[Libre-soc-isa] [Bug 688] New: Look through RISC-V cryptography extensions for ideas worth borrowing

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=688

            Bug ID: 688
           Summary: Look through RISC-V cryptography extensions for ideas
                    worth borrowing
           Product: Libre-SOC's first SoC
           Version: unspecified
          Hardware: Other
                OS: Linux
            Status: CONFIRMED
          Severity: enhancement
          Priority: ---
         Component: Specification
          Assignee: lkcl at lkcl.net
          Reporter: programmerjake at gmail.com
                CC: libre-soc-isa at lists.libre-soc.org
   NLnet milestone: ---

https://github.com/riscv/riscv-crypto/releases

Particularly interesting:
* Zkt - Data Independent Execution Latency aka. "constant-time" -- exactly like
what I've been saying we should do for years...we're most the way there
already, we just need concerted effort to keep stuff from breaking rather than
the current state of mostly being "constant-time" by accident.

one really important point that I want to ensure you, Luke, (and others) know,
so I'll repeat it even though you probably already know it:
in crypto software/hardware, "constant time" doesn't literally mean "always
runs in the same amount of time no matter what", instead it means "runtime
doesn't depend on data", though it does depend on branch conditions,
instruction fetching/address, and load/store addresses (it does not depend on
load/store data for normal memory).

The RISC-V spec draft linked above states:
> The Zkt extension attests that the machine has data-independent
> execution time for a safe subset of instructions. This property
> is commonly called "constant-time" although should not be taken
> with that literal meaning.

* Zbkx - Crossbar permutation instructions -- like register gather, but
"constant-time"
* They used real executable Sail code as their specification pseudo-code

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