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| 142 | |
142 | |
| 143 | RAND is a sequence of fully random bytes, used to increase the entropy of |
143 | RAND is a sequence of fully random bytes, used to increase the entropy of |
| 144 | the data for encryption purposes. |
144 | the data for encryption purposes. |
| 145 | |
145 | |
| 146 | SEQNO is a 32-bit sequence number. It is negotiated at every connection |
146 | SEQNO is a 32-bit sequence number. It is negotiated at every connection |
| 147 | initialization and starts at some random 31 bit value. VPE currently uses |
147 | initialization and starts at some random 31 bit value. GVPE currently uses |
| 148 | a sliding window of 512 packets/sequence numbers to detect reordering, |
148 | a sliding window of 512 packets/sequence numbers to detect reordering, |
| 149 | duplication and replay attacks. |
149 | duplication and replay attacks. |
| 150 | |
150 | |
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151 | The encryption is done on RAND+SEQNO+DATA in CBC mode with zero IV (or, |
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152 | equivalently, the IV is RAND+SEQNO, encrypted with the block cipher, |
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153 | unless RAND size is decreased or increased over the default value). |
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154 | |
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155 | The random prefix itself is generated by using AES in CTR mode with a |
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156 | random key and starting value, which should make them unpredictable even |
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157 | before encrypting them again. The sequence number additionally ensures |
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158 | that the IV is unique. |
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159 | |
| 151 | =head2 The authentication protocol |
160 | =head2 The authentication/key exchange protocol |
| 152 | |
161 | |
| 153 | Before nodes can exchange packets, they need to establish authenticity of |
162 | Before nodes can exchange packets, they need to establish authenticity of |
| 154 | the other side and a key. Every node has a private RSA key and the public |
163 | the other side and a key. Every node has a private RSA key and the public |
| 155 | RSA keys of all other nodes. |
164 | RSA keys of all other nodes. |
| 156 | |
165 | |
| 157 | A host establishes a simplex connection by sending the other node an RSA |
166 | When a node wants to establish a connection to another node, it sends an |
| 158 | encrypted challenge containing a random challenge (consisting of the |
167 | RSA-OEAP-encrypted challenge and an ECDH key. The other node replies with |
| 159 | encryption and authentication keys to use when sending packets, more |
168 | it's own ECDH key and a HKDF of the challange and both ECDH keys to proof |
| 160 | random data and PKCS1_OAEP padding) and a random 16 byte "challenge-id" |
169 | it's identity. |
| 161 | (used to detect duplicate auth packets). The destination node will respond |
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| 162 | by replying with an (unencrypted) hash of the decrypted challenge, which |
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| 163 | will authenticate that node. The destination node will also set the |
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| 164 | outgoing encryption parameters as given in the packet. |
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| 165 | |
170 | |
| 166 | When the source node receives a correct auth reply (by verifying the |
171 | The remote node enganges in exactly the same protocol. When both nodes |
| 167 | hash and the id, which will expire after 120 seconds), it will start to |
172 | have exchanged their challenge and verified the response, they calculate a |
| 168 | accept data packets from the destination node. |
173 | cipher key and a HMAC key and start exchanging data packets. |
| 169 | |
174 | |
| 170 | This means that a node can only initiate a simplex connection, telling the |
175 | In detail, the challenge consist of: |
| 171 | other side the key it has to use when it sends packets. The challenge |
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| 172 | reply is only used to set the current IP address of the other side and |
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| 173 | protocol parameters. |
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| 174 | |
176 | |
| 175 | This protocol is completely symmetric, so to be able to send packets the |
177 | RSA-OAEP (SEQNO MAC CIPHER SALT EXTRA-AUTH) ECDH1 |
| 176 | destination node must send a challenge in the exact same way as already |
178 | |
| 177 | described (so, in essence, two simplex connections are created per node |
179 | That is, it encrypts (with the public key of the remote node) an initial |
| 178 | pair). |
180 | sequence number for data packets, key material for the HMAC key, key |
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181 | material for the cipher key, a salt used by the HKDF (as shown later) and |
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182 | some extra random bytes that are unused except for authentication. It also |
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183 | sends the public key of a curve25519 exchange. |
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184 | |
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185 | The remote node decrypts the RSA data, generates it's own ECDH key (ECDH2), and |
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186 | replies with: |
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187 | |
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188 | HKDF-Expand (HKDF-Extract (ECDH2, RSA), ECDH1, AUTH_DIGEST_SIZE) ECDH2 |
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189 | |
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190 | That is, it extracts from the decrypted RSA challenge, using it's ECDH |
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191 | key as salt, and then expands using the requesting node's ECDH1 key. The |
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192 | resulting has is returned as a proof that the node could decrypt the RSA |
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193 | challenge data, together with the ECDH key. |
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194 | |
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195 | After both nodes have done this to each other, they calculate the shared |
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196 | ECDH secrets, cipher and HMAC keys for the session (each |
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197 | node generates two cipher and HMAC keys, one for sending and one for |
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198 | receiving). |
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199 | |
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200 | The HMAC key for sending is generated as follow: |
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201 | |
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202 | HMAC_KEY = HKDF-Expand (HKDF-Extract (REMOTE_SALT, MAC ECDH_SECRET), info, HMAC_MD_SIZE) |
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203 | |
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204 | It extracts from MAC and ECDH_SECRET using the I<remote> SALT, then |
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205 | expands using a static info string. |
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206 | |
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207 | The cipher key is generated in the same way, except using the CIPHER part |
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208 | of the original challenge. |
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209 | |
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210 | The result of this process is to authenticate each node to the other |
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211 | node, while exchanging keys using both RSA and ECDH, the latter providing |
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212 | perfect forward secrecy. |
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213 | |
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214 | The protocol has been overdesigned where this was possible without |
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215 | increasing implementation complexity, in an attempt to protect against |
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216 | implementation or protocol failures. For example, if the ECDH challenge |
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217 | was found to be flawed, perfect forward secrecy would be lost, but |
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218 | the data would still be protected. Likewise, standard algorithms and |
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219 | implementations are used where possible. |
| 179 | |
220 | |
| 180 | =head2 Retrying |
221 | =head2 Retrying |
| 181 | |
222 | |
| 182 | When there is no response to an auth request, the node will send auth |
223 | When there is no response to an auth request, the node will send auth |
| 183 | requests in bursts with an exponential back-off. After some time it will |
224 | requests in bursts with an exponential back-off. After some time it will |
