Public key cryptography: SSL certificates
By Leonardo Giordani -
In the context of public key cryptography, certificates are a way to prove the identity of the owner of a public key.
While public key cryptography allows us to communicate securely through an insecure network, it leaves the problem of identity untouched. Once we established an encrypted communication we can be sure that the data we send and receive cannot be read or tampered with by third parties. But how can we be sure that the entity on the other side of the communication channel, with which we initiated the communication, is what it claims to be?
In other words, the messages cannot be read or modified by malicious third-parties, but what if we established communication with a malicious actor in the first place? Such a situation can arise during a man-in-the-middle attack, where the low-level network communication is hijacked by a malicious actor who pretends to be the desired recipient of the communication.
In the context of the Internet, and in particular of the World Wide Web, the main concern is that the server that provides services we log into (think of every service that has your personal or financial data like you bank, Google, Facebook, Netflix, etc.) is run by the company that we trust and not by an attacker who wants to steal our data.
In this post I will try to clarify the main components of the certificates system and to explain the meaning of the major acronyms and names that you might hear when you deal with this part of web development.
Clarification: SSL vs TLS¶
In the world of web development and infrastructure management, we normally speak of SSL protocol and of SSL certificates, but it has to be noted that SSL (Secure Sockets Layer) is the name of a deprecated protocol. The current implementation of the protocol used to secure web applications is TLS (Transport Layer Security).
The story of SSL and TLS is rich of events and spans 25 years since its inception by Taher Elgamal at Netscape. In short, SSL had 3 major versions (the first of which was never publicly used), and was replaced by TLS in 1999. TLS itself has gone through 3 revisions at the time of writing, TLS 1.3 being the latest version available.
The TLS/SSL nomenclature is one of many sources of confusion in the complicated world of security and applied cryptography. In this article I will use only the acronym TLS, but I went for SSL in the title because I wanted the subject matter to be recognisable also by developers that are not much into security and cryptography.
X.509 certificates¶
While the problem of the identity in an insecure network can be solved in several ways, the solution embraced to secure the World Wide Web is based on a standard called X.509. When we mention TLS certificates, we usually mean X.509 certificates used in a TLS connection, such as that created by HTTPS.
X.509 is the ITU-T standard used to represent certificates, and has been chosen to be the standard used in the TLS protocol. The standard doesn't only define the binary structure of the certificate itself, but it also defines procedures to revoke the certificates, and establishes a hierarchical system of certification known as certificate path, or certificate chain.
The structure of an X.509 certificate is expressed using ASN.1, a notation used natively by the PEM format (discussed here). You can read the full specification in RFC 2459, in particular Section 4 "Certificate and Certificate Extensions Profile". I will refer to this later when I will have a look at a real certificate.
How are certificates related to HTTPS?¶
Before I discuss how certificates solve the problem of identity (or ownership of a public key), let's clarify the relationship between them and HTTPS.
HTTPS stands for HTTP Secure, and the core of the protocol consists of running HTTP over TLS. When we access a web site with HTTPS the browser first establishes a TLS connection with the server and then communicates with it using pure HTTP. This means that the whole HTTP protocol is encrypted, as the secure channel is established outside it, and also means that, aside from the different URI scheme https:// instead of http://, there are no differences between the two protocols.
Certificates come into play when the browser establishes the TLS connection, which is why you need to set-up HTTPS as part of your infrastructure and not in your web application. By the time the HTTP requests reach your application they are already decrypted and accessible in plain text, as the HTTP protocol mandates. We usually say that we "terminate TLS" when a component of our infrastructure manages certificates and decrypts HTTPS into HTTP.
How do certificates work?¶
The X.509 standard establishes entities called Certificate Authorities (CAs), and creates a hierarchy of trust called chain between them. The idea is that there is a set of entities that are trusted worldwide by operating systems, browsers, and other network-related software, and that these entities can trust other entities, thus creating a trust network.
While the market of Certificate Authorities is dominated by three major commercial players (see the usage statistics) there are approximately 100 organisations operating worldwide, among which some non-profit ones. Not all of these are trusted by all operating systems or browsers, though.
The set of CAs trusted by an organisation is called root program. The Mozilla community runs a program that is independent from the hardware/software platform, aptly called Mozilla's CA Certificate Program and uses data contained in the Common CA Database (CCADB). Private companies such as Microsoft, Apple, and Oracle run their own root programs and software running on the respective platforms (Windows, macOS/iOS, Java) can decide to trust the CAs provided by those programs.
In the open-source world, the Mozilla root program is by far the most influential and important source of information, being used by other software packages and Linux distributions.
It is possible to create certificates that are not signed by any CA, and these are called self-signed certificates. Such certificates can be used with any software that relies on certificates, but it requires such a software to disable certificate checking with the Certificate Authorities. Self-signed certificates are obviously useful for testing purposes, but there are scenarios in which it might be desirable not to rely on the CAs and establish a private network of trust.
Example: CA root certificate¶
The certificates for root CAs that are part of the Mozilla root program can be retrieved from the Common CA Database web page, or can be seen in the Firefox source code directly. On a running Firefox browser you can open the Privacy & Security menu and click on "View Certificates" at the bottom of the page. The CAs are listed under the tab "Authorities".
The interesting thing you can do here is to export a CA certificate. If you do it Firefox will save it in a file with extension .crt, that contains data in PEM format. I exported the certificate for Amazon Root CA 1 and I ended up with the file AmazonRootCA1.crt. If, instead of exporting, you view the certificate, you will end up in a page that allows you to download the certificate and the chain, both in PEM format, in files with the extension .pem. As you see, you are not the only one who is confused.
I described the PEM format in a post on RSA keys so I won't repeat here the whole discussion about it. The RFC 7468 ("Textual Encodings of PKIX, PKCS, and CMS Structures") describes certificates in section 5. Section 4 mentions the module id-pkix1-e for Certificate, CertificateList, and SubjectPublicKeyInfo RFC 5280 ("Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile").
The identifier id-pkix1-e is part of a registry of objects to be used in ASN.1 data created in the framework of the Public-Key Infrastructure using X.509 (PKIX) Working Group, that defined the infrastructure around the X.509 certificates system. Basically it's a standard way to identify binary objects and their structure. You can see a full list of all the objects in RFC 7299 ("Object Identifier Registry for the PKIX Working Group"). Not a very exciting one to read, if you ask me.
I can dump the content of the Amazon Root CA 1 certificate with OpenSSL
$ openssl asn1parse -inform pem -in amazon-root-ca-1.pem
0:d=0 hl=4 l= 833 cons: SEQUENCE
4:d=1 hl=4 l= 553 cons: SEQUENCE
8:d=2 hl=2 l= 3 cons: cont [ 0 ]
10:d=3 hl=2 l= 1 prim: INTEGER :02
13:d=2 hl=2 l= 19 prim: INTEGER :066C9FCF99BF8C0A39E2F0788A43E696365BCA
34:d=2 hl=2 l= 13 cons: SEQUENCE
36:d=3 hl=2 l= 9 prim: OBJECT :sha256WithRSAEncryption
47:d=3 hl=2 l= 0 prim: NULL
49:d=2 hl=2 l= 57 cons: SEQUENCE
51:d=3 hl=2 l= 11 cons: SET
53:d=4 hl=2 l= 9 cons: SEQUENCE
55:d=5 hl=2 l= 3 prim: OBJECT :countryName
60:d=5 hl=2 l= 2 prim: PRINTABLESTRING :US
64:d=3 hl=2 l= 15 cons: SET
66:d=4 hl=2 l= 13 cons: SEQUENCE
68:d=5 hl=2 l= 3 prim: OBJECT :organizationName
73:d=5 hl=2 l= 6 prim: PRINTABLESTRING :Amazon
81:d=3 hl=2 l= 25 cons: SET
83:d=4 hl=2 l= 23 cons: SEQUENCE
85:d=5 hl=2 l= 3 prim: OBJECT :commonName
90:d=5 hl=2 l= 16 prim: PRINTABLESTRING :Amazon Root CA 1
108:d=2 hl=2 l= 30 cons: SEQUENCE
110:d=3 hl=2 l= 13 prim: UTCTIME :150526000000Z
125:d=3 hl=2 l= 13 prim: UTCTIME :380117000000Z
140:d=2 hl=2 l= 57 cons: SEQUENCE
142:d=3 hl=2 l= 11 cons: SET
144:d=4 hl=2 l= 9 cons: SEQUENCE
146:d=5 hl=2 l= 3 prim: OBJECT :countryName
151:d=5 hl=2 l= 2 prim: PRINTABLESTRING :US
155:d=3 hl=2 l= 15 cons: SET
157:d=4 hl=2 l= 13 cons: SEQUENCE
159:d=5 hl=2 l= 3 prim: OBJECT :organizationName
164:d=5 hl=2 l= 6 prim: PRINTABLESTRING :Amazon
172:d=3 hl=2 l= 25 cons: SET
174:d=4 hl=2 l= 23 cons: SEQUENCE
176:d=5 hl=2 l= 3 prim: OBJECT :commonName
181:d=5 hl=2 l= 16 prim: PRINTABLESTRING :Amazon Root CA 1
199:d=2 hl=4 l= 290 cons: SEQUENCE
203:d=3 hl=2 l= 13 cons: SEQUENCE
205:d=4 hl=2 l= 9 prim: OBJECT :rsaEncryption
216:d=4 hl=2 l= 0 prim: NULL
218:d=3 hl=4 l= 271 prim: BIT STRING
493:d=2 hl=2 l= 66 cons: cont [ 3 ]
495:d=3 hl=2 l= 64 cons: SEQUENCE
497:d=4 hl=2 l= 15 cons: SEQUENCE
499:d=5 hl=2 l= 3 prim: OBJECT :X509v3 Basic Constraints
504:d=5 hl=2 l= 1 prim: BOOLEAN :255
507:d=5 hl=2 l= 5 prim: OCTET STRING [HEX DUMP]:30030101FF
514:d=4 hl=2 l= 14 cons: SEQUENCE
516:d=5 hl=2 l= 3 prim: OBJECT :X509v3 Key Usage
521:d=5 hl=2 l= 1 prim: BOOLEAN :255
524:d=5 hl=2 l= 4 prim: OCTET STRING [HEX DUMP]:03020186
530:d=4 hl=2 l= 29 cons: SEQUENCE
532:d=5 hl=2 l= 3 prim: OBJECT :X509v3 Subject Key Identifier
537:d=5 hl=2 l= 22 prim: OCTET STRING [HEX DUMP]:04148418CC8534ECBC0C94942E08599CC7B2104E0A08
561:d=1 hl=2 l= 13 cons: SEQUENCE
563:d=2 hl=2 l= 9 prim: OBJECT :sha256WithRSAEncryption
574:d=2 hl=2 l= 0 prim: NULL
576:d=1 hl=4 l= 257 prim: BIT STRING
Let's read part of it using the aforementioned section 4 of RFC 5280.
The signed certificate is a sequence of three main components
Certificate ::= SEQUENCE {
tbsCertificate TBSCertificate,
signatureAlgorithm AlgorithmIdentifier,
signatureValue BIT STRING }
and the TBSCertificate structure represents the unsigned certificate (TBS = To Be Signed)
TBSCertificate ::= SEQUENCE {
version [0] EXPLICIT Version DEFAULT v1,
serialNumber CertificateSerialNumber,
signature AlgorithmIdentifier,
issuer Name,
validity Validity,
subject Name,
subjectPublicKeyInfo SubjectPublicKeyInfo,
issuerUniqueID [1] IMPLICIT UniqueIdentifier OPTIONAL,
-- If present, version MUST be v2 or v3
subjectUniqueID [2] IMPLICIT UniqueIdentifier OPTIONAL,
-- If present, version MUST be v2 or v3
extensions [3] EXPLICIT Extensions OPTIONAL
-- If present, version MUST be v3
}
Comparing this with the output of OpenSSL we can find fields such as version
10:d=3 hl=2 l= 1 prim: INTEGER :02
which according to the documentation is 3 (binary 02). Many values are of type PRINTABLESTRING, so they are readable already in the ASN.1 dump.
The validity of the certificate is
110:d=3 hl=2 l= 13 prim: UTCTIME :150526000000Z
125:d=3 hl=2 l= 13 prim: UTCTIME :380117000000Z
and following section 4.1.2.5.1 of the RFC we find out that the certificate is valid between 26 May 2015 and 17 Jan 2038. You can easily read these values in the certificate page in the browser without getting an headache trying to decode ASN.1.
The CA signed the certificate using a certain algorithm. The algorithm identifier is repeated twice, first in the structure Certificate (signatureAlgorithm AlgorithmIdentifier) and then in the structure TBSCertificate (signature AlgorithmIdentifier). The two fields must have the same value.
34:d=2 hl=2 l= 13 cons: SEQUENCE
36:d=3 hl=2 l= 9 prim: OBJECT :sha256WithRSAEncryption
47:d=3 hl=2 l= 0 prim: NULL
[...]
561:d=1 hl=2 l= 13 cons: SEQUENCE
563:d=2 hl=2 l= 9 prim: OBJECT :sha256WithRSAEncryption
574:d=2 hl=2 l= 0 prim: NULL
For this certificate, the algorithm used by Amazon is sha256WithRSAEncryption. This label is described in RFC 4055 ("Additional Algorithms and Identifiers for RSA Cryptography for use in the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile") as "PKCS #1 version 1.5 signature algorithm with SHA-256". The specific algorithm can be found in RFC 2313 ("PKCS #1: RSA Encryption Version 1.5"). As the name of the algorithm suggests, the certificate is first digested with SHA-256 and then encrypted using RSA and the private key of the signer.
Speaking of keys, the public key the CA used for the certificate can be found in the field subjectpublickeyinfo, which is again made of a field type AlgorithmIdentifier and a bit string with the value of the key. In this case the fields are
205:d=4 hl=2 l= 9 prim: OBJECT :rsaEncryption
216:d=4 hl=2 l= 0 prim: NULL
218:d=3 hl=4 l= 271 prim: BIT STRING
The algorithm rsaEncryption is described in RFC 3279 ("Algorithms and Identifiers for the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile"), section 2.3.1 as
RSAPublicKey ::= SEQUENCE {
modulus INTEGER, -- n
publicExponent INTEGER } -- e
(sic) or in RFC 8017 ("PKCS #1: RSA Cryptography Specifications Version 2.2")
RSAPublicKey ::= SEQUENCE {
modulus INTEGER, -- n
publicExponent INTEGER -- e
}
We can then use the option -strparse of the module asn1parse to find the actual values
$ openssl asn1parse -inform pem -in amazon-root-ca-1.pem -strparse 218
0:d=0 hl=4 l= 266 cons: SEQUENCE
4:d=1 hl=4 l= 257 prim: INTEGER :B2788071CA78D5E371AF478050747D6ED8D78876F4
9968F7582160F97484012FAC022D86D3A0437A4EB2A4D036BA01BE8DDB48C80717364CF4EE8823C73EEB37F5B5
19F84968B0DED7B976381D619EA4FE8236A5E54A56E445E1F9FDB416FA74DA9C9B35392FFAB02050066C7AD080
B2A6F9AFEC47198F503807DCA2873958F8BAD5A9F948673096EE94785E6F89A351C0308666A14566BA54EBA3C3
91F948DCFFD1E8302D7D2D747035D78824F79EC4596EBB738717F2324628B843FAB71DAACAB4F29F240E2D4BF7
715C5E69FFEA9502CB388AAE50386FDBFB2D621BC5C71E54E177E067C80F9C8723D63F40207F2080C4804C3E3B
24268E04AE6C9AC8AA0D
265:d=1 hl=2 l= 3 prim: INTEGER :010001
As we already saw for RSA keys), OpenSSL has a specific module for important structures, and the X.509 certificates are definitely worth a module aptly called x509. using that we can easily decode any certificate
$ openssl x509 -inform pem -in amazon-root-ca-1.pem -noout -text
Certificate:
Data:
Version: 3 (0x2)
Serial Number:
06:6c:9f:cf:99:bf:8c:0a:39:e2:f0:78:8a:43:e6:96:36:5b:ca
Signature Algorithm: sha256WithRSAEncryption
Issuer: C = US, O = Amazon, CN = Amazon Root CA 1
Validity
Not Before: May 26 00:00:00 2015 GMT
Not After : Jan 17 00:00:00 2038 GMT
Subject: C = US, O = Amazon, CN = Amazon Root CA 1
Subject Public Key Info:
Public Key Algorithm: rsaEncryption
RSA Public-Key: (2048 bit)
Modulus:
00:b2:78:80:71:ca:78:d5:e3:71:af:47:80:50:74:
7d:6e:d8:d7:88:76:f4:99:68:f7:58:21:60:f9:74:
84:01:2f:ac:02:2d:86:d3:a0:43:7a:4e:b2:a4:d0:
36:ba:01:be:8d:db:48:c8:07:17:36:4c:f4:ee:88:
23:c7:3e:eb:37:f5:b5:19:f8:49:68:b0:de:d7:b9:
76:38:1d:61:9e:a4:fe:82:36:a5:e5:4a:56:e4:45:
e1:f9:fd:b4:16:fa:74:da:9c:9b:35:39:2f:fa:b0:
20:50:06:6c:7a:d0:80:b2:a6:f9:af:ec:47:19:8f:
50:38:07:dc:a2:87:39:58:f8:ba:d5:a9:f9:48:67:
30:96:ee:94:78:5e:6f:89:a3:51:c0:30:86:66:a1:
45:66:ba:54:eb:a3:c3:91:f9:48:dc:ff:d1:e8:30:
2d:7d:2d:74:70:35:d7:88:24:f7:9e:c4:59:6e:bb:
73:87:17:f2:32:46:28:b8:43:fa:b7:1d:aa:ca:b4:
f2:9f:24:0e:2d:4b:f7:71:5c:5e:69:ff:ea:95:02:
cb:38:8a:ae:50:38:6f:db:fb:2d:62:1b:c5:c7:1e:
54:e1:77:e0:67:c8:0f:9c:87:23:d6:3f:40:20:7f:
20:80:c4:80:4c:3e:3b:24:26:8e:04:ae:6c:9a:c8:
aa:0d
Exponent: 65537 (0x10001)
X509v3 extensions:
X509v3 Basic Constraints: critical
CA:TRUE
X509v3 Key Usage: critical
Digital Signature, Certificate Sign, CRL Sign
X509v3 Subject Key Identifier:
84:18:CC:85:34:EC:BC:0C:94:94:2E:08:59:9C:C7:B2:10:4E:0A:08
Signature Algorithm: sha256WithRSAEncryption
98:f2:37:5a:41:90:a1:1a:c5:76:51:28:20:36:23:0e:ae:e6:
28:bb:aa:f8:94:ae:48:a4:30:7f:1b:fc:24:8d:4b:b4:c8:a1:
97:f6:b6:f1:7a:70:c8:53:93:cc:08:28:e3:98:25:cf:23:a4:
f9:de:21:d3:7c:85:09:ad:4e:9a:75:3a:c2:0b:6a:89:78:76:
44:47:18:65:6c:8d:41:8e:3b:7f:9a:cb:f4:b5:a7:50:d7:05:
2c:37:e8:03:4b:ad:e9:61:a0:02:6e:f5:f2:f0:c5:b2:ed:5b:
b7:dc:fa:94:5c:77:9e:13:a5:7f:52:ad:95:f2:f8:93:3b:de:
8b:5c:5b:ca:5a:52:5b:60:af:14:f7:4b:ef:a3:fb:9f:40:95:
6d:31:54:fc:42:d3:c7:46:1f:23:ad:d9:0f:48:70:9a:d9:75:
78:71:d1:72:43:34:75:6e:57:59:c2:02:5c:26:60:29:cf:23:
19:16:8e:88:43:a5:d4:e4:cb:08:fb:23:11:43:e8:43:29:72:
62:a1:a9:5d:5e:08:d4:90:ae:b8:d8:ce:14:c2:d0:55:f2:86:
f6:c4:93:43:77:66:61:c0:b9:e8:41:d7:97:78:60:03:6e:4a:
72:ae:a5:d1:7d:ba:10:9e:86:6c:1b:8a:b9:59:33:f8:eb:c4:
90:be:f1:b9
Now I'm pretty sure you want to kill me because I could have shown you this from the start. But I like to understand things, and the easy path doesn't always make everything clear. At any rate, here you have a way to read an X.509 certificate in PEM format.
Please note that in this certificate the Issuer and the Subject are the same entity, as this is a root certificate, which is signed by the same entity that creates it.
Issuer: C = US, O = Amazon, CN = Amazon Root CA 1
[...]
Subject: C = US, O = Amazon, CN = Amazon Root CA 1
Moreover, one of the version 3 extensions of the self-signed certificate is a basic constraint with the boolean CA set to true. It also has the extension Key Usage set to Digital Signature, Certificate Sign, CRL Sign, which means that the certificate can be used to sign other certificates.
Example: self-signed certificate¶
You can use OpenSSL to create a self-signed certificate using the module req that you would normally use to create certificate requests. As a self-signed certificate doesn't need approval, the module can directly output the certificate.
$ openssl req -x509 -newkey rsa:2048 -keyout self-signed-key.pem -out self-signed.pem -days 365 -nodes -subj '/CN=localhost'
Generating a RSA private key
....+++++
................+++++
writing new private key to 'self-signed-key.pem'
-----
(note that for simplicity's sake I specified the option -nodes that prevents the key to be protected with a password, but this is a bad practice). This command creates the two files I mentioned, self-signed-key.pem (the private key) and self-signed.pem.
We can read the certificate using the module x509
$ openssl x509 -inform pem -in self-signed.pem -noout -text
Certificate:
Data:
Version: 3 (0x2)
Serial Number:
46:e5:2f:8e:42:82:43:b8:ac:88:cb:6d:0c:2f:71:28:a9:fe:00:ec
Signature Algorithm: sha256WithRSAEncryption
Issuer: CN = localhost
Validity
Not Before: Nov 3 00:23:34 2020 GMT
Not After : Nov 3 00:23:34 2021 GMT
Subject: CN = localhost
Subject Public Key Info:
Public Key Algorithm: rsaEncryption
RSA Public-Key: (2048 bit)
Modulus:
00:b7:14:ef:3b:eb:8b:a9:40:18:c5:d2:eb:1d:4f:
5d:e4:a3:17:f3:df:ce:b7:d3:3f:52:58:eb:61:02:
a2:68:0a:cd:0f:97:ae:e0:a5:ac:a7:88:cf:a1:15:
0a:97:ca:e7:03:8a:a5:c0:66:38:ef:bb:59:4d:48:
17:db:a7:bd:fa:4b:50:2a:be:e9:5b:bb:59:65:71:
dc:99:73:9c:bc:4d:3b:42:97:91:e9:3b:1a:8a:9d:
cc:41:38:ba:8b:8f:df:65:ff:5b:1f:ef:8a:b7:c5:
93:07:ce:15:4c:13:72:78:59:64:9a:5b:95:20:b6:
b3:8e:aa:c3:29:c3:7f:28:39:43:81:59:e4:0f:26:
7c:3f:49:d2:06:05:d9:54:ab:09:65:96:01:cc:c2:
72:be:85:1f:40:ea:94:35:04:09:9d:87:eb:a1:90:
36:ce:d2:55:f9:ee:08:db:52:78:e8:70:d0:25:89:
13:8e:0f:9d:98:98:d1:4d:67:06:8f:8a:61:9e:3a:
73:89:aa:0a:0a:1b:05:a7:52:32:ef:1b:78:5a:5f:
4b:b6:c9:a7:4e:15:10:04:50:99:00:09:2f:60:8e:
aa:20:af:6b:ee:f5:60:0b:29:da:38:1c:b2:73:14:
99:a4:ee:5e:89:e6:77:0b:ba:cf:d3:5d:d7:a3:ea:
c4:bf
Exponent: 65537 (0x10001)
X509v3 extensions:
X509v3 Subject Key Identifier:
64:7B:C1:FC:99:74:56:B7:82:D1:4F:E7:2D:94:77:1A:09:52:26:5C
X509v3 Authority Key Identifier:
keyid:64:7B:C1:FC:99:74:56:B7:82:D1:4F:E7:2D:94:77:1A:09:52:26:5C
X509v3 Basic Constraints: critical
CA:TRUE
Signature Algorithm: sha256WithRSAEncryption
43:7b:0b:c8:98:b8:6f:72:af:39:4a:d9:76:ce:e3:9d:3a:c7:
9f:14:b0:4f:20:0a:45:b3:b4:8c:e5:37:4c:bf:15:ad:8e:5c:
45:4f:3e:b7:ef:8d:60:57:bb:6f:d9:5e:6a:d3:04:05:4a:ff:
f2:66:b1:76:66:59:7e:24:89:0a:50:28:c9:d5:f5:7a:00:07:
8a:79:9c:6e:53:43:66:e5:9a:10:d8:f8:e1:f2:c1:f1:17:d0:
d2:9e:50:80:fe:2a:ca:08:b6:98:e9:b5:a4:82:23:31:45:35:
33:da:2c:e3:fe:54:f2:bd:f2:61:91:f4:32:e3:7d:4c:3a:e5:
3a:0f:cd:36:b0:8b:af:9f:8e:3d:0e:0b:a5:df:4a:3a:91:83:
b3:b2:5f:3c:47:81:73:4f:a2:c1:49:06:75:17:25:fa:5a:8d:
30:e5:55:7f:9c:3e:15:a8:b5:ab:f7:45:38:e3:76:8e:d4:0d:
60:fc:42:17:3d:85:72:41:1d:53:9d:58:b0:e9:29:0c:e4:6b:
14:c2:22:c4:d5:7b:de:36:da:df:d8:a0:4f:a4:0a:f2:3e:ca:
7e:66:a6:10:38:97:24:73:5b:db:eb:0b:6c:a8:f8:37:15:2c:
0e:b1:82:44:cc:fe:85:b0:cb:6c:26:4b:4a:70:33:dc:7e:f5:
84:ba:07:db
As you can see this certificate has the same value in Issuer and Subject, as happened before for the Amazon Root one. It also has the flag CA set to true but it doesn't have the extension Key Usage meaning that this certificate can't be used to sign other certificates.
Example: this site's certificate¶
You can see TLS certificates and the chain of trust in action in this very website. Following the documentation of you browser (instructions for Firefox are here), you can see the certificate used by The Digital Cat. At the time of writing the blog is hosted on GitHub Pages, even tough I'm using a custom domain, and GitHub partnered with Let's Encrypt to provide certificates for such a configuration (details here).
Indeed, the certificate for thedigitalcatonline.com is provided by "Let's Encrypt Authority X3", which in turn is trusted by Digital Signature Trust Co. with its root CA "DST Root CA X3".
Let's have a look at the three certificates. The one for The Digital Cat is
Certificate:
Data:
Version: 3 (0x2)
Serial Number:
03:93:02:bb:9a:c9:ed:a5:c3:d1:16:00:8b:15:76:af:e5:d9
Signature Algorithm: sha256WithRSAEncryption
Issuer: C = US, O = Let's Encrypt, CN = Let's Encrypt Authority X3
Validity
Not Before: Oct 22 04:53:28 2020 GMT
Not After : Jan 20 04:53:28 2021 GMT
Subject: CN = www.thedigitalcatonline.com
[...]
X509v3 extensions:
X509v3 Key Usage: critical
Digital Signature, Key Encipherment
X509v3 Extended Key Usage:
TLS Web Server Authentication, TLS Web Client Authentication
X509v3 Basic Constraints: critical
CA:FALSE
X509v3 Subject Key Identifier:
63:4E:15:85:56:5A:A4:94:02:C2:16:42:A4:A5:97:9A:38:02:57:97
X509v3 Authority Key Identifier:
keyid:A8:4A:6A:63:04:7D:DD:BA:E6:D1:39:B7:A6:45:65:EF:F3:A8:EC:A1
Authority Information Access:
OCSP - URI:http://ocsp.int-x3.letsencrypt.org
CA Issuers - URI:http://cert.int-x3.letsencrypt.org/
X509v3 Subject Alternative Name:
DNS:www.thedigitalcatonline.com
[...]
And you can see that this time the Subject is www.thedigitalcatonline.com, but the Issuer is Let's Encrypt Authority X3. The certificate provided by the organisation Let's Encrypt is
Certificate:
Data:
Version: 3 (0x2)
Serial Number:
0a:01:41:42:00:00:01:53:85:73:6a:0b:85:ec:a7:08
Signature Algorithm: sha256WithRSAEncryption
Issuer: O = Digital Signature Trust Co., CN = DST Root CA X3
Validity
Not Before: Mar 17 16:40:46 2016 GMT
Not After : Mar 17 16:40:46 2021 GMT
Subject: C = US, O = Let's Encrypt, CN = Let's Encrypt Authority X3
[...]
X509v3 extensions:
X509v3 Basic Constraints: critical
CA:TRUE, pathlen:0
X509v3 Key Usage: critical
Digital Signature, Certificate Sign, CRL Sign
Authority Information Access:
OCSP - URI:http://isrg.trustid.ocsp.identrust.com
CA Issuers - URI:http://apps.identrust.com/roots/dstrootcax3.p7c
X509v3 Authority Key Identifier:
keyid:C4:A7:B1:A4:7B:2C:71:FA:DB:E1:4B:90:75:FF:C4:15:60:85:89:10
X509v3 Certificate Policies:
Policy: 2.23.140.1.2.1
Policy: 1.3.6.1.4.1.44947.1.1.1
CPS: http://cps.root-x1.letsencrypt.org
X509v3 CRL Distribution Points:
Full Name:
URI:http://crl.identrust.com/DSTROOTCAX3CRL.crl
X509v3 Subject Key Identifier:
A8:4A:6A:63:04:7D:DD:BA:E6:D1:39:B7:A6:45:65:EF:F3:A8:EC:A1
[...]
Here, the Subject is Let's Encrypt Authority X3 (the Issuer of the previous certificate), and the Issuer is DST Root CA X3. Last, the certificate provided by the organisation Digital Signature Trust Co. is
Certificate:
Data:
Version: 3 (0x2)
Serial Number:
44:af:b0:80:d6:a3:27:ba:89:30:39:86:2e:f8:40:6b
Signature Algorithm: sha1WithRSAEncryption
Issuer: O = Digital Signature Trust Co., CN = DST Root CA X3
Validity
Not Before: Sep 30 21:12:19 2000 GMT
Not After : Sep 30 14:01:15 2021 GMT
Subject: O = Digital Signature Trust Co., CN = DST Root CA X3
[...]
X509v3 extensions:
X509v3 Basic Constraints: critical
CA:TRUE
X509v3 Key Usage: critical
Certificate Sign, CRL Sign
X509v3 Subject Key Identifier:
C4:A7:B1:A4:7B:2C:71:FA:DB:E1:4B:90:75:FF:C4:15:60:85:89:10
[...]
As happened for the certificate Amazon Root CA 1 that we discussed before, this one is self-signed, having the same value for Subject and Issuer.
How to verify certificates with OpenSSL¶
To verify if a certificate is valid we can use the module verify of OpenSSL. By default, OpenSSL doesn't trust anything, and verify relies on a default path in the system to find root certificates. You can see the path running
$ openssl version -d
OPENSSLDIR: "/usr/lib/ssl"
On Ubuntu 20.04, the directory /usr/lib/ssl/certs is a symbolic link to /etc/ssl/certs that is installed by the package ca-certificates which is linked to the Mozilla's CA Certificate Program (details on that package can be found in the source code).
So, if a root certificate is included in the Mozilla program, it is trusted by OpenSSL
$ openssl verify amazon-root-ca-1.pem
amazon-root-ca-1.pem: OK
while a self-signed certificate is not
$ openssl verify self-signed.pem
CN = localhost
error 18 at 0 depth lookup: self signed certificate
error self-signed.pem: verification failed
A non-root certificate can be verified specifying which root certificate signed it. So, the certificate for this website is not trusted automatically
$ openssl verify www-thedigitalcatonline-com.pem
CN = www.thedigitalcatonline.com
error 20 at 0 depth lookup: unable to get local issuer certificate
error www-thedigitalcatonline-com.pem: verification failed
But it is verified specifying the certificate for Let's Encrypt that signed it
$ openssl verify -CAfile lets-encrypt-x3.pem www-thedigitalcatonline-com.pem
www-thedigitalcatonline-com.pem: OK
because the certificate lets-encrypt-x3.pem is signed by DST_Root_CA_X3.pem which is included in the Mozilla program, and thus included in my Linux distribution.
If I remove the default certificates path OpenSSL doesn't accept the certificate for Let's Encrypt any more
$ openssl verify -no-CApath -CAfile lets-encrypt-x3.pem www-thedigitalcatonline-com.pem
C = US, O = Let's Encrypt, CN = Let's Encrypt Authority X3
error 2 at 1 depth lookup: unable to get issuer certificate
error www-thedigitalcatonline-com.pem: verification failed
Low-level certificate validation process¶
Let's have a look at the signature process for x.509 certificates. The process depends on the specific algorithm used to sign the certificate, so I will use the certificate Amazon Root CA 1 as an example, leaving to the reader the investigation about other algorithms.
A signed certificate is made of two parts, the certificate itself and the signature. The signature contains an encrypted hash of the certificate, so the verification is done in three steps:
- Decrypt the encrypted hash using the public key
- Compute the hash of the certificate using the same algorithm
- Compare the hashes
For the Amazon root certificate, we know the signature algorithm and value from the output of openssl x509
$ openssl x509 -inform pem -in amazon-root-ca-1.pem -noout -text
[...]
Signature Algorithm: sha256WithRSAEncryption
98:f2:37:5a:41:90:a1:1a:c5:76:51:28:20:36:23:0e:ae:e6:
28:bb:aa:f8:94:ae:48:a4:30:7f:1b:fc:24:8d:4b:b4:c8:a1:
97:f6:b6:f1:7a:70:c8:53:93:cc:08:28:e3:98:25:cf:23:a4:
f9:de:21:d3:7c:85:09:ad:4e:9a:75:3a:c2:0b:6a:89:78:76:
44:47:18:65:6c:8d:41:8e:3b:7f:9a:cb:f4:b5:a7:50:d7:05:
2c:37:e8:03:4b:ad:e9:61:a0:02:6e:f5:f2:f0:c5:b2:ed:5b:
b7:dc:fa:94:5c:77:9e:13:a5:7f:52:ad:95:f2:f8:93:3b:de:
8b:5c:5b:ca:5a:52:5b:60:af:14:f7:4b:ef:a3:fb:9f:40:95:
6d:31:54:fc:42:d3:c7:46:1f:23:ad:d9:0f:48:70:9a:d9:75:
78:71:d1:72:43:34:75:6e:57:59:c2:02:5c:26:60:29:cf:23:
19:16:8e:88:43:a5:d4:e4:cb:08:fb:23:11:43:e8:43:29:72:
62:a1:a9:5d:5e:08:d4:90:ae:b8:d8:ce:14:c2:d0:55:f2:86:
f6:c4:93:43:77:66:61:c0:b9:e8:41:d7:97:78:60:03:6e:4a:
72:ae:a5:d1:7d:ba:10:9e:86:6c:1b:8a:b9:59:33:f8:eb:c4:
90:be:f1:b9
You can see the signed certificate binary values with cat amazon-root-ca-1.pem | tail -n+2 | head -n-1 | base64 -di | hexdump -ve '/1 "%02x "' -e '2/8 "\n"'. While we can recognise the signature in the last 256 bytes we can't easily separate the bytes with the signature algorithm. If we open the signed certificate with an ASN.1 parser, instead, we can easily find the binary value of the certificate part
30 82 03 41 30 82 02 29 a0 03 02 01 02 02 13 06
6c 9f cf 99 bf 8c 0a 39 e2 f0 78 8a 43 e6 96 36
5b ca 30 0d 06 09 2a 86 48 86 f7 0d 01 01 0b 05
00 30 39 31 0b 30 09 06 03 55 04 06 13 02 55 53
31 0f 30 0d 06 03 55 04 0a 13 06 41 6d 61 7a 6f
6e 31 19 30 17 06 03 55 04 03 13 10 41 6d 61 7a
6f 6e 20 52 6f 6f 74 20 43 41 20 31 30 1e 17 0d
31 35 30 35 32 36 30 30 30 30 30 30 5a 17 0d 33
38 30 31 31 37 30 30 30 30 30 30 5a 30 39 31 0b
30 09 06 03 55 04 06 13 02 55 53 31 0f 30 0d 06
03 55 04 0a 13 06 41 6d 61 7a 6f 6e 31 19 30 17
06 03 55 04 03 13 10 41 6d 61 7a 6f 6e 20 52 6f
6f 74 20 43 41 20 31 30 82 01 22 30 0d 06 09 2a
86 48 86 f7 0d 01 01 01 05 00 03 82 01 0f 00 30
82 01 0a 02 82 01 01 00 b2 78 80 71 ca 78 d5 e3
71 af 47 80 50 74 7d 6e d8 d7 88 76 f4 99 68 f7
58 21 60 f9 74 84 01 2f ac 02 2d 86 d3 a0 43 7a
4e b2 a4 d0 36 ba 01 be 8d db 48 c8 07 17 36 4c
f4 ee 88 23 c7 3e eb 37 f5 b5 19 f8 49 68 b0 de
d7 b9 76 38 1d 61 9e a4 fe 82 36 a5 e5 4a 56 e4
45 e1 f9 fd b4 16 fa 74 da 9c 9b 35 39 2f fa b0
20 50 06 6c 7a d0 80 b2 a6 f9 af ec 47 19 8f 50
38 07 dc a2 87 39 58 f8 ba d5 a9 f9 48 67 30 96
ee 94 78 5e 6f 89 a3 51 c0 30 86 66 a1 45 66 ba
54 eb a3 c3 91 f9 48 dc ff d1 e8 30 2d 7d 2d 74
70 35 d7 88 24 f7 9e c4 59 6e bb 73 87 17 f2 32
46 28 b8 43 fa b7 1d aa ca b4 f2 9f 24 0e 2d 4b
f7 71 5c 5e 69 ff ea 95 02 cb 38 8a ae 50 38 6f
db fb 2d 62 1b c5 c7 1e 54 e1 77 e0 67 c8 0f 9c
87 23 d6 3f 40 20 7f 20 80 c4 80 4c 3e 3b 24 26
8e 04 ae 6c 9a c8 aa 0d 02 03 01 00 01 a3 42 30
40 30 0f 06 03 55 1d 13 01 01 ff 04 05 30 03 01
01 ff 30 0e 06 03 55 1d 0f 01 01 ff 04 04 03 02
01 86 30 1d 06 03 55 1d 0e 04 16 04 14 84 18 cc
85 34 ec bc 0c 94 94 2e 08 59 9c c7 b2 10 4e 0a
08
The signature algorithm part is
30 0d 06 09 2a 86 48 86 f7 0d 01 01 0b 05 00
03 82 01 01 00
and the ASN.1 parser tells us that those bytes represent an OBJECT IDENTIFIER which value is 2.16.840.1.101.3.4.2.1. Now, object identifiers are not complicated per se, they are just a way to identify algorithms and other well known components in ASN.1 structures. The description of the field signatureAlgorithm of an x.509 certificate mentions three other RFCs that contains descriptions of the available algorithms. In particular, RFC 4055 contains the description of PKCS #1 one-way hash functions, one of which is
id-sha256 OBJECT IDENTIFIER ::= { joint-iso-itu-t(2)
country(16) us(840) organization(1) gov(101)
csor(3) nistalgorithm(4) hashalgs(2) 1 }
You can see the values in the object identifier between parentheses. Since these are PKCS #1 (a.k.a. RSA) has functions, OpenSSL identifies it as sha256WithRSAEncryption (see again RFC 4055).
RSA encryption is described in RFC 2313 ("PKCS #1: RSA Encryption Version 1.5") and the signature algorithm based on RSA is described there in section 10. In particular, section 10.2 details the verification process, which is the one we are interested in. The steps are
- Bit-string-to-octet-string conversion of the signature
- RSA decryption
- Digest decoding (ASN.1)
- Message digesting and comparison
As for the signature conversion, the sentence
Specifically, assuming that the length in bits of the
signature S is a multiple of eight, the first bit of the signature
shall become the most significant bit of the first octet of the
encrypted data, and so on through the last bit of the signature,
which shall become the least significant bit of the last octet of the
encrypted data.
is a very verbose way to say that the signature is big-endian.
So, the hexadecimal value of the signature is
98f2375a4190a11ac57651282036230eaee628bbaaf894ae48a4307f1bfc248d
4bb4c8a197f6b6f17a70c85393cc0828e39825cf23a4f9de21d37c8509ad4e9a
753ac20b6a897876444718656c8d418e3b7f9acbf4b5a750d7052c37e8034bad
e961a0026ef5f2f0c5b2ed5bb7dcfa945c779e13a57f52ad95f2f8933bde8b5c
5bca5a525b60af14f74befa3fb9f40956d3154fc42d3c7461f23add90f48709a
d9757871d1724334756e5759c2025c266029cf2319168e8843a5d4e4cb08fb23
1143e843297262a1a95d5e08d490aeb8d8ce14c2d055f286f6c49343776661c0
b9e841d7977860036e4a72aea5d17dba109e866c1b8ab95933f8ebc490bef1b9
And reading the field Subject Public Key Info of the certificate we find the public key. Remember that this is a root certificate, so it is signed using the same key that it contains, which is not true in general.
The public key's modulus is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 the exponent is 0x10001 (default choice).
RSA public-key signature decryption is performed with signature ^ exponent mod modulus, and this operation returns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 the padding is removed, we are left with an ASN.1 binary structure that represents the digest
DigestInfo ::= SEQUENCE {
digestAlgorithm DigestAlgorithmIdentifier,
digest Digest }
(see RFC 2313 - Section 10.1.2)
The value of Digest can be extracted with an ASN.1 parser or by taking the last 256 bits and is 6fc4b8ac3d2b52c08baf56255e43d22c762962e4facab01ace16d48ec008be0a.
At this point we need to process the certificate bytes (without signature) with the SHA-256 hash function and we will find a matching value of 6fc4b8ac3d2b52c08baf56255e43d22c762962e4facab01ace16d48ec008be0a.
This process (for the specific case of this certificate) can be easily done in Python
from cryptography import x509
from hashlib import sha256
certificate_pem_file = "amazon-root-ca-1.pem"
with open(certificate_pem_file, "rb") as f:
certificate_pem = f.read()
certificate = x509.load_pem_x509_certificate(certificate_pem)
modulus = certificate.public_key().public_numbers().n
exponent = certificate.public_key().public_numbers().e
signature = int.from_bytes(certificate.signature, "big")
verification = pow(signature, exponent, modulus)
digest = bytes().fromhex(str(hex(verification))[-64:])
calculated_digest = sha256(certificate.tbs_certificate_bytes)
print(digest.hex() == calculated_digest.hexdigest())
This is arguably not the best Python code ever, but it's a simple way to demonstrate the process. As I said, this is far from being general, as it assumes the signature is sha256WithRSAEncryption, which might not be the case.
What I showed you here is what happens when we validate a root certificate. When we validate a non-root certificate the process is exactly the same (taking into account that the algorithms involved might be different), only the public key used to sign the certificate doesn't come from the certificate itself, but from the signer one. So, in the case of this blog, the certificate for www.thedigitalcat.com has a signature encrypted with the public key of Let's Encrypt. And the certificate for Let's Encrypt will be signed using the public key of Digital Signature Trust Co. This is what creates the chain of trust.
Algorithms used by root certificates¶
A quick scan of the certificates that are part of the Mozilla program reveals that the vast majority of them is using RSA to self-sign them
$ for i in /etc/ssl/certs/*.pem; do openssl x509 -inform pem -in ${i} -noout -text | grep -E "Public Key Algorithm"; done | sort | uniq -c
25 Public Key Algorithm: id-ecPublicKey
114 Public Key Algorithm: rsaEncryption
while part of them are using id-ecPublicKey which is the identifier of elliptic curves algorithms.
When it comes to signature algorithms, instead, there is more variety
$ for i in /etc/ssl/certs/*.pem; do openssl x509 -inform pem -in ${i} -noout -text | grep -E "^ Signature Algorithm"; done | sort | uniq -c
7 Signature Algorithm: ecdsa-with-SHA256
18 Signature Algorithm: ecdsa-with-SHA384
47 Signature Algorithm: sha1WithRSAEncryption
57 Signature Algorithm: sha256WithRSAEncryption
9 Signature Algorithm: sha384WithRSAEncryption
1 Signature Algorithm: sha512WithRSAEncryption
Even here, elliptic curves are slowly being adopted.
AWS components related to certificates¶
If you are using AWS, you can create certificates with ACM, the AWS Certificate Manager. Such certificates cannot be downloaded, they can only be attached to other AWS components. For this reason, the generation process requires you to create any request, as you might have to do with other authorities. Certificates created in the ACM are free.
Certificates created in the ACM can be attached to several AWS components, most notably Load Balancers, CloudFront, and API Gateway.
Traditionally, load balancers are the place where TLS is terminated for HTTPS, requiring a connection to port 443. While Application Load Balancers can do that, in 2019 AWS announced support for certificates in Network Load Balancers as well.
Let's encrypt¶
In an effort to push for HTTP encryption of any public server, the Internet Security Research Group founded in 2016 a non-profit CA named Let's Encrypt, which provides at no charge TLS certificates valid for 90 days. Such certificates can be renewed automatically as part of the setup (certbot) and represent a viable alternative to certificates issued by other CA, in particular for open source projects. This blog uses a certificate issued by Let's Encrypt (provided by GitHub Pages) and will thus expire in less than 3 months (but also automatically renewed).
Final words¶
I hope this post helped to clarify some of the most obscure points of certificates, that definitely bugged be when I first approached them. As always when standards are involved, the risk is to get lost in the myriad of documents where information is scattered, and not to realise that some (if not many) parts of the systems we run every day have a long history and thus a big burden of legacy code or nomenclature.
Resources¶
- The Wikipedia article on TLS
- The Wikipedia article on Certificate authority
- The Wikipedia article on X.509
- The Wikipedia article on Let's Encrypt
- OpenSSL documentation: asn1parse, x509, verify
- The Abstract Syntax Notation One ASN.1 interface description language
- RFC 2313 - "PKCS #1: RSA Encryption Version 1.5"
- RFC 2459 - "Internet X.509 Public Key Infrastructure Certificate and CRL Profile"
- RFC 3279 - "Algorithms and Identifiers for the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile"
- RFC 4055 - "Additional Algorithms and Identifiers for RSA Cryptography for use in the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile"
- RFC 5280 - "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile"
- RFC 7299 - "Object Identifier Registry for the PKIX Working Group"
- RFC 7468 - "Textual Encodings of PKIX, PKCS, and CMS Structures"
- RFC 8017 - "PKCS #1: RSA Cryptography Specifications Version 2.2"
- RFC 8446 - "The Transport Layer Security (TLS) Protocol Version 1.3"
- pyca/cryptography - The Python pyca/cryptography package
Feedback¶
Feel free to reach me on Twitter if you have questions. The GitHub issues page is the best place to submit corrections.
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