ASN.1:
Introduction
Zdeněk Říha

ASN.1
lAbstract Syntax Notation 1
lnotation for describing abstract types and values
lDefined in ITU-T X.680 … X.695
lUsed in many file formats, including crypto
lPublic keys, private keys
lCertificate requests, certificates
lDigital signatures, padding, encrypted files

ASN.1
lAllows format/storage/transmission of data
lCompatible among many applications
lNot dependent on HW platform
lE.g. little/big endian
lNot dependent on operating system
l
lSimple & Structured types
lMultiple encoding rules (methods)

ASN.1 – Types



ASN.1 – simple types
lInteger
lsigned integer (there’s no unsigned integer)
lBit string
lThe number of bits does not have to be a multiple of 8
lOctet string
lan arbitrary string of octets
lNULL
lNo data (used in parameters)
lPringtableString, IA5String, UTF8String, …
lStrings – the sets of characters are various
lUTCTime
lTime
l

ASN.1 – OID type
lObject identifier (OID)
lSequence of integer components that identify an object
lAssigned in a hierarchical way
lExample
lsha-1WithRSAEncryption = 1.2.840.113549.1.1.5
liso(1) member-body(2)
us(840) rsadsi(113549)
pkcs(1) pkcs-1(1) 5
l

ASN.1 – structured types
lSEQUENCE
lan ordered collection of one or more types
lSEQUENCE OF
lan ordered collection of zero or more occurrences of a given type
lSET
lan unordered collection of one or more types
lSET OF
lan unordered collection of zero or more occurrences of a given type

ASN.1 Encoding Rules
lXML – oriented formats
lXER (XML Encoding Rules)
lByte-oriented formats
lBER (Basic Encoding Rules)
lCER (Canonical Encoding Rules) – subset of BER
lDER (Distinguished Encoding Rules) – subset of BER
lUsed for crypto files
lBit-oriented formats
lPER (Packed Encoding Rules)
lVerbose, human readable formats
lGSER (Generic String Encoding Rules)

BER encoding
lTLV – Tag Length Value
lAll the data is encoded using a simple TLV format
lTag – what kind of data it is
lLength – the length of the data
lValue – the data itself
lExample
l02 01 05 [hexadecimal values]
lTag – Integer
lLength of data – 1 byte
lData: (positive integer) 5

Nested data
lSEQUENCE is similar to struct/record
l30 09 02 01 05 04 02 FF FF 05 00
l30 09 – sequence of length 9 bytes
l02 01 05 – integer 5
l04 02 FF FF – octet string FF FF
l05 00 – NULL (no data)
lupa-1884

BER tags
lTag encoding
l
l
lClass
l
l
l
lTag number
lBits 1-5
lIf all bits are 1 then the tag continues in the following byte(s)
class
class
Constructed?
Tag #

BER length
llength >=0 && length <= 127
lThe length is coded directly
lE.g. ’05’
lOtherwise the bit 8 is set, bits 1-7 code the number of bytes that specify the length
lE.g. 255 -> ‘81’ ‘FF’
lE.g. 256 -> ’82’ ‘01’ ‘00’ or also ’83’ ‘00’ ‘01’ ‘00’
lBER x DER
l‘80’ is “indefinite” length
lNot allowed in DER

BER value
lThe data itself
lDependent on data type
lInteger: signed – e.g. 128 -> ’00 80’
lOctet string: directly the data
lBit string: number of unused bits + padded bit string to a multiple of 8 bits (padding is at the
end)
lUTCTime: string of one of the forms

First look at the binary DER file
< CSCA_CZE.crt


DER vs. PEM
lPEM
lPrivacy Enhanced Mail
lPEM as such not used, but formats still used
lTextual formats
lPractical for transport channels where full 8bit data can be damaged
lPEM is base64 coded DER enveloped with
l-----BEGIN SOMETHING-----
l-----END SOMETHING-----
lWhere SOMETHING is CERTIFICATE/PKCS7/KEY…
l

Sample PEM file
< CSCA_CZE.pem


ASN.1 viewers
lUnber (part of asn1c)
lOpenssl asn1parse
lASN.1 Editor
l…
l

OpenSSL asn1parse
< CSCA_CZE.crt


unber
< CSCA_CZE.crt


Manual viewing/processing
l30 82 04 f2
lSEQUENCE
llength 1266B
l30 82 03 26
lSEQUENCE
llength 806B
lA0 03
lCONTEXT SPECIFIC 0
lLength 3B
l02 01 02
lINTEGER: 2
l
< CSCA_CZE.crt

ASN.1 Editor
< CSCA_CZE.crt


ASN.1 Grammar
lTo understand the structure (what is the meaning of particular fields) we need ASN.1 grammar
l