Resource

RF / Microwave:

• Microwave&RF: Whitepapers and articles written by well-experienced engineers. A good source for RF test equipments application notes and new methods / equipments for RF measurements. 
• Analog-Dialogue: Application notes and a periodical on RF analog circuit design.
• Mixer: An introduction on RF up/down converter circuit models.

Cryptography:

• Handbook of Applied Cryptography 
• Public-Key Cryptanalysis
• Homomorphic Crypto: Remote processing a private data set, where the computing unit only knows the encrypted data (cannot decrypt the data). Applications: in private search queries.

Mathematics:

• Clay Mathematics Institution
• Crux Mathematicorum
• Mathematicalcircles

Tools:

• syntax-highlighter