How to use the public key cryptography

Here is the case:

1. 'A' will send the public key to 'B'. Now 'B' will encrypt a file (secret.txt) with public key of 'A'
2. 'A' will receive the encrypted (secret.txt) from 'B'
3. 'A' will decrypt the encrupted file using his private key

Appreciate if you could give the working example using openssl or any any command in linux.
beer9Asked:
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Dave HoweSoftware and Hardware EngineerCommented:
That is accurate, however, openssl uses a more complex solution in practice.

A sends a certificate (Public key and other identifying info, digitally signed) to B
B symmetric encrypts text to A using a random key, then encrypts the key with the PK taken from the certificate
A decrypts the secret key using their private key, and uses the secret key to decrypt the message.

Now that we have that out of the way, Openssl *does* expose the primitives if you want them - it's a case of use at your own risk though. Here are some examples using rsa

to encrypt:
openssl rsautl -in <filename> -out <filename> -pubin -inkey <rsa-public-keyfile> -encrypt

to decrypt:
openssl rsautl -in <filename> -out <filename> -inkey <rsa-private-keyfile> -decrypt

and of course you will be wanting to generate the keys for the above functions:
openssl genpkey -algorithm RSA -out <rsa-private-keyfile> -pkeyopt rsa_keygen_bits:2048
openssl rsa -in <rsa-private-keyfile> -out <rsa-public-keyfile> -pubout

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