path: root/lib/libcrypto/man/ASN1_item_d2i.3

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.Dd $Mdocdate: January 3 2017 $
.Dt ASN1_ITEM_D2I 3
.Os
.Sh NAME
.Nm ASN1_item_d2i ,
.Nm ASN1_item_d2i_bio ,
.Nm ASN1_item_d2i_fp ,
.Nm d2i_ASN1_TYPE ,
.Nm ASN1_item_i2d ,
.Nm ASN1_item_i2d_bio ,
.Nm ASN1_item_i2d_fp ,
.Nm i2d_ASN1_TYPE ,
.Nm ASN1_item_dup ,
.Nm ASN1_item_print
.Nd decode and encode ASN.1 objects
.Sh SYNOPSIS
.In openssl/asn1.h
.Ft ASN1_VALUE *
.Fo ASN1_item_d2i
.Fa "ASN1_VALUE **val_out"
.Fa "const unsigned char **der_in"
.Fa "long length"
.Fa "const ASN1_ITEM *it"
.Fc
.Ft void *
.Fo ASN1_item_d2i_bio
.Fa "const ASN1_ITEM *it"
.Fa "BIO *in_bio"
.Fa "void *val_out"
.Fc
.Ft void *
.Fo ASN1_item_d2i_fp
.Fa "const ASN1_ITEM *it"
.Fa "FILE *in_fp"
.Fa "void *val_out"
.Fc
.Ft ASN1_TYPE *
.Fo d2i_ASN1_TYPE
.Fa "ASN1_TYPE **val_out"
.Fa "const unsigned char **der_in"
.Fa "long length"
.Fc
.Ft int
.Fo ASN1_item_i2d
.Fa "ASN1_VALUE *val_in"
.Fa "unsigned char **der_out"
.Fa "const ASN1_ITEM *it"
.Fc
.Ft int
.Fo ASN1_item_i2d_bio
.Fa "const ASN1_ITEM *it"
.Fa "BIO *out_bio"
.Fa "void *val_in"
.Fc
.Ft int
.Fo ASN1_item_i2d_fp
.Fa "const ASN1_ITEM *it"
.Fa "FILE *out_fp"
.Fa "void *val_in"
.Fc
.Ft int
.Fo i2d_ASN1_TYPE
.Fa "ASN1_TYPE *val_in"
.Fa "unsigned char **der_out"
.Fc
.Ft void *
.Fo ASN1_item_dup
.Fa "const ASN1_ITEM *it"
.Fa "void *val_in"
.Fc
.Ft int
.Fo ASN1_item_print
.Fa "BIO *out_bio"
.Fa "ASN1_VALUE *val_in"
.Fa "int indent"
.Fa "const ASN1_ITEM *it"
.Fa "const ASN1_PCTX *pctx"
.Fc
.Sh DESCRIPTION
These functions convert ASN.1 values from their BER encoding to
internal C structures
.Pq Dq d2i
and vice versa
.Pq Dq i2d .
Unlike the C structures which contain pointers to sub-objects, BER
is a serialized encoding, suitable for transfer over the network
and for storage in a file.
.Pp
.Fn ASN1_item_d2i
interpretes
.Pf * Fa der_in
as a DER- or BER-encoded byte array and decodes one value of type
.Fa it
represented by up to
.Fa length
bytes.
If successful,
.Pf * Fa der_in
is advanced to the byte following the parsed data.
.Pp
If decoding succeeds and
.Fa val_out
or
.Pf * Fa val_out
is
.Dv NULL ,
a new object is allocated.
.Pp
If decoding succeeds and
.Pf * Fa val_out
is not
.Dv NULL ,
it is assumed to point to a valid populated object and an attempt
is made to reuse it.
It must not be an empty structure such as one returned by
.Xr ASN1_item_new 3
or by one of the various type-specific
.Fn *_new
functions.
This
.Dq reuse
capability is present for backward compatibility, but its use is
strongly discouraged; see the
.Sx BUGS
section below.
.Pp
.Fn ASN1_item_d2i_bio
and
.Fn ASN1_item_d2i_fp
are similar to
.Fn ASN1_item_d2i
except that they read from a
.Vt BIO
or
.Vt FILE ,
respectively.
.Pp
.Fn d2i_ASN1_TYPE
is similar to
.Fn ASN1_item_d2i
except that it does not require a desired type to be specified by
the user, but instead returns an
.Vt ASN1_TYPE
wrapper object containing both the type and the value found in the input.
.Pp
.Fn ASN1_item_i2d
encodes the object pointed to by
.Fa val_in
into DER format.
.Pp
If
.Pf * Fa der_out
is not
.Dv NULL ,
it writes the DER-encoded data to the buffer at
.Pf * Fa der_out
and increments it to point after the data just written.
In this case, it is the responsibility of the user to make sure
that the buffer pointed to by
.Pf * Fa der_out
is long enough, such that no buffer owerflow can occur.
.Pp
If
.Pf * Fa der_out
is
.Dv NULL ,
memory is allocated for a buffer, and
.Pf * Fa der_out
is not incremented, but points to the start of the data just written.
.Pp
If
.Fa der_out
is
.Dv NULL ,
the encoded bytes are not written anywhere but discarded.
For
.Fa val_in
objects of variable encoding size, this is sometimes used to first
find the number of bytes that will be written.
Then, a sufficient amount of memory is allocated before calling
.Fn ASN1_item_i2d
again.
This explicit double-call technique is often not needed because the
auto-allocation technique described in the previous paragraph can
be used.
.Pp
.Fn ASN1_item_i2d_bio
and
.Fn ASN1_item_i2d_fp
are similar to
.Fn ASN1_item_i2d
except that they write to a
.Vt BIO
or
.Vt FILE ,
respectively.
.Pp
.Fn i2d_ASN1_TYPE
is similar to
.Fn ASN1_item_i2d
except that the type and the value are not provided separately,
but in the form of a single
.Vt ASN1_TYPE
object.
.Pp
.Fn ASN1_item_dup
creates a deep copy of
.Fa val_in
by calling
.Fn ASN1_item_i2d
and
.Fn ASN1_item_d2i .
.Sh RETURN VALUES
If successful,
.Fn ASN1_item_d2i ,
.Fn ASN1_item_d2i_bio ,
.Fn ASN1_item_d2i_fp ,
and
.Fn d2i_ASN1_TYPE
return a pointer to the decoded ASN.1 value.
In addition, if
.Fa val_out
is not
.Dv NULL ,
the pointer is also written to
.Pf * Fa val_out .
If an error occurs,
.Dv NULL
is returned.
.Pp
.Fn ASN1_item_i2d
and
.Fn i2d_ASN1_TYPE
return the number of bytes written
or a negative value if an error occurs.
.Pp
.Fn ASN1_item_i2d_bio
and
.Fn ASN1_item_i2d_fp
return 1 for success or 0 for failure.
.Pp
.Fn ASN1_item_dup
returns the new
.Vt ASN1_VALUE
object or
.Dv NULL
if an error occurs.
.Sh EXAMPLES
Many type-specific wrapper functions exist.
Using those wrappers is recommended in application code
because it restores part of the type safety that the low-level
interfaces using
.Vt ASN1_VALUE
lack.
.Pp
For example, to allocate a buffer and write the DER encoding of an
.Vt X509
object into it:
.Bd -literal -offset indent
X509		*x;
unsigned char	*buf;
int		 len;

buf = NULL;
len = i2d_X509(x, &buf);
if (len < 0)
	/* error */
.Ed
.Pp
Attempt to decode a buffer:
.Bd -literal -offset indent
X509		*x;
unsigned char	*buf, *p;
int		 len;

/* Set up buf and len to point to the input buffer. */
p = buf;
x = d2i_X509(NULL, &p, len);
if (x == NULL)
	/* error */
.Ed
.Pp
Equivalent technique:
.Bd -literal -offset indent
X509		*x;
unsigned char	*buf, *p;
int		 len;

/* Set up buf and len to point to the input buffer. */
p = buf;
x = NULL;

if (d2i_X509(&x, &p, len) == NULL)
	/* error */
.Ed
.Sh SEE ALSO
.Xr ASN1_item_new 3 ,
.Xr ASN1_TYPE_new 3
.Sh CAVEATS
If the type described by
.Fa it
fails to match the true type of
.Fa val_in
or
.Pf * Fa val_out ,
buffer overflows and segmentation faults are likely to occur.
For more details about why the type
.Vt ASN1_VALUE
constitutes dangerous user interface design, see
.Xr ASN1_item_new 3 .
.Pp
The encoded data is in binary form and may contain embedded NUL bytes.
Functions such as
.Xr strlen 3
will not return the correct length of the encoded data.
.Pp
While the way that
.Pf * Fa der_in
and
.Pf * Fa der_out
are incremented after the operation supports the typical usage
patterns of reading or writing one object after another, this
behaviour can trap the unwary.
.Pp
Using a temporary pointer into the buffer is mandatory.
A common mistake is to attempt to use a buffer directly as follows:
.Bd -literal -offset indent
X509		*x;
unsigned char	*buf;
int		 len;

len = i2d_X509(x, NULL);
buf = malloc(len);
i2d_X509(x, &buf);
/* do something with buf[] */
free(buf);
.Ed
.Pp
This code will result in
.Va buf
apparently containing garbage because it was incremented during
.Fn i2d_X509
to point after the data just written.
Also
.Va buf
will no longer contain the pointer allocated by
.Xr malloc 3
and the subsequent call to
.Xr free 3
is likely to crash.
.Pp
Another trap to avoid is misuse of the
.Fa val_out
argument:
.Bd -literal -offset indent
X509		*x;

if (d2i_X509(&x, &p, len) == NULL)
	/* error */
.Ed
.Pp
This will probably crash somewhere in
.Fn d2i_X509
because
.Va x
is uninitialized and an attempt will be made to interpret its invalid
content as an
.Vt X509
object, typically causing a segmentation violation.
If
.Va x
is set to
.Dv NULL
first, then this will not happen.
.Sh BUGS
If the
.Dq reuse
capability is used, a valid object is passed in via
.Pf * Fa val_out ,
and an error occurs, then the object is not freed and may be left
in an invalid or inconsistent state.
.Pp
In some versions of OpenSSL, the
.Dq reuse
behaviour is broken such that some parts of the reused object may
persist if they are not present in the new one.
.Pp
In many versions of OpenSSL,
.Fn ASN1_item_i2d
will not return an error if mandatory fields are not initialized
due to a programming error.
In that case, the encoded structure may contain invalid data and
some fields may be missing entirely, such that trying to parse it
with
.Fn ASN1_item_d2i
may fail.
.Pp
Any function which encodes an object may return a stale encoding
if the object has been modified after deserialization or previous
serialization.
This is because some objects cache the encoding for efficiency reasons.