Quick Answer
The answer is the sender’s private key. In asymmetric cryptography, a digital signature is created by encrypting a hash of the message with the sender’s private key, which only the sender possesses. This ensures authenticity and non-repudiation because the recipient can decrypt the signature using the sender’s public key to verify the message came from them and hasn’t been altered. On the Certified Ethical Hacker CEH exam, this concept tests your understanding of public key infrastructure (PKI) and email security standards like S/MIME and OpenPGP. A common trap is confusing the signing key with the encryption key—remember, the sender signs with their private key but encrypts the message with the recipient’s public key. For a quick memory tip, think “private to sign, public to verify,” or simply recall that your signature is yours alone, just like your private key.
This CEH practice question tests your understanding of cryptography and malware analysis. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company wants to secure its email communications using digital signatures. Which cryptographic key does the sender use to sign the email?
Sender's public key
Why wrong: Public key is used by others to verify, not to sign.
Sender's private key
The sender signs with their private key.
Recipient's private key
Why wrong: Recipient's private key is for decryption.
Recipient's public key
Why wrong: Recipient's public key is for encryption.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Sender's private key
Digital signatures use asymmetric cryptography where the sender signs the email with their private key. The recipient then verifies the signature using the sender's public key, ensuring authenticity and non-repudiation. This is defined in standards like S/MIME (RFC 5751) and OpenPGP (RFC 4880).
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Sender's public key
Why it's wrong here
Public key is used by others to verify, not to sign.
Sender's private key
Why this is correct
The sender signs with their private key.
Related concept
Read the scenario before looking for a memorised answer.
Recipient's private key
Why it's wrong here
Recipient's private key is for decryption.
Recipient's public key
Why it's wrong here
Recipient's public key is for encryption.
Common exam traps
The trap here is that candidates confuse the roles of keys in encryption vs. signing, often selecting the sender's public key because they associate 'public' with sharing, but signing requires the private key to prove the sender's identity.
Detailed technical explanation
Under the hood, the sender's private key generates a hash of the email content and encrypts that hash to create the digital signature. The recipient decrypts the hash with the sender's public key and compares it to a freshly computed hash of the email; a match confirms integrity and sender identity. In S/MIME, the sender's certificate containing the public key is often attached to the signed message for seamless verification.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
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FAQ
Cryptography and Malware Analysis — This question tests Cryptography and Malware Analysis — Read the scenario before looking for a memorised answer..
The correct answer is: Sender's private key — Digital signatures use asymmetric cryptography where the sender signs the email with their private key. The recipient then verifies the signature using the sender's public key, ensuring authenticity and non-repudiation. This is defined in standards like S/MIME (RFC 5751) and OpenPGP (RFC 4880).
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
This CEH practice question is part of Courseiva's free EC-Council certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CEH exam.
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