[openssl-users] EVP_MD_CTX and EVP_PKEY_CTX? How to init? How to free?

Blumenthal, Uri - 0553 - MITLL uri at ll.mit.edu
Fri Apr 28 19:29:52 UTC 2017

I’m playing with RSA-PSS signatures, and stumbled upon a few problems. I tried the OpenSSL manual pages, but still coming short of complete understanding. :-)


This is how I initialize the contexts (error handlers removed for brevity):


      ctx = EVP_PKEY_CTX_new(privkey, NULL);

      md_ctx = EVP_MD_CTX_create();

      const EVP_MD *md = EVP_sha256();

      rv = EVP_DigestInit_ex(md_ctx, md, NULL);

      rv = EVP_DigestSignInit(md_ctx, &ctx, md, NULL, privkey);


First question: do I need EVP_DigestInit_ex() there?


Second question: do I have to specify hash-function (EVP_MD*) twice? First when initializing EVP_MD_CTX, and second for EVP_DigestSignInit()?


At the end I need to dispose of both ctx and md_ctx. That leads to my third question/problem. The code I tried (based on what the man page says: to avoid memory leak, I need to do EVP_MD_CTX_destroy(md_ctx) crashes with SIGV:


      EVP_MD_CTX_destroy(md_ctx); // this succeeds

      EVP_PKEY_CTX_free(ctx);  // but here the code crashes


Same happens when I reverse the above order:


      EVP_PKEY_CTX_free(ctx); // this succeeds

      EVP_MD_CTX_destroy(md_ctx); // but then this one causes crash


So what’s the correct way of freeing both of them? Or is it that because they’re sort of “bound together” by EVP_DigestSignInit(md_ctx, &ctx, md, NULL, privkey); freeing one frees the other?







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