path: root/usr.sbin/snmpd/ber.3

.\" $OpenBSD: ber.3,v 1.20 2018/08/13 16:37:56 schwarze Exp $
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.\" Copyright (c) 2007, 2012 Reyk Floeter <reyk@openbsd.org>
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.Dd $Mdocdate: August 13 2018 $
.Dt BER 3
.Os
.Sh NAME
.Nm ber_get_element ,
.Nm ber_set_header ,
.Nm ber_link_elements ,
.Nm ber_unlink_elements ,
.Nm ber_replace_elements ,
.Nm ber_add_sequence ,
.Nm ber_add_set ,
.Nm ber_add_enumerated ,
.Nm ber_add_integer ,
.Nm ber_get_integer ,
.Nm ber_get_enumerated ,
.Nm ber_add_boolean ,
.Nm ber_get_boolean ,
.Nm ber_add_string ,
.Nm ber_add_nstring ,
.Nm ber_add_bitstring ,
.Nm ber_get_string ,
.Nm ber_get_nstring ,
.Nm ber_get_bitstring ,
.Nm ber_add_null ,
.Nm ber_get_null ,
.Nm ber_add_eoc ,
.Nm ber_get_eoc ,
.Nm ber_add_oid ,
.Nm ber_add_noid ,
.Nm ber_add_oidstring ,
.Nm ber_get_oid ,
.Nm ber_oid2ber ,
.Nm ber_string2oid ,
.Nm ber_oid_cmp ,
.Nm ber_printf_elements ,
.Nm ber_scanf_elements ,
.Nm ber_get_writebuf ,
.Nm ber_write_elements ,
.Nm ber_set_readbuf ,
.Nm ber_read_elements ,
.Nm ber_getpos ,
.Nm ber_free_element ,
.Nm ber_free_elements ,
.Nm ber_calc_len ,
.Nm ber_set_application ,
.Nm ber_set_writecallback ,
.Nm ber_free
.Nd encode and decode ASN.1 with Basic Encoding Rules
.Sh SYNOPSIS
.In ber.h
.Ft "struct ber_element *"
.Fn "ber_get_element" "unsigned int encoding"
.Ft "void"
.Fn "ber_set_header" "struct ber_element *elm" "int class" "unsigned int type"
.Ft "void"
.Fn "ber_link_elements" "struct ber_element *prev" "struct ber_element *elm"
.Ft "struct ber_element *"
.Fn "ber_unlink_elements" "struct ber_element *prev"
.Ft "void"
.Fn "ber_replace_elements" "struct ber_element *prev" "struct ber_element *elm"
.Ft "struct ber_element *"
.Fn "ber_add_sequence" "struct ber_element *prev"
.Ft "struct ber_element *"
.Fn "ber_add_set" "struct ber_element *prev"
.Ft "struct ber_element *"
.Fn "ber_add_integer" "struct ber_element *prev" "long long val"
.Ft "int"
.Fn "ber_get_integer" "struct ber_element *root" "long long *val"
.Ft "struct ber_element *"
.Fn "ber_add_enumerated" "struct ber_element *prev" "long long val"
.Ft "int"
.Fn "ber_get_enumerated" "struct ber_element *root" "long long *val"
.Ft "struct ber_element *"
.Fn "ber_add_boolean" "struct ber_element *prev" "int bool"
.Ft "int"
.Fn "ber_get_boolean" "struct ber_element *root" "int *bool"
.Ft "struct ber_element *"
.Fn "ber_add_string" "struct ber_element *prev" "const char *string"
.Ft "struct ber_element *"
.Fo "ber_add_nstring"
.Fa "struct ber_element *prev"
.Fa "const char *string"
.Fa "size_t size"
.Fc
.Ft "int"
.Fn "ber_get_string" "struct ber_element *root" "char **charbuf"
.Ft "int"
.Fn "ber_get_nstring" "struct ber_element *root" "void **buf" "size_t *size"
.Ft "struct ber_element *"
.Fo "ber_add_bitstring"
.Fa "struct ber_element *prev"
.Fa "const void *buf"
.Fa "size_t size"
.Fc
.Ft "int"
.Fn "ber_get_bitstring" "struct ber_element *root" "void **buf" "size_t *size"
.Ft "struct ber_element *"
.Fn "ber_add_null" "struct ber_element *prev"
.Ft "int"
.Fn "ber_get_null" "struct ber_element *root"
.Ft "struct ber_element *"
.Fn "ber_add_eoc" "struct ber_element *prev"
.Ft "int"
.Fn "ber_get_eoc" "struct ber_element *root"
.Ft "struct ber_element *"
.Fn "ber_add_oid" "struct ber_element *prev" "struct ber_oid *oid"
.Ft "struct ber_element *"
.Fn "ber_add_noid" "struct ber_element *prev" "struct ber_oid *oid" "int n"
.Ft "struct ber_element *"
.Fn "ber_add_oidstring" "struct ber_element *prev" "const char *string"
.Ft "int"
.Fn "ber_get_oid" "struct ber_element *root" "struct ber_oid *oid"
.Ft "size_t"
.Fn "ber_oid2ber" "struct ber_oid *oid" "u_int8_t *buf" "size_t size"
.Ft "int"
.Fn "ber_string2oid" "const char *string" "struct ber_oid *oid"
.Ft "int"
.Fn "ber_oid_cmp" "struct ber_oid *oid" "struct ber_oid *oid"
.Ft "struct ber_element *"
.Fn "ber_printf_elements" "struct ber_element *prev" "char *format" "..."
.Ft "int"
.Fn "ber_scanf_elements" "struct ber_element *root" "char *format" "..."
.Ft "ssize_t"
.Fn "ber_get_writebuf" "struct ber *ber" "void **buf"
.Ft "ssize_t"
.Fn "ber_write_elements" "struct ber *ber" "struct ber_element *root"
.Ft "void"
.Fn "ber_set_readbuf" "struct ber *ber" "void *buf" "size_t len"
.Ft "struct ber_element *"
.Fn "ber_read_elements" "struct ber *ber" "struct ber_element *root"
.Ft off_t
.Fn "ber_getpos" "struct ber_element *elm"
.Ft "void"
.Fn "ber_free_element" "struct ber_element *root"
.Ft "void"
.Fn "ber_free_elements" "struct ber_element *root"
.Ft "size_t"
.Fn "ber_calc_len" "struct ber_element *root"
.Ft "void"
.Fo "ber_set_application"
.Fa "struct ber *ber"
.Fa "unsigned int (*cb)(struct ber_element *)"
.Fc
.Ft "void"
.Fo "ber_set_writecallback"
.Fa "struct ber_element *elm"
.Fa "void (*cb)(void *arg, size_t offs)"
.Fa "void *arg"
.Fc
.Ft "void"
.Fn "ber_free" "struct ber *ber"
.Sh DESCRIPTION
The
.Nm ber
API provides a mechanism to read and write ASN.1 streams and buffers using the
Basic Encoding Rules.
.Pp
Encoded
.Nm ber
is stored in the following structure:
.Bd -literal
struct ber {
	off_t	 br_offs;
	u_char	*br_wbuf;
	u_char	*br_wptr;
	u_char	*br_wend;
	u_char	*br_rbuf;
	u_char	*br_rptr;
	u_char	*br_rend;

	unsigned int (*br_application)(struct ber_element *);
};
.Ed
.Pp
.Fa br_rbuf
and
.Fa br_wbuf
are the read and write buffers for a
.Nm ber
stream.
These buffers are used when reading an existing byte stream (e.g. received from
a TLS connection), or when writing a new byte stream in preparation for
subsequent operations performed by the calling application (e.g. network
transmission or export to a file).
.Pp
Intermediary storage of ber elements during decoding and encoding uses the
following structure:
.Bd -literal
struct ber_element {
	struct ber_element	*be_next;
	unsigned int		 be_type;
	unsigned int		 be_encoding;
	size_t			 be_len;
	off_t			 be_offs;
	int			 be_free;
	u_int8_t		 be_class;
	void			(*be_cb)(void *, size_t);
	void			*be_cbarg;
	union {
		struct ber_element	*bv_sub;
		void			*bv_val;
		long long		 bv_numeric;
	} be_union;
#define be_sub		be_union.bv_sub
#define be_val		be_union.bv_val
#define be_numeric	be_union.bv_numeric
};
.Ed
.Pp
A linked list containing one or more
.Vt ber_element
is created during the decoding and encoding of
.Vt ber .
.Pp
Once the
.Vt ber
and
.Vt ber_element
data structures have been declared,
.Fn ber_set_readbuf
may be called to initialize
.Fa br_rbuf
in preparation for decoding.
It is assumed that a pointer to a ber byte stream is already available to the
application, commonly obtained by
.Xr read 2 ,
.Xr recv 2 ,
or
.Xr tls_read 3 .
.Fn ber_read_elements
may then be called to parse, validate, and store the data stream into its
consituent parts for subsequent processing.
.Fn ber_read_elements
returns a pointer to a fully populated list of one or more
.Vt ber_element ,
or
.Dv NULL
on a type mismatch or read error.
.Pp
The calling application must have explicit knowledge of the expected data
types in order for correct decoding.
.Fn ber_scanf_elements
may be called to extract
.Vt ber_element
content into local variables.
The
.Fn ber_get_*
functions extract the value of a single
.Vt ber_element
instance.
.Fn ber_scanf_elements
and the
.Fn ber_get_*
functions return 0 on success and -1 on failure.
.Pp
The first step when creating new ber is to populate
.Vt ber_element
with the desired content.
This may be achieved using the
.Fn ber_add_*
and
.Fn ber_printf_elements
functions, each of which return a pointer to
.Vt ber_element
on success or
.Dv NULL
on failure.
.Pp
Once
.Vt ber_element
has been fully populated,
.Fn ber_get_writebuf
may be used to initialize
.Fa br_wbuf
for writing.
.Fn ber_write_elements
encodes
.Vt ber_element
into a compliant
.Nm ber
byte stream for subsequent use by the calling application, most commonly using
.Xr send 2 ,
.Xr write 2 ,
or
.Xr tls_write 3 .
.Sh I/O OPERATIONS
.Fn ber_get_writebuf ,
.Fn ber_write_elements ,
.Fn ber_set_readbuf ,
.Fn ber_read_elements ,
.Fn ber_getpos ,
.Fn ber_free_element ,
.Fn ber_free_elements ,
.Fn ber_set_application ,
.Fn ber_set_writecallback ,
.Fn ber_free
.Sh BER ELEMENTS
.Fn ber_get_element ,
.Fn ber_set_header ,
.Fn ber_link_elements ,
.Fn ber_unlink_elements ,
.Fn ber_replace_elements ,
.Fn ber_calc_len
.Sh BER TYPES
.Fn ber_add_sequence ,
.Fn ber_add_set ,
.Fn ber_add_integer ,
.Fn ber_get_integer ,
.Fn ber_add_enumerated ,
.Fn ber_get_enumerated ,
.Fn ber_add_boolean ,
.Fn ber_get_boolean ,
.Fn ber_add_string ,
.Fn ber_add_nstring ,
.Fn ber_add_bitstring ,
.Fn ber_get_string ,
.Fn ber_get_nstring ,
.Fn ber_get_bitstring ,
.Fn ber_add_null ,
.Fn ber_get_null ,
.Fn ber_add_eoc ,
.Fn ber_get_eoc
.Sh FORMAT STRINGS
.Fn ber_printf_elements ,
.Fn ber_scanf_elements
.Sh OBJECT IDS
Object Identifiers are commonly used in ASN.1-based protocols.
These functions provide an interface to parse OIDs.
For internal representation of OIDs, the following structure
.Vt struct ber_oid
is being used:
.Bd -literal
#define BER_MIN_OID_LEN		2
#define BER_MAX_OID_LEN		32

struct ber_oid {
	u_int32_t	bo_id[BER_MAX_OID_LEN + 1];
	size_t		bo_n;
};
.Ed
.Pp
.Fn ber_add_oid ,
.Fn ber_add_noid ,
.Fn ber_add_oidstring ,
.Fn ber_get_oid ,
.Fn ber_oid2ber ,
.Fn ber_string2oid
.Fn ber_oid_cmp ,
.Sh RETURN VALUES
Upon successful completion
.Fn ber_get_integer ,
.Fn ber_get_enumerated ,
.Fn ber_get_boolean ,
.Fn ber_get_string ,
.Fn ber_get_nstring ,
.Fn ber_get_bitstring ,
.Fn ber_get_null ,
.Fn ber_get_eoc ,
.Fn ber_get_oid ,
.Fn ber_string2oid
and
.Fn ber_scanf_elements
return 0, while
.Fn ber_write_elements
returns the number of bytes written.
Otherwise, \-1 is returned and the global variable errno is
set to indicate the error.
.Sh SEE ALSO
.Xr read 2 ,
.Xr recv 2 ,
.Xr send 2 ,
.Xr write 2 ,
.Xr tls_read 3
.Sh STANDARDS
ITU-T Recommendation X.690, also known as ISO/IEC 8825-1:
Information technology - ASN.1 encoding rules.
.Sh HISTORY
The
.Nm ber
manpage first appeared in
.Ox 4.3 .
.Sh AUTHORS
.An -nosplit
The
.Nm ber
library was written by
.An Claudio Jeker Aq Mt claudio@openbsd.org ,
.An Marc Balmer Aq Mt marc@openbsd.org
and
.An Reyk Floeter Aq Mt reyk@openbsd.org .
.Sh CAVEATS
The
.Nm ber
API is subject to the following restrictions which are common to the
Distinguished Encoding Rules as defined by X.690:
.Pp
.Bl -enum -compact
.It
Only the definite form of length encoding shall be used, encoded in the
minimum number of octets.
.It
For bitstring, octetstring and restricted character string types, the
constructed form of encoding shall not be used.
.It
If a boolean encoding represents the boolean value TRUE, its single contents
octet shall have all eight bits set to one.
.It
Each unused bit in the final octet of the encoding of a bit string value shall
be set to zero.
.It
If a bitstring value has no 1 bits, then an encoder shall encode the value with
a length of 1 and an initial octet set to 0.
.El
.Pp
In addition, set and sequence values are limited to a maximum of 65535 elements.
No alternative encodings are permitted.
.Pp
.Do
Whereas the basic encoding rules give the sender of an encoding various choices
as to how data values may be encoded, the canonical and distinguished encoding
rules select just one encoding from those allowed by the basic encoding rules.
.Dc
.Bq X.690
.Pp
The restrictions placed on this API avoid the ambiguity inherent in
.Nm ber
encoded ASN.1 thereby acting as a security mitigation.
.Sh BUGS
This manpage is a stub.