Transfer file between 2 pi pico set

I am developing an application which needs to transfer a file from a pi pico sd adapter to another via wireless safe link(eavesdrop safe).

I mind to use a ir transceiver kinda: Because it's not RF, thus dificults eavesdrop.
For error correction i am guessing to use zmodem protocol, no library?

Already tried GitHub - gdsports/XYmodem: Arduino XYMODEM file transfer protocol, which works partially, sometimes transfer fails, and i am afraid of SPI bus misbeheaviour(need pull up?). The spi bus also has a sh1106 display.

Any suggestion?

Is this a PI related problem rather than Arduino ?

Confused of Uk

No, i was planning to use IR or NFC.