Can one encrypt with a private key/decrypt with a public key?

Question

[Disclaimer: I know, if you know anything about crypto you\'re probably about to tell me why I\'m doing it wrong - I\'ve done enough Googling to know this seems to be the ty

Answer 1

The answer to your question "would the act of decrypting be evidence that it was generated with the private key", the answer is yes if the recipient can do simple validation of the data. Say you have "User name : John, timestamp : , expiry : dd/mm/yyyy". Now if a wrong public key is used to decrypt, the probability that you will get "User name : , timestamp : , expiry : ??/??/????" is zero. You can validate using a regular expression (regex) like "User name : [a-zA-Z]+, timestamp : [0-9]+, expiry : .... " and drop it validation fails. You can even check if the day is between 1 and 31, month is between 1 and 12 but you won't get to use it as regex will typically fail at "User name : " if wrong public key is used. If validation succeeds you still have to check the timestamp and ensure that you don't have a replay attack.

However, consider the following:

1. RSA public key crypto is not designed for bulk encryption of structured data as it can be exploited by attacker. public key crypto is typically used in 2 ways: 1) for securely transporting the symmetric key (which by definition has no structure) which will be used in bulk encryption; this is done by encrypting the symmetric key using the public key and 2) digitally signing a document by encrypting not the document but the hash of the document (again something which has no structure) using the private key. In your case you are encrypting the cookie which has a definite structure.
2. You are depending on the public key not getting into the hands of the wrong person or entity.
3. public key encryption is about 1000 times slower that symmetric key encryption. This may not be a factor in you case.

If you still want to use this approach, and are able to distribute the public key only to the "trusted partners", you should generate a random session key (i.e. symmetric key), encrypt it using the private key and send to all recipients who have the public key. You then encrypt the cookie using the session key. You can change the session key periodically. More infrequently you can change the public key also: you can generate a new public/private key pair and encrypt public key with the session key and send it to all recipients.