Public key cryptography`, also known as asymmetric cryptography, is a fundamental building block for protecting data and applications. It uses two keys: a public key to be shared with others and a private key to be kept secret. The public key can encrypt messages that only the corresponding private key can decrypt, while the private key can sign messages that can be verified using the public key. This encryption method is used in various scenarios such as SSH, TLS, Bitcoin, PGP, and GPG, and is also employed by Twilio in its Verify API for push authentication and SHAKEN/STIR call spoofing prevention. The security of public-key cryptography relies on trapdoor functions, which are calculations that are easy to do in one direction but hard to reverse unless a key is provided. RSA is a common algorithm used in public-key cryptography, with key sizes impacting its security strength. To ensure the security of public-key cryptography, it's recommended to use well-established libraries and frameworks that implement accepted encryption schemes, rather than rolling one's own crypto.