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BoringSSL is a modern fork of OpenSSL developed by Google for use in their own products. It is probably the library with the most similar aims to Botan. https://opensource.google.com/projects/boringssl
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Crypto++ is a C++ library of cryptographic schemes. It has been in wide use for many years. https://cryptopp.com
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libsodium offers a limited but very useful set of cryptographic tools with a C API. https://libsodium.org/