A broadcast initiation package may comprise an encrypted contents portion and an unencrypted envelope portion. The unencrypted envelope portion may contain addressing information identifying the recipient of the broadcast initiation package. The addressing information for a recipient device may comprise a user identifier associated with a recipient device and/or the user of the recipient device. The unencrypted envelope portion may contain information identifying the sender device 820 and/or the user of the sender device 820 to the messaging system 750. The unencrypted envelope portion may contain information authorizing the sender device 820 to use the messaging system 750. In some embodiments, information authorizing the sender device 820 to use the messaging system 750 may be encrypted according to credentials to authenticate the sender device 820, such as by encrypting at least some portion of the broadcast initiation package using a private key, the corresponding public key to which is stored by the messaging system 750 for identifying the sender device 820. The encrypted contents portion may include the sender broadcast encryption key 875 and the group broadcast message, if any. The encrypted contents portion may be encrypted according to the recipient-device-specific encryption key for each recipient device.