Several months ago I wrote about encrypting and decrypting RDP passwords. I left one thing open: encrypting the password up to the full 1329 bytes as mstsc does.
Many people were curious about it so I hope the answer is not a disappointment because it’s actually really simple (but I took me a while to figure that out nonetheless). In what I figure is an attempt to hide the password length mstsc always fills up the password with zeroes until it has 512 bytes length.
Then the password is encrypted like I described earlier which gives us a 1328 bytes password hash. So we have one mystery left, how to reach the 1329 bytes size which still is a strange value since the password is in Unicode which takes 2 bytes per char (so the size should be even).
As it turns out, mstsc just adds a zero!Remote Desktop Password Encryption & Decryption Tool (3961)