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At the core of things is the concept that each extension is only defined in certain context(s) -- the ClientHello, EncryptedExtensions, etc., and sometimes only for a specific protocol or protocol range; we want to enforce that we only parse or generate extensions in the context(s) for which they are defined. There is some subtlety here, in that the protocol version in use is not known when generating the ClientHello (but it is known when the ClientHello extensions are being parsed!), so the SSL_IS_TLS13() macro must be used with caution. Nonetheless, by making assertions about whether we are acting in a server role and whether the current context is (not) a ClientHello, we can consolidate almost all of the logic for determining whether an extension is permitted in a given protocol message, whether we are generating or parsing that message. The only logic that remains separate relates to generating the ClientHello, as it depends on an external factor (the maximum permitted TLS version) that is not defined in the parsing context. Reviewed-by: Matt Caswell <matt@openssl.org> (Merged from https://github.com/openssl/openssl/pull/2945) |
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| .. | ||
| extensions_clnt.c | ||
| extensions_cust.c | ||
| extensions_srvr.c | ||
| extensions.c | ||
| README | ||
| statem_clnt.c | ||
| statem_dtls.c | ||
| statem_lib.c | ||
| statem_locl.h | ||
| statem_srvr.c | ||
| statem.c | ||
| statem.h | ||
State Machine Design
====================
This file provides some guidance on the thinking behind the design of the
state machine code to aid future maintenance.
The state machine code replaces an older state machine present in OpenSSL
versions 1.0.2 and below. The new state machine has the following objectives:
- Remove duplication of state code between client and server
- Remove duplication of state code between TLS and DTLS
- Simplify transitions and bring the logic together in a single location
so that it is easier to validate
- Remove duplication of code between each of the message handling functions
- Receive a message first and then work out whether that is a valid
transition - not the other way around (the other way causes lots of issues
where we are expecting one type of message next but actually get something
else)
- Separate message flow state from handshake state (in order to better
understand each)
- message flow state = when to flush buffers; handling restarts in the
event of NBIO events; handling the common flow of steps for reading a
message and the common flow of steps for writing a message etc
- handshake state = what handshake message are we working on now
- Control complexity: only the state machine can change state: keep all
the state changes local to the state machine component
The message flow state machine is divided into a reading sub-state machine and a
writing sub-state machine. See the source comments in statem.c for a more
detailed description of the various states and transitions possible.
Conceptually the state machine component is designed as follows:
libssl
|
---------------------------|-----statem.h--------------------------------------
|
_______V____________________
| |
| statem.c |
| |
| Core state machine code |
|____________________________|
statem_locl.h ^ ^
_________| |_______
| |
_____________|____________ _____________|____________
| | | |
| statem_clnt.c | | statem_srvr.c |
| | | |
| TLS/DTLS client specific | | TLS/DTLS server specific |
| state machine code | | state machine code |
|__________________________| |__________________________|
| |_______________|__ |
| ________________| | |
| | | |
____________V_______V________ ________V______V_______________
| | | |
| statem_both.c | | statem_dtls.c |
| | | |
| Non core functions common | | Non core functions common to |
| to both servers and clients | | both DTLS servers and clients |
|_____________________________| |_______________________________|