Signed and Encrypted Mails
In the world of mails, there are two common mechanisms to add security directly into the mail content. Signed mails and encrypted mails, and they are both linked.
They both work on asymmetric encryption.
Asymmetric Encryption, public and private keys.
Obviously there is a lot to say about Asymmetric Encryption but for SOGo 6, we will keep it simple.
In Asymmetric Encryption there are two keys, let’s say A and B.
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With key A, you can encrypt data that you can only decrypt with key B.
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With key B, you can encrypt data that you can only decrypt with key A.
One of the keys will be kept secret to the user/entity: it’s the private key. The other key will be shared with other users/entities: it’s the public key.
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Keep in mind that the term private/public does not indicate what the key is used for. In some protocols, like SSH, the public key will encrypt and the private key will decrypt. But in other protocols, the public key will decrypt and the private key will encrypt |
Last thing, the term Certificate. A certificate contains a key and metadata about the key issuer, if it is trusted or not.
Signed mails
The goal is to ensure that the content of a mail received has not been altered.
User X has generated asymmetric keys. When sending a mail to User Y, a hash of the content is generated then encrypted by the private key. This encrypted data is called the signature and is part of the mail. User X has either already sent its public key to Y or added it to the mail.
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A hash is like a digital fingerprint for data. Using the same hash method, identical data will always generate the same hash. |
User Y receives the mail from X. It will decrypt the signature with X’s public key. Then, that hash is evaluated again and compared to the decrypted signature. If there’s a match, the content has not been altered. If not, something has altered the content and the mail cannot be trusted.
Why does it work
Let’s say someone, called H, intercepts User X’s mail. If H modified the mail, they can generate a new hash but they would not be able to encrypt it as they don’t have the private key.
With signed mail, all users receiving mails from X are able to verify the data integrity!
Encrypted mails
The goal is to receive encrypted emails from users that you’ve already shared your public key with
User X has generated asymmetric keys and has already shared its public key with several users, including User Y.
User Y writes an email to User X. Y will use X’s public key to encrypt the mail content. When X receives the mail, it is able to decrypt it thanks to the private key.
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If both signature and encryption methods are used, note that the public key will be used to decrypt signature received by X and to encrypt mail sent to X. |