Add threat model

SECURITY.md

@@ -12,4 +12,23 @@ Using any code prior to 0.10.1 is strongly discouraged due to a [known security
 | < 0.12  | :x:                |
 
 ## Reporting a Vulnerability
+
 Please follow [this information to report a vulnerability](https://openquantumsafe.org/liboqs/security.html#reporting-security-bugs).
+
+## Threat Model
+
+For certain algorithms and platforms, liboqs aims for "constant-time code" in the sense of no branching or memory accesses based on secret data.
+
+- Algorithms: The [algorithm datasheets](https://github.com/open-quantum-safe/liboqs/blob/main/docs/algorithms/) indicate whether each algorithm and implementation aims for constant-time behaviour. These should be read in conjunction with the [issues/passes files for each algorithm's constant-time tests](https://github.com/open-quantum-safe/liboqs/tree/main/tests/constant_time).
+- Platforms: We aim for constant-time behaviour on our [Tier 1 platforms](https://github.com/open-quantum-safe/liboqs/blob/main/PLATFORMS.md).
+
+The following types of attacks are outside the scope of our threat model:
+
+- same physical system side channel
+- CPU / hardware flaws
+- physical fault injection attacks (including Rowhammer-style attacks)
+- physical observation side channels (such as power consumption, electromagnetic emissions)
+
+Mitigations for security issues outside the stated threat model may still be applied depending on the nature of the issue and the mitigation.
+
+(Based in part on https://openssl-library.org/policies/general/security-policy/index.html)