dev0.fi

Issued by Let's Encrypt Authority X3

About this certificate

This digital certificate with serial number 04:02:51:ab:71:59:10:63:76:19:07:5e:35:8f:bc:d9:1d:a0 was issued on by Let's Encrypt.

With 2 subject alternative names this certificate can be used to secure multiple fqdn's. This certificate has already expired and will cause a warning or error message in the browser it's still listed on this site to allow you to look back on previously issued certificates. If we have found any compliance issues with this certificate they will be shown below. I hope this certificate review is providing you the detailed information in a simple form you where looking for.


We have idenified some issues with this certificate:
  • Subscriber Certificate: commonName is deprecated. (BRs: 7.1.4.2.2)

Certificate Subject

CN=dev0.fi

Let's Encrypt

Organization: Let's Encrypt
Country: US

This certificate has expire since

Certificate Details

Serial Number (hex): 04:02:51:ab:71:59:10:63:76:19:07:5e:35:8f:bc:d9:1d:a0
Serial Number (int): 349238266110272914832574546134160304053664
Serial Number lenght: 139 bits, 18 octets

SubjectKeyId: 3b:34:88:d3:74:4e:76:75:59:6d:31:a8:90:02:a4:2b:ef:12:99:6d
AuthorityKeyId: a8:4a:6a:63:04:7d:dd:ba:e6:d1:39:b7:a6:45:65:ef:f3:a8:ec:a1

Fingerprint (sha1): f6:d6:aa:7d:18:fe:cd:26:9b:b3:41:4e:b8:82:4d:06:22:6e:60:1d
Fingerprint (sha256): c2:82:59:82:8d:22:e9:15:82:4c:8a:f6:f1:19:60:b2:bd:80:ae:2e:05:77:69:a3:66:2f:6c:77:96:2f:10:9a

Issuing Certificate URL: http://cert.int-x3.letsencrypt.org/

Revocation information

OCSP Server: http://ocsp.int-x3.letsencrypt.org

Check the revocation status for certificate dev0.fi

2

DNS Names

0

Email Addresses

0

IP Addresses

Advanced Certificate Properties

Tehnical certificate details for dev0.fi

Public Key Algorithm

RSA

Key Size

4096

Signature Algorithm

SHA256 with RSA

Key Usage

Digital Signature
Key Encipherment

Extended Key Usages

Server Authentication
Client Authentication

Extensions

9 extensions
No unhandled critical extensions

CA Certificate

This is not a CA certificate

Subject Alternative Names

dev0.fi
www.dev0.fi

Other certificates including the domain name dev0.fi

(limited to 100 certificates)

Certificate

The complete raw certificate details for dev0.fi in PEM and ASN.1 format.

Certificate (PEM)

-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----

Public Key (PEM)

-----BEGIN PUBLIC KEY-----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-----END PUBLIC KEY-----

ASN.1 decoded

 [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . [c:2|t:0|true] OtherName 
 . . . . . . . . . . . . [c:0|t:2|false] INTEGER 2
 . . . . . . . . [c:0|t:2|false] INTEGER 349238266110272914832574546134160304053664
 . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . [c:0|t:6|false] OBJECT IDENTIFIER 1.2.840.113549.1.1.11 (sha256WithRSAEncryption)
 . . . . . . . . . . . . [c:0|t:5|false] NULL 
 . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . [c:0|t:17|true] SET, SET OF 
 . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . [c:0|t:6|false] OBJECT IDENTIFIER 2.5.4.6 (countryName)
 . . . . . . . . . . . . . . . . . . . . [c:0|t:19|false] PrintableString 'US'
 . . . . . . . . . . . . [c:0|t:17|true] SET, SET OF 
 . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . [c:0|t:6|false] OBJECT IDENTIFIER 2.5.4.10 (organizationName)
 . . . . . . . . . . . . . . . . . . . . [c:0|t:19|false] PrintableString 'Let's Encrypt'
 . . . . . . . . . . . . [c:0|t:17|true] SET, SET OF 
 . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . [c:0|t:6|false] OBJECT IDENTIFIER 2.5.4.3 (commonName)
 . . . . . . . . . . . . . . . . . . . . [c:0|t:19|false] PrintableString 'Let's Encrypt Authority X3'
 . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . [c:0|t:23|false] UTCTime 2019-01-16 21:14:10 +0000 UTC
 . . . . . . . . . . . . [c:0|t:23|false] UTCTime 2019-04-16 21:14:10 +0000 UTC
 . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . [c:0|t:17|true] SET, SET OF 
 . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . [c:0|t:6|false] OBJECT IDENTIFIER 2.5.4.3 (commonName)
 . . . . . . . . . . . . . . . . . . . . [c:0|t:19|false] PrintableString 'dev0.fi'
 . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . [c:0|t:6|false] OBJECT IDENTIFIER 1.2.840.113549.1.1.1 (rsaEncryption)
 . . . . . . . . . . . . . . . . [c:0|t:5|false] NULL 
 . . . . . . . . . . . . [c:0|t:3|false] BIT STRING (4208 bits)
 . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . [c:0|t:2|false] INTEGER 799461080184802498281074303405441822546375015866471447837504057698022848052024055822348271055936101233705692817157744537923068628526509254641877423732767431189087595381555884387553073759536910399869241519463970179645615434993127790535334029457160046076325043763861896840988532677331746739488844371916407898550729323901332748915202782694132584635025292129653497242990453503106576355291261320046937625185099747602154118777962523927122776944546676683742867209520667251155799475010936904569438688005499761532992322139264425300790218160194474322270920863277798067068901568845738610596349491021738734766722889841477602212606621203321074555036821392565047278749758881482655078442443453353627981047111659088501671732615486097614236740074489628520389176370380526226002875679704531921523166311471290500735106063207125173559502197623592189257181053839890826671114113765971713459679786221608935047434996032686385327341739501717730995784334347812222425109639685741594620502927820731322691915827368657004757131856864113602239720146498180973238210896812577253726570206111580812198960583568828236094963445224372942249740569051311746143178566644017136307493729640137344564649442324706916708801317654926002280133699708486021525539391604146481115497341
 . . . . . . . . . . . . . . . . . . . . [c:0|t:2|false] INTEGER 65537

 . . . . . . . . [c:2|t:3|true] ORAddress 
 . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . [c:0|t:6|false] OBJECT IDENTIFIER 2.5.29.15 (keyUsage)
 . . . . . . . . . . . . . . . . . . . . [c:0|t:1|false] BOOLEAN true
 . . . . . . . . . . . . . . . . . . . . [c:0|t:4|false] OCTET STRING (4 bytes)
 . . . . . . . . . . . . . . . . . . . . . . . . [c:0|t:3|false] BIT STRING (3 bits)
							05a0
 . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . [c:0|t:6|false] OBJECT IDENTIFIER 2.5.29.37 (extKeyUsage)
 . . . . . . . . . . . . . . . . . . . . [c:0|t:4|false] OCTET STRING (22 bytes)
 . . . . . . . . . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . [c:0|t:6|false] OBJECT IDENTIFIER 1.3.6.1.5.5.7.3.1 (serverAuth)
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . [c:0|t:6|false] OBJECT IDENTIFIER 1.3.6.1.5.5.7.3.2 (clientAuth)
 . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . [c:0|t:6|false] OBJECT IDENTIFIER 2.5.29.19 (basicConstraints)
 . . . . . . . . . . . . . . . . . . . . [c:0|t:1|false] BOOLEAN true
 . . . . . . . . . . . . . . . . . . . . [c:0|t:4|false] OCTET STRING (2 bytes)
 . . . . . . . . . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . [c:0|t:6|false] OBJECT IDENTIFIER 2.5.29.14 (subjectKeyIdentifier)
 . . . . . . . . . . . . . . . . . . . . [c:0|t:4|false] OCTET STRING (22 bytes)
 . . . . . . . . . . . . . . . . . . . . . . . . [c:0|t:4|false] OCTET STRING (20 bytes)
							3b3488d3744e7675596d31a89002a42bef12996d
 . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . [c:0|t:6|false] OBJECT IDENTIFIER 2.5.29.35 (authorityKeyIdentifier)
 . . . . . . . . . . . . . . . . . . . . [c:0|t:4|false] OCTET STRING (24 bytes)
 . . . . . . . . . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . [c:2|t:0|false] OtherName a84a6a63047dddbae6d139b7a64565eff3a8eca1
 . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . [c:0|t:6|false] OBJECT IDENTIFIER 1.3.6.1.5.5.7.1.1 (authorityInfoAccess)
 . . . . . . . . . . . . . . . . . . . . [c:0|t:4|false] OCTET STRING (99 bytes)
 . . . . . . . . . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [c:0|t:6|false] OBJECT IDENTIFIER 1.3.6.1.5.5.7.48.1 (ocsp)
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [c:2|t:6|false] IA5String 'http://ocsp.int-x3.letsencrypt.org'
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [c:0|t:6|false] OBJECT IDENTIFIER 1.3.6.1.5.5.7.48.2 (caIssuers)
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [c:2|t:6|false] IA5String 'http://cert.int-x3.letsencrypt.org/'
 . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . [c:0|t:6|false] OBJECT IDENTIFIER 2.5.29.17 (subjectAltName)
 . . . . . . . . . . . . . . . . . . . . [c:0|t:4|false] OCTET STRING (24 bytes)
 . . . . . . . . . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . [c:2|t:2|false] IA5String 'dev0.fi'
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . [c:2|t:2|false] IA5String 'www.dev0.fi'
 . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . [c:0|t:6|false] OBJECT IDENTIFIER 2.5.29.32 (certificatePolicies)
 . . . . . . . . . . . . . . . . . . . . [c:0|t:4|false] OCTET STRING (69 bytes)
 . . . . . . . . . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [c:0|t:6|false] OBJECT IDENTIFIER 2.23.140.1.2.1 (Domain Validation Certificates Policy)
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [c:0|t:6|false] OBJECT IDENTIFIER 1.3.6.1.4.1.44947.1.1.1
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [c:0|t:6|false] OBJECT IDENTIFIER 1.3.6.1.5.5.7.2.1 (cps)
 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [c:0|t:22|false] IA5String 'http://cps.letsencrypt.org'
 . . . . . . . . . . . . . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . . . . . . . . . . . . . [c:0|t:6|false] OBJECT IDENTIFIER 1.3.6.1.4.1.11129.2.4.2
 . . . . . . . . . . . . . . . . . . . . [c:0|t:4|false] OCTET STRING (245 bytes)
 . . . . . . . . . . . . . . . . . . . . . . . . [c:0|t:4|false] OCTET STRING (242 bytes)
							00f0007600e2694bae26e8e94009e8861bb63b83d43ee7fe7488fba48f2893019dddf1dbfe0000016858ba98280000040300473045022024430f3302ad84e47a389e54b2f4adce1fce08f473692904ee33c4e3c4cd270e022100e9997b7b59fcc0717630a87c13bc1bbbd5dff11bf4b56d140550685c89029f71007600293c519654c83965baaa50fc5807d4b76fbf587a2972dca4c30cf4e54547f4780000016858ba981f0000040300473045022100aae17af3e3751eb705a1a11c71e6d4efa2ba1ea0b359541451cf5411fb066fb302207d10e841bcbb2319ee5ab97a2bc4a78e9f8d39d97a04f4f1d6dfd0a50edcbae3
 . . . . [c:0|t:16|true] SEQUENCE, SEQUENCE OF 
 . . . . . . . . [c:0|t:6|false] OBJECT IDENTIFIER 1.2.840.113549.1.1.11 (sha256WithRSAEncryption)
 . . . . . . . . [c:0|t:5|false] NULL 
 . . . . [c:0|t:3|false] BIT STRING (2048 bits)
		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