cryptography - 为不同的收件人重复使用随机数？

Question

我正在生成一个新的随机对称密钥，并希望将其传递给使用crypto_box_easy. 可以为相同的消息和相同的发件人但为不同的收件人重复使用相同的（随机）随机数吗？可以将相同的随机数用于具有随机密钥和的对称加密crypto_secretbox_easy吗？

由于 nonce 必须与加密消息一起提供，它无论如何都不能被隐藏，但是在多个不同的接收者之间重复使用是个问题吗？如果他们提供了一个生成错误的公钥，那是否会以一种可以提取其他人的密钥的方式削弱加密？

非常感谢。

Answer 1

A nonce can be reused as long as a (key, nonce) tuple is not reused.

You're right that reusing a nonce with the same key would result in a catastrophic loss of privacy with a stream cipher like XSalsa20.

The thing is, crypto_box_easy uses the recipients public key to generate a shared secret that is then used with a nonce.

Thus even with a static nonce, the (nonce, key) pair for each recipient will be different.

Although, it's not acceptable to use the same (nonce, key) pair twice, you can use the same nonce for each recipient, but only once.

It's acceptable to use the same nonce once for each recipient using the crypto_box_easy construct ONCE.

It even states this in the libsodium documentation:

The nonce doesn't have to be confidential, but it should be used with just one invocation of crypto_box_easy() for a particular pair of public and secret keys.

i.e. for one message per recipient.