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A supplier is sending me very private information. For security, we are using thedecryptor.com website to encrypt the message. The website provides public and private keys.

We agreed that he will sends me a public key before payment, and a private key after payment. I would like to know if it is possible to change the content of the encrypted message while preserving the same public key (i.e, by changing the private key)

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    $\begingroup$ Warning: the website mentioned is mentioning an add server, and performs the encryption / decryption on the server; do not use or inspect. $\endgroup$
    – Maarten Bodewes
    Jul 16 at 18:30

2 Answers 2

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A supplier is sending me very private information. For security, we are using thedecryptor.com website to encrypt the message. The website provides public and private keys.

This website doesn't share any information about how the encryption / decryption happens and should not be trusted for that reason alone.

We agreed that he will sends me a public key before payment, and a private key after payment. I don't know what you are supposed to be doing with the public key. Yes, you can test if the given private key belongs to the same key pair - if you'd have known the algorithm, but nothing prevents the other party to use a different key pair altogether.

I would like to know if it is possible to change the content of the encrypted message while preserving the same public key (i.e, by changing the private key)

Normally you would expect that a change to a different private key would result in an error while decrypting, but that's not necessarily the case for every cryptosystem. As we don't know the algorithm anything can happen. I don't even see the possibility of sharing a public key on that website.

Normally you would make sure that the other party signs the message with a trusted public key, e.g. using sign-then-encrypt.

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I'm a little confused about the process you're doing there. Normally you would do this for an encrypted data transfer:

You generate a public and private key, give the supplier the public key, so that he can encrypt his message with it. After payment he sends you the encrypted data and you can decrypt the whole thing with your private key. This would be the more logical approach for me.

To your other question: As long as the message is encrypted, it is actually not possible to change the content in any meaningful way, unless an unauthorized person has access to the private key. If you would change one of the keys, it would not be possible to decrypt the message again.

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  • $\begingroup$ I think the third section is specific to specific cryptosystems. E.g. in homomorphic encryption you can perform operations on the ciphertext that are resulting in valid ciphertext with a different meaning by definition. So it is not correct to state that as a generic property, in my opinion. $\endgroup$
    – Maarten Bodewes
    Jul 17 at 16:03

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