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Krzysztof J. Obara avatar Crypto

Generating certificate using ECDSA_P256 algorithm but with sha256RSA signature

cn flag
Krzysztof J. Obara

If you have a look at a certificate encrypting google.com it advertises a 256-bit ECC key with ECDSA_P256 parameter. The signature algorithm is sha256RSA. I've been trying to achieve something similar by running the below set of commands, but since the -digest parameter I use is -sha256 the result is always sha256ECDSA signature algorithm. So the question is - how Google did that and is that achievable with OpenSSL (1.1.1k)?

Is it a result of the fact that their cert is signed by a CA cert that is RSA and not ECDSA (while mine is a self-signed ECDSA root cert)?

openssl genpkey -genparam -out ./hmca-ec.param -algorithm EC -pkeyopt ec_paramgen_curve:prime256v1

openssl genpkey -paramfile ./hmca-ec.param -out ./hmca-ec.key -pass pass:"pass" -aes-256-ecb

openssl req -x509 -new -key ./hmca-ec.key -sha256 -days 365 \
 -config ./hmca.conf -reqexts SAN -extensions SAN -out ./hmca-ec.crt

conf file (if it matters) looks like this:

[req]
distinguished_name = dirsect
x509_extensions = SAN
req_extensions = SAN
prompt = no

[SAN]
subjectKeyIdentifier=hash
subjectAltName = dirName:dirsect
extendedKeyUsage = serverAuth, clientAuth
basicConstraints = CA:TRUE
keyUsage = critical, digitalSignature, keyAgreement, keyCertSign

[dirsect]
C = US
O = a
CN = b
371
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Maarten Bodewes avatar
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Maarten Bodewes
Uh, first create a self signed RSA root certificate and then use that to sign a certificate request created using a EC(DSA) key pair? What's your question?
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dave_thompson_085 avatar
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dave_thompson_085
Actually depending on offered ciphersuites or sigalgs google.com uses either a cert with EC(DSA) P256 _or_ one with RSA-2048; both are issued using sha256-RSA signature by `GTS CA 1C3` with RSA-2048 key. `GTS CA 1C3` is itself an intermediate, not a root; its cert is issued by `GTS Root R1` which is RSA-4096 and now in most(?) truststores but also bridged to `GlobalSign Root CA` which is older (1998) and RSA-2048. See ssllabs.com/ssltest . @MaartenBodewes: so you can use _any_ RSA CA (not just a selfsigned root) to process the CSR for EC P256.
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Maarten Bodewes avatar
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Maarten Bodewes
Sure, I just indicated the shortest route :P
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