https://saltpack.org/
http://msgpack.org/
https://saltpack.org/encryption-format-v2

saltpack is a binary message format, encoded using the MessagePack format. Messages are broken up into reasonable (1MB) chunks, over which regular NaCl operations are performed. We have taken pains to address many of the shortcomings of current message formats: (1) only authenticated data is output; (2) repudiable authentication is used wherever possible; (3) chunks cannot be reordered or combined with other transmissions; (4) the public keys of senders and recipients can be hidden; and (5) message truncation is detectable.

saltpack sample:

BEGIN SALTPACK SIGNED MESSAGE. kXR7VktZdyH7rvq v5wcIkHbs7XwHpb nPtLcF6vE5yY63t aHF62jiEC1zHGqD inx5YqK0nf5W9Lp TvUmM2zBwxgd3Nw kvzZ96W7ZfDdTVg F5Y99c2l5EsCy1I xVNl0nY1TP25vsX 2cRXXPUrM2UKtWq UK2HG2ifBSOED4w xArcORHfFeiEZxF CqestMqLSCCE6lT HFcdvt1QX9JjmWL o5AAqPiECnoHiSA bPHhz2JnSCyDIOz ZET1BWzttbMDL4N pcyQLmsGqYpxhG6 uvdBxdt55w9xQvQ hDPuOsKF05Hsml6 z7h9TS2msJcNwtz vxGIQR7sbB19UOt boM1hlolmMB3loP 0KexlROFBTDC6MR nBvd9sZUxA8Z7i5 a6Dk5yFU3WEYQAo DqqjXcp0yBoHO5O KEMqkZlyMf1PKiB 2n9wE6jwxAN1xws ccthT6X3iRYk0Br gHW6QRXzAHLy6Ib LgY6b3UcQAoDo8b XyaExxinVuM5Ftk 75BJOWoyLGFhZS7 EfKR8jQQexvyjDM rJLxYtjvaLX7joS 2q1VcUlqGfZDhAa 4vxJQAyu57beOux oobLhI47iZf9bxK PmYrVQ5PsC6pY1J KTQQexvlvp2yicx K4su2AFCjihbzNI yZgKM4NHN1KZapS O3iB9SlhVfTfFcR FoQoSViTkbtDtTt 6I0jrTRHkv9XVQQ eeeuzR7qYu1Grm3 zDPyj7JgK2mDidw HchOZnfOn59QLnM nH7ErnPRXgHuWHG DBidjQPakJHuWsk 2ftpIyZd2NLYEFS Mqcbo6QeCdk7LA1 uobl4NXzpvi8amO Pe8xAl1OzUCoD34 MbCwtTAe1JNymvs okufV8lHU0jVnbj u4no9QB9aP2Wkjx PfeqIH2fEtOjmFP gPMhGWslkU0M7FL QP77gPHbgjPLSD8 yIRTrbgzpAPut5R QhIdqVlHbUOa9sI v7gSqOi0GbUlhSM 183LxZI8pIlvgn9 Ms1WNzt5Xkv0W1Q Qf419ZmuQVPQDOk 0hffDmUk71TlfVx XZCF3voC2ysgl3g YdLz4rDRzMJgd2m 01HIbfdsoZpAMty O27WtUNRLV1iyC9 tK5ApCyekI4nWcf 2OvTHnC8ma7bloW XAG. END SALTPACK SIGNED MESSAGE.