cryptlib 

Security toolkit with strong encryption and authentication. Supports many algorithms for symmetric/asymmetric encryption, authentication, and hashing. Free for non-commercial/shareware projects. Written in C. ActiveX and C++ wrappers are available.
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Xceed Encryption Library 

The Xceed Encryption library allows you to easily add strong encryption and decryption capability to your Windows applications. Supports the latest industry-standard strong encryption algorithms and offers both symmetrical and public-key cryptography
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OpenSSL 

Open SSL 0.9.7c lets applications use Secure Sockets Layer (v2/v3), Transport Layer Security (v1), and a general-purpose cryptography library, for data integrity, authenticity, and privacy security features. Perl or Cygwin needed for Win32 compile.
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