Notes

Q1.1
voir cahier + https://rietta.com/blog/2012/01/27/openssl-generating-rsa-key-from-command/

Commands>
openssl genrsa -des3 -out tomKey.pem 2048
openssl rsa -in tomKey.pem -text
openssl rsa -in tomKey.pem -outform PEM -pubout -out tomPublic.pubkey

Q1.2
We must use the public key to make sure the text is properly encrypted
We can also do it with a passphrase so it is more secure
Commands>
nano plain.txt
this is a confidential message
openssl
pkeyutl -encrypt -inkey tomKey.pem -in plain.txt -out encRSA.txt

openssl pkeyutl -encrypt -in plain -inkey rsa.key -out encRSA : This command encrypt the file plain with the key rsa.key to the file encRSA

To decrypt : openssl pkeyutl -decrypt -inkey tomKey.pem -in encRSA.txt -out decRSA.txt

test to encrypt the 2 support files

openssl pkeyutl -encrypt -inkey tomKey.pem -in 03support_rsaplain1 -out encRSAPlain
Enter pass phrase for tomKey.pem:
root@kali:~# openssl pkeyutl -encrypt -inkey tomKey.pem -in 03support_rsaplain2 -out encRSAPlain2
Enter pass phrase for tomKey.pem:
Public Key operation error
140669292762768:error:0406D06E:rsa routines:RSA_padding_add_PKCS1_type_2:data too large for key size:rsa_pk1.c:153:

Here obviously the data is too large for the key size, which mean that we should use a larger key in order to encrypt that data

Q1.3

When encrypting, you use their public key to write message and they use their private key to read it.

When signing, you use your private key to write message's signature, and they use your public key to check if it's really yours.

A signature is proof that the signer has the private key that matches some public key. To do this, it would be enough to encrypt the message with that sender's private key, and include the encrypted version alongside the plaintext version. To verify the sender, decrypt the encrypted version, and check that it is the same as the plaintext.

However, this means doubling the size of your transmission - plaintext and ciphertext together (assuming you want people who aren't interested in verifying the signature, to read the message). So instead, typically a signature is created by creating a hash of the plaintext.