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Federation Join Signature Verification Issue

Summary

FERRETCANNON can successfully perform make_join requests to Synapse, but when sending the signed event via send_join, Synapse recomputes a different event hash and rejects the signature.

Evidence

Our Canonical JSON (verified correct with PowerShell SHA-256 test):

{"auth_events":["$amxREh8Yxdn7Ajg1isWwiE8vThUUrMTMeAn9vK4iANA","$sQzYfaDpsQJg93gkc-0KBVzcKgBTjuzPUQ31aGFxEj4"],"content":{"membership":"join"},"depth":1399,"origin_server_ts":1759753025984,"prev_events":["$jtmwoL0SodCgNmjPwlPdhUVsAIA-elkCs_FI6VO78dQ"],"room_id":"!ljI2RJQdNNdO7hOrzMPrACPxwQZiKxMJhCTJtMXx-hw","sender":"@ferretcannon:ferretcannon.roflcopter.wtf","state_key":"@ferretcannon:ferretcannon.roflcopter.wtf","type":"m.room.member"}

Our Computed Hash:

x8vNFczuKAWlLMO-F7XWzAZRCS0zlplC6l7HcxihZfQ

Synapse's Recomputed Hash:

prTT9VPTdvEsf_iDhai0hAYpwSgp5ixhhwE8aXMkf18 (and different every time!)

Event Sent to Synapse:

{"auth_events":["$amxREh8Yxdn7Ajg1isWwiE8vThUUrMTMeAn9vK4iANA","$sQzYfaDpsQJg93gkc-0KBVzcKgBTjuzPUQ31aGFxEj4"],"content":{"membership":"join"},"depth":1399,"hashes":{"sha256":"x8vNFczuKAWlLMO-F7XWzAZRCS0zlplC6l7HcxihZfQ"},"origin_server_ts":1759753025984,"prev_events":["$jtmwoL0SodCgNmjPwlPdhUVsAIA-elkCs_FI6VO78dQ"],"room_id":"!ljI2RJQdNNdO7hOrzMPrACPxwQZiKxMJhCTJtMXx-hw","sender":"@ferretcannon:ferretcannon.roflcopter.wtf","signatures":{"ferretcannon.roflcopter.wtf":{"ed25519:YOLO420-1759713455":"LRLJOpeqy3KcH62tFERcuol57nP1GvBCCZtLPDxIs_XaHsj35baWXj5_dg5ipQSYvHogmm2OLMSLLpGwMN6eDQ"}},"state_key":"@ferretcannon:ferretcannon.roflcopter.wtf","type":"m.room.member"}

What We've Verified

✅ Canonical JSON Generation: Produces Matrix-compliant canonical JSON with sorted keys ✅ Number Type Handling: depth and origin_server_ts are correctly typed as Long
✅ Hash Computation: SHA-256 of our canonical JSON produces the exact hash we expect (verified with independent PowerShell script) ✅ Base64 Encoding: Using URL-safe Base64 without padding ✅ Event Structure: Correct format for Room Version 12 (no event_id in body, hash-derived event ID) ✅ Signature Computation: Signs event without hashes field, with placeholder signatures structure ✅ UTF-8 Encoding: Verified byte-for-byte UTF-8 encoding is correct

What's Different

❌ Synapse computes a DIFFERENT hash from the same event every single time ❌ The hash difference causes signature verification to fail

Possible Causes

  1. JSON Parsing Differences: When Synapse (Python) parses our JSON string, it might create a slightly different internal representation that produces different canonical JSON

  2. Character Encoding: Some subtle UTF-8 encoding difference (though we verified this is correct)

  3. Number Serialization: Python's JSON handling might serialize large integers differently (though 1759753025984 is well within safe range)

  4. Whitespace/Escaping: Some difference in how strings are escaped or whitespace is handled (though canonical JSON should eliminate this)

  5. Nested Object Handling: The content object {"membership":"join"} might be serialized differently

What We Need

  1. Synapse Logs: Access to the target Synapse server logs to see what canonical JSON it's computing

  2. Synapse Source Review: Compare our implementation with Synapse's exact canonical JSON implementation

  3. Test with Another Server: Try the same join against a different Matrix server to see if it's Synapse-specific

Implementation Details

  • Server: ferretcannon.roflcopter.wtf
  • Target: matrix.geraghty.family:8448 (Synapse)
  • Room Version: 12
  • Room ID: !ljI2RJQdNNdO7hOrzMPrACPxwQZiKxMJhCTJtMXx-hw
  • Ed25519 Key ID: ed25519:YOLO420-1759713455
  • Canonical JSON Implementation: Custom Kotlin implementation following Matrix Spec v1.16
  • JSON Library: kotlinx.serialization

Next Steps

  1. Get Synapse logs from geraghty.family to see what it's receiving/computing
  2. Compare byte-for-byte what we send vs what Synapse receives
  3. Test with Python Matrix SDK to verify our event structure
  4. Review Synapse source code for canonical JSON implementation
  5. Consider using a Matrix compliance test suite

Files to Review

  • src/main/kotlin/utils/MatrixAuth.kt - Canonical JSON and hashing logic
  • src/main/kotlin/routes/client-server/client/room/FederationJoinHandler.kt - Join flow implementation
  • test-hash.ps1 - Independent hash verification script (proves our hash is correct)