.\" $OpenBSD: PEM_ASN1_read.3,v 1.2 2020/07/23 17:34:53 schwarze Exp $
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.\" Copyright (c) 2020 Ingo Schwarze <schwarze@openbsd.org>
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.Dd $Mdocdate: July 23 2020 $
.Dt PEM_ASN1_READ 3
.Os
.Sh NAME
.Nm d2i_of_void ,
.Nm PEM_ASN1_read ,
.Nm PEM_ASN1_read_bio
.Nd PEM and DER decode an arbitrary ASN.1 value
.Sh SYNOPSIS
.In openssl/pem.h
.Ft typedef void *
.Fo d2i_of_void
.Fa "void **val_out"
.Fa "const unsigned char **der_in"
.Fa "long length"
.Fc
.Ft void *
.Fo PEM_ASN1_read
.Fa "d2i_of_void *d2i"
.Fa "const char *name"
.Fa "FILE *in_fp"
.Fa "void **val_out"
.Fa "pem_password_cb *cb"
.Fa "void *u"
.Fc
.Ft void *
.Fo PEM_ASN1_read_bio
.Fa "d2i_of_void *d2i"
.Fa "const char *name"
.Fa "BIO *in_bp"
.Fa "void **val_out"
.Fa "pem_password_cb *cb"
.Fa "void *u"
.Fc
.Sh DESCRIPTION
These functions read one object from
.Fa in_fp
or
.Fa in_bp
and perform both PEM and DER decoding.
They are needed when more specific decoding functions
like those documented in
.Xr PEM_read_bio_PrivateKey 3
and
.Xr PEM_read_SSL_SESSION 3
are inadequate for the type
.Fa name .
.Pp
For PEM decoding,
.Xr PEM_bytes_read_bio 3
is called internally.
Consequently, the first object of type
.Fa name
is returned and preceding objects of other types are discarded.
If necessary, data is decrypted, using
.Fa cb
and/or
.Fa u
if they are not
.Dv NULL ,
as described in the
.Xr pem_password_cb 3
manual page.
.Pp
For subsequent DER decoding, pass a
.Fa d2i
callback function that is adequate for the type
.Fa name ,
typically returning a pointer of a type more specific than
.Ft void * .
For example,
.Xr d2i_ASN1_TYPE 3
can always be used and its manual page describes the required
behaviour of the callback function to be passed.
Normally, passing a more specific function is more useful;
candidate functions can be found with
.Ql man -k Nm~^d2i_ .
.Pp
For the
.Fa name
argument, the
.Dv PEM_STRING_*
string constants defined in
.In openssl/pem.h
can be used.
.Pp
The
.Fa val_out
argument is useless and its many dangers are described in detail in the
.Xr d2i_ASN1_TYPE 3
manual page.
To reduce the risk of bugs, always passing
.Dv NULL
is recommended.
.Sh RETURN VALUES
These functions return a pointer to the decoded object or
.Dv NULL
if an error occurs.
They fail if
.Xr PEM_bytes_read_bio 3
fails, for example because of invalid syntax in the input, an unknown
encryption, or an invalid passphrase entered by the user.
They also fail if
.Fa d2i
returns
.Dv NULL ,
for example due to DER decoding errors.
.Pp
.Fn PEM_ASN1_read
may also fail if memory is exhausted.
.Sh EXAMPLES
Typical usage of
.Fn PEM_ASN1_read
is demonstrated by the implementation of the more specific function
to PEM and DER decode an X.509 certificate:
.Bd -literal -offset 2n
X509 *
PEM_read_X509(FILE *fp, X509 **val_out, pem_password_cb *cb, void *u)
{
	return PEM_ASN1_read((d2i_of_void *)d2i_X509, PEM_STRING_X509,
	    fp, (void **)val_out, cb, u);
}
.Ed
.Sh ERRORS
Diagnostics that can be retrieved with
.Xr ERR_get_error 3 ,
.Xr ERR_GET_REASON 3 ,
and
.Xr ERR_reason_error_string 3
include:
.Bl -tag -width Ds
.It Dv ERR_R_BUF_LIB Qq "BUF lib"
.Fn PEM_ASN1_read
failed to set up a temporary BIO,
for example because memory was exhausted.
.It Dv ERR_R_ASN1_LIB Qq "ASN1 lib"
.Fa d2i
returned
.Dv NULL ,
for example due to a DER syntax error.
.El
.Pp
Additional types of errors can result from
.Xr PEM_bytes_read_bio 3 .
.Sh SEE ALSO
.Xr BIO_new 3 ,
.Xr d2i_ASN1_TYPE 3 ,
.Xr PEM_bytes_read_bio 3 ,
.Xr PEM_read 3 ,
.Xr PEM_read_bio_PrivateKey 3 ,
.Xr PEM_read_SSL_SESSION 3 ,
.Xr PEM_X509_INFO_read 3
.Sh HISTORY
These functions first appeared in SSLeay 0.5.1
and have been available since
.Ox 2.4 .