Function to duplicate context state

[chemodax]

It would be nice to have function to duplicate hash context state. I.e. xdig_[ALGO]_ctx_dup().

It will be usefull to pre-cook state and then duplicate it. It also useful to restore hash state.

[rinrab]

I agree that this is something that library consumers might want. However I think adding a set of functions to duplicate context might be an overkill. Users could could do that on their own by allocating memory for a new context and memcpy-ing data there.

xdig_sha256_ctx_t *ctx1 = malloc(xdig_sha256_ctx_size());
xdig_sha256_ctx_init(ctx1);
xdig_sha256_ctx_update(ctx1, "123", 3);

xdig_sha256_ctx_t *ctx2 = malloc(xdig_sha256_ctx_size());
memcpy(ctx2, ctx1, xdig_sha256_ctx_size());

This might look a bit hacky. But xdig_sha256_ctx_dup(ctx2, ctx1); still not much better IMO.

It is looking odd to me when memory is managed from outside. This might be a reason to consider managing memory on the library's side. However, this conflicts with its concepts of providing simplest possible API usage. My point is that this is overcomplecation.

(btw lz4 does provide functions for creating/freeing encryption stream on heap)

https://github.com/lz4/lz4/blob/1519f46a3a11f312be8f5796e8fa4779140277f0/lib/lz4.h#L333-L334