Ouroboros Flow Allocation Protocol

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Header

OAP Header Structure
====================

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                                                               +
|                                                               |
+                       id (128 bits)                           +
|                  Unique flow allocation ID                    |
+                                                               +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                     timestamp (64 bits)                       +
|                 UTC nanoseconds since epoch                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+
|         crt_len (16 bits)     |                               |    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +    |
|                                                               |    |
+                                                               +    |
|                  certificate (variable)                       |    |
+               X.509 certificate, DER encoded                  +    |
|                                                               |    |
+                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |
|                               |         eph_len (16 bits)     |    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |
|                                                               |    |
+                                                               +    |
|                 ephemeral_key (variable)                      |    |  Signed
+              ECDHE P-384 public key, DER encoded              +    |  Region
|                                                               |    |
+                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |
|                               |        data_len (16 bits)     |    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |
|                                                               |    |
+                     data (variable)                           +    |
|              Piggybacked application data                     |    |
+                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |
|                               |         sig_len (16 bits)     |    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---+
|                                                               |
+                                                               +
|                   signature (variable)                        |
+                 ECDSA signature over signed region            +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Field Summary:
+---------------+----------+----------------------------------------+
| Field         | Size     | Description                            |
+---------------+----------+----------------------------------------+
| id            | 16 bytes | Random 128-bit flow allocation ID      |
| timestamp     |  8 bytes | UTC time in nanoseconds (replay prot.) |
| crt_len       |  2 bytes | Certificate length (0 = no auth)       |
| certificate   | variable | X.509 cert signed by CA (DER encoded)  |
| eph_len       |  2 bytes | Ephemeral key length (0 = no encrypt)  |
| ephemeral_key | variable | ECDHE P-384 public key (91 bytes typ.) |
| data_len      |  2 bytes | Application data length                |
| data          | variable | Piggybacked app data (future use)      |
| sig_len       |  2 bytes | Signature length (0 = unsigned)        |
| signature     | variable | ECDSA-SHA384 sig (~103 bytes typ.)     |
+---------------+----------+----------------------------------------+

Minimum header size: 16 + 8 + 2 + 2 + 2 + 2 = 32 bytes (no optional fields)

Typical sizes:
  - No auth, no encrypt:  32 bytes
  - Encrypt only:         32 + 91 = 123 bytes  
  - Auth only:            32 + ~560 + ~103 = ~695 bytes
  - Auth + encrypt:       32 + ~560 + 91 + ~103 = ~786 bytes

Operation

Client (IRMd)                              Server (IRMd)
     |                                           |
     | 1. Load client cert/key                   |
     | 2. Generate ephemeral keypair             |
     | 3. Build OAP_HDR (id, ts, crt, eph)       |
     | 4. Sign header with client key            |
     |                                           |
     |-------- FLOW_REQ (OAP_HDR) ------------->|
     |                                           |
     |                                           | 5. Load server cert/key
     |                                           | 6. Verify client cert against CA
     |                                           | 7. Verify client signature
     |                                           | 8. Generate ephemeral keypair
     |                                           | 9. Derive symmetric key (ECDHE)
     |                                           | 10. Build response OAP_HDR
     |                                           | 11. Sign with server key
     |                                           |
     |<------- FLOW_REPLY (OAP_HDR) ------------|
     |                                           |
     | 12. Verify server cert against CA         |
     | 13. Verify server signature               |
     | 14. Derive symmetric key (ECDHE)          |
     |                                           |
     |===========================================|
     |         Encrypted data channel            |
     |===========================================|