[AT&T HTC One (M9)] nkk71:
I'm still curious if anybody was able to flash the 2.6.502.18 AT&T RUU and confirm a proper hash

for me both decryption (as mentioned before, shows an error in system_01.img), and I just tried flashing the signed/encrypted versions, which also show an error:

C:\ADB_HTC>htc_fastboot_3.0.9.2.exe oem rebootRUU
...
OKAY [ 0.047s]
finished. total time: 0.047s
htc_fastboot finished. total time: 0.671s

C:\ADB_HTC>htc_fastboot_3.0.9.2.exe flash zip W:\zUBUNTU\m9encRUU\systemfiles\rom.zip_01.zip
htc_fastboot v3.0.9.2 (2015-05-29)
target reported max download size of 800000000 bytes
sending 'zip' (455182 KB)...
OKAY [ 29.671s]
writing 'zip'...
(bootloader) HOSD CL#576582
(bootloader) start@1
(bootloader) system.img_00@8%
(bootloader) system.img_00@16%
(bootloader) system.img_00@25%
(bootloader) system.img_00@33%
(bootloader) system.img_00@41%
(bootloader) system.img_00@50%
(bootloader) system.img_00@58%
(bootloader) system.img_00@66%
(bootloader) system.img_00@75%
(bootloader) system.img_00@83%
(bootloader) system.img_00@91%
(bootloader) system.img_00@100%
(bootloader) Update zip file OK
(bootloader) end@Done
OKAY [ 12.199s]
finished. total time: 41.870s
htc_fastboot finished. total time: 59.545s

C:\ADB_HTC>htc_fastboot_3.0.9.2.exe flash zip W:\zUBUNTU\m9encRUU\systemfiles\rom.zip_02.zip
htc_fastboot v3.0.9.2 (2015-05-29)
target reported max download size of 800000000 bytes
sending 'zip' (452217 KB)...
OKAY [ 35.318s]
writing 'zip'...
(bootloader) HOSD CL#576582
(bootloader) start@1
(bootloader) system.img_01@8%
(bootloader) system.img_01@16%
(bootloader) system.img_01@24%
(bootloader) system.img_01@32%
(bootloader) system.img_01@41%
(bootloader) system.img_01@49%
(bootloader) system.img_01@57%
(bootloader) system.img_01@65%
(bootloader) system.img_01@74%
(bootloader) ERR SPARSE data size=-1
(bootloader) system.img_01@FAIL
(bootloader) ERR inflate fail: uncompressed size(838779556) is not equal
(bootloader) total out size(838779557)
(bootloader) ERR inflate fail, zerr=-3, total_out=838779557
(bootloader) ERR zerr=-3, Z_STREAM_END=1, total_out=838779557
(bootloader) ERR [SD_UPDATE_ERR] unzip_and_update_partition: failed to u
(bootloader) nzip image or update partition[system][err 21]
(bootloader) system.img_01@FAIL
(bootloader) ERR [SD_UPDATE_ERR] do_update_zip_buf: unzip_and_flash_imag
(bootloader) es fail, err 21
(bootloader) ERR [SD_UPDATE_ERR] sddownload failed: 21
(bootloader) ERR Update zip file failed.
(bootloader) system.img_01@FAIL
(bootloader) Update zip file OK
(bootloader) ERR ZIP ERROR CODE, 21
(bootloader) end@Done
FAILED (remote: 21: fail to flash via downloadzip)
finished. total time: 46.472s
htc_fastboot finished. total time: 63.913s

C:\ADB_HTC>htc_fastboot_3.0.9.2.exe flash zip W:\zUBUNTU\m9encRUU\systemfiles\rom.zip_03.zip
htc_fastboot v3.0.9.2 (2015-05-29)
target reported max download size of 800000000 bytes
sending 'zip' (497478 KB)...
OKAY [ 38.438s]
writing 'zip'...
(bootloader) HOSD CL#576582
(bootloader) start@1
(bootloader) system.img_02@8%
(bootloader) system.img_02@17%
(bootloader) system.img_02@25%
(bootloader) system.img_02@34%
(bootloader) system.img_02@42%
(bootloader) system.img_02@51%
(bootloader) system.img_02@59%
(bootloader) system.img_02@68%
(bootloader) system.img_02@76%
(bootloader) system.img_02@85%
(bootloader) system.img_02@93%
(bootloader) system.img_02@100%
(bootloader) Update zip file OK
(bootloader) end@Done
OKAY [ 12.230s]
finished. total time: 50.669s
htc_fastboot finished. total time: 69.670s

C:\ADB_HTC>htc_fastboot_3.0.9.2.exe flash zip W:\zUBUNTU\m9encRUU\systemfiles\rom.zip_04.zip
htc_fastboot v3.0.9.2 (2015-05-29)
target reported max download size of 800000000 bytes
sending 'zip' (420135 KB)...
OKAY [ 32.791s]
writing 'zip'...
(bootloader) HOSD CL#576582
(bootloader) start@1
(bootloader) system.img_03@8%
(bootloader) system.img_03@16%
(bootloader) system.img_03@24%
(bootloader) system.img_03@33%
(bootloader) system.img_03@41%
(bootloader) system.img_03@49%
(bootloader) system.img_03@58%
(bootloader) system.img_03@66%
(bootloader) system.img_03@74%
(bootloader) system.img_03@83%
(bootloader) system.img_03@91%
(bootloader) system.img_03@99%
(bootloader) system.img_03@100%
(bootloader) Update zip file OK
(bootloader) end@Done
OKAY [ 12.355s]
finished. total time: 45.146s
htc_fastboot finished. total time: 60.981s

C:\ADB_HTC>htc_fastboot_3.0.9.2.exe flash zip W:\zUBUNTU\m9encRUU\systemfiles\rom.zip_05.zip
htc_fastboot v3.0.9.2 (2015-05-29)
target reported max download size of 800000000 bytes
sending 'zip' (401687 KB)...
OKAY [ 30.935s]
writing 'zip'...
(bootloader) HOSD CL#576582
(bootloader) start@1
(bootloader) system.img_04@8%
(bootloader) system.img_04@16%
(bootloader) system.img_04@24%
(bootloader) system.img_04@33%
(bootloader) system.img_04@41%
(bootloader) system.img_04@49%
(bootloader) system.img_04@57%
(bootloader) system.img_04@66%
(bootloader) system.img_04@74%
(bootloader) system.img_04@82%
(bootloader) system.img_04@90%
(bootloader) system.img_04@99%
(bootloader) system.img_04@100%
(bootloader) Update zip file OK
(bootloader) end@Done
OKAY [ 15.024s]
finished. total time: 45.959s
htc_fastboot finished. total time: 61.013s

^^ those are the signed&encrypted parts of the RUU (but only the system_xx.img parts)