.TH LIBAR2_DECODE_PARAMS 3 LIBAR2
.SH NAME
libar2_decode_params - Decode 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;
};

struct libar2_context {
    /* Parts of this struct that are not relevant to \fBlibar2_decode_params\fP() have been omitted. */
    void *\fIuser_data\fP;
    void *(*\fIallocate\fP)(size_t \fInum\fP, size_t \fIsize\fP, size_t \fIalignment\fP, struct libar2_context *\fIctx\fP);
    void (*\fIdeallocate\fP)(void *\fIptr\fP, struct libar2_context *\fIctx\fP);
};

size_t libar2_decode_params(const char *\fIstr\fP, const struct libar2_argon2_parameters *\fIparams\fP,
                            char **\fIbuf\fP, truct libar2_context *\fIctx\fP);
.fi
.PP
Link with
.IR -lar2 .

.SH DESCRIPTION
The
.BR libar2_decode_params ()
function decodes a string, provided via the
.I str
parameter, that encodes Argon2 hashing parameters,
and stores the decoded parameters in
.I param
and return the number of bytes read from, up to
but exclude the first byte that was determine not
to be part of the encoded data. For more
information about
.IR param ,
see
.BR libar2_hash (3).
.PP
The input string,
.IR str ,
does not encode the \(dqsecret\(dq (pepper) or
\(dqassociated data\(dq, therefore these will
be set to zero-length. The tag (message/password
hash) length will be inferred from the portion
of the input string (specifically from the number
of bytes that make up a valid base64 string) after
the data that the
.BR libar2_encode_params (3)
function encodes,
.B however
the number of bytes read from this part of the
string will not be included in the functions
return value (the return value marks the end of
the parameter string, not the hash string which
also includes the tag).
.PP
The
.BR libar2_decode_params ()
function may use the
.I *ctx->allocate
function to dynamically allocate memory
and the
.I *ctx->deallocate
function to deallocate memory it has
allocated with
.IR *ctx->allocate .
The function may call
.I *ctx->allocate
once, and on failure if
.I *ctx->allocate
was called,
.I *ctx->deallocate
once. If
.I *ctx->allocatew
was called, but not
.IR *ctx->deallocate ,
the memory allocated with
.I *ctx->allocate
will be stored in
.I *bufp
and may be deallocated by the user with
.I *ctx->deallocate
once the result stored in
.I params
is not longer needed.
See more information about
.I ctx->allocate
and
.IR ctx->deallocate ,
as well as
.IR ctx->user_data ,
see
.BR libar2_hash (3).
.PP
None of the arguments may be
.IR NULL .
.PP
Upon successful completion, if
.I str
contains the a tag,
.IR &str[n] ,
where
.I n
is the return value of the function,
will point to the beginning of the tag.

.SH RETURN VALUES
The
.BR libar2_decode_params ()
returns the number of bytes in
.I str
that make up the decoded parameter, including
the final dollar-sign
.RB ( $ ),
but excluding the tag, if any, at the end of
the string which is used to infer the tag length,
and stores the decoded parameters in
.IR params ,
and potentially dynamically allocated data in
.IR *bufp ,
upon successful completion
On error, 0 is returned (not a valid return
value on success completion) and
.I errno
is set to describe the error.

.SH ERRORS
The
.BR libar2_decode_params ()
function will fail if:
.TP
.B EINVAL
.I str
is improperly formatted or contains
an unrecognised primitive type.
.TP
.B ERANGE
.I str
contains an integer that is too large to
be represented by the field in
.I params
that it shall be stored in.
.PP
The
.BR libar2_decode_params ()
function will also fail if the
.I *ctx->allocate
function fails, and will, in that case,
not modify
.IR errno .

.SH NOTES
The encoded parameters will not be
validate by the
.BR libar2_decode_params ()
function beyond what is needed to ensure that
the parameters can be accurately parsed and
represented. 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_encode_params (3),
.BR libar2_decode_base64 (3),
.BR libar2_hash (3)