uacme.sh can generate invalid challenge due to echo -n

[s-u]

uacme.sh uses /bin/sh in the shebang which often forces shells such as bash into compatibility mode which interprets echo -n as literal output and thus creates an invalid challenge:

uacme: the server reported the following error:
{
    "type": "urn:ietf:params:acme:error:unauthorized",
    "detail": "The key authorization file from the server did not match this challenge \"tt[...]Wuo\" != \"-n tt[...]Wuo\"",
    "status": 403
}

above example was produced with

$ /bin/sh --version
GNU bash, version 3.2.57(1)-release (arm64-apple-darwin20)

and the echo behavior can be checked with

$ /bin/sh -c 'echo -n'
-n

The specs say that servers SHOULD accept challenges with trailing whitespace so the easiest fix is to remove -n. A more advanced fix would be to first check that -n is safe and not use it if it's not.

[s-u]

@ndilieto thanks for the prompt response! I can confirm this fixes the issue on macOS where printf isn an external utility so is not impacted by the shell choice (quick check suggests same is true on Linux and OpenWrt).