ASN.1:
Introduction
Zdeněk Říha
ASN.1
 Abstract Syntax Notation 1
 notation for describing abstract types and
values
 Defined in ITU-T X.680 … X.695
 Used in many file formats, including crypto
 Public keys, private keys
 Certificate requests, certificates
 Digital signatures, padding, encrypted files
ASN.1
 Allows format/storage/transmission of data
 Compatible among many applications
 Not dependent on HW platform
 E.g. little/big endian
 Not dependent on operating system
 Simple & Structured types
 Multiple encoding rules (methods)
ASN.1 – Types
ASN.1 – simple types
 Integer
 signed integer (there’s no unsigned integer)
 Bit string
 The number of bits does not have to be a multiple of 8
 Octet string
 an arbitrary string of octets
 NULL
 No data (used in parameters)
 PringtableString, IA5String, UTF8String, …
 Strings – the sets of characters are various
 UTCTime
 Time
ASN.1 – OID type
 Object identifier (OID)
 Sequence of integer components that identify an
object
 Assigned in a hierarchical way
 Example
 sha-1WithRSAEncryption = 1.2.840.113549.1.1.5
 iso(1) member-body(2)
us(840) rsadsi(113549)
pkcs(1) pkcs-1(1) 5
ASN.1 – structured types
 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
ASN.1 Encoding Rules
 XML – oriented formats
 XER (XML Encoding Rules)
 Byte-oriented formats
 BER (Basic Encoding Rules)
 CER (Canonical Encoding Rules) – subset of BER
 DER (Distinguished Encoding Rules) – subset of BER
 Used for crypto files
 Bit-oriented formats
 PER (Packed Encoding Rules)
 Verbose, human readable formats
 GSER (Generic String Encoding Rules)
BER encoding
 TLV – Tag Length Value
 All the data is encoded using a simple TLV format
 Tag – what kind of data it is
 Length – the length of the data
 Value – the data itself
 Example
 02 01 05 [hexadecimal values]
 Tag – Integer
 Length of data – 1 byte
 Data: (positive integer) 5
Nested data
 SEQUENCE is similar to struct/record
 30 09 02 01 05 04 02 FF FF 05 00
 30 09 – sequence of length 9 bytes
 02 01 05 – integer 5
 04 02 FF FF – octet string FF FF
 05 00 – NULL (no data)
BER tags
 Tag encoding
 Class
 Tag number
 Bits 1-5
 If all bits are 1 then the tag continues in the following byte(s)
class class
Constr
ucted?
Tag #
BER length
 length >=0 && length <= 127
 The length is coded directly
 E.g. ’05’
 Otherwise the bit 8 is set, bits 1-7 code the
number of bytes that specify the length
 E.g. 255 -> ‘81’ ‘FF’
 E.g. 256 -> ’82’ ‘01’ ‘00’ or also ’83’ ‘00’ ‘01’ ‘00’
 BER x DER
 ‘80’ is “indefinite” length
 Not allowed in DER
BER value
 The data itself
 Dependent on data type
 Integer: signed – e.g. 128 -> ’00 80’
 Octet string: directly the data
 Bit string: number of unused bits + padded bit
string to a multiple of 8 bits (padding is at the end)
 UTCTime: string of one of the forms
First look at the binary DER file
 CSCA_CZE.crt
DER vs. PEM
 PEM
 Privacy Enhanced Mail
 PEM as such not used, but formats still used
 Textual formats
 Practical for transport channels where full 8bit data can
be damaged
 PEM is base64 coded DER enveloped with
 -----BEGIN SOMETHING-----
 -----END SOMETHING-----
 Where SOMETHING is CERTIFICATE/PKCS7/KEY…
Sample PEM file
 CSCA_CZE.pem
ASN.1 viewers
 Unber (part of asn1c)
 Openssl asn1parse
 ASN.1 Editor
 …
OpenSSL asn1parse
 CSCA_CZE.crt
unber  CSCA_CZE.crt
Manual viewing/processing
 30 82 04 f2
 SEQUENCE
 length 1266B
 30 82 03 26
 SEQUENCE
 length 806B
 A0 03
 CONTEXT
SPECIFIC 0
 Length 3B
 02 01 02
 INTEGER: 2
 CSCA_CZE.crt
ASN.1 Editor  CSCA_CZE.crt
ASN.1 Grammar
 To understand the structure (what is the
meaning of particular fields) we need ASN.1
grammar