What Is Asymmetric Encryption? Encrypting with a Public Key
A legal department sends a confidential contract to an outside partner without first exchanging a shared secret. The sender encrypts the document with the partner's public key so that only the partner can decrypt it with the matching private key. Which cryptographic approach is being used?
Quick Answer
Asymmetric encryption is the correct cryptographic approach because it uses a public key to encrypt data and a corresponding private key to decrypt it, eliminating the need to pre-share a secret. In the scenario, the legal department encrypts the contract with the partner’s public key, ensuring that only the partner’s private key—never transmitted or shared—can unlock the document. This concept is central to the Security+ SY0-701 exam, where you must distinguish asymmetric encryption from symmetric encryption, which relies on a single shared key. A common trap is confusing the roles: remember that encrypting with a public key guarantees confidentiality, while encrypting with a private key provides non-repudiation. For the exam, think of asymmetric encryption as a “lockbox” where anyone can lock it with a public key, but only the owner’s private key can open it. A helpful mnemonic is “Public to Protect, Private to Prove”—the public key protects data in transit, and the private key proves ownership.
⚠ Common exam trap
Watch out — candidates often confuse the use of a public key for encryption with a digital signature, which uses the private key for signing, or mistakenly think that any use of a key pair is symmetric encryption.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Asymmetric encryption
B is correct because the scenario describes encrypting a document with the recipient's public key, which can only be decrypted by the recipient's private key. This is the defining characteristic of asymmetric encryption (also known as public-key cryptography), where two different but mathematically related keys are used for encryption and decryption.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Symmetric encryption
Why it's wrong here
Symmetric encryption would require both parties to already share the same secret key, which the scenario explicitly says they do not.
- ✓
Asymmetric encryption
Why this is correct
Asymmetric encryption uses a public key to encrypt data and a corresponding private key to decrypt it. That makes it ideal when two parties have not yet shared a secret. In this scenario, the sender uses the partner's public key so only the partner's private key can open the contract, preserving confidentiality across an untrusted network.
- ✗
Hashing
Why it's wrong here
Hashing can confirm whether data changed, but it does not encrypt the contract or allow the recipient to recover the contents.
- ✗
Digital signatures
Why it's wrong here
Digital signatures verify origin and integrity, but the scenario focuses on keeping the contract confidential from anyone except the intended recipient.
Quick reference
Symmetric Encryption Algorithm Comparison
| Algorithm | Key Size | Block Size | Status | Notes |
|---|---|---|---|---|
| AES-128 | 128-bit | 128-bit | Current standard | NIST approved; WPA3, TLS |
| AES-256 | 256-bit | 128-bit | Current standard | Preferred for sensitive / govt data |
| 3DES | 112-bit effective | 64-bit | Deprecated (2023) | Replaced by AES |
| DES | 56-bit | 64-bit | Broken | Cracked in < 24 h; never deploy |
| ChaCha20 | 256-bit | Stream cipher | Current | TLS 1.3, WireGuard |
Go deeper
Related to this question
Learn chapter
Cryptography Fundamentals
Key term
PSK
A pre-shared key (PSK) is a secret string of characters shared in advance between two parties to authenticate and encrypt wireless or VPN communications.
Key term
Encryption
Encryption is the process of converting readable data into a secret code to prevent unauthorized access.
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Same concept, more angles
1 more way this is tested on SY0-701
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A legal team must send a confidential contract to a partner so only the intended recipient can read it, and the partner also needs assurance the file really came from your company. Which approach best meets both needs?
medium- A.Hash the contract and email the hash value separately.
- ✓ B.Encrypt the file with the recipient's public key and sign it with the sender's private key.
- C.Use a shared symmetric key and send the key in the same email message.
- D.Compress the file and password-protect the archive with a simple passphrase.
Why B: It uses asymmetric encryption to ensure confidentiality (encrypting with the recipient's public key ensures only the intended recipient can decrypt it with their private key) and digital signing (signing with the sender's private key provides non-repudiation and authenticity, proving the file came from the sender). This combination directly addresses both requirements: only the partner can read the contract, and the partner can verify the sender's identity.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This SY0-701 practice question is part of Courseiva's free CompTIA 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 SY0-701 exam.