Merge bitcoin/bitcoin#29200: net: create I2P sessions using both ECIES-X25519 and ElGamal encryption

9d72891 net: create I2P sessions with both ECIES-X25519 and ElGamal encryption (Jon Atack)

Pull request description:

  A Bitcoin Core node may only connect to a peer destination via I2P if both sides have sessions with the same encryption type.  Encryption type is a property of the session, not the destination.  Sessions may support multiple encryption types.

  As Bitcoin Core is not currently setting the encryption type when creating I2P sessions, it uses the older default, ElGamal (type 0).

  This pull updates our I2P session creation to use both ECIES-X25519 and ElGamal (types 4 and 0, respectively). This allows to connect to I2P peers of either type, and the newer, faster ECIES-X25519 will be preferred.

  See also:

  - discussion around qbittorrent/qBittorrent#19625 (comment)
  - recently updated "Signature and Encryption Types" in https://geti2p.net/en/docs/api/samv3

  Thank you and credit to zzzi2p for reporting and to vort for the patch.

  Closes bitcoin/bitcoin#29197.

ACKs for top commit:
  zzzi2p:
    ACK 9d72891
  recursive-rat4:
    ACK 9d72891
  kristapsk:
    cr utACK 9d72891
  brunoerg:
    crACK 9d72891
  shaavan:
    crACK 9d72891

Tree-SHA512: 0912fc01af9706914a7854f7479b9d82fc86c9530466cad8674e30f7eb4894d90d514efbc1aee8b7ea690faa6ff4a23b62cf5de8737cffdbc463300082c9b917

Author: fanquake

src/i2p.cpp

@@ -427,7 +427,7 @@ void Session::CreateIfNotCreatedAlready()
         const Reply& reply = SendRequestAndGetReply(
             *sock,
             strprintf("SESSION CREATE STYLE=STREAM ID=%s DESTINATION=TRANSIENT SIGNATURE_TYPE=7 "
-                      "inbound.quantity=1 outbound.quantity=1",
+                      "i2cp.leaseSetEncType=4,0 inbound.quantity=1 outbound.quantity=1",
                       session_id));
 
         m_private_key = DecodeI2PBase64(reply.Get("DESTINATION"));
@@ -445,7 +445,7 @@ void Session::CreateIfNotCreatedAlready()
 
         SendRequestAndGetReply(*sock,
                                strprintf("SESSION CREATE STYLE=STREAM ID=%s DESTINATION=%s "
-                                         "inbound.quantity=3 outbound.quantity=3",
+                                         "i2cp.leaseSetEncType=4,0 inbound.quantity=3 outbound.quantity=3",
                                          session_id,
                                          private_key_b64));
     }