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RFC 9420 §12.4.3.1 specifies the HPKE context for encrypted_group_secrets
must be the encrypted_group_info field of the Welcome:
encrypted_group_secrets = EncryptWithLabel(init_key, "Welcome",
encrypted_group_info, group_secrets)
Both our buildWelcome and processWelcome were passing an empty context,
which let Amethyst↔Amethyst round-trips work, but made Welcome messages
sent by RFC-compliant implementations (MDK/OpenMLS, used by whitenoise)
fail with BAD_DECRYPT inside the HPKE AEAD open.
https://claude.ai/code/session_01HfHdd5S5rvxUW2ihEpLGJr