Private/public key for decrypt/encrypt reversed

[guydingmike]

I think there is a typo in the following two sections:

https://cryptobook.nakov.com/encryption-symmetric-and-asymmetric.html?q=#private-keys

Message encryption and signing is done by a private key.

Shouldn't this be "Message decryption and signing is done by a private key." ?

https://cryptobook.nakov.com/encryption-symmetric-and-asymmetric.html?q=#public-keys

Message decryption and signature verification is done by the public key.

And shouldn't this be "Message encryption and signature verification is done by the public key." ?

[azadkuh]

same mistake in: key exchange and DHKE:

• In a key transport scheme only one of the parties contributes to the shared secret and the other party obtains the secret from it. Key transport schemes are typically implemented through public-key cryptography, e.g. in the RSA key exchange the client encrypts a random session key by ~its private key and sends it to the server, where it is decrypted using the client's public key~

regardless of algorithm, encrypting by a private key is useless as anybody has access to the public key, so anyone can easily decrypt and reads the original message (or session key)

[ceduliocezar]

@guydingmike just realized the problem that u reported.

Since you reportd it a long time ago and the problem is still there I created a PR for it.

https://github.com/nakov/Practical-Cryptography-for-Developers-Book/pull/29

[Sejal-G]

same mistake in: key exchange and DHKE:

• In a key transport scheme only one of the parties contributes to the shared secret and the other party obtains the secret from it. Key transport schemes are typically implemented through public-key cryptography, e.g. in the RSA key exchange the client encrypts a random session key by ~its private key and sends it to the server, where it is decrypted using the client's public key~

regardless of algorithm, encrypting by a private key is useless as anybody has access to the public key, so anyone can easily decrypt and reads the original message (or session key)

with reference to the last line!! @azadkuh messge encrypted with a public key can be decrypted with private key, and not vice versa! Correct me if I am wrong, I am a newbie at cryptosystems.

[azadkuh]

@Sejal-G you're absolutely right.

some libs even issue an error if a private key is used for encryption, although it may be possible to extract the public key from the private key efficiently (as in RSA)

more ever, it may be mathematically impossible to decrypt a ciphered text by a public key as it does not contain all required bits for decryption.

regardless of mathematical possibility, using a private key for encryption is semantically wrong and compromises the security as everyone can obtain the public key.

ps. I'm not a guru either.