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#113 in Embedded development

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cmov is an abbreviation of conditional move. A conditional operation which takes a source value, a destination value, and a boolean, and overwrites the destination with the source if the flag is true. CMOV is the name of an x86 CPU instruction which does this for two registers.

CMOV is mainly interesting in cryptographic code because CMOV are not "predicted" by any x86 hardware, and conform to Intel's constant-time coding principles even when the condition value for the move is supposed to be a secret.

This functionality is a crticial building block for ORAM. ORAM requires performing many conditional move operations on large (~4k sized) blocks of memory repeatedly. This is expected to be the performance bottleneck if not done well. The security requirement is that these operations should not be predicted by the CPU, and should be conducted in a "side-channel resistant way" -- an attacker in the SGX threat model should not be able to observe if the move happened or not, if it happened inside an enclave.

This crate provides a trait called CMov which is meant to implement this operation, and to provide it on several simple datatypes.

Because the scope of mc-oblivious is only to support Intel x86-64 inside of SGX, on relatively recent (>= skylake) CPUs, we provide inline assembly which does the optimal thing for the datatypes that we care about.

Comparison to subtle

The subtle crate is the most obvious other crate in the same genre.

This crate builds on subtle, but it introduces a new trait for conditional assignment:

pub trait CMov {
    fn cmov(&mut self, condition: subtle::Choice, src: &self);

We chose not to use subtle::ConditionallySelectable for this because that trait defines its functionality in terms of

    fn conditional_select(a: &Self, b: &Self, choice: Choice) -> Self;

an API which necessitates a copy. Since we need to do CMOVs of very large values, implementing it this way would create a lot of extra copies on the stack. Even if those are eliminated in release mode, they won't be eliminated in debug mode, and we usually test in debug mode when iterating locally. Making things faster in debug mode allows us to get better test coverage without hurting iteration times.

Besides this, aesthetically we feel that CMov API is better, because it lines up more naturally with how the hardware actually works, which makes it easier to reason about performance.

The other main difference between us and subtle is that subtle uses no assembly, builds on stable rust, and is portable. For our purpose, we only care about x86-64 targets that are relatively recent and support SGX, and we don't mind using the nightly compiler. We specifically want to use assembly to go faster.

Additionally, using assembly may improve the security, in the sense that, the compiler explicitly promises not to modify or introspect on "volatile" inline assembly blocks. But future optimization passes introduced into llvm may in principle enable optimizations such that the indirection in "rust timing shield" and subtle doesn't work anymore. So there is some trade-off happening here between portability of the code and correct assembly generation.

That said, we still rely on subtle for a "shielded boolean" type that we need to be the argument of cmov.

Future directions

In the long run, it might be nice to get functionality like this in subtle crate itself.

For example, the rust-crypto aes crate uses platform detection in its Cargo.toml to select at compile-time between:

  • A portable implementation of aes (aes-soft)
  • A hardware-accelerated implementation of aes using x86 aesni instructions (aesni)

If rust inline assembly is stabilized, we could imagine that there is a version of subtle using platform-specific assembly for x86, and the software-based version is the fallback. Then code like in this crate could belong there.

It's possible that subtle maintainers don't want to maintain the aligned-cmov code with subtle though, because, there are not really any applications of "fast 4096 byte conditional moves" besides oblivious RAM. There are no other cryptographic primitives that require that AFAIK. Since subtle is dependend on by ALOT of cryptographic implementations now, adding this kind of functionality may be scope creep and it's not clear it's desirable to have this extra stuff in the dependency tree of many other cryptographic libraries.


Constant-time code and side-channel resistance:

Using AVX instructions for cryptographic implementations

x86-64 assembly: