Extract requested validity period from a Certificate Signing Request using OpenSSL

Question

We use a private certificate authority powered by OpenSSL to authenticate our customers. We provide a simple web-based utility which allows them to upload a CSR file for the certificate authority to sign.

At the moment, we can only issue certificates for a fixed period, currently 365 days. However, our customers have asked if they can specify the validity period of their certificates instead.

I would prefer not to have to ask the user what validity period they want, since they have to specify a validity period when they generate their CSR, and it makes sense to extract this period from the CSR when signing the certificate. However I can't work out how to do it: the normal things that OpenSSL lets you do to debug CSRs, certificates and keys don't show the relevant information: here's an example of the output of "openssl req -text -noout < csrfile":

$ openssl req -text -noout < my.csr 
Certificate Request:
    Data:
        Version: 0 (0x0)
        Subject: C=GB, L=London, O=example.com, CN=customer/[email protected]
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
            RSA Public Key: (1024 bit)
                Modulus (1024 bit):
                    00:c4:3b:11:7f:61:31:19:97:b6:26:19:01:e7:c6:
                    c3:d5:03:a5:f6:5a:4d:e2:03:d0:4e:76:49:d0:7f:
                    59:92:bf:5e:12:b3:b0:7e:20:5b:d8:a2:3f:cb:50:
                    c1:64:e5:48:04:c3:b2:04:e3:f2:4c:2f:0e:e2:a6:
                    c3:7c:36:24:dc:97:c9:f0:ba:ad:87:0f:71:45:9c:
                    6a:7f:d4:4c:d5:31:8e:49:a8:e4:3d:c4:ec:5e:54:
                    bf:f9:ba:ce:21:4c:11:15:7d:f0:d3:7a:77:f6:66:
                    5d:07:4e:4a:d3:0e:f0:52:0d:d9:cf:81:86:fe:9b:
                    c8:f8:e4:8d:d6:d1:d0:85:7f
                Exponent: 65537 (0x10001)
        Attributes:
            a0:00
    Signature Algorithm: sha1WithRSAEncryption
        5e:4c:38:59:95:e5:11:b4:a3:d5:88:1f:3c:c0:33:67:cb:b2:
        14:85:73:c3:5a:b8:23:bf:1d:25:2b:a9:38:93:da:fb:67:17:
        26:6c:79:07:dd:7f:3c:3f:b0:33:17:d1:c2:41:f7:c9:ce:1e:
        32:1c:a1:a0:a3:50:67:56:1b:58:d9:b4:48:56:70:00:43:22:
        a9:0c:17:be:67:42:f4:98:d6:d8:c0:d0:4f:6a:73:d1:a8:57:
        91:3c:02:dc:dc:8f:e3:fb:48:28:06:a2:8e:8e:27:b2:39:d7:
        3e:ce:63:ae:66:9b:ec:38:ee:09:77:dc:0f:91:40:ab:28:0f:
        ae:a9

No mention of the requested validity period anywhere.

Any suggestions?

Answer 1

I've been trying to figure out how to request a specific validity period in a CSR, and as far as I can tell, the CSR simply doesn't carry that information. The CSR's structure is defined in PKCS#10 / RFC2986, and it doesn't have a field specifically for a requested validity period. The attributes and extensions that can be put in the CSR are listed in PKCS#9, and there's nothing there about validity periods. And finally, I can do a openssl asn1parse on my generated CSRs and find that there's no validity-period-related information included regardless of what I pass to openssl req.

Answer 2

I stumbled across you question while researching the validity of a CSR. As other have mentioned, the validity period is not included in the CSR, but I was curious about the -days option a lot of people include in their examples for creating a CSR. After reading the documentation of OpenSSL it is quite clear:

-days n

when the -x509 option is being used this specifies the number of days to certify the certificate for. The default is 30 days.

and the -x509 option outputs a self signed certificate instead of a certificate request

-x509

this option outputs a self signed certificate instead of a certificate request. This is typically used to generate a test certificate or a self signed root CA. The extensions added to the certificate (if any) are specified in the configuration file. Unless specified using the set_serial option, a large random number will be used for the serial number.

Answer 3

Though you request for a certain validity period for your certificate, while generating the CSR, its uncertain to expect that validity to be acceptable by CA. Most CA's would prefer a predefined validity period and few CA's are OK with the requested validity period and generate the CSR accordingly. Now coming to the point, the CSR ASN.1 structure according to PKCS#10 standard does not specify the validity period. And thus you cannot extract that information from the CSR.