php Function sodium_crypto_Box_Secure

php function sodium_crypto_box_secretkey is part of the libsodium cryptography library, which is widely used by modern web applications for security. The function provides low-level access to libsodium's crypto_pwhash key derivation function. This function is intended for special purpose applications and should not be used in general code. Rather, use the sodium_pwhash() or password_hash() functions.

Libsodium is an opinionated cryptography library with secure and sensible defaults. Unlike older libraries, such as mcrypt and OpenSSL, it provides support for authenticated asymmetric encryption/decryption.

For this to work, both parties involved must exchange their public keys securely. They must also generate a nonce value for each message, and store it securely as well. If the key and nonce values are not identical, then the decrypted message will not match the original.

The php function sodium_crypto_box_secretkey() encrypts a message so that only the recipient can read it. The message is padded with a unique authentication tag, which is stored as an additional field in the encrypted string. The function requires the sender's secret key, and the recipient's public key. It also requires a unique nonce value for the message, which must be generated at the time of encryption.

The function uses the elliptic curve Diffie-Hellman algorithm over the Montgomery curve, Curve25519; commonly abbreviated as X25519. The ciphertext is then returned as a raw binary string, which must be converted to text by using functions such as bin2hex() or base64_encode() before it can be printed or transmitted. The ciphertext is 16 bytes longer than the plaintext, and contains an authentication tag at the end.