docs(security): strengthen crypto guidance

shared/rules/security/references/crypto.md

@@ -1,6 +1,9 @@
 # Cryptography and secrets security
 
 * Never implement custom cryptographic primitives or use obsolete algorithms.
-* Use established libraries, authenticated encryption, secure randomness, and verified signatures and certificates.
-* Treat tokens, private keys, connection strings, and credentials as secrets; keep them out of source, URLs, command lines, logs, fixtures, and frontend code.
-* Use environment-appropriate secret storage, separate environments, and rotate compromised credentials.
+* Use maintained platform cryptography with authenticated encryption, secure randomness, and verified signatures and certificates. Use safe, documented parameter choices rather than inventing formats or defaults.
+* Keep key generation, storage, access, rotation, revocation, and backup separate from application data where practical. Restrict each key to its intended purpose.
+* Treat tokens, private keys, connection strings, and credentials as secrets; keep them out of source, URLs, command lines, logs, fixtures, error messages, and frontend code.
+* Use environment-appropriate secret storage, separate environments, and rotate compromised or overexposed credentials. Do not silently fall back to a weaker key or algorithm when configuration is missing.
+* Compare secret-derived values with established constant-time APIs when the comparison is security-sensitive.
+* Do not log plaintext or retain decrypted sensitive data longer than required. Apply secure disposal only where the runtime and data type provide meaningful guarantees.