ASN.1:
Cryptographic files
Zdeněk Říha
ASN.1 Grammar
 To understand the structure (what is the
meaning of particular fields) we need ASN.1
grammar
ASN.1 – RSA keys
Source:
PKCS#1 RSA.key
ASN.1 – RSA padding
 PKCS#1 v1.5
 m = 0x00 || 0x01 || 0xFF … 0xFF || 0x00 || T
 Where T is defined as DER encoding of
 In practice:
Source:
PKCS#1
ASN.1 – RSA signature
 RSA signature is the number s = md mod n
 TSA.crt
ASN.1 – signature OIDs
Source:
BSI TR-03105 Part 5.1
ASN.1 – RSA PSS params
RSASSA-PSS
SHA256
SHA256
MGF1
Source:
PKCS#1
 CSCA_CZE.crt
ASN.1 – DSA keys
DSAPrivateKey is an INTEGER, usually denoted as X
Source:
RFC 5480
Source:
OpenSSL
 DSA.key
ASN.1 – DSA signature
Source:
RFC 5480
 DSA.crt
ASN.1 – DSA - OIDs
Source: RFC 5480
ASN.1 – ECDSA keys
ECPoint
INTEGER
Source: RFC 5915
ASN.1 - ECDSA public key
 CSCA_Switzerland.crt
ASN.1 – ECDSA signatures
1.2.840.10045.4.1 - ecdsa-with-SHA1
Source:
RFC 5480
 CSCA_Switzerland.crt
ASN.1 – ECDSA signature OID
Source:
BSI TR-03105 Part 5.1
ASN.1 - certificates
Source:
RFC 5280
ASN.1 – certificates - pubkey
Source:
RFC 5280
 CSCA_CZE.crt
ASN.1 – certificates - times
 Until 2049: UTCTime
 YYMMDDHHMMSSZ
 From 2050: GeneralizedTime
 YYYYMMDDHHMMSSZ
Source:
RFC 5280
 CSCA_CZE.crt
ASN.1 – certificates - names
Source:
RFC 5280
ASN.1 – certificate - names
 CSCA_CZE.crt
ASN.1 – certificate - names
Source:
ITU-T X.520
ASN.1 – certificate - names
Source:
ITU-T X.520
Certificate profiles
 For particular areas/purposes there exist certificate
profiles which prescribe what kind of attributes will
be used in Names
 E.g. for electronic passports ICAO Doc. 9303 states:
Source:
ICAO Doc. 9303
ASN.1 – certificates – v3
 Critical x non-critical extensions
Source:
RFC 5280
ASN.1 – certs – extensions
 CSCA_CZE.crt
X509v3 cert extensions
 Authority Key Identifier
 Identification of the issuing CA
 Non critical
 Similarly “Subject Key Identifier” Source:
RFC 5280
X509v3 cert extensions
 Key Usage
 Restrictions of the use of the key
Source:
RFC 5280
X509v3 cert extensions
 Extended Key Usage
 Purposes of the certified key
Source:
RFC 5280
X509v3 cert extensions
 Certificate Policies
 Policy relevant for
the issue and use of
the certificate
 Preferably only an
OID
Source:
RFC 5280
X509v3 cert extensions
 Subject Alternative Name
 Issuer Alternative Name
 “Internet style identities”
 Email
 DNS name
 IP address
 URL
 Must be verified by CA
X509v3 cert extensions
 Basic Constraints
 Is Subject a CA?
 Max. length/depth of the certificate chain/path
 A pathLenConstraint of zero indicates that no
non-self-issued intermediate CA certificates may
follow in a valid certification path.
Source:
RFC 5280
X509v3 cert extensions
 Name Constraints
 Only for CA certificates
 “indicates a name space within which all
subject names in subsequent certificates in a
certification path MUST be located”
Source:
RFC 5280
X509v3 cert extensions
 Policy Constraints
 Must be critical
 For CA certificates
 Constraints path validation
 Prohibit policy mapping (or)
 Require acceptable policy OID in each certificate
Source:
RFC 5280
X509v3 cert extensions
 CRL Distribution Points
 How to obtain CRL
Source:
RFC 5280
ASN.1 – certificate request
Source:
RFC 5280
ASN.1 - CRL
Source:
RFC 5280
ASN.1 – PKCS#7 / CMS
Source:
RFC 5652
ASN.1 - PKCS#7 / CMS
Source:
RFC 5652
PKCS#7 Sample  France.p7s
ASN.1 – PKCS#8
Source:
PKCS#8