Hash Value vs Digital Signature: Difference, Law and Evidence Explained

The two instruments travel together and are constantly confused: the hash value, a fingerprint that proves a record unchanged, and the digital signature, which takes that fingerprint and binds it to a person by encrypting it with a private key. One proves integrity alone; the other adds identity and non-repudiation, which is why the statute builds authentication on the signature while forensics runs on the bare hash. The signature law was built in Topics 37 and 51, the forensic hash in Topics 24 and 82; this note, as asked, is the dedicated comparison.

1. The Two Instruments

• The hash value. A hash function maps data of any size to a fixed-length string, the message digest or hash: the same input always yields the same digest, any alteration yields a visibly different one, the function is one-way so the data cannot be recovered from the digest, and finding two inputs with the same digest is computationally infeasible. The hash therefore proves one thing with great force: this data is unchanged.
• The digital signature. s.3's creation: authentication of an electronic record by an asymmetric crypto system and hash function which envelop and transform the record into another record, in practice, the signer's software hashes the record and encrypts the digest with the signer's private key; that encrypted digest, travelling with the record, is the digital signature, verifiable by anyone holding the corresponding public key listed in the signer's Digital Signature Certificate (Topic 37)

Figure 1: How the signature is built from the hash

• Verification. The verifier decrypts the signature with the public key, recovering the digest the signer computed, re-hashes the received record, and compares: matching digests prove both that the record is unaltered and that it was signed by the holder of the private key, whom the CA's certificate, up the trust chain to the Controller, identifies (Topic 51)

2. The Comparison

Figure 2: Fingerprint against signature

• What each proves. The hash proves integrity alone; the signature proves authentication (who signed), integrity (the enveloped hash), and non-repudiation (only the private key holder could have made it). A bare hash names no author, anyone can compute the digest of any file, so it can never show origin or assent.
• Keys and identity. The hash needs no key and involves no identity; the signature exists only within a key pair, and its legal force runs through the certification hierarchy, the DSC binding the public key to a person, the CA to the Controller, ss.35 to 39 and the subscriber's private key duties completing the chain (Topics 51, 52)
• Containment, not equivalence. Every digital signature contains a hash, the digest is what the private key encrypts, but a hash alone is not a signature: stripping the encryption strips the identity, leaving only the fingerprint, the single sentence that answers most comparison questions.
• Statutory homes. The hash function lives inside s.3(2)'s definition and in evidentiary practice; the digital signature is s.2(1)(p) and s.3, a species of the wider electronic signature of s.3A, with legal recognition through s.5 and the presumption for secure electronic signatures in the evidence law aiding proof (Topics 37, 42)

3. Their Evidentiary Work

• The hash in forensics. Computed at seizure and recorded in the memo, re-computed at imaging and at production: matching hashes prove the exhibit unchanged from the scene to the courtroom, the discipline of Topic 24's chain of custody and the s.79A examination, and the s.63 BSA certificate schedule itself asks for the hash of the electronic record produced, making the fingerprint part of admissibility (Topics 9, 24, 82)
• The signature in authentication. The digital signature authenticates records ex ante, contracts, filings, certificates, with s.85B-line presumptions attaching to secure electronic signatures and the forgery and misuse consequences of ss.71 to 74 and 66C policing the system (Topics 42, 65)
• Together in practice. An e-signed contract later produced in court engages both at once: the signature proves who executed the record, the hash in the certificate proves the copy produced is the record, provenance and integrity, each instrument on its own question.

4. Frequently Asked Questions

What is the difference between a hash value and a digital signature?

A hash value is a fixed-length digest computed from data by a one-way hash function: identical data yields an identical digest and any change yields a different one, so it proves integrity, but it involves no key and identifies no one. A digital signature under Section 3 of the IT Act is that digest encrypted with the signer's private key within an asymmetric crypto system, verifiable through the public key listed in the signer's Digital Signature Certificate, so it proves authentication and non-repudiation in addition to integrity. Every digital signature contains a hash; a hash alone is not a signature.

Where do hash values and digital signatures matter in evidence?

The hash is the integrity instrument of digital forensics: recorded at seizure, verified at imaging and production, deposed to in the chain of custody, and required by the Section 63 BSA certificate for electronic records, so that matching digests prove the exhibit unchanged. The digital signature is the authentication instrument of transactions: it executes records with legal recognition under Section 5, attracts the evidentiary presumptions for secure electronic signatures, and its misuse is policed by the certificate offences and Section 66C.

5. Related Topics

• Topic 37: Sections 3 and 3A. The signature technology and its law.
• Topic 82: Section 79A Examiner. The hash at work in forensic practice.