If you have an encrypted DMG on Mac and you need to access the contents on a Windows machine, you can use HFSExplorer!
Similar to Microsoft's Bitlocker, you can also encrypt entire drives/partitions with a high-grade security algorithm. And just like Bitlocker can encrypt virtual harddisks (VHD), Mac can encrypt volumes stored in a DMG. Out of the box, the two technologies are not at all compatible. But there are some third-party tools out there which allow you to access encrypted DMGs from Windows.
HFSExplorer is an open-source utility on Windows which can open any type of DMG and also supports opening encrypted DMGs. To install the software, you will need to have 32-bit Java installed (doesn't have to be the most recent version).
After installation, simply double-click the DMG from Explorer (if you haven't turned off file association during the install). If the DMG is encrypted, you will be prompted to enter the passphrase to unlock it. With the correct password entered, you will be able to browse the contents of the DMG.
The only downside to HFSExplorer is that if you wish to access a file, you will have to "extract" it: i.e. copying the file to a location outside of the DMG.
If you have a DMG which is encrypted with a strong encryption algorithm, such as AES-256, you will need to install the Java Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy Files for the installed Java version. Link to JCE for Java 8. Instructions are in the enclosed README.