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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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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Encryption Toolkit 2003 Trial Edition 

The Encryption Toolkit includes all the tools you need to add file encryption and data encryption to your applications. The toolkit includes COM encryption objects with a standard interface for both the Pocket PC and Windows® desktop platforms.
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QuickCrypt Library 

Fast, highly optimized implementation of symmetric (private-key) encryption algorithms: DES, Triple DES (3DES), DESX, BLOWFISH, RIJNDAEL (AES-Advanced Encryption Standard),GOST. Allows to encrypt/decrypt files, memory blocks, strings, streaming data.
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