Problem - Kevin Tafuro

• Let the salt be public, in which case the attacker can try out passwords by
attempting to authenticate with the server.
The server
The first thing the server needs to be able to do is create accounts for users. User credential
information is stored in objects of type AX_CRED. To compute credentials, use
the following function:
void AX_compute_credentials(char *user, size_t ulen, char *pass, size_t plen,
size_t ic, size_t pksz, size_t minkl, size_t maxkl,
size_t public_salt, size_t saltlen, AX_CRED *out);
This function has the following arguments:
user
Arbitrary binary string representing the unique login ID of the user.
ulen
Length of the username.
pass
The password, an arbitrary binary string.
plen
Length of the password in bytes.
ic
Iteration count to be used in the internal secret derivation function. See Recipe 4.10
for recommendations on setting this value (AX uses the derivation function from
that recipe).
pksz
Determines whether PAX credentials or SAX credentials should be computed. If
you are using PAX, the value specifies the length of the modulus of the public
key in bits, which must be 1,024, 2,048, 4,096, or 8,192. If you are using SAX,
set this value to 0.
minkl
Minimum key length we will allow the client to request when doing an
exchange, in bytes. We recommend 16 bytes (128 bits).
maxkl
Maximum key length we will allow the client to request when doing an
exchange, in bytes. Often, the protocol you use will only want a single fixed-size
key (and not give the client the option to choose), in which case, this should be
the same value as minkl.
public_salt
If this is nonzero, the server will give out the user’s salt value when requested.
Otherwise, the server should print out the salt at account creation time and have
the user enter it on first login from a new client machine.
Performing Password-Based Authentication and Key Exchange | 423
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