DDR-0026: Cryptography Library Choice and Usage

Status: Accepted

Context
The E27 schema-0 envelope requires AES encryption/decryption with precise
control over block handling, padding, and byte-level behavior.

Decision
This implementation uses the Python `cryptography` library (hazmat layer)
for all E27 cryptographic operations.

Rationale
- Actively maintained and widely audited
- Explicit control over cipher modes and padding
- Avoids ambiguous or deprecated APIs
- Suitable for deterministic protocol behavior

Scope
- Used only in presentation/envelope code
- No cryptographic logic embedded in session or framing layers
- Keys are passed explicitly; no storage or caching is performed

Consequences
- Cryptographic correctness is isolated and testable
- Future schema versions can swap implementations without
  affecting session or framing logic
