Plugin alters file on upload, breaking koreader sync.

[dppattison]

Koreader sync depends on a binary hash to match identical book files across devices. This calibre plugin is altering the epub on upload, making sync impossible. It would be nice if there was a toggle for this, although it's not clear why it's necessary at all.

After using the plugin to upload files, a test:

finn@lc:~/book/Calibre/Cormac McCarthy/Suttree (39)$ wget http://crosspoint.local/download?path=%2Fbook%2FMcCarthy%2C%20Cormac%2FSuttree%20-%20Cormac%20McCarthy.epub -O ~/Downloads/tmp/sutt.epub
--2026-04-17 22:38:44--  http://crosspoint.local/download?path=%2Fbook%2FMcCarthy%2C%20Cormac%2FSuttree%20-%20Cormac%20McCarthy.epub
Resolving crosspoint.local (crosspoint.local)... 192.168.50.249
Connecting to crosspoint.local (crosspoint.local)|192.168.50.249|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 2144076 (2.0M) [application/epub+zip]
Saving to: ‘/home/finn/Downloads/tmp/sutt.epub’

/home/finn/Downloads/tm 100%[============================>]   2.04M   331KB/s    in 6.3s    

2026-04-17 22:38:51 (331 KB/s) - ‘/home/finn/Downloads/tmp/sutt.epub’ saved [2144076/2144076]

finn@lc:~/book/Calibre/Cormac McCarthy/Suttree (39)$ md5sum ~/Downloads/tmp/sutt.epub 
1b7c7c1a63d7c45c590eb60da5734459  /home/finn/Downloads/tmp/sutt.epub
finn@lc:~/book/Calibre/Cormac McCarthy/Suttree (39)$ md5sum Suttree\ -\ Cormac\ McCarthy.epub 
a932769d8ae169e300fc6b1bd18ab74b  Suttree - Cormac McCarthy.epub
finn@lc:~/book/Calibre/Cormac McCarthy/Suttree (39)$

[dppattison]

This actually seems to be regular old file corruption on upload rather than something being done intentionally. Around 100 bytes out of 2.1mb changed in the epub (zip file) on upload, which was below the threshold to be obscured by deflation in this case, but still breaks hashing.

Comparing the two different-hash files, archive as downloaded and post-extraction:

finn@lc:~/Downloads/tmp$ md5sum suttorig.epub ; md5sum suttx4.epub 
a932769d8ae169e300fc6b1bd18ab74b  suttorig.epub
1b7c7c1a63d7c45c590eb60da5734459  suttx4.epub
finn@lc:~/Downloads/tmp$ diff <(xxd suttorig.epub) <(xxd suttx4.epub) | wc -l
472
finn@lc:~/Downloads/tmp$ ls -la suttorig.epub 
-rw-rw-r-- 1 finn finn 2144076 Apr 14 21:05 suttorig.epub
finn@lc:~/Downloads/tmp$ ls -lah suttorig.epub 
-rw-rw-r-- 1 finn finn 2.1M Apr 14 21:05 suttorig.epub
finn@lc:~/Downloads/tmp$ diff <(xxd suttorig.epub) <(xxd suttx4.epub) | wc -l
472
finn@lc:~/Downloads/tmp$ diff <(xxd suttorig.epub) <(xxd suttx4.epub) | head -n 22
1c1
< 00000000: 504b 0304 1400 0008 0000 bda8 8e5c 6f61  PK...........\oa
---
> 00000000: 504b 0304 1400 1608 0000 bda8 8e5c 6f61  PK...........\oa
5c5
< 00000040: 0008 0800 cf98 7c46 1a56 5883 cc03 0000  ......|F.VX.....
---
> 00000040: 1608 0800 cf98 7c46 1a56 5883 cc03 0000  ......|F.VX.....
68c68
< 00000430: 0000 0808 00cf 987c 464b 514a 4f9b 0100  .......|FKQJO...
---
> 00000430: 0016 0808 00cf 987c 464b 514a 4f9b 0100  .......|FKQJO...
96c96
< 000005f0: acfe 0350 4b03 0414 0000 0808 00cf 987c  ...PK..........|
---
> 000005f0: acfe 0350 4b03 0414 0016 0808 00cf 987c  ...PK..........|
135c135
< 00000860: 7dc6 7156 fda8 4bff 0050 4b03 0414 0000  }.qV..K..PK.....
---
> 00000860: 7dc6 7156 fda8 4bff 0050 4b03 0414 0016  }.qV..K..PK.....
148c148
< 00000930: 0050 4b03 0414 0000 0808 00cf 987c 4621  .PK..........|F!
finn@lc:~/Downloads/tmp$ diff -ra suttorig/ suttx4/
finn@lc:~/Downloads/tmp$ 

[itsthisjustin]

As far as I understand it, Calibre changes the file on upload to apply metadata and such and that's why it's recommended to use the same Calibre instance to send to other KOReader devices so that the hash is the same