ADR 0001: Cryptographic suite

Select libsodium, XChaCha20-Poly1305-IETF, and Argon2id13 for Ownfold's initial authenticated-encryption and recovery suite.

Status: accepted for v0.1.

Use libsodium’s XChaCha20-Poly1305-IETF for record encryption, data-key wrapping, and Recovery Kit encryption. Use Argon2id13 for password derivation. Generate keys, salts, and nonces with libsodium. Do not implement cryptographic primitives.

The extended nonce makes independent random nonces practical. The suite identifier and format version make future migrations explicit rather than silently changing algorithms.