Extracts the secret key from a crypto_box keypair

# sodium_crypto_box_secretkey

(PHP 7 >= 7.2.0, PHP 8)

sodium_crypto_box_secretkey — Extracts the secret key from a crypto_box keypair

### Description

sodium_crypto_box_secretkey(#[\SensitiveParameter] string `$key_pair`): string

Given a keypair, fetch only the secret key.

### Parameters

`key_pair`
  A keypair, such as one generated by sodium_crypto_box_keypair() or sodium_crypto_box_seed_keypair()

### Return Values

X25519 secret key.

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