xref: /freebsd/crypto/heimdal/doc/layman.asc (revision b528cefc6b8f9670b31a865051741d946cb37085)

A Layman's Guide to a Subset of ASN.1, BER, and DER

An RSA Laboratories Technical Note
Burton S. Kaliski Jr.
Revised November 1, 1993


Supersedes June 3, 1991 version, which was also published as
NIST/OSI Implementors' Workshop document SEC-SIG-91-17.
PKCS documents are available by electronic mail to
<pkcs@rsa.com>.

Copyright (C) 1991-1993 RSA Laboratories, a division of RSA
Data Security, Inc. License to copy this document is granted
provided that it is identified as "RSA Data Security, Inc.
Public-Key Cryptography Standards (PKCS)" in all material
mentioning or referencing this document.
003-903015-110-000-000


Abstract. This note gives a layman's introduction to a
subset of OSI's Abstract Syntax Notation One (ASN.1), Basic
Encoding Rules (BER), and Distinguished Encoding Rules
(DER). The particular purpose of this note is to provide
background material sufficient for understanding and
implementing the PKCS family of standards.


1. Introduction

It is a generally accepted design principle that abstraction
is a key to managing software development. With abstraction,
a designer can specify a part of a system without concern
for how the part is actually implemented or represented.
Such a practice leaves the implementation open; it
simplifies the specification; and it makes it possible to
state "axioms" about the part that can be proved when the
part is implemented, and assumed when the part is employed
in another, higher-level part. Abstraction is the hallmark
of most modern software specifications.

One of the most complex systems today, and one that also
involves a great deal of abstraction, is Open Systems
Interconnection (OSI, described in X.200). OSI is an
internationally standardized architecture that governs the
interconnection of computers from the physical layer up to
the user application layer. Objects at higher layers are
defined abstractly and intended to be implemented with
objects at lower layers. For instance, a service at one
layer may require transfer of certain abstract objects
between computers; a lower layer may provide transfer
services for strings of ones and zeroes, using encoding
rules to transform the abstract objects into such strings.
OSI is called an open system because it supports many
different implementations of the services at each layer.

OSI's method of specifying abstract objects is called ASN.1
(Abstract Syntax Notation One, defined in X.208), and one
set of rules for representing such objects as strings of
ones and zeros is called the BER (Basic Encoding Rules,
defined in X.209). ASN.1 is a flexible notation that allows
one to define a variety data types, from simple types such
as integers and bit strings to structured types such as sets
and sequences, as well as complex types defined in terms of
others. BER describes how to represent or encode values of
each ASN.1 type as a string of eight-bit octets. There is
generally more than one way to BER-encode a given value.
Another set of rules, called the Distinguished Encoding
Rules (DER), which is a subset of BER, gives a unique
encoding to each ASN.1 value.

The purpose of this note is to describe a subset of ASN.1,
BER and DER sufficient to understand and implement one OSI-
based application, RSA Data Security, Inc.'s Public-Key
Cryptography Standards. The features described include an
overview of ASN.1, BER, and DER and an abridged list of
ASN.1 types and their BER and DER encodings. Sections 2-4
give an overview of ASN.1, BER, and DER, in that order.
Section 5 lists some ASN.1 types, giving their notation,
specific encoding rules, examples, and comments about their
application to PKCS. Section 6 concludes with an example,
X.500 distinguished names.

Advanced features of ASN.1, such as macros, are not
described in this note, as they are not needed to implement
PKCS. For information on the other features, and for more
detail generally, the reader is referred to CCITT
Recommendations X.208 and X.209, which define ASN.1 and BER.

Terminology and notation. In this note, an octet is an eight-
bit unsigned integer. Bit 8 of the octet is the most
significant and bit 1 is the least significant.

The following meta-syntax is used for in describing ASN.1
notation:

     BIT  monospace denotes literal characters in the type
          and value notation; in examples, it generally
          denotes an octet value in hexadecimal

     n1   bold italics denotes a variable

     []   bold square brackets indicate that a term is
          optional

     {}   bold braces group related terms

     |    bold vertical bar delimits alternatives with a
          group

     ...  bold ellipsis indicates repeated occurrences

     =    bold equals sign expresses terms as subterms


2. Abstract Syntax Notation One

Abstract Syntax Notation One, abbreviated ASN.1, is a
notation for describing abstract types and values.

In ASN.1, a type is a set of values. For some types, there
are a finite number of values, and for other types there are
an infinite number. A value of a given ASN.1 type is an
element of the type's set. ASN.1 has four kinds of type:
simple types, which are "atomic" and have no components;
structured types, which have components; tagged types, which
are derived from other types; and other types, which include
the CHOICE type and the ANY type. Types and values can be
given names with the ASN.1 assignment operator (::=) , and
those names can be used in defining other types and values.

Every ASN.1 type other than CHOICE and ANY has a tag, which
consists of a class and a nonnegative tag number. ASN.1
types are abstractly the same if and only if their tag
numbers are the same. In other words, the name of an ASN.1
type does not affect its abstract meaning, only the tag
does. There are four classes of tag:

     Universal, for types whose meaning is the same in all
          applications; these types are only defined in
          X.208.

     Application, for types whose meaning is specific to an
          application, such as X.500 directory services;
          types in two different applications may have the
          same application-specific tag and different
          meanings.

     Private, for types whose meaning is specific to a given
          enterprise.

     Context-specific, for types whose meaning is specific
          to a given structured type; context-specific tags
          are used to distinguish between component types
          with the same underlying tag within the context of
          a given structured type, and component types in
          two different structured types may have the same
          tag and different meanings.

The types with universal tags are defined in X.208, which
also gives the types' universal tag numbers. Types with
other tags are defined in many places, and are always
obtained by implicit or explicit tagging (see Section 2.3).
Table 1 lists some ASN.1 types and their universal-class
tags.

    Type                     Tag number     Tag number
                             (decimal)      (hexadecimal)
    INTEGER                  2              02
    BIT STRING               3              03
    OCTET STRING             4              04
    NULL                     5              05
    OBJECT IDENTIFIER        6              06
    SEQUENCE and SEQUENCE OF 16             10
    SET and SET OF           17             11
    PrintableString          19             13
    T61String                20             14
    IA5String                22             16
    UTCTime                  23             17

     Table 1. Some types and their universal-class tags.

ASN.1 types and values are expressed in a flexible,
programming-language-like notation, with the following
special rules:

     o    Layout is not significant; multiple spaces and
          line breaks can be considered as a single space.

     o    Comments are delimited by pairs of hyphens (--),
          or a pair of hyphens and a line break.

     o    Identifiers (names of values and fields) and type
          references (names of types) consist of upper- and
          lower-case letters, digits, hyphens, and spaces;
          identifiers begin with lower-case letters; type
          references begin with upper-case letters.

The following four subsections give an overview of simple
types, structured types, implicitly and explicitly tagged
types, and other types. Section 5 describes specific types
in more detail.


2.1 Simple types

Simple types are those not consisting of components; they
are the "atomic" types. ASN.1 defines several; the types
that are relevant to the PKCS standards are the following:

     BIT STRING, an arbitrary string of bits (ones and
          zeroes).

     IA5String, an arbitrary string of IA5 (ASCII)
          characters.

     INTEGER, an arbitrary integer.

     NULL, a null value.

     OBJECT IDENTIFIER, an object identifier, which is a
          sequence of integer components that identify an
          object such as an algorithm or attribute type.

     OCTET STRING, an arbitrary string of octets (eight-bit
          values).

     PrintableString, an arbitrary string of printable
          characters.

     T61String, an arbitrary string of T.61 (eight-bit)
          characters.

     UTCTime, a "coordinated universal time" or Greenwich
          Mean Time (GMT) value.

Simple types fall into two categories: string types and non-
string types. BIT STRING, IA5String, OCTET STRING,
PrintableString, T61String, and UTCTime are string types.

String types can be viewed, for the purposes of encoding, as
consisting of components, where the components are
substrings. This view allows one to encode a value whose
length is not known in advance (e.g., an octet string value
input from a file stream) with a constructed, indefinite-
length encoding (see Section 3).

The string types can be given size constraints limiting the
length of values.


2.2 Structured types

Structured types are those consisting of components. ASN.1
defines four, all of which are relevant to the PKCS
standards:

     SEQUENCE, an ordered collection of one or more types.

     SEQUENCE OF, an ordered collection of zero or more
          occurrences of a given type.

     SET, an unordered collection of one or more types.

     SET OF, an unordered collection of zero or more
          occurrences of a given type.

The structured types can have optional components, possibly
with default values.


2.3 Implicitly and explicitly tagged types

Tagging is useful to distinguish types within an
application; it is also commonly used to distinguish
component types within a structured type. For instance,
optional components of a SET or SEQUENCE type are typically
given distinct context-specific tags to avoid ambiguity.

There are two ways to tag a type: implicitly and explicitly.

Implicitly tagged types are derived from other types by
changing the tag of the underlying type. Implicit tagging is
denoted by the ASN.1 keywords [class number] IMPLICIT (see
Section 5.1).

Explicitly tagged types are derived from other types by
adding an outer tag to the underlying type. In effect,
explicitly tagged types are structured types consisting of
one component, the underlying type. Explicit tagging is
denoted by the ASN.1 keywords [class number] EXPLICIT (see
Section 5.2).

The keyword [class number] alone is the same as explicit
tagging, except when the "module" in which the ASN.1 type is
defined has implicit tagging by default. ("Modules" are
among the advanced features not described in this note.)

For purposes of encoding, an implicitly tagged type is
considered the same as the underlying type, except that the
tag is different. An explicitly tagged type is considered
like a structured type with one component, the underlying
type. Implicit tags result in shorter encodings, but
explicit tags may be necessary to avoid ambiguity if the tag
of the underlying type is indeterminate (e.g., the
underlying type is CHOICE or ANY).


2.4 Other types

Other types in ASN.1 include the CHOICE and ANY types. The
CHOICE type denotes a union of one or more alternatives; the
ANY type denotes an arbitrary value of an arbitrary type,
where the arbitrary type is possibly defined in the
registration of an object identifier or integer value.


3. Basic Encoding Rules

The Basic Encoding Rules for ASN.1, abbreviated BER, give
one or more ways to represent any ASN.1 value as an octet
string. (There are certainly other ways to represent ASN.1
values, but BER is the standard for interchanging such
values in OSI.)

There are three methods to encode an ASN.1 value under BER,
the choice of which depends on the type of value and whether
the length of the value is known. The three methods are
primitive, definite-length encoding; constructed, definite-
length encoding; and constructed, indefinite-length
encoding. Simple non-string types employ the primitive,
definite-length method; structured types employ either of
the constructed methods; and simple string types employ any
of the methods, depending on whether the length of the value
is known. Types derived by implicit tagging employ the
method of the underlying type and types derived by explicit
tagging employ the constructed methods.

In each method, the BER encoding has three or four parts:

     Identifier octets. These identify the class and tag
          number of the ASN.1 value, and indicate whether
          the method is primitive or constructed.

     Length octets. For the definite-length methods, these
          give the number of contents octets. For the
          constructed, indefinite-length method, these
          indicate that the length is indefinite.

     Contents octets. For the primitive, definite-length
          method, these give a concrete representation of
          the  value. For the constructed methods, these
          give the concatenation of the BER encodings of the
          components of the value.

     End-of-contents octets. For the constructed, indefinite-
          length method, these denote the end of the
          contents. For the other methods, these are absent.

The three methods of encoding are described in the following
sections.


3.1 Primitive, definite-length method

This method applies to simple types and types derived from
simple types by implicit tagging. It requires that the
length of the value be known in advance. The parts of the
BER encoding are as follows:

Identifier octets. There are two forms: low tag number (for
tag numbers between 0 and 30) and high tag number (for tag
numbers 31 and greater).

     Low-tag-number form. One octet. Bits 8 and 7 specify
          the class (see Table 2), bit 6 has value "0,"
          indicating that the encoding is primitive, and
          bits 5-1 give the tag number.

                  Class            Bit  Bit
                                   8    7
                  universal        0    0
                  application      0    1
                  context-specific 1    0
                  private          1    1

        Table 2. Class encoding in identifier octets.

     High-tag-number form. Two or more octets. First octet
          is as in low-tag-number form, except that bits 5-1
          all have value "1." Second and following octets
          give the tag number, base 128, most significant
          digit first, with as few digits as possible, and
          with the bit 8 of each octet except the last set
          to "1."

Length octets. There are two forms: short (for lengths
between 0 and 127), and long definite (for lengths between 0
and 21008-1).

     Short form. One octet. Bit 8 has value "0" and bits 7-1
          give the length.

     Long form. Two to 127 octets. Bit 8 of first octet has
          value "1" and bits 7-1 give the number of
          additional length octets. Second and following
          octets give the length, base 256, most significant
          digit first.

Contents octets. These give a concrete representation of the
value (or the value of the underlying type, if the type is
derived by implicit tagging). Details for particular types
are given in Section 5.


3.2 Constructed, definite-length method

This method applies to simple string types, structured
types, types derived simple string types and structured
types by implicit tagging, and types derived from anything
by explicit tagging. It requires that the length of the
value be known in advance. The parts of the BER encoding are
as follows:

Identifier octets. As described in Section 3.1, except that
bit 6 has value "1," indicating that the encoding is
constructed.

Length octets. As described in Section 3.1.

Contents octets. The concatenation of the BER encodings of
the components of the value:

     o    For simple string types and types derived from
          them by implicit tagging, the concatenation of the
          BER encodings of consecutive substrings of the
          value (underlying value for implicit tagging).

     o    For structured types and types derived from them
          by implicit tagging, the concatenation of the BER
          encodings of components of the value (underlying
          value for implicit tagging).

     o    For types derived from anything by explicit
          tagging, the BER encoding of the underlying value.

Details for particular types are given in Section 5.


3.3 Constructed, indefinite-length method

This method applies to simple string types, structured
types, types derived simple string types and structured
types by implicit tagging, and types derived from anything
by explicit tagging. It does not require that the length of
the value be known in advance. The parts of the BER encoding
are as follows:

Identifier octets. As described in Section 3.2.

Length octets. One octet, 80.

Contents octets. As described in Section 3.2.

End-of-contents octets. Two octets, 00 00.

Since the end-of-contents octets appear where an ordinary
BER encoding might be expected (e.g., in the contents octets
of a sequence value), the 00 and 00 appear as identifier and
length octets, respectively. Thus the end-of-contents octets
is really the primitive, definite-length encoding of a value
with universal class, tag number 0, and length 0.


4. Distinguished Encoding Rules

The Distinguished Encoding Rules for ASN.1, abbreviated DER,
are a subset of BER, and give exactly one way to represent
any ASN.1 value as an octet string. DER is intended for
applications in which a unique octet string encoding is
needed, as is the case when a digital signature is computed
on an ASN.1 value. DER is defined in Section 8.7 of X.509.

DER adds the following restrictions to the rules given in
Section 3:

     1.   When the length is between 0 and 127, the short
          form of length must be used

     2.   When the length is 128 or greater, the long form
          of length must be used, and the length must be
          encoded in the minimum number of octets.

     3.   For simple string types and implicitly tagged
          types derived from simple string types, the
          primitive, definite-length method must be
          employed.

     4.   For structured types, implicitly tagged types
          derived from structured types, and explicitly
          tagged types derived from anything, the
          constructed, definite-length method must be
          employed.

Other restrictions are defined for particular types (such as
BIT STRING, SEQUENCE, SET, and SET OF), and can be found in
Section 5.


5. Notation and encodings for some types

This section gives the notation for some ASN.1 types and
describes how to encode values of those types under both BER
and DER.

The types described are those presented in Section 2. They
are listed alphabetically here.

Each description includes ASN.1 notation, BER encoding, and
DER encoding. The focus of the encodings is primarily on the
contents octets; the tag and length octets follow Sections 3
and 4. The descriptions also explain where each type is used
in PKCS and related standards. ASN.1 notation is generally
only for types, although for the type OBJECT IDENTIFIER,
value notation is given as well.


5.1 Implicitly tagged types

An implicitly tagged type is a type derived from another
type by changing the tag of the underlying type.

Implicit tagging is used for optional SEQUENCE components
with underlying type other than ANY throughout PKCS, and for
the extendedCertificate alternative of PKCS #7's
ExtendedCertificateOrCertificate type.

ASN.1 notation:

[[class] number] IMPLICIT Type

class = UNIVERSAL  |  APPLICATION  |  PRIVATE

where Type is a type, class is an optional class name, and
number is the tag number within the class, a nonnegative
integer.

In ASN.1 "modules" whose default tagging method is implicit
tagging, the notation [[class] number] Type is also
acceptable, and the keyword IMPLICIT is implied. (See
Section 2.3.) For definitions stated outside a module, the
explicit inclusion of the keyword IMPLICIT is preferable to
prevent ambiguity.

If the class name is absent, then the tag is context-
specific. Context-specific tags can only appear in a
component of a structured or CHOICE type.

Example: PKCS #8's PrivateKeyInfo type has an optional
attributes component with an implicit, context-specific tag:

PrivateKeyInfo ::= SEQUENCE {
  version Version,
  privateKeyAlgorithm PrivateKeyAlgorithmIdentifier,
  privateKey PrivateKey,
  attributes [0] IMPLICIT Attributes OPTIONAL }

Here the underlying type is Attributes, the class is absent
(i.e., context-specific), and the tag number within the
class is 0.

BER encoding. Primitive or constructed, depending on the
underlying type. Contents octets are as for the BER encoding
of the underlying value.

Example: The BER encoding of the attributes component of a
PrivateKeyInfo value is as follows:

     o    the identifier octets are 80 if the underlying
          Attributes value has a primitive BER encoding and
          a0 if the underlying Attributes value has a
          constructed BER encoding

     o    the length and contents octets are the same as the
          length and contents octets of the BER encoding of
          the underlying Attributes value

DER encoding. Primitive or constructed, depending on the
underlying type. Contents octets are as for the DER encoding
of the underlying value.


5.2 Explicitly tagged types

Explicit tagging denotes a type derived from another type by
adding an outer tag to the underlying type.

Explicit tagging is used for optional SEQUENCE components
with underlying type ANY throughout PKCS, and for the
version component of X.509's Certificate type.

ASN.1 notation:

[[class] number] EXPLICIT Type

class = UNIVERSAL  |  APPLICATION  |  PRIVATE

where Type is a type, class is an optional class name, and
number is the tag number within the class, a nonnegative
integer.

If the class name is absent, then the tag is context-
specific. Context-specific tags can only appear in a
component of a SEQUENCE, SET or CHOICE type.

In ASN.1 "modules" whose default tagging method is explicit
tagging, the notation [[class] number] Type is also
acceptable, and the keyword EXPLICIT is implied. (See
Section 2.3.) For definitions stated outside a module, the
explicit inclusion of the keyword EXPLICIT is preferable to
prevent ambiguity.

Example 1: PKCS #7's ContentInfo type has an optional
content component with an explicit, context-specific tag:

ContentInfo ::= SEQUENCE {
  contentType ContentType,
  content
    [0] EXPLICIT ANY DEFINED BY contentType OPTIONAL }

Here the underlying type is ANY DEFINED BY contentType, the
class is absent (i.e., context-specific), and the tag number
within the class is 0.

Example 2: X.509's Certificate type has a version component
with an explicit, context-specific tag, where the EXPLICIT
keyword is omitted:

Certificate ::= ...
  version [0] Version DEFAULT v1988,
...

The tag is explicit because the default tagging method for
the ASN.1 "module" in X.509 that defines the Certificate
type is explicit tagging.

BER encoding. Constructed. Contents octets are the BER
encoding of the underlying value.

Example: the BER encoding of the content component of a
ContentInfo value is as follows:

     o    identifier octets are a0

     o    length octets represent the length of the BER
          encoding of the underlying ANY DEFINED BY
          contentType value

     o    contents octets are the BER encoding of the
          underlying ANY DEFINED BY contentType value

DER encoding. Constructed. Contents octets are the DER
encoding of the underlying value.


5.3 ANY

The ANY type denotes an arbitrary value of an arbitrary
type, where the arbitrary type is possibly defined in the
registration of an object identifier or associated with an
integer index.

The ANY type is used for content of a particular content
type in PKCS #7's ContentInfo type, for parameters of a
particular algorithm in X.509's AlgorithmIdentifier type,
and for attribute values in X.501's Attribute and
AttributeValueAssertion types. The Attribute type is used by
PKCS #6, #7, #8, #9 and #10, and the AttributeValueAssertion
type is used in X.501 distinguished names.

ASN.1 notation:

ANY [DEFINED BY identifier]

where identifier is an optional identifier.

In the ANY form, the actual type is indeterminate.

The ANY DEFINED BY identifier form can only appear in a
component of a SEQUENCE or SET type for which identifier
identifies some other component, and that other component
has type INTEGER or OBJECT IDENTIFIER (or a type derived
from either of those by tagging). In that form, the actual
type is determined by the value of the other component,
either in the registration of the object identifier value,
or in a table of integer values.

Example: X.509's AlgorithmIdentifier type has a component of
type ANY:

AlgorithmIdentifier ::= SEQUENCE {
  algorithm OBJECT IDENTIFIER,
  parameters ANY DEFINED BY algorithm OPTIONAL }

Here the actual type of the parameter component depends on
the value of the algorithm component. The actual type would
be defined in the registration of object identifier values
for the algorithm component.

BER encoding. Same as the BER encoding of the actual value.

Example: The BER encoding of the value of the parameter
component is the BER encoding of the value of the actual
type as defined in the registration of object identifier
values for the algorithm component.

DER encoding. Same as the DER encoding of the actual value.


5.4 BIT STRING

The BIT STRING type denotes an arbitrary string of bits
(ones and zeroes). A BIT STRING value can have any length,
including zero. This type is a string type.

The BIT STRING type is used for digital signatures on
extended certificates in PKCS #6's ExtendedCertificate type,
for digital signatures on certificates in X.509's
Certificate type, and for public keys in certificates in
X.509's SubjectPublicKeyInfo type.

ASN.1 notation:

BIT STRING

Example: X.509's SubjectPublicKeyInfo type has a component
of type BIT STRING:

SubjectPublicKeyInfo ::= SEQUENCE {
  algorithm AlgorithmIdentifier,
  publicKey BIT STRING }

BER encoding. Primitive or constructed. In a primitive
encoding, the first contents octet gives the number of bits
by which the length of the bit string is less than the next
multiple of eight (this is called the "number of unused
bits"). The second and following contents octets give the
value of the bit string, converted to an octet string. The
conversion process is as follows:

     1.   The bit string is padded after the last bit with
          zero to seven bits of any value to make the length
          of the bit string a multiple of eight. If the
          length of the bit string is a multiple of eight
          already, no padding is done.

     2.   The padded bit string is divided into octets. The
          first eight bits of the padded bit string become
          the first octet, bit 8 to bit 1, and so on through
          the last eight bits of the padded bit string.

In a constructed encoding, the contents octets give the
concatenation of the BER encodings of consecutive substrings
of the bit string, where each substring except the last has
a length that is a multiple of eight bits.

Example: The BER encoding of the BIT STRING value
"011011100101110111" can be any of the following, among
others, depending on the choice of padding bits, the form of
length octets, and whether the encoding is primitive or
constructed:

03 04 06 6e 5d c0                               DER encoding

03 04 06 6e 5d e0                       padded with "100000"

03 81 04 06 6e 5d c0              long form of length octets

23 09        constructed encoding: "0110111001011101" + "11"
   03 03 00 6e 5d
   03 02 06 c0

DER encoding. Primitive. The contents octects are as for a
primitive BER encoding, except that the bit string is padded
with zero-valued bits.

Example: The DER encoding of the BIT STRING value
"011011100101110111" is

03 04 06 6e 5d c0


5.5 CHOICE

The CHOICE type denotes a union of one or more alternatives.

The CHOICE type is used to represent the union of an
extended certificate and an X.509 certificate in PKCS #7's
ExtendedCertificateOrCertificate type.

ASN.1 notation:

CHOICE {
  [identifier1] Type1,
  ...,
  [identifiern] Typen }

where identifier1 , ..., identifiern are optional, distinct
identifiers for the alternatives, and Type1, ..., Typen are
the types of the alternatives. The identifiers are primarily
for documentation; they do not affect values of the type or
their encodings in any way.

The types must have distinct tags. This requirement is
typically satisfied with explicit or implicit tagging on
some of the alternatives.

Example: PKCS #7's ExtendedCertificateOrCertificate type is
a CHOICE type:

ExtendedCertificateOrCertificate ::= CHOICE {
  certificate Certificate, -- X.509
  extendedCertificate [0] IMPLICIT ExtendedCertificate
}

Here the identifiers for the alternatives are certificate
and extendedCertificate, and the types of the alternatives
are Certificate and [0] IMPLICIT ExtendedCertificate.

BER encoding. Same as the BER encoding of the chosen
alternative. The fact that the alternatives have distinct
tags makes it possible to distinguish between their BER
encodings.

Example: The identifier octets for the BER encoding are 30
if the chosen alternative is certificate, and a0 if the
chosen alternative is extendedCertificate.

DER encoding. Same as the DER encoding of the chosen
alternative.


5.6 IA5String

The IA5String type denotes an arbtrary string of IA5
characters. IA5 stands for International Alphabet 5, which
is the same as ASCII. The character set includes non-
printing control characters. An IA5String value can have any
length, including zero. This type is a string type.

The IA5String type is used in PKCS #9's electronic-mail
address, unstructured-name, and unstructured-address
attributes.

ASN.1 notation:

IA5String

BER encoding. Primitive or constructed. In a primitive
encoding, the contents octets give the characters in the IA5
string, encoded in ASCII. In a constructed encoding, the
contents octets give the concatenation of the BER encodings
of consecutive substrings of the IA5 string.

Example: The BER encoding of the IA5String value
"test1@rsa.com" can be any of the following, among others,
depending on the form of length octets and whether the
encoding is primitive or constructed:

16 0d 74 65 73 74 31 40 72 73 61 2e 63 6f 6d DER encoding

16 81 0d                       long form of length octets
   74 65 73 74 31 40 72 73 61 2e 63 6f 6d

36 13     constructed encoding: "test1" + "@" + "rsa.com"
   16 05 74 65 73 74 31
   16 01 40
   16 07 72 73 61 2e 63 6f 6d

DER encoding. Primitive. Contents octets are as for a
primitive BER encoding.

Example: The DER encoding of the IA5String value
"test1@rsa.com" is

16 0d 74 65 73 74 31 40 72 73 61 2e 63 6f 6d


5.7 INTEGER

The INTEGER type denotes an arbitrary integer. INTEGER
values can be positive, negative, or zero, and can have any
magnitude.

The INTEGER type is used for version numbers throughout
PKCS, cryptographic values such as modulus, exponent, and
primes in PKCS #1's RSAPublicKey and RSAPrivateKey types and
PKCS #3's DHParameter type, a message-digest iteration count
in PKCS #5's PBEParameter type, and version numbers and
serial numbers in X.509's Certificate type.

ASN.1 notation:

INTEGER [{ identifier1(value1) ... identifiern(valuen) }]

where identifier1, ..., identifiern are optional distinct
identifiers and value1, ..., valuen are optional integer
values. The identifiers, when present, are associated with
values of the type.

Example: X.509's Version type is an INTEGER type with
identified values:

Version ::= INTEGER { v1988(0) }

The identifier v1988 is associated with the value 0. X.509's
Certificate type uses the identifier v1988 to give a default
value of 0 for the version component:

Certificate ::= ...
  version Version DEFAULT v1988,
...

BER encoding. Primitive. Contents octets give the value of
the integer, base 256, in two's complement form, most
significant digit first, with the minimum number of octets.
The value 0 is encoded as a single 00 octet.

Some example BER encodings (which also happen to be DER
encodings) are given in Table 3.

                    Integer   BER encoding
                    value
                    0         02 01 00
                    127       02 01 7F
                    128       02 02 00 80
                    256       02 02 01 00
                    -128      02 01 80
                    -129      02 02 FF 7F

      Table 3. Example BER encodings of INTEGER values.

DER encoding. Primitive. Contents octets are as for a
primitive BER encoding.


5.8 NULL

The NULL type denotes a null value.

The NULL type is used for algorithm parameters in several
places in PKCS.

ASN.1 notation:

NULL

BER encoding. Primitive. Contents octets are empty.

Example: The BER encoding of a NULL value can be either of
the following, as well as others, depending on the form of
the length octets:

05 00

05 81 00

DER encoding. Primitive. Contents octets are empty; the DER
encoding of a NULL value is always 05 00.


5.9 OBJECT IDENTIFIER

The OBJECT IDENTIFIER type denotes an object identifier, a
sequence of integer components that identifies an object
such as an algorithm, an attribute type, or perhaps a
registration authority that defines other object
identifiers. An OBJECT IDENTIFIER value can have any number
of components, and components can generally have any
nonnegative value. This type is a non-string type.

OBJECT IDENTIFIER values are given meanings by registration
authorities. Each registration authority is responsible for
all sequences of components beginning with a given sequence.
A registration authority typically delegates responsibility
for subsets of the sequences in its domain to other
registration authorities, or for particular types of object.
There are always at least two components.

The OBJECT IDENTIFIER type is used to identify content in
PKCS #7's ContentInfo type, to identify algorithms in
X.509's AlgorithmIdentifier type, and to identify attributes
in X.501's Attribute and AttributeValueAssertion types. The
Attribute type is used by PKCS #6, #7, #8, #9, and #10, and
the AttributeValueAssertion type is used in X.501
distinguished names. OBJECT IDENTIFIER values are defined
throughout PKCS.

ASN.1 notation:

OBJECT IDENTIFIER

The ASN.1 notation for values of the OBJECT IDENTIFIER type
is

{ [identifier] component1 ... componentn }

componenti = identifieri | identifieri (valuei) | valuei

where identifier, identifier1, ..., identifiern are
identifiers, and value1, ..., valuen are optional integer
values.

The form without identifier is the "complete" value with all
its components; the form with identifier abbreviates the
beginning components with another object identifier value.
The identifiers identifier1, ..., identifiern are intended
primarily for documentation, but they must correspond to the
integer value when both are present. These identifiers can
appear without integer values only if they are among a small
set of identifiers defined in X.208.

Example: The following values both refer to the object
identifier assigned to RSA Data Security, Inc.:

{ iso(1) member-body(2) 840 113549 }
{ 1 2 840 113549 }

(In this example, which gives ASN.1 value notation, the
object identifier values are decimal, not hexadecimal.)
Table 4 gives some other object identifier values and their
meanings.

 Object identifier value       Meaning
 { 1 2 }                       ISO member bodies
 { 1 2 840 }                   US (ANSI)
 { 1 2 840 113549 }            RSA Data Security, Inc.
 { 1 2 840 113549 1 }          RSA Data Security, Inc. PKCS
 { 2 5 }                       directory services (X.500)
 { 2 5 8 }                     directory services-algorithms

 Table 4. Some object identifier values and their meanings.

BER encoding. Primitive. Contents octets are as follows,
where value1, ..., valuen denote the integer values of the
components in the complete object identifier:

     1.   The first octet has value 40 * value1 + value2.
          (This is unambiguous, since value1 is limited to
          values 0, 1, and 2; value2 is limited to the range
          0 to 39 when value1 is 0 or 1; and, according to
          X.208, n is always at least 2.)

     2.   The following octets, if any, encode value3, ...,
          valuen. Each value is encoded base 128, most
          significant digit first, with as few digits as
          possible, and the most significant bit of each
          octet except the last in the value's encoding set
          to "1."

Example: The first octet of the BER encoding of RSA Data
Security, Inc.'s object identifier is 40 * 1 + 2 = 42 =
2a16. The encoding of 840 = 6 * 128 + 4816 is 86 48 and the
encoding of 113549 = 6 * 1282 + 7716 * 128 + d16 is 86 f7
0d. This leads to the following BER encoding:

06 06 2a 86 48 86 f7 0d

DER encoding. Primitive. Contents octets are as for a
primitive BER encoding.


5.10 OCTET STRING

The OCTET STRING type denotes an arbitrary string of octets
(eight-bit values). An OCTET STRING value can have any
length, including zero. This type is a string type.

The OCTET STRING type is used for salt values in PKCS #5's
PBEParameter type, for message digests, encrypted message
digests, and encrypted content in PKCS #7, and for private
keys and encrypted private keys in PKCS #8.

ASN.1 notation:

OCTET STRING [SIZE ({size | size1..size2})]

where size, size1, and size2 are optional size constraints.
In the OCTET STRING SIZE (size) form, the octet string must
have size octets. In the OCTET STRING SIZE (size1..size2)
form, the octet string must have between size1 and size2
octets. In the OCTET STRING form, the octet string can have
any size.

Example: PKCS #5's PBEParameter type has a component of type
OCTET STRING:

PBEParameter ::= SEQUENCE {
  salt OCTET STRING SIZE(8),
  iterationCount INTEGER }

Here the size of the salt component is always eight octets.

BER encoding. Primitive or constructed. In a primitive
encoding, the contents octets give the value of the octet
string, first octet to last octet. In a constructed
encoding, the contents octets give the concatenation of the
BER encodings of substrings of the OCTET STRING value.

Example: The BER encoding of the OCTET STRING value 01 23 45
67 89 ab cd ef can be any of the following, among others,
depending on the form of length octets and whether the
encoding is primitive or constructed:

04 08 01 23 45 67 89 ab cd ef                   DER encoding

04 81 08 01 23 45 67 89 ab cd ef  long form of length octets

24 0c            constructed encoding: 01 ... 67 + 89 ... ef
   04 04 01 23 45 67
   04 04 89 ab cd ef

DER encoding. Primitive. Contents octets are as for a
primitive BER encoding.

Example: The BER encoding of the OCTET STRING value 01 23 45
67 89 ab cd ef is

04 08 01 23 45 67 89 ab cd ef


5.11 PrintableString

The PrintableString type denotes an arbitrary string of
printable characters from the following character set:

                         A, B, ..., Z
                         a, b, ..., z
                         0, 1, ..., 9
               (space) ' ( ) + , - . / : = ?

This type is a string type.

The PrintableString type is used in PKCS #9's challenge-
password and unstructuerd-address attributes, and in several
X.521 distinguished names attributes.

ASN.1 notation:

PrintableString

BER encoding. Primitive or constructed. In a primitive
encoding, the contents octets give the characters in the
printable string, encoded in ASCII. In a constructed
encoding, the contents octets give the concatenation of the
BER encodings of consecutive substrings of the string.

Example: The BER encoding of the PrintableString value "Test
User 1" can be any of the following, among others, depending
on the form of length octets and whether the encoding is
primitive or constructed:

13 0b 54 65 73 74 20 55 73 65 72 20 31          DER encoding

13 81 0b                          long form of length octets
   54 65 73 74 20 55 73 65 72 20 31

33 0f               constructed encoding: "Test " + "User 1"
   13 05 54 65 73 74 20
   13 06 55 73 65 72 20 31

DER encoding. Primitive. Contents octets are as for a
primitive BER encoding.

Example: The DER encoding of the PrintableString value "Test
User 1" is

13 0b 54 65 73 74 20 55 73 65 72 20 31


5.12 SEQUENCE

The SEQUENCE type denotes an ordered collection of one or
more types.

The SEQUENCE type is used throughout PKCS and related
standards.

ASN.1 notation:

SEQUENCE {
  [identifier1] Type1 [{OPTIONAL | DEFAULT value1}],
  ...,
  [identifiern] Typen [{OPTIONAL | DEFAULT valuen}]}

where identifier1 , ..., identifiern are optional, distinct
identifiers for the components, Type1, ..., Typen are the
types of the components, and value1, ..., valuen are optional
default values for the components. The identifiers are
primarily for documentation; they do not affect values of
the type or their encodings in any way.

The OPTIONAL qualifier indicates that the value of a
component is optional and need not be present in the
sequence. The DEFAULT qualifier also indicates that the
value of a component is optional, and assigns a default
value to the component when the component is absent.

The types of any consecutive series of components with the
OPTIONAL or DEFAULT qualifier, as well as of any component
immediately following that series, must have distinct tags.
This requirement is typically satisfied with explicit or
implicit tagging on some of the components.

Example: X.509's Validity type is a SEQUENCE type with two
components:

Validity ::= SEQUENCE {
  start UTCTime,
  end UTCTime }

Here the identifiers for the components are start and end,
and the types of the components are both UTCTime.

BER encoding. Constructed. Contents octets are the
concatenation of the BER encodings of the values of the
components of the sequence, in order of definition, with the
following rules for components with the OPTIONAL and DEFAULT
qualifiers:

     o    if the value of a component with the OPTIONAL or
          DEFAULT qualifier is absent from the sequence,
          then the encoding of that component is not
          included in the contents octets

     o    if the value of a component with the DEFAULT
          qualifier is the default value, then the encoding
          of that component may or may not be included in
          the contents octets

DER encoding. Constructed. Contents octets are the same as
the BER encoding, except that if the value of a component
with the DEFAULT qualifier is the default value, the
encoding of that component is not included in the contents
octets.


5.13 SEQUENCE OF

The SEQUENCE OF type denotes an ordered collection of zero
or more occurrences of a given type.

The SEQUENCE OF type is used in X.501 distinguished names.

ASN.1 notation:

SEQUENCE OF Type

where Type is a type.

Example: X.501's RDNSequence type consists of zero or more
occurences of the RelativeDistinguishedName type, most
significant occurrence first:

RDNSequence ::= SEQUENCE OF RelativeDistinguishedName

BER encoding. Constructed. Contents octets are the
concatenation of the BER encodings of the values of the
occurrences in the collection, in order of occurence.

DER encoding. Constructed. Contents octets are the
concatenation of the DER encodings of the values of the
occurrences in the collection, in order of occurence.


5.14 SET

The SET type denotes an unordered collection of one or more
types.

The SET type is not used in PKCS.

ASN.1 notation:

SET {
  [identifier1] Type1 [{OPTIONAL | DEFAULT value1}],
  ...,
  [identifiern] Typen [{OPTIONAL | DEFAULT valuen}]}

where identifier1, ..., identifiern are optional, distinct
identifiers for the components, Type1, ..., Typen are the
types of the components, and value1, ..., valuen are
optional default values for the components. The identifiers
are primarily for documentation; they do not affect values
of the type or their encodings in any way.

The OPTIONAL qualifier indicates that the value of a
component is optional and need not be present in the set.
The DEFAULT qualifier also indicates that the value of a
component is optional, and assigns a default value to the
component when the component is absent.

The types must have distinct tags. This requirement is
typically satisfied with explicit or implicit tagging on
some of the components.

BER encoding. Constructed. Contents octets are the
concatenation of the BER encodings of the values of the
components of the set, in any order, with the following
rules for components with the OPTIONAL and DEFAULT
qualifiers:

     o    if the value of a component with the OPTIONAL or
          DEFAULT qualifier is absent from the set, then the
          encoding of that component is not included in the
          contents octets

     o    if the value of a component with the DEFAULT
          qualifier is the default value, then the encoding
          of that component may or may not be included in
          the contents octets

DER encoding. Constructed. Contents octets are the same as
for the BER encoding, except that:

     1.   If the value of a component with the DEFAULT
          qualifier is the default value, the encoding of
          that component is not included.

     2.   There is an order to the components, namely
          ascending order by tag.


5.15 SET OF

The SET OF type denotes an unordered collection of zero or
more occurrences of a given type.

The SET OF type is used for sets of attributes in PKCS #6,
#7, #8, #9 and #10, for sets of message-digest algorithm
identifiers, signer information, and recipient information
in PKCS #7, and in X.501 distinguished names.

ASN.1 notation:

SET OF Type

where Type is a type.

Example: X.501's RelativeDistinguishedName type consists of
zero or more occurrences of the AttributeValueAssertion
type, where the order is unimportant:

RelativeDistinguishedName ::=
  SET OF AttributeValueAssertion

BER encoding. Constructed. Contents octets are the
concatenation of the BER encodings of the values of the
occurrences in the collection, in any order.

DER encoding. Constructed. Contents octets are the same as
for the BER encoding, except that there is an order, namely
ascending lexicographic order of BER encoding. Lexicographic
comparison of two different BER encodings is done as
follows: Logically pad the shorter BER encoding after the
last octet with dummy octets that are smaller in value than
any normal octet. Scan the BER encodings from left to right
until a difference is found. The smaller-valued BER encoding
is the one with the smaller-valued octet at the point of
difference.


5.16 T61String

The T61String type denotes an arbtrary string of T.61
characters. T.61 is an eight-bit extension to the ASCII
character set. Special "escape" sequences specify the
interpretation of subsequent character values as, for
example, Japanese; the initial interpretation is Latin. The
character set includes non-printing control characters. The
T61String type allows only the Latin and Japanese character
interepretations, and implementors' agreements for directory
names exclude control characters [NIST92]. A T61String value
can have any length, including zero. This type is a string
type.

The T61String type is used in PKCS #9's unstructured-address
and challenge-password attributes, and in several X.521
attributes.

ASN.1 notation:

T61String

BER encoding. Primitive or constructed. In a primitive
encoding, the contents octets give the characters in the
T.61 string, encoded in ASCII. In a constructed encoding,
the contents octets give the concatenation of the BER
encodings of consecutive substrings of the T.61 string.

Example: The BER encoding of the T61String value "cl'es
publiques" (French for "public keys") can be any of the
following, among others, depending on the form of length
octets and whether the encoding is primitive or constructed:

14 0f                                           DER encoding
   63 6c c2 65 73 20 70 75 62 6c 69 71 75 65 73

14 81 0f                          long form of length octets
   63 6c c2 65 73 20 70 75 62 6c 69 71 75 65 73

34 15      constructed encoding: "cl'es" + " " + "publiques"
   14 05 63 6c c2 65 73
   14 01 20
   14 09 70 75 62 6c 69 71 75 65 73

The eight-bit character c2 is a T.61 prefix that adds an
acute accent (') to the next character.

DER encoding. Primitive. Contents octets are as for a
primitive BER encoding.

Example: The DER encoding of the T61String value "cl'es
publiques" is

14 0f 63 6c c2 65 73 20 70 75 62 6c 69 71 75 65 73


5.17 UTCTime

The UTCTime type denotes a "coordinated universal time" or
Greenwich Mean Time (GMT) value. A UTCTime value includes
the local time precise to either minutes or seconds, and an
offset from GMT in hours and minutes. It takes any of the
following forms:

YYMMDDhhmmZ
YYMMDDhhmm+hh'mm'
YYMMDDhhmm-hh'mm'
YYMMDDhhmmssZ
YYMMDDhhmmss+hh'mm'
YYMMDDhhmmss-hh'mm'

where:

     YY is the least significant two digits of the year

     MM is the month (01 to 12)

     DD is the day (01 to 31)

     hh is the hour (00 to 23)

     mm are the minutes (00 to 59)

     ss are the seconds (00 to 59)

     Z indicates that local time is GMT, + indicates that
          local time is later than GMT, and - indicates that
          local time is earlier than GMT

     hh' is the absolute value of the offset from GMT in
          hours

     mm' is the absolute value of the offset from GMT in
          minutes

This type is a string type.

The UTCTime type is used for signing times in PKCS #9's
signing-time attribute and for certificate validity periods
in X.509's Validity type.

ASN.1 notation:

UTCTime

BER encoding. Primitive or constructed. In a primitive
encoding, the contents octets give the characters in the
string, encoded in ASCII. In a constructed encoding, the
contents octets give the concatenation of the BER encodings
of consecutive substrings of the string. (The constructed
encoding is not particularly interesting, since UTCTime
values are so short, but the constructed encoding is
permitted.)

Example: The time this sentence was originally written was
4:45:40 p.m. Pacific Daylight Time on May 6, 1991, which can
be represented with either of the following UTCTime values,
among others:

"910506164540-0700"

"910506234540Z"

These values have the following BER encodings, among others:

17 0d 39 31 30 35 30 36 32 33 34 35 34 30 5a

17 11 39 31 30 35 30 36 31 36 34 35 34 30 2D 30 37 30
      30

DER encoding. Primitive. Contents octets are as for a
primitive BER encoding.


6. An example

This section gives an example of ASN.1 notation and DER
encoding: the X.501 type Name.


6.1 Abstract notation

This section gives the ASN.1 notation for the X.501 type
Name.

Name ::= CHOICE {
  RDNSequence }

RDNSequence ::= SEQUENCE OF RelativeDistinguishedName

RelativeDistinguishedName ::=
  SET OF AttributeValueAssertion

AttributeValueAssertion ::= SEQUENCE {
   AttributeType,
   AttributeValue }

AttributeType ::= OBJECT IDENTIFIER

AttributeValue ::= ANY

The Name type identifies an object in an X.500 directory.
Name is a CHOICE type consisting of one alternative:
RDNSequence. (Future revisions of X.500 may have other
alternatives.)

The RDNSequence type gives a path through an X.500 directory
tree starting at the root. RDNSequence is a SEQUENCE OF type
consisting of zero or more occurences of
RelativeDistinguishedName.

The RelativeDistinguishedName type gives a unique name to an
object relative to the object superior to it in the
directory tree. RelativeDistinguishedName is a SET OF type
consisting of zero or more occurrences of
AttributeValueAssertion.

The AttributeValueAssertion type assigns a value to some
attribute of a relative distinguished name, such as country
name or common name. AttributeValueAssertion is a SEQUENCE
type consisting of two components, an AttributeType type and
an AttributeValue type.

The AttributeType type identifies an attribute by object
identifier. The AttributeValue type gives an arbitrary
attribute value. The actual type of the attribute value is
determined by the attribute type.


6.2 DER encoding

This section gives an example of a DER encoding of a value
of type Name, working from the bottom up.

The name is that of the Test User 1 from the PKCS examples
[Kal93]. The name is represented by the following path:

                           (root)
                              |
                     countryName = "US"
                              |
          organizationName = "Example Organization"
                              |
                 commonName = "Test User 1"

Each level corresponds to one RelativeDistinguishedName
value, each of which happens for this name to consist of one
AttributeValueAssertion value. The AttributeType value is
before the equals sign, and the AttributeValue value (a
printable string for the given attribute types) is after the
equals sign.

The countryName, organizationName, and commonUnitName are
attribute types defined in X.520 as:

attributeType OBJECT IDENTIFIER ::=
  { joint-iso-ccitt(2) ds(5) 4 }

countryName OBJECT IDENTIFIER ::= { attributeType 6 }
organizationName OBJECT IDENTIFIER ::=
  { attributeType 10 }
commonUnitName OBJECT IDENTIFIER ::=
  { attributeType 3 }


6.2.1 AttributeType

The three AttributeType values are OCTET STRING values, so
their DER encoding follows the primitive, definite-length
method:

06 03 55 04 06                                   countryName

06 03 55 04 0a                              organizationName

06 03 55 04 03                                    commonName

The identifier octets follow the low-tag form, since the tag
is 6 for OBJECT IDENTIFIER. Bits 8 and 7 have value "0,"
indicating universal class, and bit 6 has value "0,"
indicating that the encoding is primitive. The length octets
follow the short form. The contents octets are the
concatenation of three octet strings derived from
subidentifiers (in decimal): 40 * 2 + 5 = 85 = 5516; 4; and
6, 10, or 3.


6.2.2 AttributeValue

The three AttributeValue values are PrintableString values,
so their encodings follow the primitive, definite-length
method:

13 02 55 53                                             "US"

13 14                                 "Example Organization"
   45 78 61 6d 70 6c 65 20 4f 72 67 61 6e 69 7a 61
   74 69 6f 6e

13 0b                                          "Test User 1"
   54 65 73 74 20 55 73 65 72 20 31

The identifier octets follow the low-tag-number form, since
the tag for PrintableString, 19 (decimal), is between 0 and
30. Bits 8 and 7 have value "0" since PrintableString is in
the universal class. Bit 6 has value "0" since the encoding
is primitive. The length octets follow the short form, and
the contents octets are the ASCII representation of the
attribute value.


6.2.3 AttributeValueAssertion

The three AttributeValueAssertion values are SEQUENCE
values, so their DER encodings follow the constructed,
definite-length method:

30 09                                     countryName = "US"
   06 03 55 04 06
   13 02 55 53

30 1b              organizationName = "Example Organizaiton"
   06 03 55 04 0a
   13 14 ... 6f 6e

30 12                             commonName = "Test User 1"
   06 03 55 04 0b
   13 0b ... 20 31

The identifier octets follow the low-tag-number form, since
the tag for SEQUENCE, 16 (decimal), is between 0 and 30.
Bits 8 and 7 have value "0" since SEQUENCE is in the
universal class. Bit 6 has value "1" since the encoding is
constructed. The length octets follow the short form, and
the contents octets are the concatenation of the DER
encodings of the attributeType and attributeValue
components.


6.2.4 RelativeDistinguishedName

The three RelativeDistinguishedName values are SET OF
values, so their DER encodings follow the constructed,
definite-length method:

31 0b
   30 09 ... 55 53

31 1d
   30 1b ... 6f 6e

31 14
   30 12 ... 20 31

The identifier octets follow the low-tag-number form, since
the tag for SET OF, 17 (decimal), is between 0 and 30. Bits
8 and 7 have value "0" since SET OF is in the universal
class Bit 6 has value "1" since the encoding is constructed.
The lengths octets follow the short form, and the contents
octets are the DER encodings of the respective
AttributeValueAssertion values, since there is only one
value in each set.


6.2.5 RDNSequence

The RDNSequence value is a SEQUENCE OF value, so its DER
encoding follows the constructed, definite-length method:

30 42
   31 0b ... 55 53
   31 1d ... 6f 6e
   31 14 ... 20 31

The identifier octets follow the low-tag-number form, since
the tag for SEQUENCE OF, 16 (decimal), is between 0 and 30.
Bits 8 and 7 have value "0" since SEQUENCE OF is in the
universal class. Bit 6 has value "1" since the encoding is
constructed. The lengths octets follow the short form, and
the contents octets are the concatenation of the DER
encodings of the three RelativeDistinguishedName values, in
order of occurrence.


6.2.6 Name

The Name value is a CHOICE value, so its DER encoding is the
same as that of the RDNSequence value:

30 42
   31 0b
      30 09
         06 03 55 04 06          attributeType = countryName
         13 02 55 53                   attributeValue = "US"
   31 1d
      30 1b
         06 03 55 04 0a     attributeType = organizationName
         13 14       attributeValue = "Example Organization"
            45 78 61 6d 70 6c 65 20 4f 72 67 61 6e 69 7a 61
            74 69 6f 6e

   31 14
      30 12
         06 03 55 04 03           attributeType = commonName
         13 0b                attributeValue = "Test User 1"
            54 65 73 74 20 55 73 65 72 20 31


References

PKCS #1   RSA Laboratories. PKCS #1: RSA Encryption
          Standard. Version 1.5, November 1993.

PKCS #3   RSA Laboratories. PKCS #3: Diffie-Hellman Key-
          Agreement Standard. Version 1.4, November 1993.

PKCS #5   RSA Laboratories. PKCS #5: Password-Based
          Encryption Standard. Version 1.5, November 1993.

PKCS #6   RSA Laboratories. PKCS #6: Extended-Certificate
          Syntax Standard. Version 1.5, November 1993.

PKCS #7   RSA Laboratories. PKCS #7: Cryptographic Message
          Syntax Standard. Version 1.5, November 1993.

PKCS #8   RSA Laboratories. PKCS #8: Private-Key Information
          Syntax Standard. Version 1.2, November 1993.

PKCS #9   RSA Laboratories. PKCS #9: Selected Attribute
          Types. Version 1.1, November 1993.

PKCS #10  RSA Laboratories. PKCS #10: Certification Request
          Syntax Standard. Version 1.0, November 1993.

X.200     CCITT. Recommendation X.200: Reference Model of
          Open Systems Interconnection for CCITT
          Applications. 1984.

X.208     CCITT. Recommendation X.208: Specification of
          Abstract Syntax Notation One (ASN.1). 1988.

X.209     CCITT. Recommendation X.209: Specification of
          Basic Encoding Rules for Abstract Syntax Notation
          One (ASN.1). 1988.

X.500     CCITT. Recommendation X.500: The
          Directory--Overview of Concepts, Models and
          Services. 1988.

X.501     CCITT. Recommendation X.501: The Directory--
          Models. 1988.

X.509     CCITT. Recommendation X.509: The Directory--
          Authentication Framework. 1988.

X.520     CCITT. Recommendation X.520: The Directory--
          Selected Attribute Types. 1988.

[Kal93]   Burton S. Kaliski Jr. Some Examples of the PKCS
          Standards. RSA Laboratories, November 1993.

[NIST92]  NIST. Special Publication 500-202: Stable
          Implementation Agreements for Open Systems
          Interconnection Protocols. Part 11 (Directory
          Services Protocols). December 1992.


Revision history


June 3, 1991 version

The June 3, 1991 version is part of the initial public
release of PKCS. It was published as NIST/OSI Implementors'
Workshop document SEC-SIG-91-17.


November 1, 1993 version

The November 1, 1993 version incorporates several editorial
changes, including the addition of a revision history. It is
updated to be consistent with the following versions of the
PKCS documents:

     PKCS #1: RSA Encryption Standard. Version 1.5, November
          1993.

     PKCS #3: Diffie-Hellman Key-Agreement Standard. Version
          1.4, November 1993.

     PKCS #5: Password-Based Encryption Standard. Version
          1.5, November 1993.

     PKCS #6: Extended-Certificate Syntax Standard. Version
          1.5, November 1993.

     PKCS #7: Cryptographic Message Syntax Standard. Version
          1.5, November 1993.

     PKCS #8: Private-Key Information Syntax Standard.
          Version 1.2, November 1993.

     PKCS #9: Selected Attribute Types. Version 1.1,
          November 1993.

     PKCS #10: Certification Request Syntax Standard.
          Version 1.0, November 1993.

The following substantive changes were made:

     Section 5: Description of T61String type is added.

     Section 6: Names are changed, consistent with other
          PKCS examples.


Author's address

Burton S. Kaliski Jr., Ph.D.
Chief Scientist
RSA Laboratories              (415) 595-7703
100 Marine Parkway            (415) 595-4126 (fax)
Redwood City, CA  94065  USA  burt@rsa.com