Is a hash the same as encryption?

No. Encryption can be reversed with a key. A hash is one-way: NIST’s blockchain overview says it is computationally infeasible to compute the input from the output, and that even the smallest change to the input gives a completely different digest.

Which hash function should a log use?

SHA-256, defined in NIST’s FIPS 180-4, is widely used and is the function in the example above. Use a standard function rather than inventing a scheme of your own.

What is the previous hash for the first entry?

A fixed starting value that the writer and the verifier both know, such as the word START in the example. Some designs sign it so that the start of the log is also fixed.

Can a hash chain be rewritten?

Yes, by anyone who can write the whole log and recompute every hash. That is why the head should be signed, or stored where the writer cannot change it.

ANVE: the private AI office, on the record. See what that means

ANVE

Private AI glossary · updated 2026-10-07

Hash chain
Each record carries a fingerprint of the one before it.

Short answer. A hash chain links records so that each one depends on the one before it. Every entry stores a hash, a fixed-length fingerprint, calculated from its own content plus the previous entry’s hash. Change any past entry and every later hash stops matching, so the edit is detectable. Crosby and Wallach write the classic form as C(i) = H(C(i-1) joined to X(i)). With SHA-256, defined in NIST’s FIPS 180-4, each fingerprint is 32 bytes, shown as 64 hexadecimal characters. A hash chain proves order and detects edits. It does not prove the first entry was true, and by itself it cannot stop someone with write access from rebuilding the whole chain.

A worked example

Here are four lines of an AI activity log, chained with SHA-256. Each hash is calculated from the previous hash, a vertical bar and the entry text, and the first starts from the word START. Only the first 8 of the 64 hexadecimal characters are shown, so you can reproduce the values with any SHA-256 tool.

Entry	Text	Hash (first 8 characters)
0	09:02 asha summarised the board minutes, model-a, local	0375e2d3
1	09:15 ravi searched the fee schedule, model-a, local	f6c42700
2	09:40 asha drafted a client letter, model-b, cloud	dee8078c
3	10:05 meera approved action item 3	2d335d08

What happens when someone edits an entry

Now change the name in entry 1 from ravi to meera and recompute. Entry 0 is untouched, but every hash from entry 1 onwards is different, because each one depends on the one before.

Entry	Original hash	Hash after the edit
0	0375e2d3	0375e2d3 (unchanged)
1	f6c42700	d7132ccf
2	dee8078c	8d3653b5
3	2d335d08	95a6a424

Where a plain hash chain falls short

Hiding the edit is easy for someone who controls the whole log: recompute every later hash and replace the stored values. A chain defends you only if the latest hash, the head, is held where the writer cannot alter it, for example signed with a key kept elsewhere or copied to a separate system.

Crosby and Wallach also describe a scaling problem. With a classic hash chain, proving that one event is in the log requires every intermediate event. They write that a classic chain might need an 800 MB trace to prove that a randomly chosen event is in a log of 80 million events, where their tree-based prototype returns a 3 KB proof. Certificate Transparency (RFC 6962) uses Merkle trees for a similar reason: its append-only logs let anyone check that a log has only grown, by comparing tree roots and consistency proofs.

Hash chain, signed log, Merkle tree and blockchain

Design	What links the records	What it adds
Hash chain	Each record includes the previous record’s hash	Detects edits, deletions and reordering after the fact
Signed hash chain	The same, plus a signature on the latest hash	Shows who vouched for the log at that point; NIST SP 800-53 AU-9(3) names signed hash functions for audit integrity
Merkle-tree log	Hashes combined in a tree	Short proofs that an entry is present and that the log only grew (RFC 6962)
Blockchain	Blocks cryptographically linked to the previous one	A consensus decision among many parties, so no single party controls the record (NIST IR 8202)

Where ANVE stands

ANVE logs every model call with the user, the app, the model and whether it ran locally or in the cloud. The example above is generic. How ANVE protects its log is a question to settle in writing before a paid pilot.

Questions people ask

Is a hash the same as encryption?: No. Encryption can be reversed with a key. A hash is one-way: NIST’s blockchain overview says it is computationally infeasible to compute the input from the output, and that even the smallest change to the input gives a completely different digest.
Which hash function should a log use?: SHA-256, defined in NIST’s FIPS 180-4, is widely used and is the function in the example above. Use a standard function rather than inventing a scheme of your own.
What is the previous hash for the first entry?: A fixed starting value that the writer and the verifier both know, such as the word START in the example. Some designs sign it so that the start of the log is also fixed.
Can a hash chain be rewritten?: Yes, by anyone who can write the whole log and recompute every hash. That is why the head should be signed, or stored where the writer cannot change it.

What is not real yet

No sealed ANVE unit exists; every image of the appliance on this site is a labelled render. The software runs today on a development machine. No customers yet, no certifications, no delivery date, and the final price is not announced.

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Hash chainEach record carries a fingerprint of the one before it.