Digital signatures on accounting documents
A digital fiscal document (a tax invoice, transaction invoice, or receipt) produced by a computerized system must, under Israeli law, be digitally signed to be considered valid. It isn't enough for it to just "look like" an invoice PDF.
- Can be opened, edited and re-saved leaving no trace
- No way to verify it matches the original
- Does not meet the legal requirement for a fiscal document
- The content locks at the moment of issue
- Any change, down to one character, invalidates the signature
- The customer can verify it themselves
What separates a regular PDF from a digital signature
A PDF generated from a Word or Excel template can be edited again with no trace: you can open it, change an amount, and save it again without anyone knowing. A digital signature ties the exact content of the document to a cryptographic signature at the moment it's issued. Any change to the content, even the smallest one, will cause the signature to fail verification. That's what makes the document trustworthy, both to the Israel Tax Authority and to the customer who receives it.
What happens under the hood
At the moment of issue, the system computes a hash of the document's content and encrypts it with a private key belonging to the issuing system. Anyone holding the matching public key can recompute the hash of the document they received and compare. If the results match, nothing has touched the document since it was issued. If a single digit in an amount changed, the hash changes completely and the comparison fails.
It's the same principle that secures HTTPS connections. The difference is what gets signed: here it's the fiscal document itself, not network traffic.
When the document gets signed
Signing happens at the moment of issuing, not when the draft is created and not when it's printed. Once a document is signed, it's locked: it can't be edited, deleted, or changed in any way.
That's why the split between a draft and an issued document matters so much. In a draft you can fix anything, as many times as you need. At issue the door closes, and fixing a mistake happens only through a credit note.
What actually gets stored
Along with the signature, a permanent snapshot of the document is also stored, exactly as it was when signed, including the business's details, the customer's details, the line items, and the amounts as they were at that moment.
Why this matters when settings change
Say the business moved offices and updated its address, or swapped its logo. Without a snapshot, reopening last year's invoice would show the new address on a historical document, which is a different document from the one the customer received and the one that was reported. With a snapshot, older documents keep showing the details that were correct on the day they were issued.
It's also why there's no point "fixing" old documents so they reflect current settings. A historical document is supposed to reflect history.
How the customer verifies
A digitally signed document usually includes a QR code or a verification link, letting anyone who receives it confirm that it's genuinely identical to what was originally issued and hasn't been altered along the way. For the customer it's a few seconds of checking, and for the issuer it's the simplest way to prove a document is authentic without sending anything else.
What happens without a proper digital signature
Issuing fiscal documents in a format that doesn't meet the signature requirements exposes the business to having those documents rejected in an audit, even if the amounts on them are completely correct. A document's legal validity depends on meeting the signature requirement, not just on its content.
That rejection cuts both ways. For the issuer it can lead to an assessment that isn't based on what was actually reported, and for the customer who received the document it can void the input VAT deduction. A business issuing invoices out of a spreadsheet is putting its customers at risk too.
What the business itself has to do
The signature belongs to the system, not to you, so there is nothing to install and no smart card to hold. What the business does have to do is make sure documents really are issued through that system rather than written on the side. One invoice issued from Word "just this once" is an unsigned document, and it breaks the numbering sequence too.
What to check before trusting a system
Three simple tests. First: open an issued document, change a digit in a PDF editor, and try to verify it. Verification that still passes means there is no real signature behind it.
Second: check that the document sent to the customer matches what the system stored. A system that re-renders the PDF on every open, from current settings, will show a new address or logo on an old document, and that is no longer the document the customer received.
Third: confirm the signature applies to every document type, not only tax invoices. A receipt and a credit note are fiscal documents in every respect.
The full list of fields each document type has to carry is in the document templates guide.
How this connects to the other requirements
The signature is one piece of what a computerized document has to carry. Alongside it sit the seven-year retention duty, the ability to export a valid Open Format file, and, for tax invoices above the threshold, an allocation number from the Tax Authority. A system that handles only one of these leaves the rest to you.
How Slate signs digitally
Every document issued on Slate (an invoice, receipt, or credit note) is automatically digitally signed at the moment of issuing, and stored as a permanent, unchangeable snapshot. There's no separate setting to turn this on or opt out of it.
This is general information, not tax advice. For a specific situation, talk to an accountant or tax adviser.
The signature card and the QR code sit at the foot of the document. The samples show them on a complete document.
Common questions
Does a PDF invoice count as digitally signed?
No. A PDF produced from Word or Excel can be edited without leaving a trace, so it does not meet the signing requirement. A digital signature binds the exact content to a cryptographic signature at the moment of issue.
When does the document get signed?
At issue, not at the draft stage and not at printing. From that moment the document is locked, and a correction goes through a credit note.
Can a signed invoice be corrected?
No. A signed document cannot be edited or deleted. The only route is to issue a credit note cancelling it, then issue a new, correct document.
How does a customer verify a document is genuine?
A signed document usually carries a QR code or verification link. The customer scans it, the system recomputes the document's fingerprint and compares, and the answer comes back in seconds.
What happens if you issue without a digital signature?
The documents can be disqualified in an audit even where the amounts are right. For the issuer that risks a best-judgment assessment, and for the customer it can cancel their input-VAT deduction.
Do I need a smart card or a personal signing device?
Not for invoices. A fiscal document is signed by the issuing system with its own key, unlike a certified personal electronic signature required in other contexts.