EVP_KEYEXCH_fetch
exchange = EVP_KEYEXCH_fetch(ctx->libctx, supported_exch, ctx->propquery);
EVP_KEYEXCH *EVP_KEYEXCH_fetch(OSSL_LIB_CTX *ctx, const char *algorithm,
kex = EVP_KEYEXCH_fetch(ctx->libctx, "DH", ctx->propq);
kex = EVP_KEYEXCH_fetch(ctx->libctx, "ECDH", ctx->propq);