life_cycle-digest - The digest algorithm life-cycle All message digests (MDs) go through a number of stages in their life-cycle: This state represents the MD before it has been allocated. It is the starting state for any life-cycle transitions. This state represents the MD after it has been allocated. This state represents the MD when it is set up and capable of processing input. This state represents the MD when it is set up and capable of processing additional input or generating output. This state represents the MD when it has generated output. For an XOF digest, this state represents the MD when it has generated a single-shot output. For an XOF digest, this state represents the MD when it has generated output. It can be called multiple times to generate more output. The output length is variable for each call. This state is entered when the MD is freed. It is the terminal state for all life-cycle transitions.

State Transition Diagram
The usual life-cycle of a MD is illustrated: +--------------------+ | start | +--------------------+ | EVP_MD_CTX_reset | EVP_MD_CTX_new +-------------------------------------------------+ v v | EVP_MD_CTX_reset + - - - - - - - - - - - - - - - - - - - - - - + EVP_MD_CTX_reset | +-------------------> ' newed ' <--------------------+ | | + - - - - - - - - - - - - - - - - - - - - - - + | | | | | | | | EVP_DigestInit | | | v | | | EVP_DigestInit + - - - - - - - - - - - - - - - - - - - - - - + | | +----+-------------------> ' initialised ' <+ EVP_DigestInit | | | | + - - - - - - - - - - - - - - - - - - - - - - + | | | | | | ^ | | | | | | EVP_DigestUpdate | EVP_DigestInit | | | | | v | | | | | | +---------------------------------------------+ | | | | +-------------------- | | | | | | | | | | | | EVP_DigestUpdate | | | | | | +-------------------- | | | | | | | | updated | | | | | +-------------------> | | | | | | | | | | | | | | | | | +----+------------------------- | | -+-------------------+----+ | | | +---------------------------------------------+ | | | | | | | | | | | | | | EVP_DigestSqueeze +-------------------+ | | | | | v | | | | | | EVP_DigestSqueeze +---------------------------------------------+ | | | | | +-------------------- | | | | | | | | | squeezed | | | | | | +-------------------> | | ---------------------+ | | | | +---------------------------------------------+ | | | | | | | | | +---------------------------------------+ | | | | | | | | | +---------------------------------------------+ EVP_DigestFinalXOF | | | | +------------------------- | finaled | <--------------------+----+ | | +---------------------------------------------+ | | | EVP_DigestFinal ^ | | | | +---------------------------------+ | | EVP_MD_CTX_free | | | v | | | +------------------+ EVP_MD_CTX_free | | | | freed | <--------------------+ | | +------------------+ | | | +------------------------------------------------------+

Formal State Transitions
This section defines all of the legal state transitions. This is the canonical list. Function Call --------------------- Current State ----------------------------------- start newed initialised updated finaled squeezed freed EVP_MD_CTX_new newed EVP_DigestInit initialised initialised initialised initialised initialised EVP_DigestUpdate updated updated EVP_DigestFinal finaled EVP_DigestFinalXOF finaled EVP_DigestSqueeze squeezed squeezed EVP_MD_CTX_free freed freed freed freed freed EVP_MD_CTX_reset newed newed newed newed EVP_MD_CTX_get_params newed initialised updated EVP_MD_CTX_set_params newed initialised updated EVP_MD_CTX_gettable_params newed initialised updated EVP_MD_CTX_settable_params newed initialised updated EVP_MD_CTX_copy_ex newed initialised updated squeezed At some point the EVP layer will begin enforcing the transitions described herein. provider-digest(7), EVP_DigestInit(3) Copyright 2021-2023 The OpenSSL Project Authors. All Rights Reserved. Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.