To ensure both confidentiality and non-repudiation for secure email communication, you need to use a combination of symmetric and asymmetric cryptography. Symmetric encryption is a method of encrypting and decrypting data using the same secret key, which is faster and more efficient than asymmetric encryption. Asymmetric encryption is a method of encrypting and decrypting data using a pair of keys: a public key and a private key, which are mathematically related but not identical. Asymmetric encryption can provide authentication, integrity, and non-repudiation, as well as key distribution.
The cryptographic technique that would best serve the purpose is to apply asymmetric encryption with RSA and use the private key for signing. RSA is a widely used algorithm for asymmetric encryption, which is based on the difficulty of factoring large numbers. RSA can be used to encrypt data, as well as to generate digital signatures, which are a way of proving the identity and authenticity of the sender and the integrity of the message.
The steps to implement this technique are as follows1:
Generate a pair of keys for each user: a public key and a private key. The public key can be shared with anyone, while the private key must be kept secret and protected by the user.
When a user wants to send an email to another user, they first encrypt the email content with a symmetric key, such as AES, which is a strong and efficient algorithm for symmetric encryption. The symmetric key is then encrypted with the recipient’s public key, using RSA. The encrypted email and the encrypted symmetric key are then sent to the recipient.
The sender also generates a digital signature for the email, using their private key and a hash function, such as SHA-256, which is a secure and widely used algorithm for generating hashes. A hash function is a mathematical function that takes any input and produces a fixed-length output, called a hash or a digest, that uniquely represents the input. A digital signature is a hash of the email that is encrypted with the sender’s private key, using RSA. The digital signature is then attached to the email and sent to the recipient.
When the recipient receives the email, they first decrypt the symmetric key with their private key, using RSA. They then use the symmetric key to decrypt the email content, using AES. They also verify the digital signature by decrypting it with the sender’s public key, using RSA, and comparing the resulting hash with the hash of the email, using the same hash function. If the hashes match, it means that the email is authentic and has not been tampered with.
Using this technique, the email communication is secure because:
The confidentiality of the email content is ensured by the symmetric encryption with AES, which is hard to break without knowing the symmetric key.
The symmetric key is also protected by the asymmetric encryption with RSA, which is hard to break without knowing the recipient’s private key.
The non-repudiation of the email is ensured by the digital signature with RSA, which is hard to forge without knowing the sender’s private key.
The digital signature also provides authentication and integrity of the email, as it proves that the email was sent by the sender and has not been altered in transit.
References:
How to Encrypt Email (Gmail, Outlook, iOS, Yahoo, Android, AOL)
