How is encryption performed when sending an email using a digital signature?

When sending an email with a digital signature, encryption and signing are performed using different cryptographic operations to ensure the integrity and authenticity of the message.

1. Digital Signature: The sender creates a hash (checksum) of the email content and encrypts this hash with their private key. This results in a digital signature, which is attached to the email. The recipient, having the sender's public key, can verify the signature by decrypting the hash and comparing it with their own computed hash of the email.

2. Email Encryption: To ensure confidentiality, the email itself is encrypted. Typically, hybrid encryption is used:

   • A random symmetric key (e.g., AES) is generated.
   • The email is encrypted with this symmetric key.
   • The symmetric key is encrypted with the recipient's public key.

Thus, when sending an email with a digital signature, a signature is first created to verify authenticity, and the email content is encrypted to protect its contents. These operations are often implemented in standards such as S/MIME or PGP.

A simplified process example:

• Sender:

  • Creates a hash of the email.
  • Signs the hash with their private key.
  • Encrypts the email with a symmetric key.
  • Encrypts the symmetric key with the recipient's public key.
  • Sends the encrypted email and signature.

• Recipient:

  • Decrypts the symmetric key with their private key.
  • Decrypts the email with the symmetric key.
  • Verifies the signature using the sender's public key.