Enables non-members to join a group without a Welcome message by using
an external commit with HPKE key encapsulation.
HPKE extensions (Hpke.kt):
- deriveKeyPair(): DHKEM(X25519) key derivation from seed
- setupBaseSExport(): HPKE sender with export-only context
- setupBaseRExport(): HPKE receiver with export-only context
- keyScheduleFull(): Full key schedule returning exporter_secret
- HpkeExportContext: Export secrets via labeled expand
MlsGroup joiner side:
- externalJoin(groupInfoBytes, identity): Static method for joining
via external commit. Performs HPKE encapsulation to external_pub,
derives init_secret, adds self to tree, creates Commit with
ExternalInit proposal and UpdatePath.
MlsGroup member side:
- externalPub(): Returns the group's HPKE public key for external joins
- groupInfo(): Returns GroupInfo with ratchet_tree + external_pub extensions
- deriveExternalInitSecret(): Derives init_secret from ExternalInit kem_output
- processCommit handles ExternalInit proposals by overriding init_secret
RatchetTree fix:
- setLeaf() now expands _leafCount when setting a leaf beyond current size,
fixing external joins where the new member extends the tree
Test: testExternalJoin verifies Alice creates group, Zara joins via external
commit, Alice processes the commit, both at epoch 1 with 2 members.
All 121 MLS tests pass (41 interop + 80 unit), 0 failures.
https://claude.ai/code/session_01NocQDWj2Y92FugjfgazzL3
Claude
477d560d18