.TH LIBAR2_ENCODE_PARAMS 3 LIBAR2
.SH NAME
libar2_encode_params - Encode Argon2 hashing parameters

.SH SYNOPSIS
.nf
#include <libar2.h>

enum libar2_argon2_version {
    LIBAR2_ARGON2_VERSION_10 = 0x10,
    LIBAR2_ARGON2_VERSION_13 = 0x13
};

enum libar2_argon2_type {
    LIBAR2_ARGON2D = 0,
    LIBAR2_ARGON2I = 1,
    LIBAR2_ARGON2ID = 2,
    LIBAR2_ARGON2DS = 4
};

struct libar2_argon2_parameters {
    enum libar2_argon2_type \fItype\fP;
    enum libar2_argon2_version \fIversion\fP;
    uint_least32_t \fIt_cost\fP;
    uint_least32_t \fIm_cost\fP;
    uint_least32_t \fIlanes\fP;
    unsigned char *\fIsalt\fP;
    size_t \fIsaltlen\fP;
    unsigned char *\fIkey\fP;
    size_t \fIkeylen\fP;
    unsigned char *\fIad\fP;
    size_t \fIadlen\fP;
    size_t \fIhashlen\fP;
};

size_t libar2_encode_params(char *\fIbuf\fP, const struct libar2_argon2_parameters *\fIparams\fP);
.fi
.PP
Link with
.IR -lar2 .

.SH DESCRIPTION
The
.BR libar2_encode_params ()
function encodes the Argon2 hashing parameters
provided via the
.I param
parameter, as a string, in a standardised format,
and stores the string in
.I buf
(unless
.I buf
is
.IR NULL )
and return the number of bytes that was (or would
have been) written to
.IR buf .
.PP
It is recommended that the
.BR libar2_encode_params ()
function is called twice: first with
.I buf
set to
.IR NULL ,
to get how large
.I buf
shall be, and then (with the same, unmodified,
.IR params
and) with a
.I buf
with an allocation size of at least the number
of bytes that was returned by the function in
the previous call to it.
.PP
The created string will have the following format

.RS
.B \(dq$%s$v=%i$m=%lu,t=%lu,p=%lu$%s$\(dq,
.RI < type >\fB,\fP
.RI < version >\fB,\fP
.RI < "memory cost" >\fB,\fP
.RI < "time cost" >\fB,\fP
.RI < "parallelism" >\fB,\fP
.RI < "base64 encoded salt" >
.RE

if the version is explicitly specified, and otherwise

.RS
.B \(dq$%s$m=%lu,t=%lu,p=%lu$%s$\(dq,
.RI < type >\fB,\fP
.RI < "memory cost" >\fB,\fP
.RI < "time cost" >\fB,\fP
.RI < "parallelism" >\fB,\fP
.RI < "base64 encoded salt" >
.RE

The string does not encode the \(dqsecret\(dq
(pepper), \(dqassociated data\(dq, or the
\(dqtag\(dq (message hash) length. This string
is the Argon2 hash string minus the \(dqtag\(dq
(the hash of the message (the password)), which
is encoded in base64 and appended to the string
formatted by this function. For information
about the expected contents of the
.I params
argument, see 
.BR libar2_hash (3).
.PP
.I params
may not be
.IR NULL .

.SH RETURN VALUES
The
.BR libar2_encode_params ()
function returns the number of bytes required
to encode the parameter string, plus one extra
byte for the NUL byte that is added to the to
terminate the string; that is, the number of
bytes written to
.I buf
or the required allocation size of
.IR buf .
If
.I buf
is
.RI non- NULL ,
a string will be stored in it according to the
specifications in the
.B DESCRIPTION
section.

.SH ERRORS
The
.BR libar2_encode_params ()
function cannot fail.

.SH EXAMPLES
The following example demonstrates how to
encode the hashing parameters into a dynamically
allocated string.
.PP
.nf
#include <libar2.h>
#include <stdlib.h>

static char *
encode_params(const struct libar2_argon2_parameters *params)
{
    size_t n = libar2_encode_params(NULL, params);
    char *buf = malloc(n);
    if (!buf)
        return NULL;
    if (libar2_encode_params(buf, params) > n)
        abort();
    return buf;
}
.fi

.SH NOTES
The
.BR libar2_encode_params ()
function till note validate its input.
This has to be done using the
.BR libar2_validate_params (3)
function.

.SH SEE ALSO
.BR libar2 (7),
.BR libar2_validate_params (3),
.BR libar2_decode_params (3),
.BR libar2_encode_base64 (3),
.BR libar2_hash (3)