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

    /// HKDF-SHA256
    pub const HkdfSha256 = Hkdf(hmac.sha2.HmacSha256);

    /// HKDF-SHA512
    pub const HkdfSha512 = Hkdf(hmac.sha2.HmacSha512);

    /// The Hkdf construction takes some source of initial keying material and
    /// derives one or more uniform keys from it.
    pub fn Hkdf(comptime Hmac: type) type {
        return struct {
            /// Length of a master key, in bytes.
            pub const prk_length = Hmac.mac_length;

            /// Return a master key from a salt and initial keying material.
            pub fn extract(salt: []const u8, ikm: []const u8) [prk_length]u8 {
                var prk: [prk_length]u8 = undefined;
                Hmac.create(&prk, ikm, salt);
                return prk;
            }

            /// Initialize the creation of a master key from a salt
            /// and keying material that can be added later, possibly in chunks.
            /// Example:
            /// ```
            /// var prk: [hkdf.prk_length]u8 = undefined;
            /// var hkdf = HkdfSha256.extractInit(salt);
            /// hkdf.update(ikm1);
            /// hkdf.update(ikm2);
            /// hkdf.final(&prk);
            /// ```
            pub fn extractInit(salt: []const u8) Hmac {
                return Hmac.init(salt);
            }

            /// Derive a subkey from a master key `prk` and a subkey description `ctx`.
            pub fn expand(out: []u8, ctx: []const u8, prk: [prk_length]u8) void {
                assert(out.len <= prk_length * 255); // output size is too large for the Hkdf construction

                var i: usize = 0;
                var counter = [1]u8{1};
                while (i + prk_length <= out.len) : (i += prk_length) {
                    var st = Hmac.init(&prk);
                    if (i != 0) {
                        st.update(out[i - prk_length ..][0..prk_length]);
                    }
                    st.update(ctx);
                    st.update(&counter);
                    st.final(out[i..][0..prk_length]);
                    counter[0] +%= 1;
                    assert(counter[0] != 1);
                }
                const left = out.len % prk_length;
                if (left > 0) {
                    var st = Hmac.init(&prk);
                    if (i != 0) {
                        st.update(out[i - prk_length ..][0..prk_length]);
                    }
                    st.update(ctx);
                    st.update(&counter);
                    var tmp: [prk_length]u8 = undefined;
                    st.final(tmp[0..prk_length]);
                    @memcpy(out[i..][0..left], tmp[0..left]);
                }
            }
        };
    }

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

    test "Hkdf" {
        const ikm = [_]u8{0x0b} ** 22;
        const salt = [_]u8{ 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c };
        const context = [_]u8{ 0xf0, 0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8, 0xf9 };
        const kdf = HkdfSha256;
        const prk = kdf.extract(&salt, &ikm);
        try htest.assertEqual("077709362c2e32df0ddc3f0dc47bba6390b6c73bb50f9c3122ec844ad7c2b3e5", &prk);
        var out: [42]u8 = undefined;
        kdf.expand(&out, &context, prk);
        try htest.assertEqual("3cb25f25faacd57a90434f64d0362f2a2d2d0a90cf1a5a4c5db02d56ecc4c5bf34007208d5b887185865", &out);

        var hkdf = kdf.extractInit(&salt);
        hkdf.update(&ikm);
        var prk2: [kdf.prk_length]u8 = undefined;
        hkdf.final(&prk2);
        try htest.assertEqual("077709362c2e32df0ddc3f0dc47bba6390b6c73bb50f9c3122ec844ad7c2b3e5", &prk2);
    }