I2P implementation interoperability

i2pbox is an I2P data-format toolkit, not a router: it creates and inspects the standard I2P data structures (keys, destinations, addresses, records) that every router implementation must speak. Only autoconf_i2pd is i2pd-specific (it generates an i2pd.conf). Everything else operates on I2P network standards and is expected to interoperate with any I2P implementation.

Router implementations covered

Implementation	Language	Role
i2pd	C++	bundled as a submodule; i2pbox is built on its libi2pd
i2p-java	Java	the reference implementation (I2P Project)
go-i2p	Go	independent implementation
emissary	Rust	independent implementation (protocol stack)

Per-command compatibility

✅ verified by tests/interop/run_interop.sh; ⬜ supported by the implementation but not exercised here; — not applicable / not supported.

i2pbox command	i2pd	i2p-java	go-i2p	emissary
keygen (EdDSA identity)	✅	⬜	✅ destination/b32/hash	⬜ X25519-only identities
keyinfo	✅	⬜	✅ destination/b32/hash	⬜
i2pbase64	✅	✅	✅	⬜
b33address	✅	⬜	⬜	⬜
routerinfo	✅	⬜	✅ parse+verify	⬜ X25519-only identities
regaddr / verifyhost	✅	⬜	⬜	⬜
regaddr_3ld / regaddralias	✅	⬜	⬜	⬜
offlinekeys	✅	⬜	⬜ (format differs)	⬜
famtool (X.509 family certs)	✅	⬜	⬜	⬜
x25519 (encrypted LeaseSet)	✅	⬜	⬜	⬜
vain	✅	⬜	⬜	⬜
autoconf_i2pd	✅	—	—	—

Interop verification matrix (run_interop.sh)

Check	go-i2p	i2p-java	emissary
I2P base64 encode (fixed vector)	✅	✅	—
I2P base64 decode (fixed vector)	✅	✅	—
Destination parse → sha256 → b32/hash	✅	✅	—
router.info parse + signature verify	✅	—	⬜ SKIP (ElGamal identity)

Private-key file formats differ per implementation

Router identity keys are not a single cross-implementation file format. The signing key material itself is standard (Ed25519), but the file wrappers differ:

Implementation	File format	Reads i2pd router.keys?
i2pd	binary PrivateKeys buffer	yes (native)
i2p-java	text properties (signingPrivateKey=...)	needs conversion
go-i2p	raw Ed25519 bytes (32/64) + separate X25519 key	no — import the standard private key bytes
emissary	no private-key file loader (parses public identities only)	no

Practical import path from i2pbox:

1. i2pbox keygen router.keys 7 generates an EdDSA-SHA512 router identity.
2. i2pbox keyinfo -p router.keys prints the standard private key (destination + private bytes, I2P base64).
3. go-i2p: feed the Ed25519 private bytes to its keystore (loadExistingKey accepts 32/64-byte Ed25519 keys).
4. i2p-java: convert with its router.keys tooling (text format).
5. The network data derived from the key (destination, b32, hash) is byte-identical across implementations — verified by run_interop.sh.

Known capability limits (as of 2026-08)

• emissary only parses X25519-encrypted identities (InvalidPublicKey(0) on ElGamal). i2pbox/i2pd generate ElGamal by default, so emissary interop on default keys is SKIPped, not failing.
• go-i2p offline keys: ReadOfflineSignature expects the offline signature block, not the i2pd PrivateKeys buffer that offlinekeys writes; both carry the same I2P Offline Signature standard but in different containers.

Running the verification

make test               # i2pd regression suite (bundled)
./tests/interop/run_interop.sh ./i2pbox   # go + rust + java cross-validation

CI runs the interop matrix on every push (see .github/workflows/ci.yml, interop job); i2p-java runs with continue-on-error because its build is heavy.