+ EVP_MD_CTX *mctx = EVP_MD_CTX_new();
+ unsigned char hash[EVP_MAX_MD_SIZE];
+
+ /* The pre-extract derive step uses a hash of no messages */
+ if (mctx == NULL
+ || EVP_DigestInit_ex(mctx, md, NULL) <= 0
+ || EVP_DigestFinal_ex(mctx, hash, NULL) <= 0) {
+ EVP_MD_CTX_free(mctx);
+ return 0;
+ }
+ EVP_MD_CTX_free(mctx);
+
+ /* Generate the pre-extract secret */
+ if (!tls13_hkdf_expand(s, md, prevsecret,
+ (unsigned char *)derived_secret_label,
+ sizeof(derived_secret_label) - 1, hash,
+ preextractsec, mdlen))
+ return 0;
+
+ prevsecret = preextractsec;