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feat(chat-cipher): wrap and unwrap a private group's key - #1745
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ChatCipher gains newGroupKey, wrapGroupKey and unwrapGroupKey for the chat.v1.KeyEnvelope X25519_XCHACHA20POLY1305 scheme, following chat/v1/model.proto steps 1-4. The wrap reuses the DM key's X25519 + HKDF-SHA256 steps under the flipcash-group-key-wrap-v1 label; the AAD is the label, chat id, wrapper key, then recipient key. There are no shared vectors for the wrap yet, so GroupKeyWrapTest checks round trips and rejections (another chat, another wrapper or recipient, a short key), and that the wrapping key differs from the DM chat key, rather than fixed bytes. FakeChatCipher implements the new methods so the services test fixtures still compile.
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Private groups store their chat key as a
chat.v1.KeyEnvelope, and iOS needs the wrap from shared-core before it can build create or approve. This adds it toChatCipheron its own so shared-core can release it; the Android create and approve flows stay onfeat/private-groups.The wrap follows the
X25519_XCHACHA20POLY1305steps 1-4 inchat/v1/model.proto. It shares the DM key's X25519 + HKDF-SHA256 derivation (salt is both Ed25519 public keys in bytewise order) under theflipcash-group-key-wrap-v1label. The AAD is label, chat id, wrapper key, then recipient key. A creator wraps for themselves by passing their own public key asrecipientPk.There are no shared vectors for the wrap yet, so
GroupKeyWrapTestchecks round trips and rejections (another chat, another wrapper or recipient, a short key), and that the wrapping key differs from the DM chat key, rather than fixed bytes. Once iOS has an implementation, a cross-platform vector would pin it.FakeChatCipherin the services test fixtures implements the new methods so that module still compiles.