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

    const std = @import("../std.zig");
    const builtin = @import("builtin");
    const math = std.math;
    const mem = std.mem;
    const assert = std.debug.assert;
    const testing = std.testing;
    const maxInt = math.maxInt;
    const Poly1305 = std.crypto.onetimeauth.Poly1305;
    const AuthenticationError = std.crypto.errors.AuthenticationError;

    /// IETF-variant of the ChaCha20 stream cipher, as designed for TLS.
    pub const ChaCha20IETF = ChaChaIETF(20);

    /// IETF-variant of the ChaCha20 stream cipher, reduced to 12 rounds.
    /// Reduced-rounds versions are faster than the full-round version, but have a lower security margin.
    /// However, ChaCha is still believed to have a comfortable security even with only 8 rounds.
    pub const ChaCha12IETF = ChaChaIETF(12);

    /// IETF-variant of the ChaCha20 stream cipher, reduced to 8 rounds.
    /// Reduced-rounds versions are faster than the full-round version, but have a lower security margin.
    /// However, ChaCha is still believed to have a comfortable security even with only 8 rounds.
    pub const ChaCha8IETF = ChaChaIETF(8);

    /// Original ChaCha20 stream cipher.
    pub const ChaCha20With64BitNonce = ChaChaWith64BitNonce(20);

    /// Original ChaCha20 stream cipher, reduced to 12 rounds.
    /// Reduced-rounds versions are faster than the full-round version, but have a lower security margin.
    /// However, ChaCha is still believed to have a comfortable security even with only 8 rounds.
    pub const ChaCha12With64BitNonce = ChaChaWith64BitNonce(12);

    /// Original ChaCha20 stream cipher, reduced to 8 rounds.
    /// Reduced-rounds versions are faster than the full-round version, but have a lower security margin.
    /// However, ChaCha is still believed to have a comfortable security even with only 8 rounds.
    pub const ChaCha8With64BitNonce = ChaChaWith64BitNonce(8);

    /// XChaCha20 (nonce-extended version of the IETF ChaCha20 variant) stream cipher
    pub const XChaCha20IETF = XChaChaIETF(20);

    /// XChaCha20 (nonce-extended version of the IETF ChaCha20 variant) stream cipher, reduced to 12 rounds
    /// Reduced-rounds versions are faster than the full-round version, but have a lower security margin.
    /// However, ChaCha is still believed to have a comfortable security even with only 8 rounds.
    pub const XChaCha12IETF = XChaChaIETF(12);

    /// XChaCha20 (nonce-extended version of the IETF ChaCha20 variant) stream cipher, reduced to 8 rounds
    /// Reduced-rounds versions are faster than the full-round version, but have a lower security margin.
    /// However, ChaCha is still believed to have a comfortable security even with only 8 rounds.
    pub const XChaCha8IETF = XChaChaIETF(8);

    /// ChaCha20-Poly1305 authenticated cipher, as designed for TLS
    pub const ChaCha20Poly1305 = ChaChaPoly1305(20);

    /// ChaCha20-Poly1305 authenticated cipher, reduced to 12 rounds
    /// Reduced-rounds versions are faster than the full-round version, but have a lower security margin.
    /// However, ChaCha is still believed to have a comfortable security even with only 8 rounds.
    pub const ChaCha12Poly1305 = ChaChaPoly1305(12);

    /// ChaCha20-Poly1305 authenticated cipher, reduced to 8 rounds
    /// Reduced-rounds versions are faster than the full-round version, but have a lower security margin.
    /// However, ChaCha is still believed to have a comfortable security even with only 8 rounds.
    pub const ChaCha8Poly1305 = ChaChaPoly1305(8);

    /// XChaCha20-Poly1305 authenticated cipher
    pub const XChaCha20Poly1305 = XChaChaPoly1305(20);

    /// XChaCha20-Poly1305 authenticated cipher
    /// Reduced-rounds versions are faster than the full-round version, but have a lower security margin.
    /// However, ChaCha is still believed to have a comfortable security even with only 8 rounds.
    pub const XChaCha12Poly1305 = XChaChaPoly1305(12);

    /// XChaCha20-Poly1305 authenticated cipher
    /// Reduced-rounds versions are faster than the full-round version, but have a lower security margin.
    /// However, ChaCha is still believed to have a comfortable security even with only 8 rounds.
    pub const XChaCha8Poly1305 = XChaChaPoly1305(8);

    // Vectorized implementation of the core function

    fn ChaChaVecImpl(comptime rounds_nb: usize, comptime degree: comptime_int) type {
        return struct {
            const Lane = @Vector(4 * degree, u32);
            const BlockVec = [4]Lane;

            fn initContext(key: [8]u32, d: [4]u32) BlockVec {
                const c = "expand 32-byte k";
                switch (degree) {
                    1 => {
                        const constant_le = Lane{
                            mem.readIntLittle(u32, c[0..4]),
                            mem.readIntLittle(u32, c[4..8]),
                            mem.readIntLittle(u32, c[8..12]),
                            mem.readIntLittle(u32, c[12..16]),
                        };
                        return BlockVec{
                            constant_le,
                            Lane{ key[0], key[1], key[2], key[3] },
                            Lane{ key[4], key[5], key[6], key[7] },
                            Lane{ d[0], d[1], d[2], d[3] },
                        };
                    },
                    2 => {
                        const constant_le = Lane{
                            mem.readIntLittle(u32, c[0..4]),
                            mem.readIntLittle(u32, c[4..8]),
                            mem.readIntLittle(u32, c[8..12]),
                            mem.readIntLittle(u32, c[12..16]),
                            mem.readIntLittle(u32, c[0..4]),
                            mem.readIntLittle(u32, c[4..8]),
                            mem.readIntLittle(u32, c[8..12]),
                            mem.readIntLittle(u32, c[12..16]),
                        };
                        const n1 = @addWithOverflow(d[0], 1);
                        return BlockVec{
                            constant_le,
                            Lane{ key[0], key[1], key[2], key[3], key[0], key[1], key[2], key[3] },
                            Lane{ key[4], key[5], key[6], key[7], key[4], key[5], key[6], key[7] },
                            Lane{ d[0], d[1], d[2], d[3], n1[0], d[1] +% n1[1], d[2], d[3] },
                        };
                    },
                    4 => {
                        const n1 = @addWithOverflow(d[0], 1);
                        const n2 = @addWithOverflow(d[0], 2);
                        const n3 = @addWithOverflow(d[0], 3);
                        const constant_le = Lane{
                            mem.readIntLittle(u32, c[0..4]),
                            mem.readIntLittle(u32, c[4..8]),
                            mem.readIntLittle(u32, c[8..12]),
                            mem.readIntLittle(u32, c[12..16]),
                            mem.readIntLittle(u32, c[0..4]),
                            mem.readIntLittle(u32, c[4..8]),
                            mem.readIntLittle(u32, c[8..12]),
                            mem.readIntLittle(u32, c[12..16]),
                            mem.readIntLittle(u32, c[0..4]),
                            mem.readIntLittle(u32, c[4..8]),
                            mem.readIntLittle(u32, c[8..12]),
                            mem.readIntLittle(u32, c[12..16]),
                            mem.readIntLittle(u32, c[0..4]),
                            mem.readIntLittle(u32, c[4..8]),
                            mem.readIntLittle(u32, c[8..12]),
                            mem.readIntLittle(u32, c[12..16]),
                        };
                        return BlockVec{
                            constant_le,
                            Lane{ key[0], key[1], key[2], key[3], key[0], key[1], key[2], key[3], key[0], key[1], key[2], key[3], key[0], key[1], key[2], key[3] },
                            Lane{ key[4], key[5], key[6], key[7], key[4], key[5], key[6], key[7], key[4], key[5], key[6], key[7], key[4], key[5], key[6], key[7] },
                            Lane{ d[0], d[1], d[2], d[3], n1[0], d[1] +% n1[1], d[2], d[3], n2[0], d[1] +% n2[1], d[2], d[3], n3[0], d[1] +% n3[1], d[2], d[3] },
                        };
                    },
                    else => @compileError("invalid degree"),
                }
            }

            inline fn chacha20Core(x: *BlockVec, input: BlockVec) void {
                x.* = input;

                const m0 = switch (degree) {
                    1 => [_]i32{ 3, 0, 1, 2 },
                    2 => [_]i32{ 3, 0, 1, 2 } ++ [_]i32{ 7, 4, 5, 6 },
                    4 => [_]i32{ 3, 0, 1, 2 } ++ [_]i32{ 7, 4, 5, 6 } ++ [_]i32{ 11, 8, 9, 10 } ++ [_]i32{ 15, 12, 13, 14 },
                    else => @compileError("invalid degree"),
                };
                const m1 = switch (degree) {
                    1 => [_]i32{ 2, 3, 0, 1 },
                    2 => [_]i32{ 2, 3, 0, 1 } ++ [_]i32{ 6, 7, 4, 5 },
                    4 => [_]i32{ 2, 3, 0, 1 } ++ [_]i32{ 6, 7, 4, 5 } ++ [_]i32{ 10, 11, 8, 9 } ++ [_]i32{ 14, 15, 12, 13 },
                    else => @compileError("invalid degree"),
                };
                const m2 = switch (degree) {
                    1 => [_]i32{ 1, 2, 3, 0 },
                    2 => [_]i32{ 1, 2, 3, 0 } ++ [_]i32{ 5, 6, 7, 4 },
                    4 => [_]i32{ 1, 2, 3, 0 } ++ [_]i32{ 5, 6, 7, 4 } ++ [_]i32{ 9, 10, 11, 8 } ++ [_]i32{ 13, 14, 15, 12 },
                    else => @compileError("invalid degree"),
                };

                var r: usize = 0;
                while (r < rounds_nb) : (r += 2) {
                    x[0] +%= x[1];
                    x[3] ^= x[0];
                    x[3] = math.rotl(Lane, x[3], 16);

                    x[2] +%= x[3];
                    x[1] ^= x[2];
                    x[1] = math.rotl(Lane, x[1], 12);

                    x[0] +%= x[1];
                    x[3] ^= x[0];
                    x[0] = @shuffle(u32, x[0], undefined, m0);
                    x[3] = math.rotl(Lane, x[3], 8);

                    x[2] +%= x[3];
                    x[3] = @shuffle(u32, x[3], undefined, m1);
                    x[1] ^= x[2];
                    x[2] = @shuffle(u32, x[2], undefined, m2);
                    x[1] = math.rotl(Lane, x[1], 7);

                    x[0] +%= x[1];
                    x[3] ^= x[0];
                    x[3] = math.rotl(Lane, x[3], 16);

                    x[2] +%= x[3];
                    x[1] ^= x[2];
                    x[1] = math.rotl(Lane, x[1], 12);

                    x[0] +%= x[1];
                    x[3] ^= x[0];
                    x[0] = @shuffle(u32, x[0], undefined, m2);
                    x[3] = math.rotl(Lane, x[3], 8);

                    x[2] +%= x[3];
                    x[3] = @shuffle(u32, x[3], undefined, m1);
                    x[1] ^= x[2];
                    x[2] = @shuffle(u32, x[2], undefined, m0);
                    x[1] = math.rotl(Lane, x[1], 7);
                }
            }

            inline fn hashToBytes(comptime dm: usize, out: *[64 * dm]u8, x: BlockVec) void {
                for (0..dm) |d| {
                    for (0..4) |i| {
                        mem.writeIntLittle(u32, out[64 * d + 16 * i + 0 ..][0..4], x[i][0 + 4 * d]);
                        mem.writeIntLittle(u32, out[64 * d + 16 * i + 4 ..][0..4], x[i][1 + 4 * d]);
                        mem.writeIntLittle(u32, out[64 * d + 16 * i + 8 ..][0..4], x[i][2 + 4 * d]);
                        mem.writeIntLittle(u32, out[64 * d + 16 * i + 12 ..][0..4], x[i][3 + 4 * d]);
                    }
                }
            }

            inline fn contextFeedback(x: *BlockVec, ctx: BlockVec) void {
                x[0] +%= ctx[0];
                x[1] +%= ctx[1];
                x[2] +%= ctx[2];
                x[3] +%= ctx[3];
            }

            fn chacha20Xor(out: []u8, in: []const u8, key: [8]u32, nonce_and_counter: [4]u32, comptime count64: bool) void {
                var ctx = initContext(key, nonce_and_counter);
                var x: BlockVec = undefined;
                var buf: [64 * degree]u8 = undefined;
                var i: usize = 0;
                inline for ([_]comptime_int{ 4, 2, 1 }) |d| {
                    while (degree >= d and i + 64 * d <= in.len) : (i += 64 * d) {
                        chacha20Core(x[0..], ctx);
                        contextFeedback(&x, ctx);
                        hashToBytes(d, buf[0 .. 64 * d], x);

                        var xout = out[i..];
                        const xin = in[i..];
                        for (0..64 * d) |j| {
                            xout[j] = xin[j];
                        }
                        for (0..64 * d) |j| {
                            xout[j] ^= buf[j];
                        }
                        inline for (0..d) |d_| {
                            if (count64) {
                                const next = @addWithOverflow(ctx[3][4 * d_], d);
                                ctx[3][4 * d_] = next[0];
                                ctx[3][4 * d_ + 1] +%= next[1];
                            } else {
                                ctx[3][4 * d_] +%= d;
                            }
                        }
                    }
                }
                if (i < in.len) {
                    chacha20Core(x[0..], ctx);
                    contextFeedback(&x, ctx);
                    hashToBytes(1, buf[0..64], x);

                    var xout = out[i..];
                    const xin = in[i..];
                    for (0..in.len % 64) |j| {
                        xout[j] = xin[j] ^ buf[j];
                    }
                }
            }

            fn chacha20Stream(out: []u8, key: [8]u32, nonce_and_counter: [4]u32, comptime count64: bool) void {
                var ctx = initContext(key, nonce_and_counter);
                var x: BlockVec = undefined;
                var i: usize = 0;
                inline for ([_]comptime_int{ 4, 2, 1 }) |d| {
                    while (degree >= d and i + 64 * d <= out.len) : (i += 64 * d) {
                        chacha20Core(x[0..], ctx);
                        contextFeedback(&x, ctx);
                        hashToBytes(d, out[i..][0 .. 64 * d], x);
                        inline for (0..d) |d_| {
                            if (count64) {
                                const next = @addWithOverflow(ctx[3][4 * d_], d);
                                ctx[3][4 * d_] = next[0];
                                ctx[3][4 * d_ + 1] +%= next[1];
                            } else {
                                ctx[3][4 * d_] +%= d;
                            }
                        }
                    }
                }
                if (i < out.len) {
                    chacha20Core(x[0..], ctx);
                    contextFeedback(&x, ctx);

                    var buf: [64]u8 = undefined;
                    hashToBytes(1, buf[0..], x);
                    @memcpy(out[i..], buf[0 .. out.len - i]);
                }
            }

            fn hchacha20(input: [16]u8, key: [32]u8) [32]u8 {
                var c: [4]u32 = undefined;
                for (c, 0..) |_, i| {
                    c[i] = mem.readIntLittle(u32, input[4 * i ..][0..4]);
                }
                const ctx = initContext(keyToWords(key), c);
                var x: BlockVec = undefined;
                chacha20Core(x[0..], ctx);
                var out: [32]u8 = undefined;
                mem.writeIntLittle(u32, out[0..4], x[0][0]);
                mem.writeIntLittle(u32, out[4..8], x[0][1]);
                mem.writeIntLittle(u32, out[8..12], x[0][2]);
                mem.writeIntLittle(u32, out[12..16], x[0][3]);
                mem.writeIntLittle(u32, out[16..20], x[3][0]);
                mem.writeIntLittle(u32, out[20..24], x[3][1]);
                mem.writeIntLittle(u32, out[24..28], x[3][2]);
                mem.writeIntLittle(u32, out[28..32], x[3][3]);
                return out;
            }
        };
    }

    // Non-vectorized implementation of the core function

    fn ChaChaNonVecImpl(comptime rounds_nb: usize) type {
        return struct {
            const BlockVec = [16]u32;

            fn initContext(key: [8]u32, d: [4]u32) BlockVec {
                const c = "expand 32-byte k";
                const constant_le = comptime [4]u32{
                    mem.readIntLittle(u32, c[0..4]),
                    mem.readIntLittle(u32, c[4..8]),
                    mem.readIntLittle(u32, c[8..12]),
                    mem.readIntLittle(u32, c[12..16]),
                };
                return BlockVec{
                    constant_le[0], constant_le[1], constant_le[2], constant_le[3],
                    key[0],         key[1],         key[2],         key[3],
                    key[4],         key[5],         key[6],         key[7],
                    d[0],           d[1],           d[2],           d[3],
                };
            }

            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,
                };
            }

            inline fn chacha20Core(x: *BlockVec, input: BlockVec) void {
                x.* = input;

                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 < rounds_nb) : (j += 2) {
                    inline for (rounds) |r| {
                        x[r.a] +%= x[r.b];
                        x[r.d] = math.rotl(u32, x[r.d] ^ x[r.a], @as(u32, 16));
                        x[r.c] +%= x[r.d];
                        x[r.b] = math.rotl(u32, x[r.b] ^ x[r.c], @as(u32, 12));
                        x[r.a] +%= x[r.b];
                        x[r.d] = math.rotl(u32, x[r.d] ^ x[r.a], @as(u32, 8));
                        x[r.c] +%= x[r.d];
                        x[r.b] = math.rotl(u32, x[r.b] ^ x[r.c], @as(u32, 7));
                    }
                }
            }

            inline fn hashToBytes(out: *[64]u8, x: BlockVec) void {
                for (0..4) |i| {
                    mem.writeIntLittle(u32, out[16 * i + 0 ..][0..4], x[i * 4 + 0]);
                    mem.writeIntLittle(u32, out[16 * i + 4 ..][0..4], x[i * 4 + 1]);
                    mem.writeIntLittle(u32, out[16 * i + 8 ..][0..4], x[i * 4 + 2]);
                    mem.writeIntLittle(u32, out[16 * i + 12 ..][0..4], x[i * 4 + 3]);
                }
            }

            inline fn contextFeedback(x: *BlockVec, ctx: BlockVec) void {
                for (0..16) |i| {
                    x[i] +%= ctx[i];
                }
            }

            fn chacha20Xor(out: []u8, in: []const u8, key: [8]u32, nonce_and_counter: [4]u32, comptime count64: bool) void {
                var ctx = initContext(key, nonce_and_counter);
                var x: BlockVec = undefined;
                var buf: [64]u8 = undefined;
                var i: usize = 0;
                while (i + 64 <= in.len) : (i += 64) {
                    chacha20Core(x[0..], ctx);
                    contextFeedback(&x, ctx);
                    hashToBytes(buf[0..], x);

                    var xout = out[i..];
                    const xin = in[i..];
                    for (0..64) |j| {
                        xout[j] = xin[j];
                    }
                    for (0..64) |j| {
                        xout[j] ^= buf[j];
                    }
                    if (count64) {
                        const next = @addWithOverflow(ctx[12], 1);
                        ctx[12] = next[0];
                        ctx[13] +%= next[1];
                    } else {
                        ctx[12] +%= 1;
                    }
                }
                if (i < in.len) {
                    chacha20Core(x[0..], ctx);
                    contextFeedback(&x, ctx);
                    hashToBytes(buf[0..], x);

                    var xout = out[i..];
                    const xin = in[i..];
                    for (0..in.len % 64) |j| {
                        xout[j] = xin[j] ^ buf[j];
                    }
                }
            }

            fn chacha20Stream(out: []u8, key: [8]u32, nonce_and_counter: [4]u32, comptime count64: bool) void {
                var ctx = initContext(key, nonce_and_counter);
                var x: BlockVec = undefined;
                var i: usize = 0;
                while (i + 64 <= out.len) : (i += 64) {
                    chacha20Core(x[0..], ctx);
                    contextFeedback(&x, ctx);
                    hashToBytes(out[i..][0..64], x);
                    if (count64) {
                        const next = @addWithOverflow(ctx[12], 1);
                        ctx[12] = next[0];
                        ctx[13] +%= next[1];
                    } else {
                        ctx[12] +%= 1;
                    }
                }
                if (i < out.len) {
                    chacha20Core(x[0..], ctx);
                    contextFeedback(&x, ctx);

                    var buf: [64]u8 = undefined;
                    hashToBytes(buf[0..], x);
                    @memcpy(out[i..], buf[0 .. out.len - i]);
                }
            }

            fn hchacha20(input: [16]u8, key: [32]u8) [32]u8 {
                var c: [4]u32 = undefined;
                for (c, 0..) |_, i| {
                    c[i] = mem.readIntLittle(u32, input[4 * i ..][0..4]);
                }
                const ctx = initContext(keyToWords(key), c);
                var x: BlockVec = undefined;
                chacha20Core(x[0..], ctx);
                var out: [32]u8 = undefined;
                mem.writeIntLittle(u32, out[0..4], x[0]);
                mem.writeIntLittle(u32, out[4..8], x[1]);
                mem.writeIntLittle(u32, out[8..12], x[2]);
                mem.writeIntLittle(u32, out[12..16], x[3]);
                mem.writeIntLittle(u32, out[16..20], x[12]);
                mem.writeIntLittle(u32, out[20..24], x[13]);
                mem.writeIntLittle(u32, out[24..28], x[14]);
                mem.writeIntLittle(u32, out[28..32], x[15]);
                return out;
            }
        };
    }

    fn ChaChaImpl(comptime rounds_nb: usize) type {
        switch (builtin.cpu.arch) {
            .x86_64 => {
                const has_avx2 = std.Target.x86.featureSetHas(builtin.cpu.features, .avx2);
                const has_avx512f = std.Target.x86.featureSetHas(builtin.cpu.features, .avx512f);
                if (has_avx512f) return ChaChaVecImpl(rounds_nb, 4);
                if (has_avx2) return ChaChaVecImpl(rounds_nb, 2);
                return ChaChaVecImpl(rounds_nb, 1);
            },
            .aarch64 => {
                const has_neon = std.Target.aarch64.featureSetHas(builtin.cpu.features, .neon);
                if (has_neon) return ChaChaVecImpl(rounds_nb, 4);
                return ChaChaNonVecImpl(rounds_nb);
            },
            else => return ChaChaNonVecImpl(rounds_nb),
        }
    }

    fn keyToWords(key: [32]u8) [8]u32 {
        var k: [8]u32 = undefined;
        for (0..8) |i| {
            k[i] = mem.readIntLittle(u32, key[i * 4 ..][0..4]);
        }
        return k;
    }

    fn extend(key: [32]u8, nonce: [24]u8, comptime rounds_nb: usize) struct { key: [32]u8, nonce: [12]u8 } {
        var subnonce: [12]u8 = undefined;
        @memset(subnonce[0..4], 0);
        subnonce[4..].* = nonce[16..24].*;
        return .{
            .key = ChaChaImpl(rounds_nb).hchacha20(nonce[0..16].*, key),
            .nonce = subnonce,
        };
    }

    fn ChaChaIETF(comptime rounds_nb: usize) type {
        return struct {
            /// Nonce length in bytes.
            pub const nonce_length = 12;
            /// Key length in bytes.
            pub const key_length = 32;
            /// Block length in bytes.
            pub const block_length = 64;

            /// Add the output of the ChaCha20 stream cipher to `in` and stores the result into `out`.
            /// WARNING: This function doesn't provide authenticated encryption.
            /// Using the AEAD or one of the `box` versions is usually preferred.
            pub fn xor(out: []u8, in: []const u8, counter: u32, key: [key_length]u8, nonce: [nonce_length]u8) void {
                assert(in.len == out.len);
                assert(in.len <= 64 * (@as(u39, 1 << 32) - counter));

                var d: [4]u32 = undefined;
                d[0] = counter;
                d[1] = mem.readIntLittle(u32, nonce[0..4]);
                d[2] = mem.readIntLittle(u32, nonce[4..8]);
                d[3] = mem.readIntLittle(u32, nonce[8..12]);
                ChaChaImpl(rounds_nb).chacha20Xor(out, in, keyToWords(key), d, false);
            }

            /// Write the output of the ChaCha20 stream cipher into `out`.
            pub fn stream(out: []u8, counter: u32, key: [key_length]u8, nonce: [nonce_length]u8) void {
                assert(out.len <= 64 * (@as(u39, 1 << 32) - counter));

                var d: [4]u32 = undefined;
                d[0] = counter;
                d[1] = mem.readIntLittle(u32, nonce[0..4]);
                d[2] = mem.readIntLittle(u32, nonce[4..8]);
                d[3] = mem.readIntLittle(u32, nonce[8..12]);
                ChaChaImpl(rounds_nb).chacha20Stream(out, keyToWords(key), d, false);
            }
        };
    }

    fn ChaChaWith64BitNonce(comptime rounds_nb: usize) type {
        return struct {
            /// Nonce length in bytes.
            pub const nonce_length = 8;
            /// Key length in bytes.
            pub const key_length = 32;
            /// Block length in bytes.
            pub const block_length = 64;

            /// Add the output of the ChaCha20 stream cipher to `in` and stores the result into `out`.
            /// WARNING: This function doesn't provide authenticated encryption.
            /// Using the AEAD or one of the `box` versions is usually preferred.
            pub fn xor(out: []u8, in: []const u8, counter: u64, key: [key_length]u8, nonce: [nonce_length]u8) void {
                assert(in.len == out.len);
                assert(in.len <= 64 * (@as(u71, 1 << 64) - counter));

                const k = keyToWords(key);
                var c: [4]u32 = undefined;
                c[0] = @as(u32, @truncate(counter));
                c[1] = @as(u32, @truncate(counter >> 32));
                c[2] = mem.readIntLittle(u32, nonce[0..4]);
                c[3] = mem.readIntLittle(u32, nonce[4..8]);
                ChaChaImpl(rounds_nb).chacha20Xor(out, in, k, c, true);
            }

            /// Write the output of the ChaCha20 stream cipher into `out`.
            pub fn stream(out: []u8, counter: u32, key: [key_length]u8, nonce: [nonce_length]u8) void {
                assert(out.len <= 64 * (@as(u71, 1 << 64) - counter));

                const k = keyToWords(key);
                var c: [4]u32 = undefined;
                c[0] = @as(u32, @truncate(counter));
                c[1] = @as(u32, @truncate(counter >> 32));
                c[2] = mem.readIntLittle(u32, nonce[0..4]);
                c[3] = mem.readIntLittle(u32, nonce[4..8]);
                ChaChaImpl(rounds_nb).chacha20Stream(out, k, c, true);
            }
        };
    }

    fn XChaChaIETF(comptime rounds_nb: usize) type {
        return struct {
            /// Nonce length in bytes.
            pub const nonce_length = 24;
            /// Key length in bytes.
            pub const key_length = 32;
            /// Block length in bytes.
            pub const block_length = 64;

            /// Add the output of the XChaCha20 stream cipher to `in` and stores the result into `out`.
            /// WARNING: This function doesn't provide authenticated encryption.
            /// Using the AEAD or one of the `box` versions is usually preferred.
            pub fn xor(out: []u8, in: []const u8, counter: u32, key: [key_length]u8, nonce: [nonce_length]u8) void {
                const extended = extend(key, nonce, rounds_nb);
                ChaChaIETF(rounds_nb).xor(out, in, counter, extended.key, extended.nonce);
            }

            /// Write the output of the XChaCha20 stream cipher into `out`.
            pub fn stream(out: []u8, counter: u32, key: [key_length]u8, nonce: [nonce_length]u8) void {
                const extended = extend(key, nonce, rounds_nb);
                ChaChaIETF(rounds_nb).xor(out, counter, extended.key, extended.nonce);
            }
        };
    }

    fn ChaChaPoly1305(comptime rounds_nb: usize) type {
        return struct {
            pub const tag_length = 16;
            pub const nonce_length = 12;
            pub const key_length = 32;

            /// c: ciphertext: output buffer should be of size m.len
            /// tag: authentication tag: output MAC
            /// m: message
            /// ad: Associated Data
            /// npub: public nonce
            /// k: private key
            pub fn encrypt(c: []u8, tag: *[tag_length]u8, m: []const u8, ad: []const u8, npub: [nonce_length]u8, k: [key_length]u8) void {
                assert(c.len == m.len);
                assert(m.len <= 64 * (@as(u39, 1 << 32) - 1));

                var polyKey = [_]u8{0} ** 32;
                ChaChaIETF(rounds_nb).xor(polyKey[0..], polyKey[0..], 0, k, npub);

                ChaChaIETF(rounds_nb).xor(c[0..m.len], m, 1, k, npub);

                var mac = Poly1305.init(polyKey[0..]);
                mac.update(ad);
                if (ad.len % 16 != 0) {
                    const zeros = [_]u8{0} ** 16;
                    const padding = 16 - (ad.len % 16);
                    mac.update(zeros[0..padding]);
                }
                mac.update(c[0..m.len]);
                if (m.len % 16 != 0) {
                    const zeros = [_]u8{0} ** 16;
                    const padding = 16 - (m.len % 16);
                    mac.update(zeros[0..padding]);
                }
                var lens: [16]u8 = undefined;
                mem.writeIntLittle(u64, lens[0..8], ad.len);
                mem.writeIntLittle(u64, lens[8..16], m.len);
                mac.update(lens[0..]);
                mac.final(tag);
            }

            /// m: message: output buffer should be of size c.len
            /// c: ciphertext
            /// tag: authentication tag
            /// ad: Associated Data
            /// npub: public nonce
            /// k: private key
            /// NOTE: the check of the authentication tag is currently not done in constant time
            pub fn decrypt(m: []u8, c: []const u8, tag: [tag_length]u8, ad: []const u8, npub: [nonce_length]u8, k: [key_length]u8) AuthenticationError!void {
                assert(c.len == m.len);

                var polyKey = [_]u8{0} ** 32;
                ChaChaIETF(rounds_nb).xor(polyKey[0..], polyKey[0..], 0, k, npub);

                var mac = Poly1305.init(polyKey[0..]);

                mac.update(ad);
                if (ad.len % 16 != 0) {
                    const zeros = [_]u8{0} ** 16;
                    const padding = 16 - (ad.len % 16);
                    mac.update(zeros[0..padding]);
                }
                mac.update(c);
                if (c.len % 16 != 0) {
                    const zeros = [_]u8{0} ** 16;
                    const padding = 16 - (c.len % 16);
                    mac.update(zeros[0..padding]);
                }
                var lens: [16]u8 = undefined;
                mem.writeIntLittle(u64, lens[0..8], ad.len);
                mem.writeIntLittle(u64, lens[8..16], c.len);
                mac.update(lens[0..]);
                var computedTag: [16]u8 = undefined;
                mac.final(computedTag[0..]);

                var acc: u8 = 0;
                for (computedTag, 0..) |_, i| {
                    acc |= computedTag[i] ^ tag[i];
                }
                if (acc != 0) {
                    return error.AuthenticationFailed;
                }
                ChaChaIETF(rounds_nb).xor(m[0..c.len], c, 1, k, npub);
            }
        };
    }

    fn XChaChaPoly1305(comptime rounds_nb: usize) type {
        return struct {
            pub const tag_length = 16;
            pub const nonce_length = 24;
            pub const key_length = 32;

            /// c: ciphertext: output buffer should be of size m.len
            /// tag: authentication tag: output MAC
            /// m: message
            /// ad: Associated Data
            /// npub: public nonce
            /// k: private key
            pub fn encrypt(c: []u8, tag: *[tag_length]u8, m: []const u8, ad: []const u8, npub: [nonce_length]u8, k: [key_length]u8) void {
                const extended = extend(k, npub, rounds_nb);
                return ChaChaPoly1305(rounds_nb).encrypt(c, tag, m, ad, extended.nonce, extended.key);
            }

            /// m: message: output buffer should be of size c.len
            /// c: ciphertext
            /// tag: authentication tag
            /// ad: Associated Data
            /// npub: public nonce
            /// k: private key
            pub fn decrypt(m: []u8, c: []const u8, tag: [tag_length]u8, ad: []const u8, npub: [nonce_length]u8, k: [key_length]u8) AuthenticationError!void {
                const extended = extend(k, npub, rounds_nb);
                return ChaChaPoly1305(rounds_nb).decrypt(m, c, tag, ad, extended.nonce, extended.key);
            }
        };
    }

    test "chacha20 AEAD API" {
        const aeads = [_]type{ ChaCha20Poly1305, XChaCha20Poly1305 };
        const m = "Ladies and Gentlemen of the class of '99: If I could offer you only one tip for the future, sunscreen would be it.";
        const ad = "Additional data";

        inline for (aeads) |aead| {
            const key = [_]u8{69} ** aead.key_length;
            const nonce = [_]u8{42} ** aead.nonce_length;
            var c: [m.len]u8 = undefined;
            var tag: [aead.tag_length]u8 = undefined;
            var out: [m.len]u8 = undefined;

            aead.encrypt(c[0..], tag[0..], m, ad, nonce, key);
            try aead.decrypt(out[0..], c[0..], tag, ad[0..], nonce, key);
            try testing.expectEqualSlices(u8, out[0..], m);
            c[0] +%= 1;
            try testing.expectError(error.AuthenticationFailed, aead.decrypt(out[0..], c[0..], tag, ad[0..], nonce, key));
        }
    }

    // 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 m = "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.xor(result[0..], m[0..], 1, key, nonce);
        try testing.expectEqualSlices(u8, &expected_result, &result);

        var m2: [114]u8 = undefined;
        ChaCha20IETF.xor(m2[0..], result[0..], 1, key, nonce);
        try testing.expect(mem.order(u8, m, &m2) == .eq);
    }

    // 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 m = [_]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.xor(result[0..], m[0..], 0, key, nonce);
        try 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 m = [_]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.xor(result[0..], m[0..], 0, key, nonce);
        try 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 m = [_]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.xor(result[0..], m[0..], 0, key, nonce);
        try 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 m = [_]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.xor(result[0..], m[0..], 0, key, nonce);
        try 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 m = [_]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.xor(result[0..], m[0..], 0, key, nonce);
        try testing.expectEqualSlices(u8, &expected_result, &result);
    }

    test "seal" {
        {
            const m = "";
            const ad = "";
            const key = [_]u8{
                0x80, 0x81, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87, 0x88, 0x89, 0x8a, 0x8b, 0x8c, 0x8d, 0x8e, 0x8f,
                0x90, 0x91, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97, 0x98, 0x99, 0x9a, 0x9b, 0x9c, 0x9d, 0x9e, 0x9f,
            };
            const nonce = [_]u8{ 0x7, 0x0, 0x0, 0x0, 0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47 };
            const exp_out = [_]u8{ 0xa0, 0x78, 0x4d, 0x7a, 0x47, 0x16, 0xf3, 0xfe, 0xb4, 0xf6, 0x4e, 0x7f, 0x4b, 0x39, 0xbf, 0x4 };

            var out: [exp_out.len]u8 = undefined;
            ChaCha20Poly1305.encrypt(out[0..m.len], out[m.len..], m, ad, nonce, key);
            try testing.expectEqualSlices(u8, exp_out[0..], out[0..]);
        }
        {
            const m = [_]u8{
                0x4c, 0x61, 0x64, 0x69, 0x65, 0x73, 0x20, 0x61, 0x6e, 0x64, 0x20, 0x47, 0x65, 0x6e, 0x74, 0x6c,
                0x65, 0x6d, 0x65, 0x6e, 0x20, 0x6f, 0x66, 0x20, 0x74, 0x68, 0x65, 0x20, 0x63, 0x6c, 0x61, 0x73,
                0x73, 0x20, 0x6f, 0x66, 0x20, 0x27, 0x39, 0x39, 0x3a, 0x20, 0x49, 0x66, 0x20, 0x49, 0x20, 0x63,
                0x6f, 0x75, 0x6c, 0x64, 0x20, 0x6f, 0x66, 0x66, 0x65, 0x72, 0x20, 0x79, 0x6f, 0x75, 0x20, 0x6f,
                0x6e, 0x6c, 0x79, 0x20, 0x6f, 0x6e, 0x65, 0x20, 0x74, 0x69, 0x70, 0x20, 0x66, 0x6f, 0x72, 0x20,
                0x74, 0x68, 0x65, 0x20, 0x66, 0x75, 0x74, 0x75, 0x72, 0x65, 0x2c, 0x20, 0x73, 0x75, 0x6e, 0x73,
                0x63, 0x72, 0x65, 0x65, 0x6e, 0x20, 0x77, 0x6f, 0x75, 0x6c, 0x64, 0x20, 0x62, 0x65, 0x20, 0x69,
                0x74, 0x2e,
            };
            const ad = [_]u8{ 0x50, 0x51, 0x52, 0x53, 0xc0, 0xc1, 0xc2, 0xc3, 0xc4, 0xc5, 0xc6, 0xc7 };
            const key = [_]u8{
                0x80, 0x81, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87, 0x88, 0x89, 0x8a, 0x8b, 0x8c, 0x8d, 0x8e, 0x8f,
                0x90, 0x91, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97, 0x98, 0x99, 0x9a, 0x9b, 0x9c, 0x9d, 0x9e, 0x9f,
            };
            const nonce = [_]u8{ 0x7, 0x0, 0x0, 0x0, 0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47 };
            const exp_out = [_]u8{
                0xd3, 0x1a, 0x8d, 0x34, 0x64, 0x8e, 0x60, 0xdb, 0x7b, 0x86, 0xaf, 0xbc, 0x53, 0xef, 0x7e, 0xc2,
                0xa4, 0xad, 0xed, 0x51, 0x29, 0x6e, 0x8,  0xfe, 0xa9, 0xe2, 0xb5, 0xa7, 0x36, 0xee, 0x62, 0xd6,
                0x3d, 0xbe, 0xa4, 0x5e, 0x8c, 0xa9, 0x67, 0x12, 0x82, 0xfa, 0xfb, 0x69, 0xda, 0x92, 0x72, 0x8b,
                0x1a, 0x71, 0xde, 0xa,  0x9e, 0x6,  0xb,  0x29, 0x5,  0xd6, 0xa5, 0xb6, 0x7e, 0xcd, 0x3b, 0x36,
                0x92, 0xdd, 0xbd, 0x7f, 0x2d, 0x77, 0x8b, 0x8c, 0x98, 0x3,  0xae, 0xe3, 0x28, 0x9,  0x1b, 0x58,
                0xfa, 0xb3, 0x24, 0xe4, 0xfa, 0xd6, 0x75, 0x94, 0x55, 0x85, 0x80, 0x8b, 0x48, 0x31, 0xd7, 0xbc,
                0x3f, 0xf4, 0xde, 0xf0, 0x8e, 0x4b, 0x7a, 0x9d, 0xe5, 0x76, 0xd2, 0x65, 0x86, 0xce, 0xc6, 0x4b,
                0x61, 0x16, 0x1a, 0xe1, 0xb,  0x59, 0x4f, 0x9,  0xe2, 0x6a, 0x7e, 0x90, 0x2e, 0xcb, 0xd0, 0x60,
                0x6,  0x91,
            };

            var out: [exp_out.len]u8 = undefined;
            ChaCha20Poly1305.encrypt(out[0..m.len], out[m.len..], m[0..], ad[0..], nonce, key);
            try testing.expectEqualSlices(u8, exp_out[0..], out[0..]);
        }
    }

    test "open" {
        {
            const c = [_]u8{ 0xa0, 0x78, 0x4d, 0x7a, 0x47, 0x16, 0xf3, 0xfe, 0xb4, 0xf6, 0x4e, 0x7f, 0x4b, 0x39, 0xbf, 0x4 };
            const ad = "";
            const key = [_]u8{
                0x80, 0x81, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87, 0x88, 0x89, 0x8a, 0x8b, 0x8c, 0x8d, 0x8e, 0x8f,
                0x90, 0x91, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97, 0x98, 0x99, 0x9a, 0x9b, 0x9c, 0x9d, 0x9e, 0x9f,
            };
            const nonce = [_]u8{ 0x7, 0x0, 0x0, 0x0, 0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47 };
            const exp_out = "";

            var out: [exp_out.len]u8 = undefined;
            try ChaCha20Poly1305.decrypt(out[0..], c[0..exp_out.len], c[exp_out.len..].*, ad[0..], nonce, key);
            try testing.expectEqualSlices(u8, exp_out[0..], out[0..]);
        }
        {
            const c = [_]u8{
                0xd3, 0x1a, 0x8d, 0x34, 0x64, 0x8e, 0x60, 0xdb, 0x7b, 0x86, 0xaf, 0xbc, 0x53, 0xef, 0x7e, 0xc2,
                0xa4, 0xad, 0xed, 0x51, 0x29, 0x6e, 0x8,  0xfe, 0xa9, 0xe2, 0xb5, 0xa7, 0x36, 0xee, 0x62, 0xd6,
                0x3d, 0xbe, 0xa4, 0x5e, 0x8c, 0xa9, 0x67, 0x12, 0x82, 0xfa, 0xfb, 0x69, 0xda, 0x92, 0x72, 0x8b,
                0x1a, 0x71, 0xde, 0xa,  0x9e, 0x6,  0xb,  0x29, 0x5,  0xd6, 0xa5, 0xb6, 0x7e, 0xcd, 0x3b, 0x36,
                0x92, 0xdd, 0xbd, 0x7f, 0x2d, 0x77, 0x8b, 0x8c, 0x98, 0x3,  0xae, 0xe3, 0x28, 0x9,  0x1b, 0x58,
                0xfa, 0xb3, 0x24, 0xe4, 0xfa, 0xd6, 0x75, 0x94, 0x55, 0x85, 0x80, 0x8b, 0x48, 0x31, 0xd7, 0xbc,
                0x3f, 0xf4, 0xde, 0xf0, 0x8e, 0x4b, 0x7a, 0x9d, 0xe5, 0x76, 0xd2, 0x65, 0x86, 0xce, 0xc6, 0x4b,
                0x61, 0x16, 0x1a, 0xe1, 0xb,  0x59, 0x4f, 0x9,  0xe2, 0x6a, 0x7e, 0x90, 0x2e, 0xcb, 0xd0, 0x60,
                0x6,  0x91,
            };
            const ad = [_]u8{ 0x50, 0x51, 0x52, 0x53, 0xc0, 0xc1, 0xc2, 0xc3, 0xc4, 0xc5, 0xc6, 0xc7 };
            const key = [_]u8{
                0x80, 0x81, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87, 0x88, 0x89, 0x8a, 0x8b, 0x8c, 0x8d, 0x8e, 0x8f,
                0x90, 0x91, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97, 0x98, 0x99, 0x9a, 0x9b, 0x9c, 0x9d, 0x9e, 0x9f,
            };
            const nonce = [_]u8{ 0x7, 0x0, 0x0, 0x0, 0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47 };
            const exp_out = [_]u8{
                0x4c, 0x61, 0x64, 0x69, 0x65, 0x73, 0x20, 0x61, 0x6e, 0x64, 0x20, 0x47, 0x65, 0x6e, 0x74, 0x6c,
                0x65, 0x6d, 0x65, 0x6e, 0x20, 0x6f, 0x66, 0x20, 0x74, 0x68, 0x65, 0x20, 0x63, 0x6c, 0x61, 0x73,
                0x73, 0x20, 0x6f, 0x66, 0x20, 0x27, 0x39, 0x39, 0x3a, 0x20, 0x49, 0x66, 0x20, 0x49, 0x20, 0x63,
                0x6f, 0x75, 0x6c, 0x64, 0x20, 0x6f, 0x66, 0x66, 0x65, 0x72, 0x20, 0x79, 0x6f, 0x75, 0x20, 0x6f,
                0x6e, 0x6c, 0x79, 0x20, 0x6f, 0x6e, 0x65, 0x20, 0x74, 0x69, 0x70, 0x20, 0x66, 0x6f, 0x72, 0x20,
                0x74, 0x68, 0x65, 0x20, 0x66, 0x75, 0x74, 0x75, 0x72, 0x65, 0x2c, 0x20, 0x73, 0x75, 0x6e, 0x73,
                0x63, 0x72, 0x65, 0x65, 0x6e, 0x20, 0x77, 0x6f, 0x75, 0x6c, 0x64, 0x20, 0x62, 0x65, 0x20, 0x69,
                0x74, 0x2e,
            };

            var out: [exp_out.len]u8 = undefined;
            try ChaCha20Poly1305.decrypt(out[0..], c[0..exp_out.len], c[exp_out.len..].*, ad[0..], nonce, key);
            try testing.expectEqualSlices(u8, exp_out[0..], out[0..]);

            // corrupting the ciphertext, data, key, or nonce should cause a failure

            var bad_c = c;
            bad_c[0] ^= 1;
            try testing.expectError(error.AuthenticationFailed, ChaCha20Poly1305.decrypt(out[0..], bad_c[0..out.len], bad_c[out.len..].*, ad[0..], nonce, key));
            var bad_ad = ad;
            bad_ad[0] ^= 1;
            try testing.expectError(error.AuthenticationFailed, ChaCha20Poly1305.decrypt(out[0..], c[0..out.len], c[out.len..].*, bad_ad[0..], nonce, key));
            var bad_key = key;
            bad_key[0] ^= 1;
            try testing.expectError(error.AuthenticationFailed, ChaCha20Poly1305.decrypt(out[0..], c[0..out.len], c[out.len..].*, ad[0..], nonce, bad_key));
            var bad_nonce = nonce;
            bad_nonce[0] ^= 1;
            try testing.expectError(error.AuthenticationFailed, ChaCha20Poly1305.decrypt(out[0..], c[0..out.len], c[out.len..].*, ad[0..], bad_nonce, key));
        }
    }

    test "crypto.xchacha20" {
        const key = [_]u8{69} ** 32;
        const nonce = [_]u8{42} ** 24;
        const m = "Ladies and Gentlemen of the class of '99: If I could offer you only one tip for the future, sunscreen would be it.";
        {
            var c: [m.len]u8 = undefined;
            XChaCha20IETF.xor(c[0..], m[0..], 0, key, nonce);
            var buf: [2 * c.len]u8 = undefined;
            try testing.expectEqualStrings(try std.fmt.bufPrint(&buf, "{s}", .{std.fmt.fmtSliceHexUpper(&c)}), "E0A1BCF939654AFDBDC1746EC49832647C19D891F0D1A81FC0C1703B4514BDEA584B512F6908C2C5E9DD18D5CBC1805DE5803FE3B9CA5F193FB8359E91FAB0C3BB40309A292EB1CF49685C65C4A3ADF4F11DB0CD2B6B67FBC174BC2E860E8F769FD3565BBFAD1C845E05A0FED9BE167C240D");
        }
        {
            const ad = "Additional data";
            var c: [m.len + XChaCha20Poly1305.tag_length]u8 = undefined;
            XChaCha20Poly1305.encrypt(c[0..m.len], c[m.len..], m, ad, nonce, key);
            var out: [m.len]u8 = undefined;
            try XChaCha20Poly1305.decrypt(out[0..], c[0..m.len], c[m.len..].*, ad, nonce, key);
            var buf: [2 * c.len]u8 = undefined;
            try testing.expectEqualStrings(try std.fmt.bufPrint(&buf, "{s}", .{std.fmt.fmtSliceHexUpper(&c)}), "994D2DD32333F48E53650C02C7A2ABB8E018B0836D7175AEC779F52E961780768F815C58F1AA52D211498DB89B9216763F569C9433A6BBFCEFB4D4A49387A4C5207FBB3B5A92B5941294DF30588C6740D39DC16FA1F0E634F7246CF7CDCB978E44347D89381B7A74EB7084F754B90BDE9AAF5A94B8F2A85EFD0B50692AE2D425E234");
            try testing.expectEqualSlices(u8, out[0..], m);
            c[0] +%= 1;
            try testing.expectError(error.AuthenticationFailed, XChaCha20Poly1305.decrypt(out[0..], c[0..m.len], c[m.len..].*, ad, nonce, key));
        }
    }