Question 5 (b)

Creation and verification of a digital signature

6 marks

Time to write
About 12 minutes

From an older CBCS paper

Question

Question 5 (b) · 6 marks

State the procedure of creation and verification of digital signature.

Reworded from the university paper; the task is the same. The original paper belongs to the University of Delhi. See DU's question bank.

Before you write

Read off the question and its marks

12 minfor 6 marks

Read from the question

  • procedureA process: the steps in order, one heading each.

What the paper leaves open

The only copy of this paper found is cut off at the right edge of the page; the few words lost there are read from the sense and from the Hindi version of the question. The mark for this part is not legible on the copy; it is reckoned here at 6 marks, the paper's own split for its other questions. This older-course question examines the Information Technology Act, 2000, which the current UGCF Business Laws syllabus does not name; it is answered as printed.

Write it in this order

  1. Read; underline the asks; jot your points 1 min
  2. 1 Section 3: authentication by digital signature 2 marks 3 min
  3. 2 Creation: asymmetric crypto system, hash function, private key 2 marks 3 min
  4. 3 Verification: public key; meaning of "verify" 2 marks 3 min
  5. Check: every ask answered, key terms underlined 2 min

Short on time? Trim steps 1–3 evenly rather than skip one: each is capped at its own marks.

Try it first. About 12 minutes for 6 marks. Write it out, then open the solution.

Solution

Hover or tap a marked figure, like 72,000, to see how it is made and where else it appears.

Provision. Under section 3 of the Information Technology Act, 2000, a subscriber may authenticate an electronic record by affixing his digital signature. The Act prescribes the technique: an asymmetric crypto system together with a hash function, which together convert the original record into another electronic record (s. 3(2)).

Creation of the digital signature:

  1. Digest: the hash function is applied to the record. It maps the record, of any length, to a short fixed string of bits, the "hash result". The same record always gives the same hash result; it is computationally infeasible to work back from the hash result to the record, or to find two records with the same hash result.
  2. Signing: the subscriber applies his private key, one key of his unique key pair, to that hash result. The result is the digital signature, which is attached to the record and sent with it.

Verification (s. 3(3) and s. 2(1)(zh)):

  1. The recipient applies the same hash function to the record as received, producing a fresh hash result.
  2. He uses the subscriber's public key on the digital signature to check it against that fresh hash result.
  3. If they match, two things are established: the record was signed with the private key that pairs with this public key, and the record has not been altered since it was signed. If they do not match, verification fails: either the record was changed or it was not signed with that key.

Conclusion. The signer hashes the record and signs the hash with his private key; the recipient re-hashes the record and checks the signature with the signer's public key, which confirms both who signed and that the record is intact.

Where the 6 marks sit

#ComponentMarks
1Section 3: authentication by digital signature2
2Creation: asymmetric crypto system, hash function, private key2
3Verification: public key; meaning of "verify"2
Total6
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