simplexmq/spec/modules/Simplex/FileTransfer/Transport.md

Simplex.FileTransfer.Transport

XFTP protocol types, version negotiation, and encrypted file streaming with integrity verification.

Source: FileTransfer/Transport.hs

xftpClientHandshakeStub — XFTP doesn't use TLS handshake

xftpClientHandshakeStub always fails with throwE TEVersion. The source comment states: "XFTP protocol does not use this handshake method." The XFTP handshake is performed at the HTTP/2 layer — XFTPServerHandshake and XFTPClientHandshake are sent as HTTP/2 request/response bodies (see FileTransfer/Client.hs and FileTransfer/Server.hs).

receiveSbFile — constant-time auth tag verification

receiveSbFile validates the authentication tag using BA.constEq (constant-time byte comparison). The auth tag is collected from the stream after all file data — if the file data ends mid-chunk, the remaining bytes of that chunk are used first, and a follow-up read provides the rest of the tag if needed.

receiveFile_ — two-phase integrity verification

File reception has two verification phases:

1. During receive: either size checking (plaintext via hReceiveFile) or auth tag validation (encrypted via receiveSbFile)
2. After receive: LC.sha256Hash of the entire received file is compared to chunkDigest

sendEncFile — auth tag appended after all chunks

sendEncFile streams encrypted chunks via LC.sbEncryptChunk, then sends LC.sbAuth sbState (the authentication tag) as a final frame when the remaining size reaches zero.