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Nicola...my apologies for the typo...
From: openssl-users <openssl-users-bounces@xxxxxxxxxxx> on behalf of Jason Schultz <jetson23@xxxxxxxxxxx>
Sent: Friday, February 21, 2020 1:05 PM To: Nicola Tuveri <nic.tuv@xxxxxxxxx> Cc: openssl-users <openssl-users@xxxxxxxxxxx> Subject: Re: Questions about using Elliptic Curve ciphers in OpenSSL
Nicole-
This was very helpful, thank you for taking the time to respond. I was confused about the parameters files, I understand why they are not needed.
Also, I should have been more clear, the creation of these cert/key pairs is strictly for testing purposes (and to give our users an easy way to test before they have their own certificate, signed by a CA).
Thanks again.
From: Nicola Tuveri <nic.tuv@xxxxxxxxx>
Sent: Wednesday, February 19, 2020 9:42 PM To: Jason Schultz <jetson23@xxxxxxxxxxx> Cc: Kyle Hamilton <aerowolf@xxxxxxxxx>; openssl-users <openssl-users@xxxxxxxxxxx> Subject: Re: Questions about using Elliptic Curve ciphers in OpenSSL I think there might be some confusion.
The "parameters" files are a legacy from when cryptosystems using "custom" domains were not widely deprecated. Such parameter files were required for any instance of key generation, to make sure that a key was generated in the defined custom domain, and were part of any key serialization because in cryptosystems that define domain parameters a keypair is generally void of operational meaning if it isn't associated with a domain in which that keypair can be used to perform operations and also because when two or more peers are involved we need to make sure that exchanged keys belonged to the same domain in which we chose to operate. Nowadays the experts discourage "custom" domains (see e.g. RFC 8422), as they bring way more disadvantages than advantages, especially considering that the disadvantages include potential serious security pitfalls. Historically you needed to pregenerate a domain parameters file for ephemeral DH used in the key exchange part of the TLS handshake, because key generation is a relatively cheap operation, but generating the big random primes required for creating new domain parameters is a quite demanding process: this was the params file that was provided to the SSL/TLS backend and needed to be saved alongside keys and certificates. With ECDH, parameter generation for custom domains is even more involved, error prone, and the validation of custom parameters received from a peer is very expensive and littered with risks
for the overall security if not done properly.
That being said, recommending "use named curves" just means to use well-established and studied set of parameters that standardizing bodies deemed recommendable for secure use: this way both peers refer to a set of parameters with a given common name rather than explicit parameters, and the clients can trust the evaluation done by experts rather than having to verify the received parameters for complex mathematical properties. Now, to the commands in your email, it must be clear that there are a few cryptosystems involved in a generic TLS handshake: 1. key exchange: usually ephemeral ECDH 2. digital signature (to validate the handshake with the server credentials): commonly RSA, ECDSA or EdDSA (depending on the server key type) 3. digital signature (to validate the certificate where the CA states "this public key belongs to this subject"): commonly RSA, ECDSA or EdDSA (depending on the CA key type)
(We should note that 3 does not necessarily require a `verify()` operation for every handshake, because both the issuer and the subject credentials are static, so a certificate for a server could be validated once and cached for later use). 1) Ephemeral ECDH generates a new keypair for every handshake, so the parties need to agree on which domain parameters to use.
We negotiate named curves rather than explicit parameters, and that is what `status = SSL_CTX_set1_curves_list(ctx, "P-521:P-384:P-256");` does: both parties specify a list of supported curves, and one in common is picked (preference on multiple hits is an irrelevant detail here). So no need for a parameters file here, we use a list of names, and this is independent from the cryptosystem picked for the two digital signature operations. 2) The server needs to have its own keypair, this means a one-time-only keygen operation for which parameters are necessary if we pick ECDSA as the cryptosystem of our choice. You can do this using explicit parameters or a named curve, and the latter is preferred. In any case there is no need to store a parameters file after the key has been generated, as the key
parameters are saved in the key serialization anyway, both for named and for custom curves.
There is no harm in generating an intermediary params file, but it is superfluous, and also the fact that there is no need to create such a file should answer to your original question about where/how it is best to store the parameter file. To generate such a private key without the need for an intermediate params file you could run: ~~~sh curve_name=prime256v1 privkey_file=mykeyout.pem openssl genpkey \
-algorithm EC -pkeyopt ec_paramgen_curve:$curve_name \ -pkeyopt ec_param_enc:named_curve \ -outform pem -out $privkey_file ~~~ Once you have a private key you could generate a certificate signing request (CSR) for your CA to issue a certificate for your server: ~~~sh csr_file=mycsrout.pem csr_config=/etc/ssl/openssl.cnf openssl req -sha256 \
-key $privkey_file \ -new -out $csr_file -outform pem \ -config $csr_config ~~~ Your CA upon receiving your CSR would do its due diligence to verify this is a valid request, that you are indeed entitled to request such a certificate for the requested subject, and all kinds of checks. If the CSR meets all the CA criteria, they will proceed to generate a certificate out of your CSR. The CA key type is often RSA, so it wouldn't be uncommon to obtain a certificate in which the issuer (the CA) owns an RSA key, and the subject is associated with an EC key. As an example I created my own fake CA key+certificate and signed a CSR for "Internet Widgits Pty Ltd". The CA could sign your CSR using openssl like this: ~~~sh ca_cert=ca_cert.pem ca_privkey=ca_privkey.pem ca_config=/etc/ssl/openssl.cnf days=30 cert_file=mycertfileout.pem openssl x509 \ -req -in $csr_file \ -CA $ca_cert -CAkey $ca_privkey \ -CAcreateserial -config $ca_config \ -days $days -sha256 \ -out $cert_file ~~~
The resulting certificate could be inspected by: ~~~sh openssl x509 -in $cert_file -noout -text Certificate: Data: Version: 1 (0x0) Serial Number: 5a:59:46:22:0c:71:16:24:31:52:bc:9c:d7:ac:39:11:36:44:97:d7 Signature Algorithm: sha256WithRSAEncryption Issuer: C = AU, ST = Some-State, O = Fake CA, OU = CA, CN = ca.fake.example.com Validity Not Before: Feb 19 20:50:47 2020 GMT Not After : Mar 20 20:50:47 2020 GMT Subject: C = AU, ST = Some-State, O = Internet Widgits Pty Ltd Subject Public Key Info: Public Key Algorithm: id-ecPublicKey Public-Key: (256 bit) pub: 04:6c:2d:42:83:8a:17:dc:8f:9f:c4:b4:e6:84:eb: ac:90:7a:98:23:a2:9c:cf:35:da:cf:f7:79:bd:5d: 56:4d:de:5a:2a:72:7b:82:6c:bb:fa:ee:14:d7:2c: 45:d1:bf:a5:7c:ec:e2:30:01:9c:98:c9:42:0c:ae: fd:84:a4:91:c4 ASN1 OID: prime256v1 NIST CURVE: P-256 Signature Algorithm: sha256WithRSAEncryption c0:02:28:4e:3d:f0:b8:2a:a1:31:30:ec:f8:4f:8c:c3:d1:e2: c4:57:1e:ff:ce:d7:49:ad:44:68:71:00:43:f5:49:2e:2d:c0: 06:0e:92:fa:d0:64:5c:b7:d9:ff:69:0b:0f:7b:7d:b5:32:48: 1a:e4:87:8d:38:b5:01:1d:0a:3f:d1:f9:14:d5:27:c1:ad:54: c1:03:12:e7:ed:7e:35:af:db:82:25:e6:c5:29:84:52:45:5a: ed:8d:9c:d8:16:56:d5:3e:14:5c:5d:94:88:f5:b2:c1:d1:48: 95:a2:c7:dc:17:a8:37:39:51:c5:42:a0:2a:76:7d:86:6c:cb: c9:84:63:df:86:bb:87:2e:17:b5:da:9b:08:ff:d6:2a:92:c0: 9b:8b:29:cf:c4:73:c9:83:3a:e7:89:81:37:4d:52:1f:a2:f4: f4:01:4f:7d:67:d5:d1:50:44:f1:6c:f9:04:62:89:29:0e:40: 4b:f2:04:c4:bc:78:02:25:05:e8:41:47:10:36:43:19:54:6f: 77:1e:02:a9:63:c3:4a:23:7f:86:c9:88:f8:aa:57:89:49:87: d8:26:de:32:54:f1:8b:72:ac:75:08:ec:2c:cc:cc:c6:d9:66: 00:a0:4d:a4:f1:d8:ae:c7:5e:b8:fd:62:6d:2a:ba:ce:63:17: 30:bd:0c:cb ~~~ As you can see the signature on the certificate is an RSA signature (because the CA key in this example was an RSA key), while the Subject Public Key that the certificate authenticates is an EC key (over the NIST P-256 curve). Normally you would distribute such certificate alongside a chain of certificates that can validate a trust link up to some trusted third party that your clients will recognize as trustworthy. Now in your first command in the last email you are merging all the 3 steps (keygen, CSR generation, CSR signing) into a single command to generate a key and a self signed certificate where the subject/issuer/signer is certifying itself. This kind of certificate is not particularly useful if not for testing purposes or unless you are bootstrapping your own private PKI and control all your clients so that you can force them to include your new root CA certificate among their trusted certificates. Compared to your original command, even here there is no need to create a separate param file as long as you are using named curves: ~~~sh openssl req -sha256 \ -newkey ec -pkeyopt ec_paramgen_curve:$curve_name -pkeyopt ec_param_enc:named_curve \ -keyout $privkey_file \ -new -x509 -out $cert_file -outform pem \ -config $ca_config -days $days ~~~ This can be inspected with: ~~~sh openssl x509 -noout -text -in $cert_file Certificate: Data: Version: 3 (0x2) Serial Number: 1d:ba:c3:dc:35:db:3f:3a:ca:21:ca:06:fe:4b:2b:3d:78:f8:e4:a7 Signature Algorithm: ecdsa-with-SHA256 Issuer: C = AU, ST = Some-State, O = Internet Widgits Pty Ltd, OU = private root CA, CN = root_ca.example.com Validity Not Before: Feb 19 21:27:58 2020 GMT Not After : Mar 20 21:27:58 2020 GMT Subject: C = AU, ST = Some-State, O = Internet Widgits Pty Ltd, OU = private root CA, CN = root_ca.example.com Subject Public Key Info: Public Key Algorithm: id-ecPublicKey Public-Key: (256 bit) pub: 04:77:ce:fb:29:ed:1d:25:27:09:bd:27:56:95:88: 42:71:82:49:2f:ab:be:be:45:99:22:a8:1c:43:74: af:f9:07:1b:49:b7:91:d1:5a:ea:bd:f1:b9:12:65: dd:42:e2:83:ec:0b:96:03:b8:1d:5b:ea:81:02:28: 6d:a3:53:6a:2f ASN1 OID: prime256v1 NIST CURVE: P-256 X509v3 extensions: X509v3 Subject Key Identifier: D8:87:52:0C:DE:12:5F:2F:04:22:7B:EE:CF:E9:A2:4B:18:E2:AE:FD X509v3 Authority Key Identifier: keyid:D8:87:52:0C:DE:12:5F:2F:04:22:7B:EE:CF:E9:A2:4B:18:E2:AE:FD X509v3 Basic Constraints: critical CA:TRUE Signature Algorithm: ecdsa-with-SHA256 30:45:02:21:00:8c:9c:39:a1:70:0b:27:69:c9:2c:7d:52:7f: 31:3d:b1:73:bf:15:9d:6c:df:73:98:58:2b:14:15:2d:87:63: 25:02:20:2a:3f:b2:c0:f5:cd:83:8c:92:5d:e5:69:ad:34:33: 7e:2a:ca:b1:2a:c0:21:2a:82:a1:51:f8:1c:07:7b:50:c5 ~~~ >From which it is evident that Issuer and Subject are identical, that the Subject key identifier matches the Authority key identifier, ans where the certificate signature is ECDSA because the Issuer key is an EC key. I hope this long email clarified the doubts you expressed. Cheers, Nicola Tuveri On Tue, 18 Feb 2020 at 19:45, Jason Schultz <jetson23@xxxxxxxxxxx> wrote:
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