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feat(chat-cipher): encrypt and decrypt private group messages and blobs - #1755
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Private groups send scheme 2 (CHAT_KEY_XCHACHA20POLY1305): XChaCha20-Poly1305 under the shared group key, with aad "flipcash-group-e2ee-v1" || chat_id || sender_id for messages and "flipcash-group-e2ee-blob-v1" || chat_id || blob_id || sender_id for blobs (nonce || ct). sender_id is the raw UserId.value, not a public key. group_cipher.json holds fixed vectors for messages, blobs and the group key wrap, generated by scripts/gen_group_cipher.py against libsodium rather than DefaultChatCipher, so another implementation can match bytes.
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Adds the scheme 2 (
CHAT_KEY_XCHACHA20POLY1305) cipher for private group messages and blobs toChatCipher, split out of the private groups branch so shared-core can ship it before the app side lands, as with #1745.encryptGroup/decryptGroup: XChaCha20-Poly1305 under the shared group key, aad"flipcash-group-e2ee-v1" || chat_id || sender_id.encryptGroupBlob/decryptGroupBlob: same key, aad"flipcash-group-e2ee-blob-v1" || chat_id || blob_id || sender_id, outputnonce || ct.sender_idis the rawUserId.value, not a public key.group_cipher.jsonholds fixed vectors for messages, blobs and the group key wrap.scripts/gen_group_cipher.pygenerates them against libsodium rather thanDefaultChatCipher, so the iOS side can match bytes.GroupCipherVectorTestchecksDefaultChatCipheragainst them.FakeChatCiphergets stubs for the new methods.No app or UI code.