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

    pub const HmacMd5 = Hmac(crypto.hash.Md5);
    pub const HmacSha1 = Hmac(crypto.hash.Sha1);

    pub const sha2 = struct {
        pub const HmacSha224 = Hmac(crypto.hash.sha2.Sha224);
        pub const HmacSha256 = Hmac(crypto.hash.sha2.Sha256);
        pub const HmacSha384 = Hmac(crypto.hash.sha2.Sha384);
        pub const HmacSha512 = Hmac(crypto.hash.sha2.Sha512);
    };

    pub fn Hmac(comptime Hash: type) type {
        return struct {
            const Self = @This();
            pub const mac_length = Hash.digest_length;
            pub const key_length_min = 0;
            pub const key_length = mac_length; // recommended key length

            o_key_pad: [Hash.block_length]u8,
            hash: Hash,

            // HMAC(k, m) = H(o_key_pad || H(i_key_pad || msg)) where || is concatenation

            pub fn create(out: *[mac_length]u8, msg: []const u8, key: []const u8) void {
                var ctx = Self.init(key);
                ctx.update(msg);
                ctx.final(out);
            }

            pub fn init(key: []const u8) Self {
                var ctx: Self = undefined;
                var scratch: [Hash.block_length]u8 = undefined;
                var i_key_pad: [Hash.block_length]u8 = undefined;

                // Normalize key length to block size of hash

                if (key.len > Hash.block_length) {
                    Hash.hash(key, scratch[0..mac_length], .{});
                    @memset(scratch[mac_length..Hash.block_length], 0);
                } else if (key.len < Hash.block_length) {
                    @memcpy(scratch[0..key.len], key);
                    @memset(scratch[key.len..Hash.block_length], 0);
                } else {
                    @memcpy(&scratch, key);
                }

                for (&ctx.o_key_pad, 0..) |*b, i| {
                    b.* = scratch[i] ^ 0x5c;
                }

                for (&i_key_pad, 0..) |*b, i| {
                    b.* = scratch[i] ^ 0x36;
                }

                ctx.hash = Hash.init(.{});
                ctx.hash.update(&i_key_pad);
                return ctx;
            }

            pub fn update(ctx: *Self, msg: []const u8) void {
                ctx.hash.update(msg);
            }

            pub fn final(ctx: *Self, out: *[mac_length]u8) void {
                var scratch: [mac_length]u8 = undefined;
                ctx.hash.final(&scratch);
                var ohash = Hash.init(.{});
                ohash.update(&ctx.o_key_pad);
                ohash.update(&scratch);
                ohash.final(out);
            }
        };
    }

    const htest = @import("test.zig");

    test "hmac md5" {
        var out: [HmacMd5.mac_length]u8 = undefined;
        HmacMd5.create(out[0..], "", "");
        try htest.assertEqual("74e6f7298a9c2d168935f58c001bad88", out[0..]);

        HmacMd5.create(out[0..], "The quick brown fox jumps over the lazy dog", "key");
        try htest.assertEqual("80070713463e7749b90c2dc24911e275", out[0..]);
    }

    test "hmac sha1" {
        var out: [HmacSha1.mac_length]u8 = undefined;
        HmacSha1.create(out[0..], "", "");
        try htest.assertEqual("fbdb1d1b18aa6c08324b7d64b71fb76370690e1d", out[0..]);

        HmacSha1.create(out[0..], "The quick brown fox jumps over the lazy dog", "key");
        try htest.assertEqual("de7c9b85b8b78aa6bc8a7a36f70a90701c9db4d9", out[0..]);
    }

    test "hmac sha256" {
        var out: [sha2.HmacSha256.mac_length]u8 = undefined;
        sha2.HmacSha256.create(out[0..], "", "");
        try htest.assertEqual("b613679a0814d9ec772f95d778c35fc5ff1697c493715653c6c712144292c5ad", out[0..]);

        sha2.HmacSha256.create(out[0..], "The quick brown fox jumps over the lazy dog", "key");
        try htest.assertEqual("f7bc83f430538424b13298e6aa6fb143ef4d59a14946175997479dbc2d1a3cd8", out[0..]);
    }