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Date: Sat, 8 Mar 2014 13:48:09 -0500
From: Bill Cox <>
Subject: Upgrade HKDF to HKDF2?

All the current KDF primitives I have read about have serious
limitations.  I don't want to inherit those limitations in my PHC
entry.  Rather than simply pre-processing and post-processing around
an existing KDF as a work-around, why not upgrade HKDF (the best I've
seen so far for a memory hard KDF) and make it work really well for
password hashing?  I propose to call this HKDF2, if the HKDF authors
allow it (and preferably this whole thing is done with their blessing
and review).

Digging into HMAC, PBKDF2, HKDF, and what Blake2 does with it's key
data has been very interesting.  Here's limitations of each:

HMAC: Input collisions galore!  Really?!?  Why are we using this for
hashing passwords???
PBKDF2: Inherits HMAC collisions, and suffers from chosen c attack -
PBKDF2 should be avoided
HKDF: Still new, but it works around HMAC input collisions.  However,
it potentially leaks password length
             due to no padding.  It also forces sensitive "info" data
to remain in memory too long.
Blake2: Implements secure HMAC-like functionality directly with no
input collisions, which is cool.
             It is not able to directly generate long output lengths,
and has a limited input key length.
             Blake2 isn't meant to directly be a KDF, so this isn't a
criticism, just an observation.

What I'd like to do is enhance HKDF:

1) Stop using HMAC under HKDF.  Instead, when the hash function is
like Bake2 and implements a secure HMAC interface, use it directly
rather than HMAC.  When it's not available, such as SHA-256, use the
hash function's Init, Update, and Finalize interface to hash in the
padded/compressed password, and then the data.

2) Add an "info" parameter to HKDF_Extract (credit for this idea goes
to Dennis E. Hamilton).  There is no reason to waste time hashing
input parameters into every output key, and being forced to keep
sensitive data around to the end is a security problem.

3) Pad sensitive inputs, and let them be long: password, salt, and info inputs

4) Hash in passwordSize, saltSize, and infoSize, so the user wont have to

5) Expand the counter that is hashed along with the other data in
HKDF_Expand to 32-bits.  It's currently 8 bits.

6) Add "clearPassword" and "clearInfo" flags and securely erase the
password and info if set

Upgrade 1 improves security by bypassing the current rampant mis-use
of HMAC in password hashing.  HMAC is a terrible primitive for
password hashing due to it's trivial password input collisions.  HKDF
hacks around this limitation, by abusing the HMAC interface: it passes
the password as "message" and the salt as "key".  Yuk!  Instead of
this hack, which causes HKDF to potentially leak timing information
about the password length, let's pad the password correctly, unlike

Upgrade 2 is a simple improvement that improves both speed and
security.  There is no reason, for example, to hash the passwordSize
parameter into every output block of HKDF_Expand, rather than hashing
it into the PRK generated by HKDF_Extract.  This is not just faster,
but it also enables the user to clear sensitive data like secondary
secret keys used in as info parameter while doing processing between
HKDF_Extract and HKDF_Expand.  For example, that processing might be a
memory-hard KDF that runs for a second, and hashes several GiB of
memory.  HKDF currently would force a server master key, when used as
info, to hang around until the memory hashing completes.  This is

Upgrade 3, padding sensitive inputs, provides additional security
against timing side-channel attacks that might enable an attacker to
gain some knowledge about the password length.  I think this should be
done with all variable length inputs, not just the password: password,
salt, and info.  The info parameter might be as sensitive as the
password, and as we see from how HKDF abuses HMAC by passing the
password as the message and the salt as the key, it is likely down the
road that the salt parameter might be sensitive data.  We should not
leak length information about any variable length inputs!  Is
something as basic as this controversial?

Padding should be done by simply running a multiple of a PAD_SIZE of
iterations, copying from the input into the pad.  Here's my code for

static void extractPassword(uint8_t prk[KEYSIZE], uint8_t *password,
uint32_t passwordSize, bool clearPassword) {
    uint8 pad[PAD_SIZE];
    for(uint32_t i = 0; i < passwordSize; i += PAD_SIZE) {
        for(uint32_t j = 0; j < PAD_SIZE; j++) {
            pad[j] = password[(i + j) % passwordSize]);
        // prk = Hash(message=pad, key=prk
        Hash(prk, KEYSIZE, pad, PAD_SIZE, prk, KEYSIZE);
    if(clearPassword) {
        secureZeroMemory(password, passwordSize);
    secureZeroMemory(pad, PAD_SIZE);

Upgrade 4, adding input sizes to the info parameter and hashing it in
along with everything else, can be accomplished by setting prk to 0's,
and then writing these lengths to it, before it's passed to the
padding function.  This eliminates collisions caused by password size
reduction, for example, which is a problem in HMAC.

Upgrade 5 is just a trivial expansion of the counter length to 4 bytes
to support longer output hashes.  For example, I use Blake2s to
initialize the first block of memory, which is currently 16KiB.  HKDF
can only generate 8KiB.  This is a trival upgrade that in no way
impacts security, so if we're upgrading HKDF anyway, I'd throw it in.

Upgrade 6: Add parameters to securely clear the password and info
inputs as soon as they are no longer needed.  This is not a critical
upgrade, and can be left out, but most users will wind up rolling
their own, and doing it wrong!  Here's my secureZero function,
originally copied from Blake2, but modified to use padding:

static inline void secureZeroMemory(void *v, uint32_t n) {
    volatile uint8_t *p = (volatile uint8_t *)v;
    for(uint32_t i = 0; i < n; i += PAD_SIZE) {
        for(uint32_t j = 0; j < PAD_SIZE; j++) {
            p[(i + j) % n] = 0;

Blake2 does not need the pad in their version of secure_zero_memory,
since they only zero fixed-length data, but this function would be
used to clear the password.  This function does not leak the lower m
bits of n, where PAD_SIZE = 2^m.  It does potentially leak the upper
bits of n.  I'm using PAD_SIZE == 256 at the moment, for 8 bits of
obfuscation, enough for the most cases.  There is a tradeoff here: it
is more likely to leak cache timing information the longer we loop
over the password, which could leak some password length info (like
nearest multiple of 64).  However, running with a short pad
potentially leaks more bits if an attacker gains knowledge of how long
this function runs.  The choice of PAD_SIZE is thus a compromise.  I
currently use 256.

Thoughts?  Is it time to create HKDF2, or should I just hack around
HKDF or PBKDF2 however I like, just like everyone else?

One dumb idea: allow the PHC entrants to change their initial/final
KDF schemes when "tweaks" are allowed, and allow them to collaborate
on how this is done so that preferably, they all do it in a similar
way to the extent possible.  I'm leaning towards proposing an HKDF2
implementation, but I'd like to adopt whatever the PHC judges think is


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