Questions about using Elliptic Curve ciphers in OpenSSL
Nicola Tuveri
nic.tuv at gmail.com
Wed Feb 19 21:42:06 UTC 2020
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 at hotmail.com> wrote:
> Nicola-
>
> Thanks for your response. It does help, but at the same time it also
> raises questions and maybe conflicts with what I thought I was doing
> correct earlier in this thread. I'm talking mostly about where I landed in
> this post:
>
> https://www.mail-archive.com/openssl-users@openssl.org/msg87538.html
> Re: Questions about using Elliptic Curve ciphers in OpenSSL
> <https://www.mail-archive.com/openssl-users@openssl.org/msg87538.html>
> Thank you for your response Thulasi, this helped. I'm posting this back to
> the OpenSSL users list in case it helps anyone else, and in case anyone can
> help with my additional questions.
> www.mail-archive.com
>
>
> I am only using named curves. You also said:
>
> "...you don't really need at all to generate a ecparam file (which only
> contains the name): the private key file already contains the very same
> name and fully contains what you need to perform ECDSA signatures that can
> be validated against a matching certificate."
>
> Let me apply that and start from the beginning and outline everything (I
> think) I need to do in that case:
>
> 1 - Generate a certificate and private key pair. Using the OpenSSL command
> line:
>
> openssl req -nodes -sha256 -newkey ec:<(openssl ecparam -name prime256v1)
> -keyout mykeyout.pem -new -out mycertfileout.pem -config /etc/ssl/openssl.cnf
> -x509 -days 365 -outform pem
>
>
> Note: the "ec:" parameter basically substitutes the openssl command above
> with the file I had created and used in this command. Also, the "-genkey"
> parameter I included in the ecparam command was probably not needed, or
> potentially bad?
>
> 2 - Call the SSL_CTX_use_PrivateKey_file() and
> SSL_CTX_use_certificate_file() to use the certificate and private key pair.
> (Same as before)
>
> 3 - Call the APIs to set the curves and allow the server to pick the
> appropriate curves for the client:
>
> status = SSL_CTX_set1_curves_list(ctx, "P-521:P-384:P-256");
> status = SSL_CTX_set_ecdh_auto(ctx, 1);
>
>
> Do I have this right? Is the only difference combining the two commands
> into one in Step 1 above, instead of the intermediate ecparams file? Or is
> there something else I'm missing on the generation of certificate/private
> key pairs?
>
> Thanks,
>
> Jason
>
>
>
>
> ------------------------------
> *From:* Nicola Tuveri <nic.tuv at gmail.com>
> *Sent:* Tuesday, February 18, 2020 2:50 PM
> *To:* Jason Schultz <jetson23 at hotmail.com>
> *Cc:* Kyle Hamilton <aerowolf at gmail.com>; openssl-users <
> openssl-users at openssl.org>
> *Subject:* Re: Questions about using Elliptic Curve ciphers in OpenSSL
>
> The ec parameters are public anyway, so there is no real need to store
> such files somewhere with restricted reading access.
>
> On the other hand, I want to reiterate that if you are using (and this is
> highly recommended) one of the named curves (e.g. NIST P-256) you don't
> really need at all to generate a ecparam file (which only contains the
> name): the private key file already contains the very same name and fully
> contains what you need to perform ECDSA signatures that can be validated
> against a matching certificate.
>
> In the same way, for the ECDHE part, pick curves that you want to support
> (most TLS 1.2 and 1.3 clients will be happy to support P-256 and X25519 key
> exchanges) from the named curves: also in this case there is no need to
> generate a separate ecparam file.
>
> Hope this helps!
>
> Best regards,
>
> Nicola Tuveri
>
>
> On Tue, 18 Feb 2020 at 15:27, Jason Schultz <jetson23 at hotmail.com> wrote:
>
> This comment does spark another question though. Do I need to protect the
> ecparam file I created for us in generating the private key? I know the
> private key should reside in /etc/ssl/private/ as that directory has no
> read access. Right now I have the ecparam generated file in
> /etc/ssl/dsaparams/, which is readable. Should that file also reside in
> /etc/ssl/private/ so it's protected?
>
> Thanks.
>
>
> ------------------------------
> *From:* Kyle Hamilton <aerowolf at gmail.com>
> *Sent:* Sunday, February 16, 2020 10:49 PM
> *To:* Jason Schultz <jetson23 at hotmail.com>
> *Cc:* Thulasi Goriparthi <thulasi.goriparthi at gmail.com>; openssl-users <
> openssl-users at openssl.org>
> *Subject:* Re: Questions about using Elliptic Curve ciphers in OpenSSL
>
> Be aware that you just posted your certificate's private key, and thus you
> should regenerate a new keypair/certificate to use. Otherwise, anyone who
> can manipulate traffic to your machine can execute a man-in-the-middle
> attack.
>
> -Kyle H
>
>
> On Fri, Feb 14, 2020, 07:40 Jason Schultz <jetson23 at hotmail.com> wrote:
>
>
> Thank you for your response Thulasi, this helped. I'm posting this back to
> the OpenSSL users list in case it helps anyone else, and in case anyone can
> help with my additional questions. While waiting for responses, I've been
> able to find out how my certificate and keys were generated. I'd like to
> walk through that to hopefully verify I'm handling things correctly.
>
> First, here is how my EC parameters file was generated:
>
> openssl ecparam -name prime256v1 -genkey -out myecparamsfile.pem
>
> And the resulting file:
>
> M640A-SAIL:/etc/ssl # openssl ecparam -in myecparamsfile.pem -text
>
> ASN1 OID: prime256v1
>
> NIST CURVE: P-256
>
> -----BEGIN EC PARAMETERS-----
>
> BggqhkjOPQMBBw==
>
> -----END EC PARAMETERS-----
>
> # openssl ecparam -in myecparamsfile.pem -text
>
> ASN1 OID: prime256v1
>
> NIST CURVE: P-256
>
> -----BEGIN EC PARAMETERS-----
>
> BggqhkjOPQMBBw==
>
> -----END EC PARAMETERS-----
>
> Is this good so far? Do I need the -genkey?
>
> Then I take this file and use it when I generate my certificate and
> private key pair, here is the openssl command I used:
>
> openssl req -nodes -sha256 -newkey ec:/etc/ssl/private/myecparamsfile.pem
> -keyout mykeyout.pem -new -out mycertfileout.pem -config
> /etc/ssl/openssl.cnf -x509 -days 365 -outform pem
> Generating a EC private key
> writing new private key to 'mykeyout.pem'
> <parameter input snipped>
>
> And the resulting key:
>
> # cat mykeyout.pem
> -----BEGIN PRIVATE KEY-----
> MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQgbfUwVhomun9Q5IAY
> xTOAn+sDoXZ+k4UWkvUyfshPBJ6hRANCAAQsakFVUTV4JmfVJH31XOvHVhhBodnV
> 8evYCJSd2Jgo4uOomCSh3oekKL+Tia+LOmynygfvmneOX2YadoNr9uzH
> -----END PRIVATE KEY-----
>
> # openssl ec -noout -text -in mykeyout.pem
> read EC key
> Private-Key: (256 bit)
> priv:
> 6d:f5:30:56:1a:26:ba:7f:50:e4:80:18:c5:33:80:
> 9f:eb:03:a1:76:7e:93:85:16:92:f5:32:7e:c8:4f:
> 04:9e
> pub:
> 04:2c:6a:41:55:51:35:78:26:67:d5:24:7d:f5:5c:
> eb:c7:56:18:41:a1:d9:d5:f1:eb:d8:08:94:9d:d8:
> 98:28:e2:e3:a8:98:24:a1:de:87:a4:28:bf:93:89:
> af:8b:3a:6c:a7:ca:07:ef:9a:77:8e:5f:66:1a:76:
> 83:6b:f6:ec:c7
> ASN1 OID: prime256v1
> NIST CURVE: P-256
>
> And certificate:
>
> M740A-PMM1:/etc/ssl # openssl x509 -text -in mycertfileout.pem
> Certificate:
> Data:
> Version: 3 (0x2)
> Serial Number:
> e2:2f:c6:e4:bf:f1:de:20
> Signature Algorithm: ecdsa-with-SHA256
> Issuer: C=US, ST=NY, L=Loc, O=Org, OU=test, CN=My
> Name/emailAddress=test at example.com
> Validity
> Not Before: Feb 13 16:11:39 2020 GMT
> Not After : Feb 12 16:11:39 2021 GMT
> Subject: C=US, ST=NY, L=Loc, O=Org, OU=test, CN=My
> Name/emailAddress=test at example.com
> Subject Public Key Info:
> Public Key Algorithm: id-ecPublicKey
> Public-Key: (256 bit)
> pub:
> 04:2c:6a:41:55:51:35:78:26:67:d5:24:7d:f5:5c:
> eb:c7:56:18:41:a1:d9:d5:f1:eb:d8:08:94:9d:d8:
> 98:28:e2:e3:a8:98:24:a1:de:87:a4:28:bf:93:89:
> af:8b:3a:6c:a7:ca:07:ef:9a:77:8e:5f:66:1a:76:
> 83:6b:f6:ec:c7
> ASN1 OID: prime256v1
> NIST CURVE: P-256
> X509v3 extensions:
> X509v3 Subject Key Identifier:
> D6:8A:F3:3B:4E:A1:F8:F8:34:C1:1B:7A:EC:BF:9B:58:7F:68:4A:D9
> X509v3 Authority Key Identifier:
>
> keyid:D6:8A:F3:3B:4E:A1:F8:F8:34:C1:1B:7A:EC:BF:9B:58:7F:68:4A:D9
>
> X509v3 Basic Constraints:
> CA:TRUE
> Signature Algorithm: ecdsa-with-SHA256
> 30:44:02:20:37:f0:f7:f7:4a:b4:8e:8f:64:72:e4:d1:31:9f:
> a1:36:c5:5d:f3:42:4c:24:37:75:cf:b6:55:b0:66:1b:6e:63:
> 02:20:39:18:81:f8:6c:86:3a:57:74:05:cc:99:6c:d9:dc:6a:
> a2:20:98:4c:66:a1:97:d1:c7:ea:42:b4:01:1a:f7:b2
>
> Then I call the APIs as described in my first email to use them:
>
> ctx = SSL_CTX_new(TLS_method());
>
> status = SSL_CTX_use_PrivateKey_file(ctx,<keyfile>,SSL_FILETYPE_PEM);
> status = SSL_CTX_use_certificate_file(ctx, ,<certfile>,SSL_FILETYPE_PEM);
>
>
> // Verify the cert and key are a pair
> status = SSL_CTX_check_private_key(ctx);
>
>
> Then call the APIs to set the curves and allow the server to pick the
> appropriate curve for the client:
>
> status = SSL_CTX_set1_curves_list(ctx, "P-521:P-384:P-256");
> status = SSL_CTX_set_ecdh_auto(ctx, 1);
>
>
> That should be it, right? The EC parameters file has been used to generate
> the private key, it does not need to be read in by an API call.
>
> With the steps above, I get a successful TLS connection from a client
> using ECDHE-ECDSA-AES256-GCM-SHA384.
>
> And yes, I think my main confusion was on what to do with the DH
> parameters file. I thought using ECDHE key exchange was similar to DSA with
> DH. With ECDHE, I don't need to read in a parameters file at all.
>
> If there's anything wrong above, please let me know, otherwise, thanks for
> all the help!
>
>
> ------------------------------
> *From:* Thulasi Goriparthi <thulasi.goriparthi at gmail.com>
> *Sent:* Wednesday, February 12, 2020 8:29 AM
> *To:* jetson23 at hotmail.com <jetson23 at hotmail.com>
> *Cc:* rsalz at akamai.com <rsalz at akamai.com>
> *Subject:* Re: Questions about using Elliptic Curve ciphers in OpenSSL
>
> To clarify further, EC keys can be generated from either explicit (group)
> parameters or named curves which are standardized numbers to specific group
> parameters.
>
> Explicit/Custom EC parameters are not recommended/convenient/usual. Your
> key contains parameters in the form of a named curve (p-256).
>
> You are probably getting confused between dhparam used to generate
> ephemeral keys for DHE based key exchange and EC curve selection for ECDHE
> based key exchange.
>
> Curve selection for ECDHE will be done from the list of curves offered by
> the client and can be different from the curve used in the server's
> certificate(ECDSA).
>
> Thanks,
> Thulasi.
>
>
> On Tue, 11 Feb, 2020, 23:24 Salz, Rich via openssl-users, <
> openssl-users at openssl.org> wrote:
>
> I believe you just load your ECDSA cert and the other stuff – Dhparams!! –
> is not needed.
>
>
>
>
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