// Based on public domain Supercop by Daniel J. Bernstein

const std = @import("../index.zig");
const mem = std.mem;
const endian = std.endian;
const assert = std.debug.assert;
const testing = std.testing;
const builtin = @import("builtin");
const maxInt = std.math.maxInt;

const QuarterRound = struct {
    a: usize,
    b: usize,
    c: usize,
    d: usize,
};

fn Rp(a: usize, b: usize, c: usize, d: usize) QuarterRound {
    return QuarterRound{
        .a = a,
        .b = b,
        .c = c,
        .d = d,
    };
}

// The chacha family of ciphers are based on the salsa family.
fn salsa20_wordtobyte(out: []u8, input: [16]u32) void {
    assert(out.len >= 64);

    var x: [16]u32 = undefined;

    for (x) |_, i|
        x[i] = input[i];

    const rounds = comptime []QuarterRound{
        Rp(0, 4, 8, 12),
        Rp(1, 5, 9, 13),
        Rp(2, 6, 10, 14),
        Rp(3, 7, 11, 15),
        Rp(0, 5, 10, 15),
        Rp(1, 6, 11, 12),
        Rp(2, 7, 8, 13),
        Rp(3, 4, 9, 14),
    };

    comptime var j: usize = 0;
    inline while (j < 20) : (j += 2) {
        // two-round cycles
        inline for (rounds) |r| {
            x[r.a] +%= x[r.b];
            x[r.d] = std.math.rotl(u32, x[r.d] ^ x[r.a], u32(16));
            x[r.c] +%= x[r.d];
            x[r.b] = std.math.rotl(u32, x[r.b] ^ x[r.c], u32(12));
            x[r.a] +%= x[r.b];
            x[r.d] = std.math.rotl(u32, x[r.d] ^ x[r.a], u32(8));
            x[r.c] +%= x[r.d];
            x[r.b] = std.math.rotl(u32, x[r.b] ^ x[r.c], u32(7));
        }
    }

    for (x) |_, i| {
        // TODO https://github.com/ziglang/zig/issues/863
        mem.writeIntSliceLittle(u32, out[4 * i .. 4 * i + 4], x[i] +% input[i]);
    }
}

fn chaCha20_internal(out: []u8, in: []const u8, key: [8]u32, counter: [4]u32) void {
    var ctx: [16]u32 = undefined;
    var remaining: usize = if (in.len > out.len) in.len else out.len;
    var cursor: usize = 0;

    const c = "expand 32-byte k";
    const constant_le = []u32{
        mem.readIntSliceLittle(u32, c[0..4]),
        mem.readIntSliceLittle(u32, c[4..8]),
        mem.readIntSliceLittle(u32, c[8..12]),
        mem.readIntSliceLittle(u32, c[12..16]),
    };

    mem.copy(u32, ctx[0..], constant_le[0..4]);
    mem.copy(u32, ctx[4..12], key[0..8]);
    mem.copy(u32, ctx[12..16], counter[0..4]);

    while (true) {
        var buf: [64]u8 = undefined;
        salsa20_wordtobyte(buf[0..], ctx);

        if (remaining < 64) {
            var i: usize = 0;
            while (i < remaining) : (i += 1)
                out[cursor + i] = in[cursor + i] ^ buf[i];
            return;
        }

        var i: usize = 0;
        while (i < 64) : (i += 1)
            out[cursor + i] = in[cursor + i] ^ buf[i];

        cursor += 64;
        remaining -= 64;

        ctx[12] += 1;
    }
}

/// ChaCha20 avoids the possibility of timing attacks, as there are no branches
/// on secret key data.
///
/// in and out should be the same length.
/// counter should generally be 0 or 1
///
/// ChaCha20 is self-reversing. To decrypt just run the cipher with the same
/// counter, nonce, and key.
pub fn chaCha20IETF(out: []u8, in: []const u8, counter: u32, key: [32]u8, nonce: [12]u8) void {
    assert(in.len >= out.len);
    assert((in.len >> 6) + counter <= maxInt(u32));

    var k: [8]u32 = undefined;
    var c: [4]u32 = undefined;

    k[0] = mem.readIntSliceLittle(u32, key[0..4]);
    k[1] = mem.readIntSliceLittle(u32, key[4..8]);
    k[2] = mem.readIntSliceLittle(u32, key[8..12]);
    k[3] = mem.readIntSliceLittle(u32, key[12..16]);
    k[4] = mem.readIntSliceLittle(u32, key[16..20]);
    k[5] = mem.readIntSliceLittle(u32, key[20..24]);
    k[6] = mem.readIntSliceLittle(u32, key[24..28]);
    k[7] = mem.readIntSliceLittle(u32, key[28..32]);

    c[0] = counter;
    c[1] = mem.readIntSliceLittle(u32, nonce[0..4]);
    c[2] = mem.readIntSliceLittle(u32, nonce[4..8]);
    c[3] = mem.readIntSliceLittle(u32, nonce[8..12]);
    chaCha20_internal(out, in, k, c);
}

/// This is the original ChaCha20 before RFC 7539, which recommends using the
/// orgininal version on applications such as disk or file encryption that might
/// exceed the 256 GiB limit of the 96-bit nonce version.
pub fn chaCha20With64BitNonce(out: []u8, in: []const u8, counter: u64, key: [32]u8, nonce: [8]u8) void {
    assert(in.len >= out.len);
    assert(counter +% (in.len >> 6) >= counter);

    var cursor: u64 = 0;
    var k: [8]u32 = undefined;
    var c: [4]u32 = undefined;

    k[0] = mem.readIntSliceLittle(u32, key[0..4]);
    k[1] = mem.readIntSliceLittle(u32, key[4..8]);
    k[2] = mem.readIntSliceLittle(u32, key[8..12]);
    k[3] = mem.readIntSliceLittle(u32, key[12..16]);
    k[4] = mem.readIntSliceLittle(u32, key[16..20]);
    k[5] = mem.readIntSliceLittle(u32, key[20..24]);
    k[6] = mem.readIntSliceLittle(u32, key[24..28]);
    k[7] = mem.readIntSliceLittle(u32, key[28..32]);

    c[0] = @truncate(u32, counter);
    c[1] = @truncate(u32, counter >> 32);
    c[2] = mem.readIntSliceLittle(u32, nonce[0..4]);
    c[3] = mem.readIntSliceLittle(u32, nonce[4..8]);

    const block_size = (1 << 6);
    const big_block = (block_size << 32);

    // first partial big block
    if (((@intCast(u64, maxInt(u32) - @truncate(u32, counter)) + 1) << 6) < in.len) {
        chaCha20_internal(out[cursor..big_block], in[cursor..big_block], k, c);
        cursor = big_block - cursor;
        c[1] += 1;
        if (comptime @sizeOf(usize) > 4) {
            // A big block is giant: 256 GiB, but we can avoid this limitation
            var remaining_blocks: u32 = @intCast(u32, (in.len / big_block));
            var i: u32 = 0;
            while (remaining_blocks > 0) : (remaining_blocks -= 1) {
                chaCha20_internal(out[cursor .. cursor + big_block], in[cursor .. cursor + big_block], k, c);
                c[1] += 1; // upper 32-bit of counter, generic chaCha20_internal() doesn't know about this.
                cursor += big_block;
            }
        }
    }

    chaCha20_internal(out[cursor..], in[cursor..], k, c);
}

// https://tools.ietf.org/html/rfc7539#section-2.4.2
test "crypto.chacha20 test vector sunscreen" {
    const expected_result = []u8{
        0x6e, 0x2e, 0x35, 0x9a, 0x25, 0x68, 0xf9, 0x80,
        0x41, 0xba, 0x07, 0x28, 0xdd, 0x0d, 0x69, 0x81,
        0xe9, 0x7e, 0x7a, 0xec, 0x1d, 0x43, 0x60, 0xc2,
        0x0a, 0x27, 0xaf, 0xcc, 0xfd, 0x9f, 0xae, 0x0b,
        0xf9, 0x1b, 0x65, 0xc5, 0x52, 0x47, 0x33, 0xab,
        0x8f, 0x59, 0x3d, 0xab, 0xcd, 0x62, 0xb3, 0x57,
        0x16, 0x39, 0xd6, 0x24, 0xe6, 0x51, 0x52, 0xab,
        0x8f, 0x53, 0x0c, 0x35, 0x9f, 0x08, 0x61, 0xd8,
        0x07, 0xca, 0x0d, 0xbf, 0x50, 0x0d, 0x6a, 0x61,
        0x56, 0xa3, 0x8e, 0x08, 0x8a, 0x22, 0xb6, 0x5e,
        0x52, 0xbc, 0x51, 0x4d, 0x16, 0xcc, 0xf8, 0x06,
        0x81, 0x8c, 0xe9, 0x1a, 0xb7, 0x79, 0x37, 0x36,
        0x5a, 0xf9, 0x0b, 0xbf, 0x74, 0xa3, 0x5b, 0xe6,
        0xb4, 0x0b, 0x8e, 0xed, 0xf2, 0x78, 0x5e, 0x42,
        0x87, 0x4d,
    };
    const input = "Ladies and Gentlemen of the class of '99: If I could offer you only one tip for the future, sunscreen would be it.";
    var result: [114]u8 = undefined;
    const key = []u8{
        0, 1, 2, 3, 4, 5, 6, 7,
        8, 9, 10, 11, 12, 13, 14, 15,
        16, 17, 18, 19, 20, 21, 22, 23,
        24, 25, 26, 27, 28, 29, 30, 31,
    };
    const nonce = []u8{
        0, 0, 0, 0,
        0, 0, 0, 0x4a,
        0, 0, 0, 0,
    };

    chaCha20IETF(result[0..], input[0..], 1, key, nonce);
    testing.expectEqualSlices(u8, expected_result, result);

    // Chacha20 is self-reversing.
    var plaintext: [114]u8 = undefined;
    chaCha20IETF(plaintext[0..], result[0..], 1, key, nonce);
    testing.expect(mem.compare(u8, input, plaintext) == mem.Compare.Equal);
}

// https://tools.ietf.org/html/draft-agl-tls-chacha20poly1305-04#section-7
test "crypto.chacha20 test vector 1" {
    const expected_result = []u8{
        0x76, 0xb8, 0xe0, 0xad, 0xa0, 0xf1, 0x3d, 0x90,
        0x40, 0x5d, 0x6a, 0xe5, 0x53, 0x86, 0xbd, 0x28,
        0xbd, 0xd2, 0x19, 0xb8, 0xa0, 0x8d, 0xed, 0x1a,
        0xa8, 0x36, 0xef, 0xcc, 0x8b, 0x77, 0x0d, 0xc7,
        0xda, 0x41, 0x59, 0x7c, 0x51, 0x57, 0x48, 0x8d,
        0x77, 0x24, 0xe0, 0x3f, 0xb8, 0xd8, 0x4a, 0x37,
        0x6a, 0x43, 0xb8, 0xf4, 0x15, 0x18, 0xa1, 0x1c,
        0xc3, 0x87, 0xb6, 0x69, 0xb2, 0xee, 0x65, 0x86,
    };
    const input = []u8{
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
    };
    var result: [64]u8 = undefined;
    const key = []u8{
        0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0,
    };
    const nonce = []u8{ 0, 0, 0, 0, 0, 0, 0, 0 };

    chaCha20With64BitNonce(result[0..], input[0..], 0, key, nonce);
    testing.expectEqualSlices(u8, expected_result, result);
}

test "crypto.chacha20 test vector 2" {
    const expected_result = []u8{
        0x45, 0x40, 0xf0, 0x5a, 0x9f, 0x1f, 0xb2, 0x96,
        0xd7, 0x73, 0x6e, 0x7b, 0x20, 0x8e, 0x3c, 0x96,
        0xeb, 0x4f, 0xe1, 0x83, 0x46, 0x88, 0xd2, 0x60,
        0x4f, 0x45, 0x09, 0x52, 0xed, 0x43, 0x2d, 0x41,
        0xbb, 0xe2, 0xa0, 0xb6, 0xea, 0x75, 0x66, 0xd2,
        0xa5, 0xd1, 0xe7, 0xe2, 0x0d, 0x42, 0xaf, 0x2c,
        0x53, 0xd7, 0x92, 0xb1, 0xc4, 0x3f, 0xea, 0x81,
        0x7e, 0x9a, 0xd2, 0x75, 0xae, 0x54, 0x69, 0x63,
    };
    const input = []u8{
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
    };
    var result: [64]u8 = undefined;
    const key = []u8{
        0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 1,
    };
    const nonce = []u8{ 0, 0, 0, 0, 0, 0, 0, 0 };

    chaCha20With64BitNonce(result[0..], input[0..], 0, key, nonce);
    testing.expectEqualSlices(u8, expected_result, result);
}

test "crypto.chacha20 test vector 3" {
    const expected_result = []u8{
        0xde, 0x9c, 0xba, 0x7b, 0xf3, 0xd6, 0x9e, 0xf5,
        0xe7, 0x86, 0xdc, 0x63, 0x97, 0x3f, 0x65, 0x3a,
        0x0b, 0x49, 0xe0, 0x15, 0xad, 0xbf, 0xf7, 0x13,
        0x4f, 0xcb, 0x7d, 0xf1, 0x37, 0x82, 0x10, 0x31,
        0xe8, 0x5a, 0x05, 0x02, 0x78, 0xa7, 0x08, 0x45,
        0x27, 0x21, 0x4f, 0x73, 0xef, 0xc7, 0xfa, 0x5b,
        0x52, 0x77, 0x06, 0x2e, 0xb7, 0xa0, 0x43, 0x3e,
        0x44, 0x5f, 0x41, 0xe3,
    };
    const input = []u8{
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00,
    };
    var result: [60]u8 = undefined;
    const key = []u8{
        0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0,
    };
    const nonce = []u8{ 0, 0, 0, 0, 0, 0, 0, 1 };

    chaCha20With64BitNonce(result[0..], input[0..], 0, key, nonce);
    testing.expectEqualSlices(u8, expected_result, result);
}

test "crypto.chacha20 test vector 4" {
    const expected_result = []u8{
        0xef, 0x3f, 0xdf, 0xd6, 0xc6, 0x15, 0x78, 0xfb,
        0xf5, 0xcf, 0x35, 0xbd, 0x3d, 0xd3, 0x3b, 0x80,
        0x09, 0x63, 0x16, 0x34, 0xd2, 0x1e, 0x42, 0xac,
        0x33, 0x96, 0x0b, 0xd1, 0x38, 0xe5, 0x0d, 0x32,
        0x11, 0x1e, 0x4c, 0xaf, 0x23, 0x7e, 0xe5, 0x3c,
        0xa8, 0xad, 0x64, 0x26, 0x19, 0x4a, 0x88, 0x54,
        0x5d, 0xdc, 0x49, 0x7a, 0x0b, 0x46, 0x6e, 0x7d,
        0x6b, 0xbd, 0xb0, 0x04, 0x1b, 0x2f, 0x58, 0x6b,
    };
    const input = []u8{
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
    };
    var result: [64]u8 = undefined;
    const key = []u8{
        0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0,
    };
    const nonce = []u8{ 1, 0, 0, 0, 0, 0, 0, 0 };

    chaCha20With64BitNonce(result[0..], input[0..], 0, key, nonce);
    testing.expectEqualSlices(u8, expected_result, result);
}

test "crypto.chacha20 test vector 5" {
    const expected_result = []u8{
        0xf7, 0x98, 0xa1, 0x89, 0xf1, 0x95, 0xe6, 0x69,
        0x82, 0x10, 0x5f, 0xfb, 0x64, 0x0b, 0xb7, 0x75,
        0x7f, 0x57, 0x9d, 0xa3, 0x16, 0x02, 0xfc, 0x93,
        0xec, 0x01, 0xac, 0x56, 0xf8, 0x5a, 0xc3, 0xc1,
        0x34, 0xa4, 0x54, 0x7b, 0x73, 0x3b, 0x46, 0x41,
        0x30, 0x42, 0xc9, 0x44, 0x00, 0x49, 0x17, 0x69,
        0x05, 0xd3, 0xbe, 0x59, 0xea, 0x1c, 0x53, 0xf1,
        0x59, 0x16, 0x15, 0x5c, 0x2b, 0xe8, 0x24, 0x1a,

        0x38, 0x00, 0x8b, 0x9a, 0x26, 0xbc, 0x35, 0x94,
        0x1e, 0x24, 0x44, 0x17, 0x7c, 0x8a, 0xde, 0x66,
        0x89, 0xde, 0x95, 0x26, 0x49, 0x86, 0xd9, 0x58,
        0x89, 0xfb, 0x60, 0xe8, 0x46, 0x29, 0xc9, 0xbd,
        0x9a, 0x5a, 0xcb, 0x1c, 0xc1, 0x18, 0xbe, 0x56,
        0x3e, 0xb9, 0xb3, 0xa4, 0xa4, 0x72, 0xf8, 0x2e,
        0x09, 0xa7, 0xe7, 0x78, 0x49, 0x2b, 0x56, 0x2e,
        0xf7, 0x13, 0x0e, 0x88, 0xdf, 0xe0, 0x31, 0xc7,

        0x9d, 0xb9, 0xd4, 0xf7, 0xc7, 0xa8, 0x99, 0x15,
        0x1b, 0x9a, 0x47, 0x50, 0x32, 0xb6, 0x3f, 0xc3,
        0x85, 0x24, 0x5f, 0xe0, 0x54, 0xe3, 0xdd, 0x5a,
        0x97, 0xa5, 0xf5, 0x76, 0xfe, 0x06, 0x40, 0x25,
        0xd3, 0xce, 0x04, 0x2c, 0x56, 0x6a, 0xb2, 0xc5,
        0x07, 0xb1, 0x38, 0xdb, 0x85, 0x3e, 0x3d, 0x69,
        0x59, 0x66, 0x09, 0x96, 0x54, 0x6c, 0xc9, 0xc4,
        0xa6, 0xea, 0xfd, 0xc7, 0x77, 0xc0, 0x40, 0xd7,

        0x0e, 0xaf, 0x46, 0xf7, 0x6d, 0xad, 0x39, 0x79,
        0xe5, 0xc5, 0x36, 0x0c, 0x33, 0x17, 0x16, 0x6a,
        0x1c, 0x89, 0x4c, 0x94, 0xa3, 0x71, 0x87, 0x6a,
        0x94, 0xdf, 0x76, 0x28, 0xfe, 0x4e, 0xaa, 0xf2,
        0xcc, 0xb2, 0x7d, 0x5a, 0xaa, 0xe0, 0xad, 0x7a,
        0xd0, 0xf9, 0xd4, 0xb6, 0xad, 0x3b, 0x54, 0x09,
        0x87, 0x46, 0xd4, 0x52, 0x4d, 0x38, 0x40, 0x7a,
        0x6d, 0xeb, 0x3a, 0xb7, 0x8f, 0xab, 0x78, 0xc9,
    };
    const input = []u8{
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,

        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    };
    var result: [256]u8 = undefined;
    const key = []u8{
        0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
        0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f,
        0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17,
        0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f,
    };
    const nonce = []u8{
        0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
    };

    chaCha20With64BitNonce(result[0..], input[0..], 0, key, nonce);
    testing.expectEqualSlices(u8, expected_result, result);
}