how to hook File operations to return a new file size to user？

[myseemylife]

firstly，thx for this amazing project，let I can hook NtCreateFile NtOpenFile NtCReadFile NtClose in ntdll.dll
i use it to dencrypt some private file ,
while the issue is. that.the source private file size is A bytes, after dencrypted. the decoded file size is B bytes. but the user always use the A to read all content. so after the read logical..the left (A-B) useless bytes are return to user buffer. how can i avoid this?
Any suggestion ? sry for my bad english.