A fellow B593 hacker Mr. Ronkainen from blog.asiantuntijakaveri.fi informed me about his findings regarding /var/curcfg.xml password encryption. This is something I did already spit-ball with him in comments, but this time he had something concrete to show.
This is for decrypting an FTP-password. Since you can set your own, you definitely know what the plaintext password is. His findings are:
exe->Data_DbDecrypt(nil, "llxYjYnY:\021\003\2324\275\241\233Wu\353$Vx;\333#", "", "" <unfinished ...>
exe->strncpy(0x7facddd8, "llxYjYnY", 8) = 0x7facddd8 (Data_DbDecrypt)
exe->strcpy(0x7facdaf8, "12345678") = 0x7facdaf8 (Data_getProductInfo)
exe->strncpy(0x7facdb01, "12345678", 9) = 0x7facdb01 (Data_getKey)
exe->strncat("12345678", "llxYjYnY") = "12345678llxYjYnY" (Data_getKey)
<... Data_DbDecrypt resumed> ) = nil
exe->strcpy(0x4ce009, "BBBB") = 0x4ce009
The first call is for the raw input data. It clearly contains 8 characters, a colon (:) and something encrypted after it. Then there is a surprising part, call to a function named Data_getProductInfo() returning hard-coded 12345678 every time. Based on the code, the "product info" is simply concatenated into the Base64-decoded 8 char prefix, forming a 16 byte encryption key.
I've already speculated, that they changed encryption in SP100+ from 3-DES to AES. Based on the function names in firmware libries, combine that with knowledge of block ciphers and give it a go with AES-128 ECB with the above keying. Hey presto! It works! 
I wrote a public tool for doing password encryptions/decryptions: http://blog.hqcodeshop.fi/B593/password_recover.php The sources for my web-thing are also there, if you want to use that by yourself.
As you can see from the form, I cannot work with the previous 3-DES stuff. It's simply because I don't know what the key/IV are. There is also another thing with web-GUI and SSH-passwords. They are not using the above keying mechanism. My speculation is, that they are using AES-256 (possibly in ECB-mode) for those, but I have no details about the key.
If you want to test the password recovery, you'll need your /var/curcfg.xml at hand. Pick an encrypted password from that, for example:
<X_FTPServiceInstance InstanceID="1" Username="test" Password="bU50RkQ1T2o6UNkuA7Bdj40/TiNehA6fDw==" FtpUserEnable="1" Privilege="2" Path="usb2_1/../.."/>
or
<WEPKeyInstance InstanceID="4" WEPKey="bU50RkQ1T2o69goRBo2nWOh00YDVCHLGDw=="/>
Select web-form Target as FTP-user, copy/paste the value from XML Password-field into Base64-encoded and klick decrypt. It should give you "test" as Plain-text value. There is another example for Wi-Fi WPA-key, it says WEP in the XML-file, but we can ignore that.
I'll keep investigating the other passwords too. Mr. Ronkainen suggested, that something in the box could be encrypted with PKCS#1, but the block size is off, at least in passwords. Stay tuned for more updates.