References only, not question text. Open Official paper ↗ where Cambridge publishes the paper,
or see the official Cambridge past-papers page; for other papers, ask your
school. “Paper 42” means Paper 4, variant 2.
Practice questions
Original questions written by HomiLabs in the style of the exam, with answers and mark schemes. They are not
Cambridge past paper questions and have not been reviewed by Cambridge. Try each one before opening the answer.
Q1Digital certificates & signatures · Core · Paper 3 style · 6 marks
(a)State what is meant by a digital certificate.[2]
(b)Give four items of data that are found in a digital certificate.[4]
Show answer
(a) An electronic document issued by a trusted certificate authority that links (binds) a public key to the identity of its owner, so that others can trust the key belongs to that person or organisation.
B1 issued/signed by a certificate authority. B1 confirms that a public key belongs to a named owner.
(b) Any four of: owner's (subject's) name/organisation or domain; owner's public key; serial number; validity period (start and expiry dates); name of the issuing CA; the CA's digital signature; the algorithm used for the signature; certificate version.
1 mark each for any four.
Q2Digital certificates & signatures · Core · Paper 3 style · 5 marks
(a)State the purpose of a digital signature.[2]
(b)State which key is used to create a digital signature, and which key is used to check it.[2]
(c)State whether a digital signature keeps the content of the message secret. Give a reason.[1]
Show answer
(a) To prove that a message really came from the stated sender (authentication / non-repudiation) and that it has not been altered since it was signed (integrity).
B1 authenticity of sender / non-repudiation. B1 integrity – message not altered.
(b) Created with the sender's private key; checked with the sender's public key.
B1 each.
(c) No – the message itself is not encrypted by signing (and the signature can be checked by anyone with the public key).
B1.
Q3Digital certificates & signatures · Core · Paper 3 style · 4 marks
(a)Describe the role of a certificate authority (CA).[3]
(b)Explain why a web browser can check a certificate signed by a CA without contacting that CA first.[1]
Show answer
(a) A CA is a trusted organisation that checks the identity of people or organisations applying for certificates, issues digital certificates that link their identity to their public key, signs the certificates with its own private key, and revokes certificates that are no longer valid.
1 mark each, max 3: trusted third party; verifies applicant's identity; issues certificates; signs them with the CA's private key; maintains revocation list / revokes certificates.
(b) The browser/operating system already stores the trusted root CAs' certificates, including their public keys, so it can check the CA's signature directly.
B1.
Q4Digital certificates & signatures · Core · Paper 3 style · 3 marks
(a)Explain the difference between a digital signature and a digital certificate.[3]
Show answer
(a) A digital signature is created by the sender for a particular message (an encrypted hash of that message) to prove who sent it and that it is unchanged. A digital certificate is issued by a CA to an owner and is used to prove that a public key belongs to that owner. The certificate itself contains the CA's digital signature, and it supplies the public key used to check the sender's signature.
B1 signature – made by sender, unique to each message, proves origin/integrity. B1 certificate – issued by CA, proves ownership of public key. B1 link: certificate provides the trusted public key used to verify signatures / certificate is signed by the CA.
Q5Digital certificates & signatures · Standard · Paper 3 style · 4 marks
Mr Khan, a contractor in Multan, sends a signed quotation by email to a client.
(a)Describe how Mr Khan's software creates the digital signature for the quotation.[3]
(b)Explain why the signature is different for every quotation he sends.[1]
Show answer
(a) The quotation is put through a hashing algorithm to produce a message digest (hash). The digest is encrypted using Mr Khan's private key; this encrypted digest is the digital signature. The signature is attached to the quotation and both are sent.
B1 hash/digest of the message produced. B1 digest encrypted with Mr Khan's private key. B1 signature sent with the message.
(b) The digest depends on the content of the message, so different quotations give different digests and therefore different signatures.
B1.
Q6Digital certificates & signatures · Standard · Paper 3 style · 6 marks
(a)A client receives a message and a digital signature. Describe how the client's software checks the signature.[4]
(b)State two possible reasons why the two digests might not match.[2]
Show answer
(a) It obtains the sender's public key (from the sender's digital certificate). It decrypts the signature with that public key to get the original message digest. It applies the same hashing algorithm to the received message to make a new digest. It compares the two digests: if they match, the message is authentic and unchanged; if not, it has been altered or did not come from the sender.
1 mark each: public key of sender obtained (certificate); signature decrypted with sender's public key; received message hashed with the same algorithm; digests compared – match means authentic and unaltered.
(b) The message was altered (tampered with or corrupted) in transmission; it was signed with a different private key (not from the claimed sender / wrong public key used).
1 mark each.
Q7Digital certificates & signatures · Standard · Paper 3 style · 5 marks
An online tutoring company in Lahore wants a digital certificate for its website.
(a)Describe the steps the company follows to obtain a digital certificate.[4]
(b)State what the company must never send to the CA.[1]
Show answer
(a) The company generates a public/private key pair. It sends a certificate request to a CA containing its public key and details of its identity (organisation name, domain). The CA checks the company's identity and that it controls the domain. The CA creates the certificate containing the company's details and public key, signs it with the CA's private key, and sends it to the company, which installs it on its web server.
1 mark each, max 4: key pair generated; request with public key and identity details sent to CA; CA verifies identity/domain; CA creates certificate and signs with its private key; certificate issued/installed on server.
(b) Its private key.
B1.
Q8Digital certificates & signatures · Standard · Paper 3 style · 4 marks
(a)When a user visits a website using HTTPS, the browser checks the website's digital certificate. Describe three checks the browser makes.[3]
(b)State what the browser does if one of the checks fails.[1]
Show answer
(a) It checks that the certificate was signed by a trusted CA (by verifying the CA's signature with the CA's public key); that the certificate is within its validity dates (not expired); that the domain name in the certificate matches the website address; and that the certificate has not been revoked.
1 mark each for any three.
(b) It displays a security warning / blocks the connection and tells the user the site may not be secure.
B1.
Q9Digital certificates & signatures · Challenge · Paper 3 style · 7 marks
A simplified example of signing uses tiny keys. The sender's private key is (d = 27, n = 55) and public key is (e = 3, n = 55).The 'digest' of a message is the sum of the ASCII codes of its characters MOD 55. The signature is digest^d MOD n; it is checked by calculating signature^e MOD n.ASCII codes: P 80, A 65, Y 89, space 32, 0 48, 5 53, 9 57.
(a)Calculate the digest of the message PAY 500.[2]
(b)You are told that 30²⁷ MOD 55 = 35, so the signature is 35. Show how the receiver checks this signature.[2]
(c)An attacker changes the message to PAY 900 but keeps the signature 35. Show how the receiver detects this.[2]
(d)Explain why the attacker cannot create a new valid signature for PAY 900.[1]
(b) 35³ = 42 875; 42 875 MOD 55 = 30, which equals the digest of the received message, so the signature is valid.
M1 35³ MOD 55 calculated = 30. A1 compared with digest of received message – match.
(c) Digest of PAY 900 = 419 MOD 55 = 34. Checking the signature still gives 30. 34 ≠ 30, so the message has been altered.
M1 new digest 34. A1 does not match 30 → tampering detected.
(d) A valid signature needs the sender's private key, which the attacker does not have.
B1.
Q10Digital certificates & signatures · Challenge · Paper 3 style · 6 marks
A software company in Lahore distributes updates for its accounting app. Each update is code-signed: it carries a digital signature and the company's digital certificate.
(a)Explain how code signing protects users who install an update.[3]
(b)The company discovers that its private key has been stolen. Describe what should happen and why.[3]
Show answer
(a) The user's computer checks the certificate (signed by a trusted CA) to get the company's genuine public key, then verifies the update's signature with it. If the update has been changed (e.g. malware inserted) or was not produced by the company, the digests will not match and the installation is blocked/warned.
B1 certificate verified to obtain trusted public key. B1 signature verified / digest comparison. B1 tampered or fake updates detected and not installed.
(b) The company tells the CA, which revokes the certificate (adds it to its revocation list), so software will no longer trust signatures made with that key. The company generates a new key pair, obtains a new certificate and re-signs its updates. Otherwise the thief could sign malware that users' computers would accept as genuine.
B1 certificate revoked by CA. B1 new key pair/certificate obtained. B1 reason – thief could sign malicious software as the company.
Q11Digital certificates & signatures · Challenge · Paper 3 style · 5 marks
(a)Explain why a digital signature encrypts a hash of the message rather than the whole message.[2]
(b)State three properties a hashing algorithm needs for use in digital signatures.[3]
Show answer
(a) A hash is short and of fixed length, so encrypting it with the slow asymmetric algorithm is much quicker than encrypting a long message; the hash still represents the whole message because any change to the message changes the hash.
B1 hash is small/fixed length – faster to encrypt/decrypt. B1 still detects any change to the message.
(b) It always gives the same digest for the same message; a tiny change in the message gives a completely different digest; it is not possible to work back from the digest to the message (one-way); it is very unlikely that two different messages give the same digest (collision resistant); it produces a fixed-length output.
1 mark each for any three.
Q12Digital certificates & signatures · Challenge · Paper 3 style · 7 marks
A company in Islamabad sends a confidential signed contract to a partner in Dubai. Each has a key pair and a digital certificate.
(a)For each of the four keys, state who uses it and for what purpose.(i) Islamabad company's private key(ii) Islamabad company's public key(iii) Dubai partner's private key(iv) Dubai partner's public key[4]
(b)For each security aim, state whether it is achieved by the signature, by the encryption, or by both: confidentiality, integrity, non-repudiation.[3]
Show answer
(a) (i) used by Islamabad to sign the contract (encrypt its hash). (ii) used by Dubai to verify the signature (decrypt the hash). (iii) used by Dubai to decrypt the contract. (iv) used by Islamabad to encrypt the contract (or the session key that encrypts it).
Q13Encryption & keys · Core · Paper 3 style · 4 marks
(a)Define each term.(i) plaintext(ii) ciphertext(iii) encryption key[3]
(b)Explain why encryption does not stop data from being intercepted.[1]
Show answer
(a) (i) The original data/message before it is encrypted (readable). (ii) The data after it has been encrypted, which cannot be understood without decrypting it. (iii) A value used by the encryption algorithm to convert plaintext into ciphertext (and/or to decrypt it).
B1 each.
(b) Encryption only makes intercepted data meaningless to someone without the key; the data can still be captured during transmission.
B1.
Q14Encryption & keys · Core · Paper 3 style · 5 marks
(a)Describe symmetric encryption.[2]
(b)Describe asymmetric encryption.[2]
(c)Give one advantage of symmetric encryption over asymmetric encryption.[1]
Show answer
(a) The same secret key is used to encrypt and to decrypt the data; the sender and receiver must both have this key.
B1 same key for both. B1 key must be shared/kept secret by both parties.
(b) A matching pair of keys is used: a public key, which can be given to anyone, and a private key, which is kept secret by its owner. Data encrypted with one key of the pair can only be decrypted with the other.
B1 public and private key pair. B1 what one key encrypts only the other can decrypt / private key never shared.
(c) It is faster / needs less processing, so it is better for large amounts of data.
B1.
Q15Encryption & keys · Core · Paper 3 style · 4 marks
Ayesha in Lahore wants to send a confidential report to Bilal in Karachi using asymmetric encryption.
(a)State which key Ayesha uses to encrypt the report, and which key Bilal uses to decrypt it.[2]
(b)Explain why nobody else can read the report even if they have Bilal's public key.[2]
Show answer
(a) Ayesha encrypts with Bilal's public key; Bilal decrypts with his own (Bilal's) private key.
B1 Bilal's public key. B1 Bilal's private key.
(b) Data encrypted with Bilal's public key can only be decrypted with the matching private key, and only Bilal has his private key; the public key cannot be used to decrypt it.
B1 only the matching private key can decrypt. B1 private key is kept secret by Bilal / public key cannot decrypt.
Q16Encryption & keys · Core · Paper 3 style · 3 marks
A single character is encrypted with a simple symmetric method: the 8-bit ASCII code of the character is combined with an 8-bit key using XOR. The receiver uses XOR with the same key to decrypt.
(a)The character 'H' has ASCII code 01001000 and the key is 10110101. Give the ciphertext.[1]
(b)Show that applying XOR with the key to the ciphertext gives back the plaintext.[1]
(c)State why this method is symmetric encryption.[1]
Show answer
(a) 11111101
B1.
(b) 11111101 XOR 10110101 = 01001000, which is 'H'.
B1 working shown.
(c) The same key is used to encrypt and decrypt.
B1.
Q17Encryption & keys · Standard · Paper 3 style · 5 marks
(a)Explain the key distribution problem that occurs with symmetric encryption.[2]
(b)Explain why the key distribution problem does not arise in the same way with asymmetric encryption.[3]
Show answer
(a) The secret key must be given to the receiver before any encrypted data can be sent. If the key is sent over the network it could be intercepted, and then anyone with it can decrypt all the messages.
B1 key must be shared with the receiver. B1 it may be intercepted in transit, compromising all data.
(b) Only the public key is ever sent to other people, and it can be shared openly because it cannot be used to decrypt messages encrypted with it. The private key, which does the decrypting, stays with its owner and never has to be transmitted, so there is no secret key to intercept.
1 mark each, max 3: public key can be published/sent openly; public key cannot decrypt what it encrypted; private key is never transmitted/kept by owner; so an interceptor gains nothing useful. (Allow: the public key must still be shown to be genuine, e.g. by a digital certificate.)
Q18Encryption & keys · Standard · Paper 3 style · 5 marks
A law firm in Islamabad has 20 staff. Every pair of staff must be able to exchange messages that nobody else can read.
(a)Calculate the number of different secret keys needed if symmetric encryption is used with a separate key for every pair of staff.[2]
(b)Calculate the number of keys needed if asymmetric encryption is used instead.[1]
(c)Five new staff join. Calculate how many extra keys each method needs.[2]
Show answer
(a) 190
M1 20 × 19 ÷ 2. A1 190.
(b) 40 (20 key pairs)
B1: one public and one private key per person = 40.
(c) Symmetric: 110 more keys (300 − 190). Asymmetric: 10 more keys.
B1 25 × 24 ÷ 2 = 300, so 110 extra. B1 10 extra (5 pairs).
Q19Encryption & keys · Standard · Paper 3 style · 5 marks
An attacker tries every possible key (a brute-force attack) at a rate of 10¹² keys per second.
(a)Calculate the time to try every key for a 64-bit key. Give your answer in days, correct to 1 decimal place.[2]
(b)State how many times longer it would take for a 128-bit key, as a power of 2.[1]
(c)Explain why increasing key length increases security, and give one drawback of using very long keys.[2]
(c) Each extra bit doubles the number of possible keys, so a brute-force attack takes far longer. Drawback: encryption and decryption need more processing time / power.
B1 more possible keys, exponential increase. B1 slower processing / more computation.
Q20Encryption & keys · Standard · Paper 3 style · 4 marks
An online pharmacy in Karachi encrypts customer orders before sending them to its warehouse.
(a)Explain how encryption protects the customers if the orders are intercepted.[2]
(b)The pharmacy's IT manager says the private key must be stored securely. Explain what could happen if the private key were stolen.[2]
Show answer
(a) Intercepted orders are ciphertext, so the attacker cannot read personal details such as names, addresses or medicines without the decryption key.
B1 data unreadable without key. B1 reference to the scenario (personal / medical / payment details).
(b) Anyone with the private key can decrypt all orders sent with the matching public key, and could also produce digital signatures pretending to be the pharmacy.
B1 decrypt messages meant for the pharmacy. B1 impersonate the pharmacy / sign as the pharmacy. Allow: key pair must be revoked/replaced.
Q21Encryption & keys · Challenge · Paper 3 style · 6 marks
A very small (insecure) example shows how asymmetric keys work. The public key is (e = 3, n = 55) and the private key is (d = 27, n = 55).To encrypt a number M: C = M^e MOD n. To decrypt: M = C^d MOD n.
(a)Encrypt the message number M = 8 using the public key.[2]
(b)Encrypt the message number M = 9 using the public key.[1]
(c)You are told that 17²⁷ MOD 55 = 8. Explain what this shows about the two keys.[2]
(d)Explain why real systems use keys that are thousands of bits long.[1]
Show answer
(a) 17
M1 8³ = 512. A1 512 MOD 55 = 17.
(b) 14
B1 729 MOD 55 = 14.
(c) Using the private key on the ciphertext 17 gives back the original message 8, so data encrypted with the public key can be decrypted with the matching private key.
B1 decryption with private key gives original message. B1 the keys form a matching pair (one undoes the other).
(d) With such a small n an attacker can easily find the private key (e.g. by factorising 55 = 5 × 11); very large keys make this computationally infeasible.
B1.
Q22Encryption & keys · Challenge · Paper 3 style · 6 marks
A hospital in Peshawar sends large files of patient scan results to a laboratory in Islamabad over the internet. Both have asymmetric key pairs.
(a)Describe a method that uses both symmetric and asymmetric encryption so that the files are sent securely and efficiently.[5]
(b)State one further measure that would let the laboratory check that the files really came from the hospital.[1]
Show answer
(a) The hospital generates a random symmetric session key. It encrypts the scan files with this symmetric key. It encrypts the session key with the laboratory's public key and sends both. The laboratory decrypts the session key with its own private key, then uses the session key to decrypt the files. Symmetric encryption is used for the files because it is fast for large data; asymmetric encryption safely delivers the session key.
1 mark each, max 5: random session/symmetric key generated; files encrypted with symmetric key; session key encrypted with lab's public key; lab decrypts session key with its private key; lab decrypts files with session key; reason – speed for large files / key distribution solved.
(b) The hospital adds a digital signature (hash encrypted with the hospital's private key) which the lab checks with the hospital's public key.
B1.
Q23Encryption & keys · Challenge · Paper 3 style · 5 marks
Zara downloads what she believes is her bank's public key from an email link. In fact an attacker has sent her the attacker's own public key.
(a)Explain how the attacker can read the messages Zara sends to the bank, without the bank or Zara noticing.[3]
(b)Explain how a digital certificate would have prevented this.[2]
Show answer
(a) Zara encrypts messages with the attacker's public key thinking it is the bank's. The attacker intercepts them and decrypts them with the attacker's private key. The attacker can then re-encrypt them with the bank's real public key and forward them, so the bank receives them normally (a man-in-the-middle attack).
B1 Zara uses attacker's public key. B1 attacker decrypts with own private key. B1 re-encrypts with the bank's key and forwards / man-in-the-middle.
(b) A certificate binds the bank's identity to its public key and is signed by a trusted certificate authority. Zara's software can check the CA's signature; the attacker cannot produce a valid certificate for the bank's name with their own key.
B1 certificate links public key to verified owner. B1 signed by trusted CA / signature can be checked, so a fake key would be detected.
Q24Encryption & keys · Challenge · Paper 3 style · 5 marks
(a)A student writes: "If I encrypt a message with my private key, nobody else can read it." Explain why this statement is wrong and state what encrypting with your private key does provide.[3]
(b)Describe how the student could send a message that is both confidential and provably from them.[2]
Show answer
(a) Your public key is available to everyone, and anything encrypted with your private key can be decrypted with your public key, so anyone can read it. What it does provide is authentication: only you could have produced it, because only you have the private key (this is the basis of a digital signature / non-repudiation).
B1 public key decrypts it. B1 public key is available to anyone so not confidential. B1 provides proof of origin/authenticity/non-repudiation.
(b) Sign the message (encrypt its hash with their own private key), then encrypt the message and signature with the receiver's public key. The receiver decrypts with their private key and checks the signature with the sender's public key.
B1 sign with sender's private key. B1 encrypt with receiver's public key (and correct decryption order described).
Q25SSL/TLS protocols · Core · Paper 3 style · 5 marks
(a)State the purpose of the Transport Layer Security (TLS) protocol.[2]
(b)Give two situations where TLS is used.[2]
(c)State the relationship between SSL and TLS.[1]
Show answer
(a) To provide secure communication between a client and a server over a network: data is encrypted so it stays private, the server (and optionally the client) is authenticated, and data integrity is checked.
1 mark each, max 2: encryption/privacy; authentication of server/client; integrity (data not altered).
(b) Online banking / shopping websites (HTTPS); email between client and mail server; instant messaging; VoIP calls; logging in to accounts.
1 mark each for two.
(c) TLS is the newer, more secure protocol that replaced SSL (it was developed from SSL).
B1.
Q26SSL/TLS protocols · Standard · Paper 3 style · 6 marks
(a)Describe the steps of a TLS handshake when a browser connects to a secure website.[5]
(b)Explain why the session key is discarded when the session ends.[1]
Show answer
(a) The browser (client) sends a 'hello' message with the TLS versions and cipher suites (encryption methods) it supports and a random number. The server replies choosing the version and cipher suite and sends its digital certificate (containing its public key). The browser checks the certificate with the CA's public key. The client and server then agree a shared session key (e.g. the client sends secret data encrypted with the server's public key, or a key-exchange method is used). Both confirm the handshake is finished, and the rest of the session is encrypted with the symmetric session key.
1 mark each, max 5: client hello with supported versions/ciphers; server hello chooses cipher; server sends certificate/public key; client verifies certificate; session key agreed/exchanged using asymmetric encryption; encrypted communication begins with symmetric key.
(b) A new key for each session means that if one session key is discovered, only that session's data is exposed, not past or future sessions.
B1.
Q27SSL/TLS protocols · Standard · Paper 3 style · 6 marks
(a)TLS is made up of two layers: the handshake protocol and the record protocol. Describe the role of each.[4]
(b)State which layer of the TCP/IP model TLS works between, and give the protocol formed when HTTP is used over TLS.[2]
Show answer
(a) Handshake protocol: lets the client and server authenticate each other, agree on the encryption algorithms to use and set up the shared session keys. Record protocol: carries the application data once the handshake is done; it divides the data into blocks (records), adds a message authentication code to check integrity, and encrypts them with the session key.
B1 handshake – authentication. B1 handshake – agree cipher/keys. B1 record – encrypts and transmits data. B1 record – integrity checking / splitting into records.
(b) Between the application layer and the transport layer; HTTPS.
B1 between application and transport (TCP). B1 HTTPS.
Q28SSL/TLS protocols · Challenge · Paper 3 style · 5 marks
A user in Karachi connects to an online shop. The browser shows https:// and a padlock.
(a)Explain why TLS uses asymmetric encryption during the handshake but symmetric encryption for the rest of the session.[3]
(b)The user assumes that the padlock means the shop is honest. Explain why this is not necessarily true.[2]
Show answer
(a) Asymmetric encryption lets the two sides agree a secret key safely over an insecure network without having shared a key before, and the certificate authenticates the server. But it is slow, so once the shared session key is set up, the much faster symmetric encryption is used for the bulk of the data.
B1 asymmetric solves key exchange/authentication. B1 asymmetric is slow / processor-intensive. B1 symmetric faster for large amounts of data.
(b) The padlock only shows that the connection is encrypted and that the certificate matches the domain; a criminal can obtain a valid certificate for their own fake domain, so the site itself may still be fraudulent.
B1 padlock = encrypted connection / certificate valid for the domain. B1 does not prove the owner is trustworthy – fake sites can have certificates.
Q29Quantum cryptography · Standard · Paper 3 style · 6 marks
(a)Describe what is meant by quantum cryptography.[2]
(b)Give two benefits and two drawbacks of quantum cryptography.[4]
Show answer
(a) It uses the properties of quantum physics – usually the polarisation of single photons of light sent through fibre-optic cable – to exchange an encryption key between two parties securely (quantum key distribution).
B1 uses quantum properties of photons (polarisation). B1 used to distribute/exchange a secret key securely.
(b) Benefits: any attempt to eavesdrop on the key changes the photons and can be detected; the key exchange is considered virtually unbreakable / not threatened by faster computers; long keys can be generated. Drawbacks: requires expensive dedicated hardware and fibre links; works only over limited distances before the signal is lost; data rate is low; photon polarisation can be altered as they travel long distances, causing errors.
1 mark each for two benefits and two drawbacks.
Q30Quantum cryptography · Challenge · Paper 3 style · 5 marks
In a simplified quantum key distribution, the sender transmits photons. Each photon carries a bit using one of two bases: rectilinear (+) or diagonal (×). The receiver chooses a basis at random for each photon. Afterwards they compare bases (not bits) publicly and keep only the bits where the bases matched.Photon: 1 2 3 4 5 6 7 8Sender's bit: 1 0 1 1 0 0 1 0Sender's basis: + × × + + × + ×Receiver's basis: + + × × + × × ×
(a)Give the shared key that both keep, assuming no eavesdropper.[2]
(b)Explain how the sender and receiver could detect that an eavesdropper had intercepted the photons.[3]
Show answer
(a) 11000 (from photons 1, 3, 5, 6, 8)
M1 bases match for photons 1, 3, 5, 6 and 8. A1 key 11000.
(b) An eavesdropper must measure each photon, and measuring in the wrong basis changes its polarisation, so the photon passed on may carry the wrong value. The two parties publicly compare a sample of their kept bits; if more of them disagree than expected, an eavesdropper is present and the key is thrown away.
B1 measuring a photon disturbs/changes its state (cannot be copied). B1 compare a sample of the key bits. B1 errors above expected level reveal eavesdropping – key discarded.
Similar questions
Questions that test the same subtopic in different years, grouped so you can work on one skill at a time.
References only: find each question in its paper (official link where available, otherwise through your school).
Digital certificates & signatures3 questions · 3/9 sessions
