Create SECURITY.md

Add formal security policy to the project.

Author: tomato42

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Only the latest released version is supported.
+Alpha and beta releases are always unsupported with security fixes.
+
+The project uses semantic versioning, as such, minor version changes are API compatible.
+
+| Version  | Supported          |
+| -------- | ------------------ |
+| 0.18.x   | :white_check_mark: |
+| < 0.18   | :x:                |
+
+## Support Scope
+
+This library was not designed with security in mind. If you are processing data that needs
+to be protected we suggest you use a quality wrapper around OpenSSL.
+[`pyca/cryptography`](https://cryptography.io/) is one example of such a wrapper.
+The primary use-case of this library is as a portable library for interoperability testing
+and as a teaching tool.
+
+**This library does not protect against side-channel attacks.**
+
+Do not allow attackers to measure how long it takes you to generate a key pair or sign a message.
+Do not allow attackers to run code on the same physical machine when key pair generation or
+signing is taking place (this includes virtual machines).
+Do not allow attackers to measure how much power your computer uses while generating the key pair
+or signing a message. Do not allow attackers to measure RF interference coming from your computer
+while generating a key pair or signing a message. Note: just loading the private key will cause
+key pair generation. Other operations or attack vectors may also be vulnerable to attacks. 
+For a sophisticated attacker observing just one operation with a private key will be sufficient
+to completely reconstruct the private key.
+
+Fixes for side-channel vulerabilities will not be developed.
+
+Please also note that any Pure-python cryptographic library will be vulnerable to the same
+side-channel attacks. This is because Python does not provide side-channel secure primitives
+(with the exception of [`hmac.compare_digest()`](https://docs.python.org/3/library/hmac.html#hmac.compare_digest)),
+making side-channel secure programming impossible.
+
+This library depends upon a strong source of random numbers. Do not use it on a system
+where `os.urandom()` does not provide cryptographically secure random numbers.
+
+## Reporting a Vulnerability
+
+If you find a security vulnerability in this library, you can report it using the "Report a vulnerability" button on the Security tab in github UI.
+Alternatively, you can contact the project maintainer at hkario at redhat dot com.