OpenSSL generate different types of self signed certificate

Question

Does anyone know how to use OpenSSL to generate certificates for the following public key types:

1. DSA - For DHE_DSS key exchange.
2. Diffie-Hellman - For DH_DSS and DH_RSA key exchange.
3. ECDH - For ECDH_ECDSA and ECDH_RSA key exchange.
4. ECDSA - For ECDHE_ECDSA key exchange.

Most that I can find online teaches you how to generate a RSA type certificate though.

Answer 1

(EC)DSA

For a DSA key pair, use this:

openssl dsaparam -out dsakey.pem -genkey 1024

where "1024" is the size in bits. The first DSA standard mandated that the size had to be a multiple of 64, in the 512..1024 range. Then another version deprecated sizes below 1024, so a valid DSA key size had length 1024 bits and nothing else. Current version specifies that a valid DSA key has length 1024, 2048 or 3072 bits. OpenSSL accepts other lengths. If you want to maximize interoperability, use 1024 bits.

For an ECDSA key pair, use this:

openssl ecparam -genkey -out eckey.pem -name prime256v1

To see what curve names are supported by OpenSSL, use: openssl ecparam -list_curves

(For optimal interoperability, stick to NIST curve P-256, that OpenSSL knows under the name "prime256v1".)

Once you have a DSA or ECDSA key pair, you can generate a self-signed certificate containing the public key, and signed with the private key:

openssl req -x509 -new -key dsakey.pem -out cert.pem

(Replace "dsakey.pem" with "eckey.pem" to use the EC key generated above.)

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(EC)DH

For Diffie-Hellman (with or without elliptic curves), things are more complex, because DH is not a signature algorithm:

• You will not be able to produce a self-signed certificate with a DH key.
• You cannot either make a PKCS#10 request for a certificate with a DH key, because a PKCS#10 request is supposed to be self-signed (this self-signature is used as a proof of possession).

While OpenSSL, the library, has the support needed for issuing a certificate which contains a DH public key; this page may contain pointers. The challenge is to convince OpenSSL, the command-line tool, to do it. In the jungle of the OpenSSL documentation, I have not found a complete way to do it. Key pairs are easy enough to generate, though.

To generate a DH key pair, with the OpenSSL command-line tool, you have to do it in two steps:

openssl dhparam -out dhparam.pem 1024
openssl genpkey -paramfile dhparam.pem -out dhkey.pem

For an ECDH key pair, use this:

openssl ecparam -out ecparam.pem -name prime256v1
openssl genpkey -paramfile ecparam.pem -out ecdhkey.pem

However, it so happens that the format for certificates containing ECDH public keys is completely identical to the format for certificates containing ECDSA public keys; indeed, the format contains "an EC public key" without indication of the intended algorithm (ECDH or ECDSA). Therefore, any private key and certificates for ECDSA (private key for generating ECDSA signatures, certificate self-signed or signed by any other CA) will be fit for ECDH-* cipher suites.

The one case that I don't know how to produce with the OpenSSL command-line tool is a static Diffie-Hellman (non-EC) certificate. Note, though, that OpenSSL does not support SSL/TLS with static DH cipher suites either, so even if you could produce the certificate, it would not work with OpenSSL.

(And, in fact, nobody uses static DH in practice.)

Answer 2

Just dredging up an old question. I recently had need to create a DH cert for test purposes. This is how to do it. First create DH parameters and private key as per Tom's answer:

openssl dhparam -out dhparam.pem 1024
openssl genpkey -paramfile dhparam.pem -out dhkey.pem

Next create the public key file:

openssl pkey -in dhkey.pem -pubout -out dhpubkey.pem

Now you need a CSR file. CSRs are self signed which obviously can't be done for DH because DH is not a signing algorithm. But create one anyway for a different key (one that can sign, e.g. RSA). To create an RSA key and an associated CSR:

openssl genrsa -out rsakey.pem 1024
openssl req -new -key rsakey.pem -out rsa.csr

Finally, you generate the DH cert from the RSA CSR and the DH public key. It is not possible to create a self signed DH cert because (as noted above) DH is not a signing algorithm. Therefore you will need to have set up a CA certificate/key. In this example I assume you have previously created a CAkey.pem and CAcert.pem:

openssl x509 -req -in rsa.csr -CAkey CAkey.pem -CA CAcert.pem -force_pubkey dhpubkey.pem -out dhcert.pem -CAcreateserial

Your DH certificate will be in dhcert.pem

Answer 3

In fact, it is generally possible to issue X.509v3 certificates for these types of key pairs, too:

• DH (Diffie-Hellman)
• ECDH (Elliptic Curve Diffie-Hellman)
• X25519 and X448 (Edwards Curves 25519 and 448, Key Agreement not signing!)

RSA is a cryptographic algorithm that allows you both, digital signing and enciphering/deciphering. To create a CSR (PKCS#10 request) for an RSA key pair (even it is used solely for encryption purposes), you can just use the private RSA key part to perform the necessary proof-of-possession, i.e. the self-signature. Obviously, this is not possible for keyagreement-only algorithms (DH, ECDH, X25519, and X448). There is just one very old RFC, RFC 2875 "Diffie-Hellman Proof-of-Possession Algorithms", which seems not to be used anywhere, though.

The standard OpenSSL command line tools are unable to accomplish the task, i.e. first using "openssl req" to generate the CSR (P10) and subsequently issuing the X.509v3 certificate using "openssl ca". Only the key generation can be performed, e.g. using "openssl genpkey" (for X25519/X448), "openssl ecparam", etc.

The ASN.1 module of PKCS#10 (see RFC 2986) states for the type CertificationRequest the digital signature as a BIT STRING without the OPTIONAL keyword, i.e. it is mandatory.

You would have to patch OpenSSL to either allow empty digital signatures (proof-of-possession) for key agreement algorithms or implement RFC 2875, which would mean a lot of work for all above mentioned algorithms.