During a penetration test you intercept S/MIME-protected e-mail traffic. Before attempting cryptanalysis, you review how S/MIME applies confidentiality. Which of the following best describes the way S/MIME encrypts an e-mail message that is being sent to a single recipient?
The message is encrypted with a randomly generated symmetric key, and that key is included in the CMS object encrypted with the recipient's public key.
A Diffie-Hellman exchange between sender and recipient negotiates a shared secret, and no key material is embedded in the message itself.
The entire MIME message is directly encrypted with the recipient's RSA public key without any symmetric key.
Each 16-byte block of the message is separately encrypted with the recipient's public key and then concatenated to form the ciphertext.
S/MIME relies on the Cryptographic Message Syntax (CMS) enveloped-data structure. The sender first generates a one-time symmetric content-encryption key (for example, an AES or 3DES key) and uses it to encrypt the MIME body. The symmetric key is then encrypted with the recipient's X.509 public key and carried inside the CMS wrapper. The recipient uses the corresponding private key to recover the session key and decrypt the content. The other options are inaccurate: S/MIME never encrypts large message bodies directly with an asymmetric algorithm, it does not perform an online Diffie-Hellman exchange, and it does not encrypt each block individually with the recipient's public key because that would be inefficient and would exceed RSA's size limits.
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What is S/MIME and its primary purpose?
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What is the Cryptographic Message Syntax (CMS) in S/MIME?
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How does S/MIME use asymmetric and symmetric encryption together?
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What is Cryptographic Message Syntax (CMS)?
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How does a symmetric key differ from an asymmetric key?
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What is the role of X.509 certificates in S/MIME?
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