diff --git a/i2p2www/spec/proposals/144-ecies-x25519-aead-ratchet.rst b/i2p2www/spec/proposals/144-ecies-x25519-aead-ratchet.rst index ee5d50c013876473494affc842f406cb72a3b679..a1777817b6f5c5156a90c552121d06351bf6972c 100644 --- a/i2p2www/spec/proposals/144-ecies-x25519-aead-ratchet.rst +++ b/i2p2www/spec/proposals/144-ecies-x25519-aead-ratchet.rst @@ -5,7 +5,7 @@ ECIES-X25519-AEAD-Ratchet :author: zzz, chisana :created: 2018-11-22 :thread: http://zzz.i2p/topics/2639 - :lastupdated: 2019-07-17 + :lastupdated: 2019-07-26 :status: Open .. contents:: @@ -949,8 +949,8 @@ new session messages. -KDF for Payload Section Encrypted Contents -`````````````````````````````````````````` +KDF for Payload Section (with Alice static key) +``````````````````````````````````````````````` .. raw:: html @@ -960,30 +960,86 @@ KDF for Payload Section Encrypted Contents bsk = GENERATE_PRIVATE() bpk = DERIVE_PUBLIC(bsk) - // Alice's X25519 static keys (if Static Key Section present) - // or X25519 ephemeral keys (if Static Key Section not present) - // or decoded one-time keys (if no Static Key Section, and ephemeral key unset in Ephemeral Key Section) + // Alice's X25519 static keys ask = GENERATE_PRIVATE() - // apk was decrypted in Static Key Section (if present) - // or Ephemeral Key Section (if Static Key Section not present) - // or decoded one-time public key (if no Static Key Section, and ephemeral key unset in Ephemeral Key Section) + // apk was decrypted in Static Key Section apk = DERIVE_PUBLIC(ask) sharedSecret = DH(ask, bpk) = DH(bsk, apk) // MixKey(DH()) // ChaChaPoly parameters to encrypt/decrypt - // chainKey from Static Key Section (if present) - // or Ephemeral Key Section (if Static Key Section not present) + // chainKey from Static Key Section k = HKDF(chainKey, sharedSecret, "Part3StaticKeyHK", 64) chainKey = keydata[0:31] k = keydata[32:64] n = message number from Ephemeral Key Section - ad = SHA-256(apk) // see above for which public key is used + ad = SHA-256(apk) + +{% endhighlight %} + + +KDF for Payload Section (without Alice static key) +`````````````````````````````````````````````````` + +.. raw:: html + + {% highlight lang='text' %} +// Bob's X25519 static keys + // bpk is published in leaseset + bsk = GENERATE_PRIVATE() + bpk = DERIVE_PUBLIC(bsk) + + // Alice's X25519 ephemeral keys + ask = GENERATE_PRIVATE() + // apk was decrypted in Ephemeral Key Section + apk = DERIVE_PUBLIC(ask) + + sharedSecret = DH(ask, bpk) = DH(bsk, apk) + + // MixKey(DH()) + // ChaChaPoly parameters to encrypt/decrypt + // chainKey from Ephemeral Key Section + k = HKDF(chainKey, sharedSecret, "Part3EphemeralHK", 64) + chainKey = keydata[0:31] + k = keydata[32:64] + n = message number from Ephemeral Key Section + ad = SHA-256(apk) {% endhighlight %} +KDF for Payload Section (one-time format) +````````````````````````````````````````` + +.. raw:: html + + {% highlight lang='text' %} +// Bob's X25519 static keys + // bpk is published in leaseset + bsk = GENERATE_PRIVATE() + bpk = DERIVE_PUBLIC(bsk) + + // Alice's X25519 ephemeral keys + // Alice's decoded one-time keys + ask = GENERATE_PRIVATE() + // Alice's decoded one-time public key + apk = DERIVE_PUBLIC(ask) + + sharedSecret = DH(ask, bpk) = DH(bsk, apk) + + // MixKey(DH()) + // ChaChaPoly parameters to encrypt/decrypt + k = HKDF(INITIAL_ROOT_KEY, sharedSecret, "Part3OneTimeKeys", 64) + chainKey = keydata[0:31] + k = keydata[32:64] + n = 0 + ad = SHA-256(apk) + +{% endhighlight %} + + + Justification ````````````` diff --git a/i2p2www/spec/proposals/152-ecies-tunnels.rst b/i2p2www/spec/proposals/152-ecies-tunnels.rst index bcc5184caa4533f022c863819245c3e0c5fd6b60..f2ded763cccd103e1ff8f3b6bf2d286f218f60e4 100644 --- a/i2p2www/spec/proposals/152-ecies-tunnels.rst +++ b/i2p2www/spec/proposals/152-ecies-tunnels.rst @@ -6,7 +6,7 @@ ECIES Tunnels :author: chisana :created: 2019-07-04 :thread: http://zzz.i2p/topics/2737 - :lastupdated: 2019-07-19 + :lastupdated: 2019-07-26 :status: Open .. contents:: @@ -41,7 +41,6 @@ Cryptographic Primitives - ChaCha20Poly1305(msg, nonce, AD, key) - as in [NTCP2]_ and [ECIES-X25519]_ - X25519(privateKey, publicKey) - as in [NTCP2]_ and [ECIES-X25519]_ - HKDF(rootKey, sharedSecret, CONTEXT, keylen) - as in [NTCP2]_ and [ECIES-X25519]_ -- Blowfish(msg, key) - used only for ECIES-only tunnel nonce encryption, sixteen rounds cipher mode ECIES for Tunnel Building ========================= @@ -89,13 +88,15 @@ bytes 0-3: tunnel ID to receive messages as, nonzero bytes 36-39: next tunnel ID, nonzero bytes 40-71: next router identity hash byte 72: flags - bytes 73-76: request time (in hours since the epoch, rounded down) - bytes 77-80: next message ID - bytes 81-464: tunnel build options / random padding + bytes 73-76: request time (in minutes since the epoch, rounded down) + bytes 77-80: request expiration (in minutes since creation, rounded down) + bytes 81-84: next message ID + bytes 85-463: tunnel build options / random padding {% endhighlight %} -The tunnel build options block will be defined by [Tunnel-Build-Options]_. +The tunnel build options block will be defined by [Tunnel-Build-Options]_, but may be +defined within this spec, TBD. Request Record Spec Encrypted (ECIES) ````````````````````````````````````` @@ -104,16 +105,17 @@ Request Record Spec Encrypted (ECIES) {% highlight lang='dataspec' %} -bytes 0-15: hop's truncated identity hash - bytes 16-47: sender's ephemeral public key +bytes 0-15: Hop's truncated identity hash + bytes 16-47: Sender's ephemeral public key bytes 48-511: ChaChaPoly AEAD encrypted BuildRequestRecord bytes 512-527: Poly1305 MAC {% endhighlight %} -After full transition to ECIES records, bytes 129-527 can be a range of included padding. +After full transition to ECIES records, random padding can be a range if variable sized records +are supported, TBD. -Random padding will be formatted using the Padding block structure from [ECIES-X25519]_ and [NTCP2]_. +Ranged random padding will be formatted using the Padding block structure from [ECIES-X25519]_ and [NTCP2]_. Flag Changes for Mixed Tunnels `````````````````````````````` @@ -126,11 +128,13 @@ Bit order: 76543210 (bit 7 is MSB) bit 7: if set, allow messages from anyone bit 6: if set, allow messages to anyone, and send the reply to the specified next hop in a Tunnel Build Reply Message - bit 5: if set, only ECIES hops in the tunnel, use Blowfish+ChaCha20 layer encryption + bit 5: if set, use new (ChaCha20) layer encryption bits 4-0: Undefined, must set to 0 for compatibility with future options {% endhighlight %} +New layer encryption flag may be moved into Tunnel Build Options, TBD. + Tunnel Reply Records for ECIES ------------------------------ @@ -291,10 +295,9 @@ Below is a description of how to derive the keys previously transmitted in reque recordKey = keydata[32:63] // AEAD key for Request Record encryption replyKey = keydata[64:95] // Hop reply key - keydata = HKDF(rootKey, sharedSecret, "TunnelLayerIVKey", 96) - rootKey = keydata[0:31] // update the root key - layerKey = keydata[32:63] // Tunnel layer key - IVKey = keydata[64:96] // Tunnel IV/nonce key + keydata = HKDF(rootKey, sharedSecret, "TunnelLayerIVKey", 64) + layerKey = keydata[0:31] // Tunnel layer key + IVKey = keydata[32:63] // Tunnel IV/nonce key {% endhighlight %} @@ -352,6 +355,20 @@ used for at most **two** hops, and the hops must be **consecutive**. **WARNING**: Using the same ephemeral keys for non-consecutive hops, or more than two hops, allows colluding hops to know they're in the same tunnel, **VERY BAD**!!! +.. raw:: html + + {% highlight lang='dataspec' %} + +// See reply key KDF for key generation + // Encrypting an ECIES hop request record + AD = Sha256(hop static key \|\| hop Identity hash) + (ciphertext, MAC) = ChaCha20-Poly1305(msg = BuildRequestRecord, nonce = 0, AD, key = hop's recordKey) + + // Encrypting an ElGamal hop request record + ciphertext = ElGamal-Encrypt(msg = BuildRequestRecord, key = hop's ElGamal public key) + +{% endhighlight %} + Reply Record Encryption for ECIES Hops -------------------------------------- @@ -458,11 +475,11 @@ Inbound tunnels: - Encrypt the IV and tunnel message(s) using ChaCha20 - Use 8-byte ``tunnelNonce`` given the lifetime of tunnels -- Destroy tunnel before 2^(64/2 - 1) messages: 2^31 = 2,147,483,648 +- Use 8-byte monotonically increasing counter for ``tunnelNonce`` encryption +- Destroy tunnel before 2^(64 - 1) messages: 2^64 - 1 = 18,446,744,073,709,551,615 - - Nonce limit in place to avoid [Sweet32]_ attack on [Blowfish]_ - - Nonce limit unlikely to ever be reached, given this would be ~3,579,139 msgs/second for 10 minute tunnels - - Nonce cannot be truncated. For shorter nonce, a different method must be used with smaller state space. + - Nonce limit in place to avoid rollover of the 64-bit counter + - Nonce limit exceedingly unlikely to ever be reached, given this would be over ~3,074,457,345,618,258 msgs/second for 10 minute tunnels The tunnel's Inbound Gateway (IBGW), processes messages received from another tunnel's Outbound Endpoint (OBEP). @@ -479,6 +496,7 @@ The IBGW preprocesses the messages into the appropriately formatted tunnel messa // For ECIES-only tunnels // IBGW generates a random nonce, ensuring no collision in its Bloom filter tunnelNonce = Random(len = 64-bits) + counter = counter + 1 // IBGW ChaCha20 "encrypts" the preprocessed tunnel messages with its tunnelNonce and layerKey encMsg = ChaCha20(msg = tunnel msg(s), nonce = tunnelNonce, key = layerKey) @@ -490,6 +508,8 @@ The IBGW preprocesses the messages into the appropriately formatted tunnel messa {% endhighlight %} Tunnel message format will slightly change, using an 8-byte nonce instead of a 16-byte IV. +The counter used for encrypting the nonce is appended to the 8-byte ``tunnelNonce``. +The counter is not advanced by tunnel participants. The rest of the format is unchanged. Outbound tunnels: @@ -503,10 +523,11 @@ Replies are directed to a zero-hop or existing inbound tunnel's IBGW. - Iteratively decrypt tunnel messages - - ECIES-only tunnel hops will encrypt using Blowfish+ChaCha20 + - ECIES-only tunnel hops will encrypt using ChaCha20 - mixed-tunnel hops will encrypt using AES256/ECB+CBC - Use the same rules for IV and layer nonces as Inbound tunnels +- For ECIES-only tunnels, advance the nonce once per set of tunnel messages sent .. raw:: html @@ -514,8 +535,11 @@ Replies are directed to a zero-hop or existing inbound tunnel's IBGW. // For ECIES-only tunnel hops - // For each hop, Blowfish-Decrypt the previous tunnelNonce with the current hop's Blowfish keys - tunnelNonce = Blowfish-Decrypt(msg = prev. tunnelNonce, key = IVKey) + // For each set of messages, increase the counter + counter = counter + 1 + // For each hop, ChaCha20 the previous tunnelNonce with the current hop's IV key + // The counter is advanced for each set of tunnel messages + tunnelNonce = ChaCha20(msg = prev. tunnelNonce, nonce = counter, key = IVKey) // For each hop, ChaCha20 "decrypt" the tunnel message with the current hop's tunnelNonce and layerKey decMsg = ChaCha20(msg = tunnel msg(s), nonce = tunnelNonce, key = hop's layerKey) @@ -544,9 +568,15 @@ IV double-encryption will still be used for mixed-tunnel hops, since they are co To validate received ``tunnelNonce``, the participant checks against its Bloom filter for duplicates. +To validate the received counter, the participant checks against it counter Bloom filter for duplicates. + +The two Bloom filters must be independent from one another. + +Participants do not advance the counter. + After validation, the participant: -- [Blowfish]_ encrypts the ``tunnelNonce`` with its ``IVKey`` +- ChaCha20 encrypts the ``tunnelNonce`` with its ``IVKey`` and received counter - Uses the encrypted ``tunnelNonce`` & its ``layerKey`` to ChaCha20 encrypt the tunnel message(s) - Sends the tuple {``tunnelId``, encrypted ``tunnelNonce``, ciphertext} to the next hop. @@ -556,8 +586,9 @@ After validation, the participant: // For ECIES-only tunnel hops // For verification, tunnel participant should check Bloom filter for received nonce uniqueness - // After verification, Blowfish encrypt the tunnelNonce with the hop's IVKey - tunnelNonce = Blowfish-Encrypt(msg = received tunnelNonce, key = IVKey) + // The counter must also be checked for uniqueness against its own independent Bloom filter + // After verification, ChaCha20 encrypt the tunnelNonce with the hop's IVKey + tunnelNonce = ChaCha20(msg = received tunnelNonce, nonce = received counter, key = IVKey) encMsg = ChaCha20(msg = received message, nonce = tunnelNonce, key = layerKey) // For ElGamal hops (unchanged) @@ -576,9 +607,9 @@ Mixed tunnels are considered unchanged for tunnel layer encryption. For ECIES-only tunnels, the following scheme will be used: -- Validate the received ``tunnelNonce`` against the Bloom filter +- Validate the received ``tunnelNonce`` and counter against the respective Bloom filters - ChaCha20 decrypt the encrypted data using the received ``tunnelNonce`` & the hop's ``layerKey`` -- [Blowfish]_ decrypt the ``tunnelNonce`` using the hop's ``IVKey`` to get the preceding ``tunnelNonce`` +- ChaCha20 decrypt the ``tunnelNonce`` using the hop's ``IVKey`` and received counter to get the preceding ``tunnelNonce`` - ChaCha20 decrypt the encrypted data using the decrypted ``tunnelNonce`` & the preceding hop's ``layerKey`` - Repeat for each hop in the tunnel, back to the IBGW @@ -589,7 +620,7 @@ For ECIES-only tunnels, the following scheme will be used: // For ECIES-only tunnel hops // Repeat for each hop in the tunnel back to the IBGW // Replace the received tunnelNonce w/ the prior round hop's decrypted tunnelNonce for subsequent hops - tunnelNonce = Blowfish-Decrypt(msg = received tunnelNonce, key = IVKey) + tunnelNonce = ChaCha20(msg = received tunnelNonce, nonce = received counter, key = IVKey) decMsg(s) = ChaCha20(msg = encrypted layer message(s), nonce = tunnelNonce, key = layerKey) // For mixed tunnel hops (unchanged) @@ -601,33 +632,26 @@ For ECIES-only tunnels, the following scheme will be used: {% endhighlight %} -Security Analysis for Blowfish+ChaCha20 Tunnel Layer Encryption +Security Analysis for ChaCha20 Tunnel Layer Encryption --------------------------------------------------------------- Switching from AES256/ECB to ChaCha20 has a number of advantages, and new security considerations. The biggest security considerations to account for, are that ChaCha20 nonces must be unique per-message, -for the life of the key being used, and [Blowfish]_ is susceptible to [Sweet32]_ birthday attacks. +for the life of the key being used. Failing to use unique nonces with the same key on different messages breaks ChaCha20. -Nonce uniqueness is main reason for using an [Blowfish]_, see [RFC-7539-S4]_. +Simple counters will be used alongside the ``tunnelNonce`` for encrypting the nonce, +since they are required for proper decryption by the IBEP. -Simple counters cannot be used, since they require syncing for proper decryption. -Syncing the counter can't be guaranteed at the IBEP, without further changes to tunnel protocols. +Using an appended counter allows the IBEP to decrypt the ``tunnelNonce`` for each hop's layer encryption, +recovering the previous nonce. -[Blowfish]_ is only used for nonce encryption to guarantee unique nonces, and prevent non-consecutive -hops in the same tunnel from colluding to know they are in the same tunnel. - -The tunnel lifetime of ten minutes and nonce limit of 2^31 messages guarantees that [Sweet32]_ attacks -are ineffective against Blowfish. Exceeding the limit would require over ~3,579,139 messages/second in each tunnel. - -Even if 2^31 messages proves to not be a strict enough limit, we can safely reduce the limit by another power of two, -without ever realistically reaching the limit. - -Even if a [Sweet32]_ attack were successful, an attacker would only gain access to the ``tunnelNonce`` -for the colliding message, which doesn't break the ChaCha20 encryption. Non-consecutive hops -would only be able to confirm they are participants in the same tunnel. +The 64-bit counter alongside the ``tunnelNonce`` doesn't reveal any new information to tunnel hops, +and cannot be used for correlation attacks. The counter also doesn't need to be private, as it only +needs to be unique per-message in a given tunnel. Uniqueness can be ensured by a second Bloom filter, +tracking which counter values have been used. The biggest security advantage is that there are no confirmation or oracle attacks against ChaCha20. @@ -642,34 +666,6 @@ double-encryption. The chosen-plaintext producing a recovered IV cannot be used to perform a padding-oracle attack against AES256/CBC layer encryption, since duplicate IVs are rejected. -Justification for Blowfish --------------------------- - -[Blowfish]_ is needed to symmetrically encrypt ChaCha20 nonces used in tunnel layer encryption. It was chosen for -its 64-bit block size, and ability to symmetrically encrypt without using a nonce. - -A 64-bit block size is needed to generate unique nonces for ChaCha20. ChaCha20 has a maximum nonce size of 96-bits, -using the IETF variant. Nonces of smaller sizes can be used, and are padded with zeroes. However, larger nonces, like -the 128-bit AES256/CBC IV cannot safely be truncated, as 96-bit segments may be identical for two otherwise -unique IVs. - -HKDF and other hashing functions cannot be used to safely truncate the received IV, since it must be possible -for Inbound Endpoints to recover the IV of preceding hops in the tunnel. - -Of the 64-bit ciphers, [Blowfish]_ is the most secure, with the widest support in well-audited cryptography libraries. - -Other alternatives like DES and 3DES, are more cumbersome, and weaker in comparison. Despite its comparative strength, -[Blowfish]_ is still vulnerable to [Sweet32]_ attacks. This means that in ~2^32 blocks, there will be a block collision, -allowing for recovery of the nonce from that block. Given the lifetime of tunnels, the restriction on unique received -nonces, and the limit of 2^31 messages, [Blowfish]_ would not vulnerable to [Sweet32]_ in the I2P ECIES Tunnels context. - -Realistically, much fewer than 2^29 nonces will ever be seen by any tunnel, since this would be over 894,784 msgs/sec. - -For comparison, Google receives ~76,000 searches per second. Even assuming 10 messages per search, a tunnel would have -to be over 100,000 msgs/sec busier than Google. Vanishingly unlikely. - -Statistics from: https://www.internetlivestats.com/one-second/ - Tunnel Message Overhead for ECIES ================================= @@ -749,9 +745,3 @@ References .. [RFC-7539-S4] https://tools.ietf.org/html/rfc7539#section-4 - -.. [Blowfish] - https://www.schneier.com/academic/blowfish/ - -.. [Sweet32] - https://sweet32.info/