macOS: ffmpeg failed to open remote file using sftp protocol over proxy (crash)

Environment:

• macOS: 10.13
• proxychains-ng: stable 4.12
• ffmpeg: stable 3.4

Behavior:

ffmpeg can directly open remote file using sftp successfully, but failed over proxychains4.

Error:

~ ❯❯❯ proxychains4 ffmpeg -i sftp://john@192.168.3.2:/home/john/downloads/input.mp4 -r 24 output.avi -v trace
[proxychains] config file found: /usr/local/etc/proxychains.conf
[proxychains] preloading /usr/local/Cellar/proxychains-ng/4.12_1/lib/libproxychains4.dylib
[proxychains] DLL init: proxychains-ng 4.12
ffmpeg version 3.4 Copyright (c) 2000-2017 the FFmpeg developers
[NULL @ 0x7fa05b800000] Opening 'sftp://john@192.168.3.2:/home/john/downloads/input.mp4' for reading
[sftp @ 0x7fa059c09860] No default whitelist set
[proxychains] Strict chain  ...  127.0.0.1:8889  ...  192.168.3.2:22  ...  OK
Assertion failed: (ctx), function digest_update, file /BuildRoot/Library/Caches/com.apple.xbs/Sources/boringssl/boringssl-109.20.5/apple/crypto/digests.c, line 49.
[1]    473 abort      proxychains4 ffmpeg -i sftp://john@192.168.3.2:/home/john/downloads/input.mp4

Debug:

I use otool to confirm ffmpeg links the libssh of homebrew instead of macOS.

~ ❯❯❯ proxychains4 otool -L /usr/local/bin/ffmpeg
[proxychains] config file found: /usr/local/etc/proxychains.conf
[proxychains] preloading /usr/local/Cellar/proxychains-ng/4.12_1/lib/libproxychains4.dylib
/usr/local/bin/ffmpeg:
......
	/usr/local/opt/libssh/lib/libssh.4.dylib (compatibility version 4.0.0, current version 4.4.2)
......

ffmpeg and proxychains4 both don’t link the libboringssl, and why is it involved in the problem? I am not sure if the error is a proxychains4 issue.

Start by reproducing the macOS crash with the documented proxychains4 ffmpeg SFTP command and compare it with direct ffmpeg access. Inspect proxychains-ng's socket-hooking path and the reported libssh linkage, then determine whether the assertion is caused by proxychains-ng or an interaction with the linked dependencies; done means identifying the component responsible and documenting or fixing the crash.