How to declare a block

Are you really sure on how to declare a block? How about all 4 variations? On iOS and macOS, it can be easy to forget Objective-C block syntax as it isn’t the most intuitive, especially now Swift is becoming the main language of new projects. Remembering the syntax is actually just a short click away but here are easy-to-read example uses with nullability tips and things to watch out for when declaring a block.
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Encrypt data using AES and 256-bit keys

AES stands for Advanced Encryption Standard and is an industry-standard algorithm for encrypting data symmetrically which even the US government has approved for SECRET documents. If you don’t know what symmetrical encryption is, it means that you use the same key or password to encrypt the data as you do to unencrypt it. So you need to keep the key extremely secret however it’s still incredibly useful. I’ll show you how to use openssl to encrypt some data and decrypt it using the Common Crypto libraries on iOS.
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