[openssl-dev] sizeof (HMAC_CTX) changes with update, breaks binary compatibility

[Timo Teras]

On Thu, 11 Jun 2015 21:09:59 -0400
Dan McDonald <danmcd at omniti.com> wrote:

> 
> > On Jun 11, 2015, at 9:07 PM, Dan McDonald <danmcd at omniti.com> wrote:
> > 
> > typedef struct hmac_ctx_st {
> >   const EVP_MD *md;
> >   EVP_MD_CTX md_ctx;
> >   EVP_MD_CTX i_ctx;
> >   EVP_MD_CTX o_ctx;
> >   unsigned int key_length;
> >   unsigned char key[HMAC_MAX_MD_CBLOCK];
> > + int key_init;
> > } HMAC_CTX;
> 
> A cheesy, but binary compatible, fix might be:
> 
> typedef struct hmac_ctx_st {
>   const EVP_MD *md;
>   EVP_MD_CTX md_ctx;
>   EVP_MD_CTX i_ctx;
>   EVP_MD_CTX o_ctx;
>   unsigned int key_init:1;   /* Ewww, cheesy use of bitfields... */
>   unsigned int key_length:31;  /* but the sizeof (HMAC_CTX) doesn't
> change! */ unsigned char key[HMAC_MAX_MD_CBLOCK];
> } HMAC_CTX;

Why is separate key_init needid? Could we not use md!=NULL or
key_length!=0 checks to see if the context is initialized?