    const std = @import("std");
    const mem = std.mem;
    const maxInt = std.math.maxInt;
    const OutputTooLongError = std.crypto.errors.OutputTooLongError;
    const WeakParametersError = std.crypto.errors.WeakParametersError;

    // RFC 2898 Section 5.2

    //

    // FromSpec:

    //

    // PBKDF2 applies a pseudorandom function (see Appendix B.1 for an

    // example) to derive keys. The length of the derived key is essentially

    // unbounded. (However, the maximum effective search space for the

    // derived key may be limited by the structure of the underlying

    // pseudorandom function. See Appendix B.1 for further discussion.)

    // PBKDF2 is recommended for new applications.

    //

    // PBKDF2 (P, S, c, dk_len)

    //

    // Options:        PRF        underlying pseudorandom function (h_len

    //                            denotes the length in octets of the

    //                            pseudorandom function output)

    //

    // Input:          P          password, an octet string

    //                 S          salt, an octet string

    //                 c          iteration count, a positive integer

    //                 dk_len      intended length in octets of the derived

    //                            key, a positive integer, at most

    //                            (2^32 - 1) * h_len

    //

    // Output:         DK         derived key, a dk_len-octet string

    // Based on Apple's CommonKeyDerivation, based originally on code by Damien Bergamini.

    /// Apply PBKDF2 to generate a key from a password.
    ///
    /// PBKDF2 is defined in RFC 2898, and is a recommendation of NIST SP 800-132.
    ///
    /// dk: Slice of appropriate size for generated key. Generally 16 or 32 bytes in length.
    ///             May be uninitialized. All bytes will be overwritten.
    ///             Maximum size is `maxInt(u32) * Hash.digest_length`
    ///             It is a programming error to pass buffer longer than the maximum size.
    ///
    /// password: Arbitrary sequence of bytes of any length, including empty.
    ///
    /// salt: Arbitrary sequence of bytes of any length, including empty. A common length is 8 bytes.
    ///
    /// rounds: Iteration count. Must be greater than 0. Common values range from 1,000 to 100,000.
    ///         Larger iteration counts improve security by increasing the time required to compute
    ///         the dk. It is common to tune this parameter to achieve approximately 100ms.
    ///
    /// Prf: Pseudo-random function to use. A common choice is `std.crypto.auth.hmac.HmacSha256`.
    pub fn pbkdf2(dk: []u8, password: []const u8, salt: []const u8, rounds: u32, comptime Prf: type) (WeakParametersError || OutputTooLongError)!void {
        if (rounds < 1) return error.WeakParameters;

        const dk_len = dk.len;
        const h_len = Prf.mac_length;
        comptime std.debug.assert(h_len >= 1);

        // FromSpec:

        //

        //   1. If dk_len > maxInt(u32) * h_len, output "derived key too long" and

        //      stop.

        //

        if (dk_len / h_len >= maxInt(u32)) {
            // Counter starts at 1 and is 32 bit, so if we have to return more blocks, we would overflow

            return error.OutputTooLong;
        }

        // FromSpec:

        //

        //   2. Let l be the number of h_len-long blocks of bytes in the derived key,

        //      rounding up, and let r be the number of bytes in the last

        //      block

        //

        const blocks_count = @intCast(u32, std.math.divCeil(usize, dk_len, h_len) catch unreachable);
        var r = dk_len % h_len;
        if (r == 0) {
            r = h_len;
        }

        // FromSpec:

        //

        //   3. For each block of the derived key apply the function F defined

        //      below to the password P, the salt S, the iteration count c, and

        //      the block index to compute the block:

        //

        //                T_1 = F (P, S, c, 1) ,

        //                T_2 = F (P, S, c, 2) ,

        //                ...

        //                T_l = F (P, S, c, l) ,

        //

        //      where the function F is defined as the exclusive-or sum of the

        //      first c iterates of the underlying pseudorandom function PRF

        //      applied to the password P and the concatenation of the salt S

        //      and the block index i:

        //

        //                F (P, S, c, i) = U_1 \xor U_2 \xor ... \xor U_c

        //

        //  where

        //

        //            U_1 = PRF (P, S || INT (i)) ,

        //            U_2 = PRF (P, U_1) ,

        //            ...

        //            U_c = PRF (P, U_{c-1}) .

        //

        //  Here, INT (i) is a four-octet encoding of the integer i, most

        //  significant octet first.

        //

        //  4. Concatenate the blocks and extract the first dk_len octets to

        //  produce a derived key DK:

        //

        //            DK = T_1 || T_2 ||  ...  || T_l<0..r-1>

        var block: u32 = 0;
        while (block < blocks_count) : (block += 1) {
            var prev_block: [h_len]u8 = undefined;
            var new_block: [h_len]u8 = undefined;

            // U_1 = PRF (P, S || INT (i))

            const block_index = mem.toBytes(mem.nativeToBig(u32, block + 1)); // Block index starts at 0001

            var ctx = Prf.init(password);
            ctx.update(salt);
            ctx.update(block_index[0..]);
            ctx.final(prev_block[0..]);

            // Choose portion of DK to write into (T_n) and initialize

            const offset = block * h_len;
            const block_len = if (block != blocks_count - 1) h_len else r;
            const dk_block: []u8 = dk[offset..][0..block_len];
            mem.copy(u8, dk_block, prev_block[0..dk_block.len]);

            var i: u32 = 1;
            while (i < rounds) : (i += 1) {
                // U_c = PRF (P, U_{c-1})

                Prf.create(&new_block, prev_block[0..], password);
                mem.copy(u8, prev_block[0..], new_block[0..]);

                // F (P, S, c, i) = U_1 \xor U_2 \xor ... \xor U_c

                for (dk_block) |_, j| {
                    dk_block[j] ^= new_block[j];
                }
            }
        }
    }

    const htest = @import("test.zig");
    const HmacSha1 = std.crypto.auth.hmac.HmacSha1;

    // RFC 6070 PBKDF2 HMAC-SHA1 Test Vectors

    test "RFC 6070 one iteration" {
        const p = "password";
        const s = "salt";
        const c = 1;
        const dk_len = 20;

        var dk: [dk_len]u8 = undefined;

        try pbkdf2(&dk, p, s, c, HmacSha1);

        const expected = "0c60c80f961f0e71f3a9b524af6012062fe037a6";

        try htest.assertEqual(expected, dk[0..]);
    }

    test "RFC 6070 two iterations" {
        const p = "password";
        const s = "salt";
        const c = 2;
        const dk_len = 20;

        var dk: [dk_len]u8 = undefined;

        try pbkdf2(&dk, p, s, c, HmacSha1);

        const expected = "ea6c014dc72d6f8ccd1ed92ace1d41f0d8de8957";

        try htest.assertEqual(expected, dk[0..]);
    }

    test "RFC 6070 4096 iterations" {
        const p = "password";
        const s = "salt";
        const c = 4096;
        const dk_len = 20;

        var dk: [dk_len]u8 = undefined;

        try pbkdf2(&dk, p, s, c, HmacSha1);

        const expected = "4b007901b765489abead49d926f721d065a429c1";

        try htest.assertEqual(expected, dk[0..]);
    }

    test "RFC 6070 16,777,216 iterations" {
        // These iteration tests are slow so we always skip them. Results have been verified.

        if (true) {
            return error.SkipZigTest;
        }

        const p = "password";
        const s = "salt";
        const c = 16777216;
        const dk_len = 20;

        var dk = [_]u8{0} ** dk_len;

        try pbkdf2(&dk, p, s, c, HmacSha1);

        const expected = "eefe3d61cd4da4e4e9945b3d6ba2158c2634e984";

        try htest.assertEqual(expected, dk[0..]);
    }

    test "RFC 6070 multi-block salt and password" {
        const p = "passwordPASSWORDpassword";
        const s = "saltSALTsaltSALTsaltSALTsaltSALTsalt";
        const c = 4096;
        const dk_len = 25;

        var dk: [dk_len]u8 = undefined;

        try pbkdf2(&dk, p, s, c, HmacSha1);

        const expected = "3d2eec4fe41c849b80c8d83662c0e44a8b291a964cf2f07038";

        try htest.assertEqual(expected, dk[0..]);
    }

    test "RFC 6070 embedded NUL" {
        const p = "pass\x00word";
        const s = "sa\x00lt";
        const c = 4096;
        const dk_len = 16;

        var dk: [dk_len]u8 = undefined;

        try pbkdf2(&dk, p, s, c, HmacSha1);

        const expected = "56fa6aa75548099dcc37d7f03425e0c3";

        try htest.assertEqual(expected, dk[0..]);
    }

    test "Very large dk_len" {
        // This test allocates 8GB of memory and is expected to take several hours to run.

        if (true) {
            return error.SkipZigTest;
        }
        const p = "password";
        const s = "salt";
        const c = 1;
        const dk_len = 1 << 33;

        var dk = try std.testing.allocator.alloc(u8, dk_len);
        defer {
            std.testing.allocator.free(dk);
        }

        // Just verify this doesn't crash with an overflow

        try pbkdf2(dk, p, s, c, HmacSha1);
    }