DH_compute_key () - replacement in 3.0

[Narayana, Sunil Kumar]

Hi Daniel ,
We do have generated the key using EVP_PKEY_gen as suggested in earlier emails, but since this was a non-ephemeral and we wanted to store the key in "raw" octet bytes, so we did extracted the whole DH priv/pub key pair out from the key generated via  EVP_PKEY_gen  ( using as suggested… EVP_PKEY_get_raw_public_key (pkey, pub, &len)  )

Now, at a later stage in application we have to compute the Secret key using the stored key’s (in above step).
As of now,  these keys are in uchar format, but are converted to BIGNUM and given to DH_compute_key as below.

   BIGNUM      *bn_publicKey;
    dh->priv_key = BN_bin2bn(privateKey, octet_len, NULL);
    bn_publicKey = BN_bin2bn(publicKey, octet_len, NULL);
    rv = DH_compute_key(sharedSecret, bn_publicKey, dh);

So in order to keep the existing frame work in place and just replace the DH_compute_key, we should be using the  dh->priv_key/ bn_publicKey  to compute shared secret key.
So we require to convert the BIGNUM key types to EVP_KEY types to use in EVP_PKEY_derive_init, EVP_PKEY_derive_set_peer, and EVP_PKEY_derive to get shared secret
Please suggest…


Regards,
Sunil

From: Sands, Daniel <dnsands at sandia.gov>
Sent: 15 December 2020 05:16
To: Narayana, Sunil Kumar <sanarayana at rbbn.com>; openssl-users at openssl.org
Subject: RE: DH_compute_key () - replacement in 3.0

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to exactly replace this we are generating “pubparam_key/priparam_key”  using   bn_publicKey/dh->priv_key  as below

OSSL_PARAM_BLD *pubparamsbld = NULL, priparamsbld = NULL;
OSSL_PARAM *pubparams = NULL, priparams = NULL;
EVP_PKEY *pubparam_key = NULL, *priparam_key = NULL;
EVP_PKEY_CTX *pubctx = NULL, *prictx = NULL;


pubparamsbld = OSSL_PARAM_BLD_new()
priparamsbld = OSSL_PARAM_BLD_new()

OSSL_PARAM_BLD_push_BN(pubparamsbld, OSSL_PKEY_PARAM_PUB_KEY, bn_publicKey)
OSSL_PARAM_BLD_push_BN(priparamsbld, OSSL_PKEY_PARAM_PRIV_KEY,bn_privateKey)

//build context
pubctx = EVP_PKEY_CTX_new_from_name(NULL, "DH", NULL);
prictx = EVP_PKEY_CTX_new_from_name(NULL, "DH", NULL);

EVP_PKEY_key_fromdata_init(pubctx)
EVP_PKEY_key_fromdata_init(prictx)

pubparams = OSSL_PARAM_BLD_to_param(pubparamsbld);
EVP_PKEY_fromdata(pubctx, &pubparam_key, pubparams))

priparams = OSSL_PARAM_BLD_to_param(priparamsbld);
EVP_PKEY_fromdata(prictx, &priparam_key, priparams))

From there, we are planning to use EVP_PKEY_derive_init, EVP_PKEY_derive_set_peer, and EVP_PKEY_derive to get shared secret




Didn’t you generate the private keys using the EVP_PKEY_gen as was suggested to your previous email inquiry?  If so, you shouldn’t have to rebuild it in such a way, since you already have a usable PKEY that has the generated keypair.  If you created a private keypair called privkey, the public key data can be sent to your peer with i2d_PUBKEY_bio(peer_bio, privkey) and received on the peer’s side with d2i_PUBKEY_bio(peer_bio, &peerkey);

Now you just need to build a new context around your private EVP_PKEY using derive_ctx = EVP_PKEY_CTX_new(privkey, NULL); and then do the EVP_PKEY_derive series of calls.

Your example code does not seem to set the P or G parameters of your keypair, so if you must do it that way, you will need to add them too.


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