nn_idbe.rpl is a library that downloads and decrypts icon databases.
The library can downloads icons for Wii U or 3DS titles.
For Wii U icons,
Replace %02X with (I think?) the first two digits of the second half of the title ID? (passing in 10 works fine) and %016llX with the 16-digit title ID. the last %d is probably the version number.
If you just want the latest icon, you can also use
e.g. the icon for Splatoon is https://idbe-wup.cdn.nintendo.net/icondata/10/0005000010176A00.idbe
For 3DS icons:
These sites use a self-signed SSL certificate, but do allow access without special client certificates (i.e. accessible from a regular browser)
The icon database is encrypted with AES128 CBC. There's a two byte header:
0: (unknown, usually zero?) 1: key index
followed by encrypted data.
nn_idbe contains hardcoded keys in one 0x50 sized block, consisting of the 16-byte hardcoded IV, and 4 hardcoded 16-byte AES128 keys.
In version 11464 of OSv11 (000500101000400A), the keys are located at $DATA+0x4c of nn_idbe.rpl.
Decrypted icon format
There's a SHA256 of the data at the beginning of the file. After the sha, there's some stuff that contains UTF-16 text of the game name and publisher, then image data begins.
The image data is given as Truevision TGA and usually starts 0x2050 bytes into the decrypted buffer.