I am new to learning about certificates & their use in cybersecurity. I was experimenting around with my browser's certificate issued by GTS which is google trust services.
Now, I am confused about what the signature algorithm field means. I tried to google search this and found that the signature algorithm refers to the algorithm used to sign the certificate. If that is the case, I don't understand why I see 3 different signature algorithm fields in my cert. Also, 2 of them have a key size associated with them while the first field does now.
The first signature algorithm is under the category of "Issuer" so I thought maybe this is the algorithm being used to sign the cert. The second & third fields, shown in the second image, are under the category of public key. So what are they being used to sign?
Also, I don't see any key associated with the first signature algorithm, so I am a bit confused with this. Any help is much appreciated! Thank you!
What exactly is meant by "Signature Algorithm" on a certificate? Which signature algorithm is being used to sign my certificate?
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1 Answers
Meta: this is not a programming issue, but I can't fit this in a comment. I am not voting to close because it is inappropriate to do so after answering, but if I am notified the question is closed I will delete (or I authorize a mod to do so) to ensure Q can be deleted or roombad.
I don't know what program you are using to get that decode, or if you have modified it beyond the black-outs, but it appears to be seriously misleading. Here is a better decode from OpenSSL, which follows the ASN.1 structure, with <<# marks added by me:
(redacted)>openssl s_client -connect www.google.com:443 <NUL 2>NUL | openssl x509 -noout -text
Certificate:
Data:
Version: 3 (0x2)
Serial Number:
45:48:e6:58:30:39:c0:ad:0a:00:00:00:00:ff:65:fa
Signature Algorithm: sha256WithRSAEncryption <<#1A
Issuer: C = US, O = Google Trust Services LLC, CN = GTS CA 1C3
Validity
Not Before: Sep 13 04:06:57 2021 GMT
Not After : Nov 20 04:06:56 2021 GMT
Subject: CN = www.google.com
Subject Public Key Info:
Public Key Algorithm: rsaEncryption
Public-Key: (2048 bit)
Modulus:
00:d7:27:92:c3:bb:e0:95:f4:20:46:a4:1a:5f:96:
78:a7:58:9d:cb:7c:2a:9c:7c:cb:2d:be:30:e9:c1:
71:80:11:da:c3:57:c4:c1:74:5c:a6:26:64:c3:49:
53:7c:44:19:f2:b3:c4:b3:5f:fc:90:30:b3:d4:31:
d1:16:09:b2:97:44:43:99:d6:13:19:20:ef:92:9e:
6e:41:44:56:32:c8:1c:5b:54:48:38:6b:5d:c5:00:
a4:62:be:7e:51:76:26:f6:5b:9c:e0:ed:b3:b8:dd:
16:eb:c6:9d:fc:b6:16:c0:60:1a:84:d8:b1:a5:d1:
5d:1f:35:eb:40:08:f0:2b:a1:a8:e8:d0:93:8f:85:
c6:25:a3:63:d0:d8:09:2e:fa:d2:6f:12:73:4e:aa:
ad:6f:c6:cb:b0:24:b4:65:e3:e3:fd:03:f9:d4:64:
07:2a:4b:6b:df:6b:ae:b2:90:eb:7e:57:f0:a8:3e:
08:d1:07:06:e8:04:dc:a6:bd:02:ee:07:97:1f:cf:
41:2c:8a:b0:15:bc:de:c9:13:b9:0a:8f:38:78:4c:
03:d1:46:36:e6:54:e4:3b:5f:eb:f4:02:14:82:09:
d9:0e:60:ea:29:b4:e3:7e:81:8d:4c:81:ee:4b:6d:
6e:a8:7f:f5:79:39:21:20:01:eb:77:4d:ea:22:d8:
15:13
Exponent: 65537 (0x10001)
X509v3 extensions:
X509v3 Key Usage: critical
Digital Signature, Key Encipherment
X509v3 Extended Key Usage:
TLS Web Server Authentication
X509v3 Basic Constraints: critical
CA:FALSE
X509v3 Subject Key Identifier:
C0:43:06:E9:20:B5:1E:51:86:CF:27:BB:3B:91:D5:0B:AE:F8:99:A6
X509v3 Authority Key Identifier:
keyid:8A:74:7F:AF:85:CD:EE:95:CD:3D:9C:D0:E2:46:14:F3:71:35:1D:27
Authority Information Access:
OCSP - URI:http://ocsp.pki.goog/gts1c3
CA Issuers - URI:http://pki.goog/repo/certs/gts1c3.der
X509v3 Subject Alternative Name:
DNS:www.google.com
X509v3 Certificate Policies:
Policy: 2.23.140.1.2.1
Policy: 1.3.6.1.4.1.11129.2.5.3
X509v3 CRL Distribution Points:
Full Name:
URI:http://crls.pki.goog/gts1c3/QqFxbi9M48c.crl
CT Precertificate SCTs:
Signed Certificate Timestamp:
Version : v1 (0x0)
Log ID : 7D:3E:F2:F8:8F:FF:88:55:68:24:C2:C0:CA:9E:52:89:
79:2B:C5:0E:78:09:7F:2E:6A:97:68:99:7E:22:F0:D7
Timestamp : Sep 13 05:06:59.644 2021 GMT
Extensions: none
Signature : ecdsa-with-SHA256 <<#2
30:45:02:21:00:84:00:48:E0:6F:E9:0F:D7:AF:A6:67:
22:C8:D3:D3:A8:E4:FB:38:11:3E:5B:C2:EF:AC:E2:54:
7A:94:AC:1A:47:02:20:1E:84:FB:69:49:C2:1B:2E:0B:
84:8C:AD:CA:13:FF:97:19:3C:57:8A:0A:AC:23:DD:61:
C2:AB:7F:07:46:45:65
Signed Certificate Timestamp:
Version : v1 (0x0)
Log ID : 94:20:BC:1E:8E:D5:8D:6C:88:73:1F:82:8B:22:2C:0D:
D1:DA:4D:5E:6C:4F:94:3D:61:DB:4E:2F:58:4D:A2:C2
Timestamp : Sep 13 05:06:59.161 2021 GMT
Extensions: none
Signature : ecdsa-with-SHA256 <<#3
30:45:02:21:00:D5:16:13:47:CE:39:C6:60:AF:11:24:
61:A3:D3:B6:50:BF:32:01:0D:6F:5F:5F:2E:37:E4:F8:
1E:60:9E:70:E6:02:20:09:6A:39:F4:15:FC:36:6C:5F:
9B:C7:E1:B5:48:64:7F:BC:FD:36:6E:1D:7B:E5:74:6A:
55:B0:6E:0F:AF:CF:FF
Signature Algorithm: sha256WithRSAEncryption <<#1B
3a:11:f4:ac:db:fe:63:eb:40:ae:09:4e:d2:3a:89:90:37:c2:
bd:f5:bf:8e:69:7b:48:4e:33:6a:35:46:35:50:bc:94:2e:c3:
87:b4:66:e4:d6:bd:2f:98:99:d4:ba:0f:56:04:de:20:44:86:
61:35:50:3f:66:95:fc:4a:2a:69:b7:3b:0c:70:0f:17:cc:60:
a4:fe:1d:b3:f8:90:0c:b9:fa:3d:69:d0:2f:a9:15:91:cd:89:
bb:92:7d:f5:c6:7f:2f:b8:89:0a:95:f3:71:93:1c:52:77:22:
e8:af:54:f1:b2:0f:9c:4f:9b:28:59:c4:de:ed:63:0f:7b:06:
69:ac:af:5d:bd:1c:52:ca:67:3a:db:52:10:f3:16:55:20:dd:
db:4c:e7:93:e5:d1:56:d1:1f:07:12:0c:da:8c:df:c8:d7:91:
98:5c:c2:f7:f4:dc:ff:66:6b:35:95:f8:b9:cc:cd:1d:0b:cf:
d1:99:5e:ce:1a:d9:97:f3:c5:85:65:e0:17:b9:88:c6:1e:5f:
51:01:97:21:4e:49:6b:a6:ed:3d:df:8d:95:b5:be:54:5a:e4:
58:0d:4c:50:64:5f:47:91:48:45:d4:2b:37:50:bf:d5:fb:cd:
54:f3:c5:a2:72:38:fd:44:da:f9:6f:6a:2a:45:2c:ac:c5:a5:
37:3f:e8:fe
#1A and #1B are the algorithm of the signature on the cert by the issuer, which is in the block following #1B. Yes, there are two copies of this AlgorithmIdentifier in the ASN.1 structure, at the places shown, because X.509 was designed back in the 1980s and people then were concerned about algorithm substitution attacks based on experience with symmetric/secret-key systems, which turned out not to be a significant problem for asymmetric/public-key systems. It is SHA256withRSA because the issuing CA, GTS CA 1C3, uses an RSA (2048-bit) key. Edit: found crossdupes https://security.stackexchange.com/questions/24788/signaturealgorithm-vs-tbscertificate-signature and https://security.stackexchange.com/questions/114746/why-is-the-signature-algorithm-listed-twice-in-an-x509-certificate .
#2 and #3 are the algorithms for signatures on the two Signed Certificate Timestamps (SCTs) embedded in the certificate to support Certificate Transparency. You can see each one is part of an indented block under a heading Signed Certificate Timestamp:. The SCTs are created and signed by various transparency log systems, identified by their logid, and the two log systems GTS CA 1C3 chose to use happen to both have used ecdsa-with-sha256 signatures with P-256 keys. (We can directly see only that the R,S values are 256 bits corresponding to some curve group with a 256-bit order, but RFC6962 confirms that the only acceptable ECDSA curve is P-256.)
Aside: I don't understand why you thought it necessary to black-out some information from a certificate that everyone in the world can easily get and look at. The entire purpose of a certificate (at least an Internet server certificate) is to be publicly known to everybody.
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