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Date: Sun, 19 Jan 2014 13:31:06 +0100
From: Christian Forler <>
Subject: Re: [PHC] Native server relief support for password hashing in browsers

On 19.01.2014 08:54, Solar Designer wrote:
> On Sat, Jan 18, 2014 at 01:14:32PM -0800, Tony Arcieri wrote:
>> On Sat, Jan 18, 2014 at 1:12 PM, Larry Bugbee <> wrote:
>>> May I assume that if the pswd is hashed client side, two conditions need
>>> to be true: 1) javascript or other hashing support is available, and 2) the
>>> hash is passed in a secure tunnel
>> Yes
> I'd say "secure tunnel" is orthogonal to "client-side hashing", as well
> as to "better password hashing" in general.  Any of these are somewhat
> nice to have even without the others, although of course a combination
> of them is usually preferable.
You are right, but in the following I want to paraphrase your statement
in a very explicit way to prevent misconception.

Secure server-relief requires a secure channel, otherwise an adversary
can just eavesdrop the transmitted hash to impersonate Alice.

Suppose F() is time and memory costly KDF (e.g, scrypt), and
H() cryptographic hash function (e.g. Blake2).

1. Alice (e.g. javascript running in a browser) computes
x = F(password || salt).

2. Alice sends x to the server over an insecure channel

3. Eve eavesdrops x.

4. Bob (server) loads Alice password hash h_alice,
and then tests if h_alice = H(x). If so Bob grants access, otherwise
denies access.

5. Eve can impersonate Alice by sending x to Bob.

A countermeasure against passive adversaries are challenges (i.e.,
random bit string).

1. Alice sent a access request to Bob.
2. Bob sends a challenge z (i.e., random bit string) to Alice
3. Alice response with y = H( H(F(password || salt)) || z)
4, Bob test if h_alice  =  H( h_alice || z).

But such a protocol is vulnerable to active attacks (e.g.,
Men-in-the-Middle attacks).

Conclusive, perform server relief over an secure channel (e.g., TLS)  if
possible, otherwise your login process is as insecure as telnet.

Best regards,

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