tessalonoldenzaal.nl

Issued by Let's Encrypt Authority X3

About this certificate

This digital certificate with serial number 04:97:19:7c:3f:e5:04:6a:35:89:eb:74:d0:51:5e:68:df:57 was issued on by Let's Encrypt.

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=tessalonoldenzaal.nl

Let's Encrypt

Organization: Let's Encrypt
Country: US

This certificate has expire since

Certificate Details

Serial Number (hex): 04:97:19:7c:3f:e5:04:6a:35:89:eb:74:d0:51:5e:68:df:57
Serial Number (int): 399865656972744196369741225183878805249879
Serial Number lenght: 139 bits, 18 octets

SubjectKeyId: 72:71:c1:9f:20:85:f0:5f:54:7e:7d:68:1f:51:2f:9c:58:f9:41:7f
AuthorityKeyId: a8:4a:6a:63:04:7d:dd:ba:e6:d1:39:b7:a6:45:65:ef:f3:a8:ec:a1

Fingerprint (sha1): 0a:fb:76:77:76:a8:eb:e5:b3:3f:be:cf:f1:c7:9d:44:18:85:ca:da
Fingerprint (sha256): f2:7c:20:40:13:79:b7:54:e1:f4:52:ed:54:68:78:89:be:b4:13:a6:cf:45:63:d7:7d:7f:20:44:e1:c4:7e:18

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 tessalonoldenzaal.nl

1

DNS Names

0

Email Addresses

0

IP Addresses

Advanced Certificate Properties

Tehnical certificate details for tessalonoldenzaal.nl

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

tessalonoldenzaal.nl

Other certificates including the domain name tessalonoldenzaal.nl

(limited to 100 certificates)

Certificate

The complete raw certificate details for tessalonoldenzaal.nl 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 399865656972744196369741225183878805249879
 . . . . . . . . [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 2020-02-02 01:14:18 +0000 UTC
 . . . . . . . . . . . . [c:0|t:23|false] UTCTime 2020-05-02 01:14:18 +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 'tessalonoldenzaal.nl'
 . . . . . . . . [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 1030403588545523121728398068588395280951885454549980589323542974240023658115694679608623843020027824076559468147814684853345027115804417738987005480973779440667511290722796117291959725697150876626612959548884824154127765998490805515699523011599066215720363072982430132605832803476340363487771250395952768751077841968405523865246351801130647938003511328320249550289368693354157535722935565460517532923052517767914723746819950076254337119403138325490941161273444845744903206504241980332336100802652904845680776566244535405542177458740560379984949914948799226139136914835784522112139145565353608874137147048301125685507135447030475834748160247940762296039847313473366821691605207169584785814306624239840350159306363888716370930081032630050416924223808239033173018616606899318556908850064520271781009626765887788067633896595566536478225043306287903031214026732316849873529538490042028957946032528342408766130107929527391813570820542069970134625417672267242774595629705405304270191389744444409303234213928123055325278210234955951477676702108856889750749043023661086380735020130692355174931183354053099083273931408215820656385600171117255800652635113966980428320014997026170237140508951985399583785155204050724396936895461478957319473276931
 . . . . . . . . . . . . . . . . . . . . [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)
							7271c19f2085f05f547e7d681f512f9c58f9417f
 . . . . . . . . . . . . . . . . [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 'tessalonoldenzaal.nl'
 . . . . . . . . . . . . . . . . [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 (244 bytes)
 . . . . . . . . . . . . . . . . . . . . . . . . [c:0|t:4|false] OCTET STRING (241 bytes)
							00ef0076006f5376ac31f03119d89900a45115ff77151c11d902c10029068db2089a37d9130000017003ad5f920000040300473045022024eb2c9a8577efe25ab43579481fdf839142f7d94d3ca0257ad0700c245b8741022100b9d72d1b84eafe43bca1915dd095c6ba27921a8bf65607006c6455d637350ac000750007b75c1be57d68fff1b0c61d2315c7bae6577c5794b76aeebc613a1a69d3a21c0000017003ad5f8f0000040300463044022055cf5e2a2687a26a7a4dd30b999f71bcacbd8f18fb74ec2bada5f47bf43f868702205fadc4cbae72ad5a077a4375f6a55f1928a47475c195539fdfc156d58a28641c
 . . . . [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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