1.0 Certificates
The ASN.1 definition of an X.509 Certificate as defined by RFC 2459 looks like this
Certificate ::= SEQUENCE {
tbsCertificate TBSCertificate,
signatureAlgorithm AlgorithmIdentifier,
signature BIT STRING
}
An X.509 Certificate is simply a signed TBSCertificate.
The definition of a TBSCertificate looks like this
TBSCertificate ::= SEQUENCE {
version [0] EXPLICIT Version DEFAULT v1(0),
serialNumber CertificateSerialNumber,
signature AlgorithmIdentifier,
issuer Name,
validity Validity,
subject Name,
subjectPublicKeyInfo SubjectPublicKeyInfo,
issuerUniqueId [1] IMPLICIT UniqueIdentifier OPTIONAL,
-- If present, version shall be v2 or v3
subjectUniqueId [2] IMPLICIT UniqueIdentifier OPTIONAL,
-- If present, version shall be v2 or v3
extensions [3] EXPLICIT Extensions OPTIONAL
-- If present, version shall be v3
}1.1 subjectPublicKeyInfo And subject
The
subjectPublicKeyInfofield specifies a public key, and the
subjectfield identifies the owner of the specified public key.
1.2 issuer and serialNumber
The
issuer
field identifies the signer of the TBSCertificate.
This together with the
serialNumberfield should uniquely identify the certificate.
1.3 Validity
The validity field specifies the period during which the certificate is valid.
The ASN.1 definition of the Validity type looks like this
Validity ::= SEQUENCE {
notBefore Time,
notAfter Time }
The public key specified by the certificate should not be used at time T if T is before the value specified by notBefore or after the value specified by notAfter.
Unfortunately, in practice certificates are often issued with such long validity periods that checking the validity really serves no useful purpose.
1.4 Self-Signed Certificates
If the subject and issuer fields are identical then the certificate is self-signed, that is, the signature of the certificate was generated using the private key corresponding to the public key specified by the certificate.
2.0 Certificate Chains
A certificate chain in simply a certificate (A) plus the certificate (B) of the issuer of certificate A, plus the certificate of the issuer of certificate B, and so on.
To put it another way
certificate[i+1].subject == certificate[i].issuerA finite certificate chain must necessarily terminate in a self-signed certificate, or terminate arbitrarily and not include all of the certificates back to a self-signed certificate.
The purpose of a certificate chain is to make it possible to verify the signature of the i’th ‘certificate in the chain using the public key specified in the i+1’th certificate.
2.1 Certificate Chains And Trust
Given a certificate chain terminated by a self-signed certificate it is always possible to verify all the certificates in the chain and hence anything signed by subject of the first certificate in the chain.
This is also possible given a chain terminated by a non-self-signed certificate plus the rest of the certificates back to a self-signed certificate obtained from some other source.
In either case what it is possible to do is to establish unequivocally is that something supposedly signed by the subject of the first certificate in the chain was in fact signed by them but that is all.
It is not possible to establish whether the subjects of any of the certificate are who they claim to be.
To do this you must trust one of the certificates in the chain and this cannot be done algorithmically it can only done by policy.
This is usually achieved by having one or more certificates known to the system defined a priori as trust roots.
Any certificate which can be verified directly or indirectly using a trust root can itself be trusted and consequently anything else signed by the subject of a trusted certificate can itself be trusted for some value of trust.
This of course assumes that the subject of a certificate considered to be a trust root is not simply signing every and any certificate that comes their way, and similarly for the subjects of those certificates and so on and so forth.
This is not necessarily a good assumption to make and the longer the certificate chain the less good it becomes but given the way certificates and the public key infrastructure work its all you can do.
3.0 Example
This is a simple example of a certificate chain of length three terminating in a self-signed certificate.
Certificate[0]
Data:
Version: 1 (0x0)
Serial Number:
8c:f3:8c:e9:05:ed:d1:7c
Signature Algorithm: sha1WithRSAEncryption
Issuer:
C=OZ, ST=Nowhere, L=Emerald City, O=ASH Two, OU=Android Applications Group, \
CN=Admin/emailAddress=admin@android.applications.group.ash2.oz
Validity
Not Before: Aug 4 10:53:06 2014 GMT
Not After : Sep 3 10:53:06 2014 GMT
Subject:
C=OZ, ST=Nowhere, L=Emerald City, O=ASH Two, OU=AndroidApp Flounder Group, \
CN=AndroidApp Flounder
Subject Public Key Info:
Public Key Algorithm: rsaEncryption
RSA Public Key: (2048 bit)
Modulus (2048 bit):
00:8a:84:e0:00:7f:ee:fe:1d:18:57:2d:be:a7:2e:
68:aa:6b:6d:26:6d:14:1b:0c:e0:b5:44:46:0c:62:
40:7a:d9:d5:46:c3:d0:38:28:29:cb:e4:8f:42:5d:
8f:7d:81:78:24:ae:61:f6:a5:9b:a6:b4:0b:b6:f4:
01:96:1b:b4:90:b7:74:c7:99:55:33:9d:fc:80:4b:
e3:fd:00:4b:99:ee:68:b5:81:6a:60:c3:f2:2e:fc:
31:7e:ff:b9:66:24:6f:6b:a0:82:e8:15:f6:cd:d1:
b1:b5:05:41:2c:19:e2:f4:92:e9:df:23:94:80:ad:
e1:45:41:31:78:52:d2:8d:18:58:c2:0a:a1:ef:9d:
5e:d6:5c:60:07:09:6d:87:51:79:42:25:2b:99:0c:
35:2d:3d:1f:09:d0:71:05:bb:a7:90:fb:70:e7:26:
d9:c6:43:53:35:60:5b:4c:1f:df:38:74:ca:a0:b0:
0e:77:9c:fa:7d:c2:32:9a:99:b2:53:6a:c8:fa:ea:
0a:3e:b7:99:50:7d:d1:94:3f:a4:2e:18:aa:ff:8a:
82:69:0e:2b:83:07:7b:fc:b0:7b:62:7a:99:1e:e7:
6b:ab:d6:0d:5e:d2:f2:19:55:af:81:82:e8:d1:db:
a9:ea:16:58:32:8a:55:ac:be:8e:cf:87:a0:ed:2d:
4f:9f
Exponent:
65537 (0x10001)
Signature Algorithm: sha1WithRSAEncryption
48:7e:34:61:08:14:18:b2:8f:66:3f:2e:58:fd:a5:4c:4f:85:
fa:d1:36:5b:b9:ba:21:1e:3e:fd:8e:a9:40:23:b6:8a:84:38:
9f:40:4f:f6:4f:73:a9:f8:79:e7:21:a1:67:32:48:1b:5f:e0:
5b:6b:13:53:ff:9a:40:41:13:99:e2:0d:eb:6b:15:c5:24:be:
71:d7:9e:98:af:11:2b:9d:9a:f1:e1:d9:27:27:47:b2:41:3a:
43:65:ce:b9:c6:7b:00:ee:77:5e:b1:a8:63:e0:f2:fc:94:32:
42:4f:9b:30:6b:8e:fe:26:e5:0a:ed:1f:ab:9d:5b:48:56:4e:
30:1d
Certificate[1]
Data:
Version: 3 (0x2)
Serial Number: 1 (0x1)
Signature Algorithm: sha1WithRSAEncryption
Issuer: C=OZ, ST=Nowhere, L=Emerald City, O=ASH Two, OU=Certs Group, \
CN=Admin/emailAddress=admin@certs.group.ash2.oz
Validity
Not Before: Aug 4 10:45:35 2014 GMT
Not After : Aug 4 10:45:35 2015 GMT
Subject: C=OZ, ST=Nowhere, L=Emerald City, O=ASH Two, OU=Android Applications Group, \
CN=Admin/emailAddress=admin@android.applications.group.ash2.oz
Subject Public Key Info:
Public Key Algorithm: rsaEncryption
RSA Public Key: (1024 bit)
Modulus (1024 bit):
00:dd:17:06:f9:6a:44:be:96:ed:01:39:e2:fe:5e:
99:9e:cf:4f:55:b5:c8:5b:27:42:b6:b5:bf:49:73:
41:fd:17:b2:b7:bd:1f:6e:28:07:fc:24:07:57:76:
80:5b:88:12:f4:09:28:1d:97:d4:b9:37:7a:2f:52:
83:9b:64:60:6f:72:b2:4e:d0:ed:15:d5:26:b2:5a:
33:e6:27:37:4f:42:7e:24:36:f1:2e:dc:a8:5d:bd:
52:8e:c0:72:62:39:fa:cb:08:f6:4e:e2:89:d0:3b:
ab:88:da:e6:a4:18:cc:e3:9d:fb:6d:2e:a1:ac:ec:
56:44:f6:8f:d1:c7:b3:e7:c5
Exponent: 65537 (0x10001)
X509v3 extensions:
X509v3 Subject Key Identifier:
7B:AB:2E:B9:61:D2:08:3E:78:42:4F:CA:EE:DF:28:3D:27:76:C4:2D
X509v3 Authority Key Identifier:
keyid:5B:ED:98:20:92:DD:B5:8F:0F:0F:32:47:29:48:38:BB:F7:52:B0:48
DirName:/C=OZ/ST=Nowhere/L=Emerald City/O=ASH Two/OU=Certs Group\
/CN=Admin/emailAddress=admin@certs.group.ash2.oz
serial:EF:55:32:3F:F5:CF:64:E5
Signature Algorithm: sha1WithRSAEncryption
98:a1:e6:be:33:06:94:1f:93:e1:c5:1a:a4:53:5c:c5:ad:f0:
b7:c4:bd:f3:dd:22:2e:f1:aa:0e:c7:08:42:1b:9a:f8:1c:f4:
05:c2:a2:96:a3:ea:d6:5d:7d:ee:65:78:4c:24:5d:35:8b:0b:
7c:fe:b3:a4:c7:5f:b6:45:f0:3b:f2:5d:8c:32:fa:8e:2d:0b:
6e:10:1c:5c:3c:55:d4:f0:f3:c3:61:53:22:6a:65:93:d0:2a:
a4:29:09:61:8e:46:9c:ab:47:0b:74:16:97:d3:30:51:4f:eb:
ad:e1:4f:6e:83:a9:f6:11:0f:7d:ab:97:73:f9:8a:cf:43:53:
f8:6d
Certificate[2]
Data:
Version: 3 (0x2)
Serial Number:
9a:32:99:e8:05:ee:62:db
Signature Algorithm: sha1WithRSAEncryption
Issuer:
C=OZ, ST=Nowhere, L=Emerald City, O=ASH Two, OU=Android Applications Group, \
CN=Admin/emailAddress=admin@android.applications.group.ash2.oz
Validity
Not Before: Aug 5 16:57:20 2014 GMT
Not After : Aug 5 16:57:20 2015 GMT
Subject:
C=OZ, ST=Nowhere, L=Emerald City, O=ASH Two, OU=Android Applications Group, \
CN=Admin/emailAddress=admin@android.applications.group.ash2.oz
Subject Public Key Info:
Public Key Algorithm: rsaEncryption
RSA Public Key: (1024 bit)
Modulus (1024 bit):
00:c6:27:36:f5:fd:33:26:14:7b:a9:1b:10:84:fc:
7b:58:e3:cf:21:b9:28:50:4b:b9:d0:9b:73:5b:e4:
81:06:be:0d:b6:ff:d2:d4:06:bd:bd:63:3d:e6:65:
f3:10:e5:39:5b:4c:e1:96:e4:fe:a3:e3:52:c1:6b:
bd:a0:e8:fb:a8:11:f8:0c:68:ee:6b:1b:e0:c8:9d:
41:0a:41:c6:dd:ff:1b:4c:c4:a1:9b:17:85:58:4a:
3d:4e:88:4f:2e:ea:d3:28:8e:23:a1:d9:92:ba:87:
b6:91:d9:07:8b:c2:9a:e3:75:18:36:3e:fc:12:ae:
ff:3a:4b:3a:e7:21:a1:08:2d
Exponent: 65537 (0x10001)
X509v3 extensions:
X509v3 Subject Key Identifier:
79:59:43:52:D9:04:7D:C0:EA:A8:14:37:BB:7B:85:92:91:44:07:D1
X509v3 Authority Key Identifier:
keyid:79:59:43:52:D9:04:7D:C0:EA:A8:14:37:BB:7B:85:92:91:44:07:D1
DirName:/C=OZ/ST=Nowhere/L=Emerald City/O=ASH Two/OU=Android Applications Group \
/CN=Admin/emailAddress=admin@android.applications.group.ash2.oz
serial:9A:32:99:E8:05:EE:62:DB
X509v3 Basic Constraints:
CA:TRUE
Signature Algorithm: sha1WithRSAEncryption
a2:7b:0f:7d:02:ee:5a:b4:e0:b7:78:33:5d:59:9b:13:be:71:
98:6b:2e:44:95:56:e5:69:78:49:99:5d:f9:14:a8:4f:1c:fc:
4d:13:f7:2a:cd:eb:cc:97:7c:41:1e:24:2b:d4:56:3d:0a:90:
cd:7b:9f:af:84:99:9a:bc:c7:63:3a:24:46:7c:f7:06:48:f9:
7c:10:9b:58:ee:2e:07:1b:a9:66:66:d0:cc:e7:cb:f2:8c:53:
6b:a8:d3:2e:a6:d2:f0:f4:ca:65:fb:66:73:18:86:8a:30:ef:
f7:09:27:56:00:5f:e2:30:d8:5e:a2:6b:d6:22:af:bd:18:59:
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