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Let's say we have an e-mail of 10MB, and we want to distribute this to 1000 users. Before sending the e-mail, I encrypt it. And all users have my public key to decrypt it.

What encryption method can I use with less computational resources and produce as less possible e-mails? But compression methods are not allowed like hashing.

schroeder
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wasilikoslow
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    Public key is used to encrypt, not decrypt. Each user should create their own private/public keypair, and send you their public key. Then, you create a random symmetric AES key, encrypt the file with that key, then host the file on a web server. Then, for each user, encrypt the AES key with that user's public key, and send them the link to download the encrypted file, along with the AES key encrypted with the user's public key. Then, the user decrypts the AES key using their private key, downloads the encrypted file, and decrypts it using the key. – mti2935 Aug 02 '22 at 17:09
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    Is your concern the performance cost? I'm not seeing a security issue here. – schroeder Aug 02 '22 at 17:12
  • *"compression methods are not allowed like hashing"* - hashing is not a compression method. With hashing the original information are lost – Steffen Ullrich Aug 02 '22 at 17:17
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    Related: https://security.stackexchange.com/questions/252745/sharing-files-with-others-encrypted-on-untrustworthy-file-storage-e-g-the-clo/ – mti2935 Aug 02 '22 at 20:07
  • I am new to security and cryptographic terms and methods, lots of stuff to grasp. Yes I realize even if I use hashing, there is no going back so that does not work. But the related link seems what I was looking for :) one little question more, if the keys are shared beforehand, does regular symmetric key method provides authenticity ? @mti2935 – wasilikoslow Aug 03 '22 at 06:47
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    @wasilikoslow I hope you find the world of information security and cryptography to be as fascinating (and challenging) as most of us on this board. If you use AES in an authenticated encryption mode such as (AES-GCM), this will provide confidentiality, as well as message authentication (also sometimes known as message integrity). – mti2935 Aug 03 '22 at 11:15

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