BOLT-04: modify Sphinx packet construction to use starting random bytes

In this commit, we modify the existing instructions to create the Sphinx
packet to no longer start out with a zero initialize set of 1366 bytes.
Instead, we now instruct the sender to use _random_ bytes derived from a
CSPRG. This fixes a recently discovered privacy leak that allows an
adversarial exit hop to ascertain a lower bound on the true path length.

Note that this doesn't affect packet processing, so this is a backwards
compatible change. Only clients need to update in order to avoid this
privacy leak.

After this change is applied, the test vectors as is don't match the
spec, as they're created using the original all zero starting bytes. We
can either update these with our specified set of random bytes, or leave
them as is, as they're fully deterministic as is.

An alternative path would be to generate more random bytes from the
shared secret as we do elsewhere (the chacha based CSPRNG).

.aspell.en.pws

@@ -374,3 +374,4 @@ snprintf
 GitHub
 IRC
 bitmasks
+CSPRNG

04-onion-routing.md

@@ -106,14 +106,17 @@ A number of encryption and verification keys are derived from the shared secret:
    to obfuscate the per-hop information
  - _mu_: used during the HMAC generation
  - _um_: used during error reporting
+ - _pad_: use to generate random filler bytes for the starting mix-header
+   packet
 
-The key generation function takes a key-type (_rho_=`0x72686F`, _mu_=`0x6d75`,
-or _um_=`0x756d`) and a 32-byte secret as inputs and returns a 32-byte key.
+The key generation function takes a key-type (_rho_=`0x72686F`, _mu_=`0x6d75`, 
+_um_=`0x756d`, or _pad_=`0x706164`) and a 32-byte secret as inputs and returns
+a 32-byte key.
 
 Keys are generated by computing an HMAC (with `SHA256` as hashing algorithm)
-using the appropriate key-type (i.e. _rho_, _mu_, or _um_) as HMAC-key and the
-32-byte shared secret as the message. The resulting HMAC is then returned as the
-key.
+using the appropriate key-type (i.e. _rho_, _mu_, _um_, or _pad_) as HMAC-key
+and the 32-byte shared secret as the message. The resulting HMAC is then
+returned as the key.
 
 Notice that the key-type does not include a C-style `0x00`-termination-byte,
 e.g. the length of the _rho_ key-type is 3 bytes, not 4.
@@ -506,9 +509,15 @@ The construction returns a single 1366-byte packet along with the first receivin
 The packet construction is performed in the reverse order of the route, i.e.
 the last hop's operations are applied first.
 
-The packet is initialized with 1366 `0x00`-bytes.
+The packet is initialized with 1366 _random_ bytes derived from a CSPRNG
+(ChaCha20). The _pad_ key referenced above is used to extract additional random
+bytes from a ChaCha20 stream, using it as a CSPRNG for this purpose.  Once the
+`paddingKey` has been obtained, ChaCha20 is used with an all zero nonce, to
+generate 1366 random bytes. Those random bytes are then used as the starting
+state of the mix-header to be created.
 
-A filler is generated (see [Filler Generation](#filler-generation)) using the shared secret.
+A filler is generated (see [Filler Generation](#filler-generation)) using the
+shared secret.
 
 For each hop in the route, in reverse order, the sender applies the
 following operations:
@@ -573,6 +582,12 @@ func NewOnionPacket(paymentPath []*btcec.PublicKey, sessionKey *btcec.PrivateKey
 	// Allocate and initialize fields to zero-filled slices
 	var mixHeader [routingInfoSize]byte
 	var nextHmac [hmacSize]byte
+        
+        // Our starting packet needs to be filled out with random bytes, we
+        // generate some determinstically using the session private key.
+        paddingKey := generateKey("pad", sessionKey.Serialize()
+        paddingBytes := generateCipherStream(paddingKey, routingInfoSize)
+        copy(mixHeader[:], paddingBytes)
 
 	// Compute the routing information for each hop along with a
 	// MAC of the routing information using the shared key for that hop.

bolt04/onion-test-multi-frame.json

@@ -31,7 +31,7 @@
 	}
     ]
   },
-  "onion": "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",
+  "onion": "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",
   "decode": [
     "4141414141414141414141414141414141414141414141414141414141414141",
     "4242424242424242424242424242424242424242424242424242424242424242",

bolt04/onion-test-v0.json

@@ -11,7 +11,7 @@
         "payload": "0000000000000000000000000000000000000000000000000000000000000000",
         "rhokey": "ce496ec94def95aadd4bec15cdb41a740c9f2b62347c4917325fcc6fb0453986",
         "mukey": "b57061dc6d0a2b9f261ac410c8b26d64ac5506cbba30267a649c28c179400eba",
-        "hmac": "65f21f9190c70217774a6fbaaa7d63ad64199f4664813b955cff954949076dcf"
+        "hmac": "b8640887e027e946df96488b47fbc4a4fadaa8beda4abe446fafea5403fae2ef"
       },
       {
         "realm": 0,
@@ -20,7 +20,7 @@
         "payload": "0101010101010101000000000000000100000001000000000000000000000000",
         "rhokey": "450ffcabc6449094918ebe13d4f03e433d20a3d28a768203337bc40b6e4b2c59",
         "mukey": "05ed2b4a3fb023c2ff5dd6ed4b9b6ea7383f5cfe9d59c11d121ec2c81ca2eea9",
-        "hmac": "9b122c79c8aee73ea2cdbc22eca15bbcc9409a4cdd73d2b3fcd4fe26a492d376"
+        "hmac": "a93aa4f40241cef3e764e24b28570a0db39af82ab5102c3a04e51bec8cca9394"
       },
       {
         "realm": 0,
@@ -29,7 +29,7 @@
         "payload": "0202020202020202000000000000000200000002000000000000000000000000",
         "rhokey": "11bf5c4f960239cb37833936aa3d02cea82c0f39fd35f566109c41f9eac8deea",
         "mukey": "caafe2820fa00eb2eeb78695ae452eba38f5a53ed6d53518c5c6edf76f3f5b78",
-        "hmac": "548e58057ab0a0e6c2d8ad8e855d89f9224279a5652895ea14f60bffb81590eb"
+        "hmac": "5d1b11f1efeaa9be32eb1c74b113c0b46f056bb49e2a35a51ceaece6bd31332c"
       },
       {
         "realm": 0,
@@ -38,7 +38,7 @@
         "payload": "0303030303030303000000000000000300000003000000000000000000000000",
         "rhokey": "cbe784ab745c13ff5cffc2fbe3e84424aa0fd669b8ead4ee562901a4a4e89e9e",
         "mukey": "5052aa1b3d9f0655a0932e50d42f0c9ba0705142c25d225515c45f47c0036ee9",
-        "hmac": "0daed5f832ef34ea8d0d2cc0699134287a2739c77152d9edc8fe5ccce7ec838f"
+        "hmac": "19ca6357b5552b28e50ae226854eec874bbbf7025cf290a34c06b4eff5d2bac0"
       },
       {
         "realm": 0,
@@ -47,11 +47,11 @@
         "payload": "0404040404040404000000000000000400000004000000000000000000000000",
         "rhokey": "034e18b8cc718e8af6339106e706c52d8df89e2b1f7e9142d996acf88df8799b",
         "mukey": "8e45e5c61c2b24cb6382444db6698727afb063adecd72aada233d4bf273d975a",
-        "hmac": "62cc962876e734e089e79eda497077fb411fac5f36afd43329040ecd1e16c6d9"
+        "hmac": "16d4553c6084b369073d259381bb5b02c16bb2c590bbd9e69346cf7ebd563229
       }
     ]
   },
-  "onion": "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",
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   "decode": [
     "4141414141414141414141414141414141414141414141414141414141414141",
     "4242424242424242424242424242424242424242424242424242424242424242",