What is it?
The Cryptographic Application Programming Interface (also known variously as CryptoAPI, Microsoft Cryptography API, MS-CAPI or simply CAPI) is an application programming interface included with Microsoft Windows operating systems that provides services to enable developers to secure Windows-based applications using cryptography.
Description of crypt32.dll from Microsoft
“Cryptography is the use of codes to convert data so that only a specific recipient will be able to read it, using a key. Microsoft cryptographic technologies include CryptoAPI, Cryptographic Service Providers (CSP), CryptoAPI Tools, CAPICOM, WinTrust, issuing and managing certificates, and developing customizable public key infrastructures. Certificate and smart card enrollment, certificate management, and custom module development are also described.”