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HasChanged

function HasChanged(aCurrentHash, aNewData: string): Boolean

Example​

procedure ScriptEvent(var Value: variant);
var
  NewHash: string;
begin
  if Hash.HasChanged('', 'new nonempty content') then
  begin
    NewHash := Hash.RowHash;
    Value := NewHash;
  end;
end;

Usage​

HasChanged hashes nonempty new data into RowHash and compares it exactly with the supplied digest, but can reuse stale state for empty data.

Critical empty-data behavior

LockBox's string hasher performs no operation for aNewData = ''. HasChanged does not burn the digest after a call. Therefore:

  • on a newly created object, empty new data produces RowHash = '';
  • after a prior nonempty HasChanged, empty new data reuses that prior digest;
  • it never computes the standard SHA-384 digest of an empty message.

Do not use HasChanged with empty new data. Handle empty as a separate state or add an unambiguous nonempty framing prefix before hashing.

Errors, security and performance

Hash allocation/algorithm failures can raise and leave the previous RowHash intact until assignment succeeds. SHA-384 is unkeyed: the comparison detects ordinary mismatch but not a malicious replacement of both content and digest. The mutable object is not thread-safe. Work and temporary UTF-8 memory are proportional to input length.

Additional Technical Info​

HasChanged hashes aNewData, stores the hexadecimal result in RowHash, then compares the supplied current hash with it.

For nonempty data the exact flow is:

  1. SHA-384 hash the UTF-8 bytes of aNewData;
  2. rewind/read the 48-byte digest and convert it to 96 uppercase hexadecimal characters;
  3. assign that text to RowHash;
  4. return aCurrentHash <> RowHash.

The comparison is ordinary case-sensitive string inequality. It does not trim, validate length/hex syntax, normalize lowercase or use constant-time comparison. A lowercase representation of the same digest therefore reports changed.

RowHash is updated even when the result is false. Copy it immediately if required; on the host-provided Hash object another script using the same action can overwrite it.

External references

Created 2026-07-15