Module grease 

Available on 32-bit or 64-bit only.

GREASE Support for MLS

This module implements GREASE (Generate Random Extensions And Sustain Extensibility) as defined in RFC 9420 Section 13.5.

GREASE values are used to ensure that implementations properly handle unknown values and maintain extensibility. The GREASE values follow a specific pattern where both bytes are of the form 0x_A (e.g., 0x0A0A, 0x1A1A, 0x2A2A, etc.).

Purpose

GREASE helps prevent extensibility failures by:

• Ensuring implementations don’t reject unknown values
• Testing that parsers properly handle unexpected values
• Maintaining forward compatibility

Usage in OpenMLS

OpenMLS provides tools for working with GREASE values:

• Recognition: GREASE values are automatically recognized during deserialization and stored in dedicated Grease variants (e.g., ProposalType::Grease, ExtensionType::Grease).

• Validation: During capability validation, GREASE values are treated the same as unknown values and filtered out appropriately.

• Injection: Library users can inject random GREASE values into capabilities using the Capabilities::with_grease method or by manually adding Grease variants.

Example

use openmls::prelude::*;
use openmls_rust_crypto::OpenMlsRustCrypto;

let provider = OpenMlsRustCrypto::default();

// Inject random GREASE values into capabilities
let capabilities = Capabilities::builder()
    .with_grease(provider.rand())
    .build();

// Capabilities now contain random GREASE values
assert!(capabilities.ciphersuites().iter().any(|cs| cs.is_grease()));

Constants

GREASE_VALUES

All valid GREASE values as defined in RFC 9420. These follow the pattern 0x_A_A where _ is 0-E.

Functions

is_grease_value

Checks if a given u16 value is a valid GREASE value.

random_grease_value

Returns a random GREASE value from the set of valid GREASE values.