truncate(s0)), @as(u8, @truncate(s3 >> 8)), @as(u8, @truncate(s2 >> 16)), @as(u8, @truncate(s1 >> 24)));
        var t0 = mem.readInt(u32, &x, .little);
        x = sbox_lookup(&sbox_decrypt, @as(u8, @truncate(s1)), @as(u8, @truncate(s0 >> 8)), @as(u8, @truncate(s3 >> 16)), @as(u8, @truncate(s2 >> 24)));
        var t1 = mem.readInt(u32, &x, .little);
        x = sbox_lookup(&sbox_decrypt, @as(u8, @truncate(s2)), @as(u8, @truncate(s1 >> 8)), @as(u8, @truncate(s0 >> 16)), @as(u8, @truncate(s3 >> 24)));
        var t2 = mem.readInt(u32, &x, .little);
        x = sbox_lookup(&sbox_decrypt, @as(u8, @truncate(s3)), @as(u8, @truncate(s2 >> 8)), @as(u8, @truncate(s1 >> 16)), @as(u8, @truncate(s0 >> 24)));
        var t3 = mem.readInt(u32, &x, .little);

        t0 ^= round_key.repr[0];
        t1 ^= round_key.repr[1];
        t2 ^= round_key.repr[2];
        t3 ^= round_key.repr[3];

        return Block{ .repr = Repr{ t0, t1, t2, t3 } };
    }

    /// Apply the bitwise XOR operation to the content of two blocks.
    pub fn xorBlocks(block1: Block, block2: Block) Block {
        var x: Repr = undefined;
        comptime var i = 0;
        inline while (i < 4) : (i += 1) {
            x[i] = block1.repr[i] ^ block2.repr[i];
        }
        return Block{ .repr = x };
    }

    /// Apply the bitwise AND operation to the content of two blocks.
    pub fn andBlocks(block1: Block, block2: Block) Block {
        var x: Repr = undefined;
        comptime var i = 0;
        inline while (i < 4) : (i += 1) {
            x[i] = block1.repr[i] & block2.repr[i];
        }
        return Block{ .repr = x };
    }

    /// Apply the bitwise OR operation to the content of two blocks.
    pub fn orBlocks(block1: Block, block2: Block) Block {
        var x: Repr = undefined;
        comptime var i = 0;
        inline while (i < 4) : (i += 1) {
            x[i] = block1.repr[i] | block2.repr[i];
        }
        return Block{ .repr = x };
    }

    /// Perform operations on multiple blocks in parallel.
    pub const parallel = struct {
        /// The recommended number of AES encryption/decryption to perform in parallel for the chosen implementation.
        pub const optimal_parallel_blocks = 1;

        /// Encrypt multiple blocks in parallel, each their own round key.
        pub fn encryptParallel(comptime count: usize, blocks: [count]Block, round_keys: [count]Block) [count]Block {
            var i = 0;
            var out: [count]Block = undefined;
            while (i < count) : (i += 1) {
                out[i] = blocks[i].encrypt(round_keys[i]);
            }
            return out;
        }

        /// Decrypt multiple blocks in parallel, each their own round key.
        pub fn decryptParallel(comptime count: usize, blocks: [count]Block, round_keys: [count]Block) [count]Block {
            var i = 0;
            var out: [count]Block = undefined;
            while (i < count) : (i += 1) {
                out[i] = blocks[i].decrypt(round_keys[i]);
            }
            return out;
        }

        /// Encrypt multiple blocks in parallel with the same round key.
        pub fn encryptWide(comptime count: usize, blocks: [count]Block, round_key: Block) [count]Block {
            var i = 0;
            var out: [count]Block = undefined;
            while (i < count) : (i += 1) {
                out[i] = blocks[i].encrypt(round_key);
            }
            return out;
        }

        /// Decrypt multiple blocks in parallel with the same round key.
        pub fn decryptWide(comptime count: usize, blocks: [count]Block, round_key: Block) [count]Block {
            var i = 0;
            var out: [count]Block = undefined;
            while (i < count) : (i += 1) {
                out[i] = blocks[i].decrypt(round_key);
            }
            return out;
        }

        /// Encrypt multiple blocks in parallel with the same last round key.
        pub fn encryptLastWide(comptime count: usize, blocks: [count]Block, round_key: Block) [count]Block {
            var i = 0;
            var out: [count]Block = undefined;
            while (i < count) : (i += 1) {
                out[i] = blocks[i].encryptLast(round_key);
            }
            return out;
        }

        /// Decrypt multiple blocks in parallel with the same last round key.
        pub fn decryptLastWide(comptime count: usize, blocks: [count]Block, round_key: Block) [count]Block {
            var i = 0;
            var out: [count]Block = undefined;
            while (i < count) : (i += 1) {
                out[i] = blocks[i].decryptLast(round_key);
            }
            return out;
        }
    };
};

/// A fixed-size vector of AES blocks.
/// All operations are performed in parallel, using SIMD instructions when available.
pub fn BlockVec(comptime blocks_count: comptime_int) type {
    return struct {
        const Self = @This();

        /// The number of AES blocks the target architecture can process with a single instruction.
        pub const native_vector_size = 1;

        /// The size of the AES block vector that the target architecture can process with a single instruction, in bytes.
        pub const native_word_size = native_vector_size * 16;

        const native_words = blocks_count;

        /// Internal representation of a block vector.
        repr: [native_words]Block,

        /// Length of the block vector in bytes.
        pub const block_length: usize = blocks_count * 16;

        /// Convert a byte sequence into an internal representation.
        pub fn fromBytes(bytes: *const [blocks_count * 16]u8) Self {
            var out: Self = undefined;
            for (0..native_words) |i| {
                out.repr[i] = Block.fromBytes(bytes[i * native_word_size ..][0..native_word_size]);
            }
            return out;
        }

        /// Convert the internal representation of a block vector into a byte sequence.
        pub fn toBytes(block_vec: Self) [blocks_count * 16]u8 {
            var out: [blocks_count * 16]u8 = undefined;
            for (0..native_words) |i| {
                out[i * native_word_size ..][0..native_word_size].* = block_vec.repr[i].toBytes();
            }
            return out;
        }

        /// XOR the block vector with a byte sequence.
        pub fn xorBytes(block_vec: Self, bytes: *const [blocks_count * 16]u8) [32]u8 {
            var out: Self = undefined;
            for (0..native_words) |i| {
                out.repr[i] = block_vec.repr[i].xorBytes(bytes[i * native_word_size ..][0..native_word_size]);
            }
            return out;
        }

        /// Apply the forward AES operation to the block vector with a vector of round keys.
        pub fn encrypt(block_vec: Self, round_key_vec: Self) Self {
            var out: Self = undefined;
            for (0..native_words) |i| {
                out.repr[i] = block_vec.repr[i].encrypt(round_key_vec.repr[i]);
            }
            return out;
        }

        /// Apply the forward AES operation to the block vector with a vector of last round keys.
        pub fn encryptLast(block_vec: Self, round_key_vec: Self) Self {
            var out: Self = undefined;
            for (0..native_words) |i| {
                out.repr[i] = block_vec.repr[i].encryptLast(round_key_vec.repr[i]);
            }
            return out;
        }

        /// Apply the inverse AES operation to the block vector with a vector of round keys.
        pub fn decrypt(block_vec: Self, inv_round_key_vec: Self) Self {
            var out: Self = undefined;
            for (0..native_words) |i| {
                out.repr[i] = block_vec.repr[i].decrypt(inv_round_key_vec.repr[i]);
            }
            return out;
        }

        /// Apply the inverse AES operation to the block vector with a vector of last round keys.
        pub fn decryptLast(block_vec: Self, inv_round_key_vec: Self) Self {
            var out: Self = undefined;
            for (0..native_words) |i| {
                out.repr[i] = block_vec.repr[i].decryptLast(inv_round_key_vec.repr[i]);
            }
            return out;
        }

        /// Apply the bitwise XOR operation to the content of two block vectors.
        pub fn xorBlocks(block_vec1: Self, block_vec2: Self) Self {
            var out: Self = undefined;
            for (0..native_words) |i| {
                out.repr[i] = block_vec1.repr[i].xorBlocks(block_vec2.repr[i]);
            }
            return out;
        }

        /// Apply the bitwise AND operation to the content of two block vectors.
        pub fn andBlocks(block_vec1: Self, block_vec2: Self) Self {
            var out: Self = undefined;
            for (0..native_words) |i| {
                out.repr[i] = block_vec1.repr[i].andBlocks(block_vec2.repr[i]);
            }
            return out;
        }

        /// Apply the bitwise OR operation to the content of two block vectors.
        pub fn orBlocks(block_vec1: Self, block_vec2: Block) Self {
            var out: Self = undefined;
            for (0..native_words) |i| {
                out.repr[i] = block_vec1.repr[i].orBlocks(block_vec2.repr[i]);
            }
            return out;
        }
    };
}

fn KeySchedule(comptime Aes: type) type {
    std.debug.assert(Aes.rounds == 10 or Aes.rounds == 14);
    const key_length = Aes.key_bits / 8;
    const rounds = Aes.rounds;

    return struct {
        const Self = @This();
        const words_in_key = key_length / 4;

        round_keys: [rounds + 1]Block,

        // Key expansion algorithm. See FIPS-197, Figure 11.
        fn expandKey(key: [key_length]u8) Self {
            const subw = struct {
                // Apply sbox_encrypt to each byte in w.
                fn func(w: u32) u32 {
                    const x = sbox_lookup(&sbox_key_schedule, @as(u8, @truncate(w)), @as(u8, @truncate(w >> 8)), @as(u8, @truncate(w >> 16)), @as(u8, @truncate(w >> 24)));
                    return mem.readInt(u32, &x, .little);
                }
            }.func;

            var round_keys: [rounds + 1]Block = undefined;
            comptime var i: usize = 0;
            inline while (i < words_in_key) : (i += 1) {
                round_keys[i / 4].repr[i % 4] = mem.readInt(u32, key[4 * i ..][0..4], .big);
            }
            inline while (i < round_keys.len * 4) : (i += 1) {
                var t = round_keys[(i - 1) / 4].repr[(i - 1) % 4];
                if (i % words_in_key == 0) {
                    t = subw(std.math.rotl(u32, t, 8)) ^ (@as(u32, powx[i / words_in_key - 1]) << 24);
                } else if (words_in_key > 6 and i % words_in_key == 4) {
                    t = subw(t);
                }
                round_keys[i / 4].repr[i % 4] = round_keys[(i - words_in_key) / 4].repr[(i - words_in_key) % 4] ^ t;
            }
            i = 0;
            inline while (i < round_keys.len * 4) : (i += 1) {
                round_keys[i / 4].repr[i % 4] = @byteSwap(round_keys[i / 4].repr[i % 4]);
            }
            return Self{ .round_keys = round_keys };
        }

        /// Invert the key schedule.
        pub fn invert(key_schedule: Self) Self {
            const round_keys = &key_schedule.round_keys;
            var inv_round_keys: [rounds + 1]Block = undefined;
            const total_words = 4 * round_keys.len;
            var i: usize = 0;
            while (i < total_words) : (i += 4) {
                const ei = total_words - i - 4;
                comptime var j: usize = 0;
                inline while (j < 4) : (j += 1) {
                    var rk = round_keys[(ei + j) / 4].repr[(ei + j) % 4];
                    if (i > 0 and i + 4 < total_words) {
                        const x = sbox_lookup(&sbox_key_schedule, @as(u8, @truncate(rk >> 24)), @as(u8, @truncate(rk >> 16)), @as(u8, @truncate(rk >> 8)), @as(u8, @truncate(rk)));
                        const y = table_lookup(&table_decrypt, x[3], x[2], x[1], x[0]);
                        rk = y[0] ^ y[1] ^ y[2] ^ y[3];
                    }
                    inv_round_keys[(i + j) / 4].repr[(i + j) % 4] = rk;
                }
            }
            return Self{ .round_keys = inv_round_keys };
        }
    };
}

/// A context to perform encryption using the standard AES key schedule.
pub fn AesEncryptCtx(comptime Aes: type) type {
    std.debug.assert(Aes.key_bits == 128 or Aes.key_bits == 256);
    const rounds = Aes.rounds;

    return struct {
        const Self = @This();
        pub const block = Aes.block;
        pub const block_length = block.block_length;
        key_schedule: KeySchedule(Aes),

        /// Create a new encryption context with the given key.
        pub fn init(key: [Aes.key_bits / 8]u8) Self {
            const key_schedule = KeySchedule(Aes).expandKey(key);
            return Self{
                .key_schedule = key_schedule,
            };
        }

        /// Encrypt a single block.
        pub fn encrypt(ctx: Self, dst: *[16]u8, src: *const [16]u8) void {
            const round_keys = ctx.key_schedule.round_keys;
            var t = Block.fromBytes(src).xorBlocks(round_keys[0]);
            comptime var i = 1;
            if (side_channels_mitigations == .full) {
                inline while (i < rounds) : (i += 1) {
                    t = t.encrypt(round_keys[i]);
                }
            } else {
                inline while (i < 5) : (i += 1) {
                    t = t.encrypt(round_keys[i]);
                }
                inline while (i < rounds - 1) : (i += 1) {
                    t = t.encryptUnprotected(round_keys[i]);
                }
                t = t.encrypt(round_keys[i]);
            }
            t = t.encryptLast(round_keys[rounds]);
            dst.* = t.toBytes();
        }

        /// Encrypt+XOR a single block.
        pub fn xor(ctx: Self, dst: *[16]u8, src: *const [16]u8, counter: [16]u8) void {
            const round_keys = ctx.key_schedule.round_keys;
            var t = Block.fromBytes(&counter).xorBlocks(round_keys[0]);
            comptime var i = 1;
            if (side_channels_mitigations == .full) {
                inline while (i < rounds) : (i += 1) {
                    t = t.encrypt(round_keys[i]);
                }
            } else {
                inline while (i < 5) : (i += 1) {
                    t = t.encrypt(round_keys[i]);
                }
                inline while (i < rounds - 1) : (i += 1) {
                    t = t.encryptUnprotected(round_keys[i]);
                }
                t = t.encrypt(round_keys[i]);
            }
            t = t.encryptLast(round_keys[rounds]);
            dst.* = t.xorBytes(src);
        }

        /// Encrypt multiple blocks, possibly leveraging parallelization.
        pub fn encryptWide(ctx: Self, comptime count: usize, dst: *[16 * count]u8, src: *const [16 * count]u8) void {
            var i: usize = 0;
            while (i < count) : (i += 1) {
                ctx.encrypt(dst[16 * i .. 16 * i + 16][0..16], src[16 * i .. 16 * i + 16][0..16]);
            }
        }

        /// Encrypt+XOR multiple blocks, possibly leveraging parallelization.
        pub fn xorWide(ctx: Self, comptime count: usize, dst: *[16 * count]u8, src: *const [16 * count]u8, counters: [16 * count]u8) void {
            var i: usize = 0;
            while (i < count) : (i += 1) {
                ctx.xor(dst[16 * i .. 16 * i + 16][0..16], src[16 * i .. 16 * i + 16][0..16], counters[16 * i .. 16 * i + 16][0..16].*);
            }
        }
    };
}

/// A context to perform decryption using the standard AES key schedule.
pub fn AesDecryptCtx(comptime Aes: type) type {
    std.debug.assert(Aes.key_bits == 128 or Aes.key_bits == 256);
    const rounds = Aes.rounds;

    return struct {
        const Self = @This();
        pub const block = Aes.block;
        pub const block_length = block.block_length;
        key_schedule: KeySchedule(Aes),

        /// Create a decryption context from an existing encryption context.
        pub fn initFromEnc(ctx: AesEncryptCtx(Aes)) Self {
            return Self{
                .key_schedule = ctx.key_schedule.invert(),
            };
        }

        /// Create a new decryption context with the given key.
        pub fn init(key: [Aes.key_bits / 8]u8) Self {
            const enc_ctx = AesEncryptCtx(Aes).init(key);
            return initFromEnc(enc_ctx);
        }

        /// Decrypt a single block.
        pub fn decrypt(ctx: Self, dst: *[16]u8, src: *const [16]u8) void {
            const inv_round_keys = ctx.key_schedule.round_keys;
            var t = Block.fromBytes(src).xorBlocks(inv_round_keys[0]);
            comptime var i = 1;
            if (side_channels_mitigations == .full) {
                inline while (i < rounds) : (i += 1) {
                    t = t.decrypt(inv_round_keys[i]);
                }
            } else {
                inline while (i < 5) : (i += 1) {
                    t = t.decrypt(inv_round_keys[i]);
                }
                inline while (i < rounds - 1) : (i += 1) {
                    t = t.decryptUnprotected(inv_round_keys[i]);
                }
                t = t.decrypt(inv_round_keys[i]);
            }
            t = t.decryptLast(inv_round_keys[rounds]);
            dst.* = t.toBytes();
        }

        /// Decrypt multiple blocks, possibly leveraging parallelization.
        pub fn decryptWide(ctx: Self, comptime count: usize, dst: *[16 * count]u8, src: *const [16 * count]u8) void {
            var i: usize = 0;
            while (i < count) : (i += 1) {
                ctx.decrypt(dst[16 * i .. 16 * i + 16][0..16], src[16 * i .. 16 * i + 16][0..16]);
            }
        }
    };
}

/// AES-128 with the standard key schedule.
pub const Aes128 = struct {
    pub const key_bits: usize = 128;
    pub const rounds = ((key_bits - 64) / 32 + 8);
    pub const block = Block;

    /// Create a new context for encryption.
    pub fn initEnc(key: [key_bits / 8]u8) AesEncryptCtx(Aes128) {
        return AesEncryptCtx(Aes128).init(key);
    }

    /// Create a new context for decryption.
    pub fn initDec(key: [key_bits / 8]u8) AesDecryptCtx(Aes128) {
        return AesDecryptCtx(Aes128).init(key);
    }
};

/// AES-256 with the standard key schedule.
pub const Aes256 = struct {
    pub const key_bits: usize = 256;
    pub const rounds = ((key_bits - 64) / 32 + 8);
    pub const block = Block;

    /// Create a new context for encryption.
    pub fn initEnc(key: [key_bits / 8]u8) AesEncryptCtx(Aes256) {
        return AesEncryptCtx(Aes256).init(key);
    }

    /// Create a new context for decryption.
    pub fn initDec(key: [key_bits / 8]u8) AesDecryptCtx(Aes256) {
        return AesDecryptCtx(Aes256).init(key);
    }
};

// constants

// Rijndael's irreducible polynomial.
const poly: u9 = 1 << 8 | 1 << 4 | 1 << 3 | 1 << 1 | 1 << 0; // x⁸ + x⁴ + x³ + x + 1

// Powers of x mod poly in GF(2).
const powx = init: {
    var array: [16]u8 = undefined;

    var value = 1;
    for (&array) |*power| {
        power.* = value;
        value = mul(value, 2);
    }

    break :init array;
};

const sbox_encrypt align(64) = generateSbox(false); // S-box for encryption
const sbox_key_schedule align(64) = generateSbox(false); // S-box only for key schedule, so that it uses distinct L1 cache entries than the S-box used for encryption
const sbox_decrypt align(64) = generateSbox(true); // S-box for decryption
const table_encrypt align(64) = generateTable(false); // 4-byte LUTs for encryption
const table_decrypt align(64) = generateTable(true); // 4-byte LUTs for decryption

// Generate S-box substitution values.
fn generateSbox(invert: bool) [256]u8 {
    @setEvalBranchQuota(10000);

    var sbox: [256]u8 = undefined;

    var p: u8 = 1;
    var q: u8 = 1;
    for (sbox) |_| {
        p = mul(p, 3);
        q = mul(q, 0xf6); // divide by 3

        var value: u8 = q ^ 0x63;
        value ^= math.rotl(u8, q, 1);
        value ^= math.rotl(u8, q, 2);
        value ^= math.rotl(u8, q, 3);
        value ^= math.rotl(u8, q, 4);

        if (invert) {
            sbox[value] = p;
        } else {
            sbox[p] = value;
        }
    }

    if (invert) {
        sbox[0x63] = 0x00;
    } else {
        sbox[0x00] = 0x63;
    }

    return sbox;
}

// Generate lookup tables.
fn generateTable(invert: bool) [4][256]u32 {
    @setEvalBranchQuota(50000);

    var table: [4][256]u32 = undefined;

    for (generateSbox(invert), 0..) |value, index| {
        table[0][index] = math.shl(u32, mul(value, if (invert) 0xb else 0x3), 24);
        table[0][index] |= math.shl(u32, mul(value, if (invert) 0xd else 0x1), 16);
        table[0][index] |= math.shl(u32, mul(value, if (invert) 0x9 else 0x1), 8);
        table[0][index] |= mul(value, if (invert) 0xe else 0x2);

        table[1][index] = math.rotl(u32, table[0][index], 8);
        table[2][index] = math.rotl(u32, table[0][index], 16);
        table[3][index] = math.rotl(u32, table[0][index], 24);
    }

    return table;
}

// Multiply a and b as GF(2) polynomials modulo poly.
fn mul(a: u8, b: u8) u8 {
    @setEvalBranchQuota(30000);

    var i: u8 = a;
    var j: u9 = b;
    var s: u9 = 0;

    while (i > 0) : (i >>= 1) {
        if (i & 1 != 0) {
            s ^= j;
        }

        j *= 2;
        if (j & 0x100 != 0) {
            j ^= poly;
        }
    }

    return @as(u8, @truncate(s));
}

const cache_line_bytes = std.atomic.cache_line;

fn sbox_lookup(sbox: *align(64) const [256]u8, idx0: u8, idx1: u8, idx2: u8, idx3: u8) [4]u8 {
    if (side_channels_mitigations == .none) {
        return [4]u8{
            sbox[idx0],
            sbox[idx1],
            sbox[idx2],
            sbox[idx3],
        };
    } else {
        const stride = switch (side_channels_mitigations) {
            .none => unreachable,
            .basic => sbox.len / 4,
            .medium => @min(sbox.len, 2 * cache_line_bytes),
            .full => @min(sbox.len, cache_line_bytes),
        };
        const of0 = idx0 % stride;
        const of1 = idx1 % stride;
        const of2 = idx2 % stride;
        const of3 = idx3 % stride;
        var t: [4][sbox.len / stride]u8 align(64) = undefined;
        var i: usize = 0;
        while (i < t[0].len) : (i += 1) {
            const tx = sbox[i * stride ..];
            t[0][i] = tx[of0];
            t[1][i] = tx[of1];
            t[2][i] = tx[of2];
            t[3][i] = tx[of3];
        }
        std.mem.doNotOptimizeAway(t);
        return [4]u8{
            t[0][idx0 / stride],
            t[1][idx1 / stride],
            t[2][idx2 / stride],
            t[3][idx3 / stride],
        };
    }
}

fn table_lookup(table: *align(64) const [4][256]u32, idx0: u8, idx1: u8, idx2: u8, idx3: u8) [4]u32 {
    if (side_channels_mitigations == .none) {
        return [4]u32{
            table[0][idx0],
            table[1][idx1],
            table[2][idx2],
            table[3][idx3],
        };
    } else {
        const table_len: usize = 256;
        const stride = switch (side_channels_mitigations) {
            .none => unreachable,
            .basic => table_len / 4,
            .medium => @max(1, @min(table_len, 2 * cache_line_bytes / 4)),
            .full => @max(1, @min(table_len, cache_line_bytes / 4)),
        };
        const of0 = idx0 % stride;
        const of1 = idx1 % stride;
        const of2 = idx2 % stride;
        const of3 = idx3 % stride;
        var t: [4][table_len / stride]u32 align(64) = undefined;
        var i: usize = 0;
        while (i < t[0].len) : (i += 1) {
            const tx = table[0][i * stride ..];
            t[0][i] = tx[of0];
            t[1][i] = tx[of1];
            t[2][i] = tx[of2];
            t[3][i] = tx[of3];
        }
        std.mem.doNotOptimizeAway(t);
        return [4]u32{
            t[0][idx0 / stride],
            math.rotl(u32, (&t[1])[idx1 / stride], 8),
            math.rotl(u32, (&t[2])[idx2 / stride], 16),
            math.rotl(u32, (&t[3])[idx3 / stride], 24),
        };
    }
}
