Protecting Seals from Forgery

Question

In my very Earth-like world, the ruling entity possesses a seal of authority.

It is essential that the validity of any seal is proven to be authentic, which had historically been achieved by having very intricate seals which would be too expensive to forge.

However, times have changed and with precision measuring and milling cheaply available, no mere seal is safe. Otherwise society is early Victorian.

Tradition is a very important part of the culture, and it would be loath to have to replace the seal with any other method of guaranteeing authenticity.

How can one modify the seal to guarantee authenticity, when any physical shape can be easily copied?

When referring to 'seal' I mean both the negative (for example a signet ring) and/or the positive (the wax seal).

Any part of the process and government policies may be adjusted to solve this problem.

Answer 1

Not sure why you'd want to do that in the same room, but a low barrier with glass or a wire mesh should keep the seals away from the forge.
(See reference picture ↓)

...wait, all the other answers interpreted this differently.

...
Alright, fine.
How about two or more negative seals (...the non-animal kind) that superimpose to create the positive seal? That way rotation matters, and maybe even detailed steps like how long to wait between the two(+) applications, and a positive can't easily be used to create the negatives.
(Of course you'd need to somehow ensure that the seal can't be made by a custom single negative somehow...)

Let's do the proper thing and attribute the picture, even though it wasn't entirely fitting.
It's from here and here.

Answer 2

Make the seal a signature

Specifically, a cryptographic signature.

The mathematics of an asymmetric cipher like RSA would be accessible to the Victorians, although they might not fully understand what they were doing (or understand why it is secure) they could probably work through the arithmetic by hand. This is also the era of Charles Babbage and mechanical computing.

Your 'seal' is actually the output device of an Analytical Engine-esque machine which calculates the digital signature of some input, like the date and title of the document being stamped, using a private key that is hidden in the configuration of the machine. The seal design might look something like a QR code: the machine clanks away and outputs by raising or lowering type blocks that cause the seal to imprint, or not, a particular bit in the code. To seal a document, you load it into the machine, input the title, and the machine stamps a seal which contains both the title and the signature.

The imprint of the seal is therefore different for every document signed, and in a way that ensures that imprints are verifiable (as coming from a source that knows the private key) and non-transferrable (an imprint sealing the 1850 Treaty of Outer Flooble cannot be copied to seal the 1865 Order Granting Me Unrivalled Wealth). The signatures can be verified by hand (or on another machine) without needing to take the document back to its source or compare it to any other imprints.

Answer 3

Actual seals are so widely used exactly because it is close to impossible to fake them completely. Of course you can fake almost any seal to, say, pass a police check on the road. But experts would distinguish most fakes with ease.

The reason is that any seal has it's own "fingerprint": scratches, wear, small features and even ink distribution (due to different smoothness of material ink has different layer thickness - like frost patterns on a window). Same thing as with bullets and rifles. There are technologies now that can forge the exact copy. But since they include, say material degradation with electron beam , they are very expensive and require a lot of material and human resources (and its hard to keep it secret) and does not guarantee a success.

In old times they used to make seals as complex as they could (as now is done for banknotes). Say in old China, they even invented a special zig-zag writing for seals (all straight lines were replaced with highly detailed patterns). And that worked fine for them.

So your question is both true and false:

• it's true, because most "seal readers" do not perform any complex analyses (in most cases it is just a glance). And thus the answer is - you can't protect seal from rough, but still good enough for casuals copying. And thus people are using "multifactor authorization", like seal on special paper, special form (with tens of other banknote-like ways of protection), verified by personal signature.
• it's false, because it is practically impossible to forge exact copy of a physical body. Even in such a precision production as gun barrels difference is still enough to identify one single barrel among millions of them. It means that seals are already "protected" enough for experts to reliably identify fakes.

Answer 4

Secretly radioactive

Secretly add to the wax a slightly radioactive substance that emits radiation at a very precise wavelength. Seal-testing equipment openly scans for imprecisions in details, but also secretly scans for this wavelength. The wax is produced every year, and the half-life of the radioactive substance means that there is also a precise amount of radiation depending on the time of year.

The signet ring(s) also contains a small amount of this substance, which is replaced every year.

Even if someone discovered that the seals and rings emit this radiation, they might not realize the time-dependency of the amount of radiation.

A good candidate for the radioactive substance is Iron-55, which has a half-life of 2.737 years. It decays into stable manganese by emitting X-rays with a power of (mostly) 5.9 keV. Iron-55 is most effectively produced by irradiation of iron with neutrons. Forgers examining old seals may discover traces of manganese and mistakenly add this to the wax.

Answer 5

TL;DR: use cryptography, and moveable elements in your seal. The seal is basically there to be pretty; the actual authentication will be done via clever mathematics.

---

Lets get down to the basics here.

What is the purpose of a seal, or signature, or official ID badge or anything similar? It is intended to enable someone who is familiar with what the article should look like to tell whether they are being shown something that is the real thing. This is one half of message authentication.

Any change to the seal that can only be verified by the office of the holder of the seal basically renders the seal useless. If everyone who sees a copy of the seal then immediately has to telegraph the office of the holder of the seal to verify it, why not just skip the whole seal malarkey in the first place and just use some other mechanism of authentication.

Obviously, not everyone needs to be able to verify a seal... Joe Schmoe, having his door kicked down by the police is not going to be in a position to telegraph the chief of police to see if this is a legitimate operation, nor is he going to have the time or ability to check the police badges to see if they are forged. Furthermore, not everyone can verify a seal... secret military orders might need to be carried out without the luxury of verifying their validity with high command.

What I suggest, then, is something a little like a Message Authentication Code. These can be computed using something a little like the predecessors of the Enigma device, called rotor machines, and seals that have elements that can be advanced or retracted according to some mechanical settings.

Firstly, the authenticator needs to construct a checksum of the message. Take your rotor machine, bash in the first letter of each line of the message. Each time you press a key, the rotors turn. When you've finished bashing it in, there is a mechanical readout... maybe 12 little flags, which can be on or off, and correspond to 12 little elements around the edge of the seal. When the seal is created, you adjust the mechanical elements appropriately. When you verify the message, you check that the output of the rotor machine matches the checksum.

By way of a bonus, not only can you not trivially forge a seal for a new document, but you also cannot modify an existing document or steal its seal because the checksums will not match.

Obviously, the rotor machine can be stolen, but now in order to create a forgery you must have three things:

1. A high quality copy of the positive seal.
2. Knowlegde of the checksum algorithm.
3. A rotor machine, suitably configured.

Your average forger is going to have to work a lot harder.

Depending on your mathematical or engineering knowhow, you might even be able to create something more complex... a mechanical calculator capable of implementing the RSA cryptosystem. Things like the Curta device postdate the Victoria era, but Babbage's difference engine would probably have been up to the job. Now, large keysizes are needed nowadays because of the existence of fast computers that could brute force smaller keys, but much more modest size keys would be suitable for your needs.

As above, you create a checksum of the message, and use a mechanical device to generate the pattern used by the seal. This time though, it doesn't matter if the verification devices are stolen... they can only be used to verify a seal, not to create it!

Answer 6

Supplement the seal with secondary authentication

How do you tell if a Banksy painting is authentic? You might check for a signature, sure, but fakes can copy the signature too - so you check Banksy's website, where genuine paintings are pictured.

And of course, even if you could fake Donald Trump's signature perfectly, you couldn't just start mailing executive orders and agreeing to trade deals - there are extra levels of validation, one would hope!

Maybe you want to apply the seal to secret documents, without posting the whole document online? In that case make your sealing wax produce a random pattern, the way glitter nail polish produces a pattern that's nigh-impossible to reproduce. Then you can post pictures of the seal without showing the rest of the document.

Random, nigh-uncopyable seal features

Perhaps every seal is carved from wood of the Macguffin tree, with a distinctive grain, coarse enough that it always transfers when stamping, but fine enough you can't duplicate it by taking a silicone cast. Seal validation is done extremely carefully, checking this grain is just right.

Per-recipient seals

Inspired by Wikipedia's description of "ginkō-in" you could have different seals for different applications. You've got a seal you use for bank documents? Only you and your bank (and maybe people who handle your cheques) know what it looks like.

Just accept mediocre security

Plenty of real-world procedures involving signatures just tolerate fraud - almost everything in our world that relies on signatures, for example. Easy to just print an authentic-looking signature straight onto a document or cheque - and everyone knows cashiers are sloppy about validating credit card receipt signatures. So long as fraud is low enough everyone can afford insurance (or self-insurance) that's good enough. And if you're a celebrity, a billionaire or a world leader, maybe people know to carry out secondary checks, like your bank manager knowing you personally.

Likewise, plenty of people in East Asia use seals right now - and while they're not 100% secure, they're good enough for most purposes.

Answer 7

You did not specify the level of technology involved, however with cheap precision milling and measuring available I assume a reasonably advanced technology level.

Whatever physical method is used to secure the seal it would be possible for someone to forge it with sufficient time, money and especially access to inside information. However it could be made extremely difficult for the forgers.

The first step is to ensure that the secret of the seal is kept absolutely secret the knowledge being held by just 2 or 3 of the most highly trusted officials. Nobody else should know how it works. Infact different aspects of the secret could be held by different officials to make it even more secure each one thinking they had the sole secret, but with only the person doing the sealing knowing the full truth and being able to cross check fully.

There are then numerous options for imbedding additional security into the seal any of which might be used:

A code could be used where by the position of the seal, its orientation or the colour of the wax used could be changed slightly according to the date of the document or the number of words or letters contained on the document or some combination of these in an elaborate manner if required.

Chemical or physical changes could be used. The wax seal might be impregnated with harmless chemicals that glowed under UV or other frequencies of light or reacted with other chemicals so that the seal could be easily tested for the presence of one or more harmless chemicals in the seal.

Radioactivity could be used with very low levels of radiation. Perhaps the wax for the official seals have a radiation level 3x that of the background radiation from the local area and half that of the granite blocks that rulers castle is built from.

A wide variety of Microscopic particles of different shapes and sizes could be added to the wax or even the ring or stamp in such a way that examination of the seal under a microscope would reveal their presence.

The above methods and many other similar devices could be used in isolation or together and could even be varied over time.

Answer 8

Price War

Since the information content of the seal can apparently be copied with ease (cryptography notwithstanding), then you are stuck with relying on the physical content to provide authentication. While using exotic materials is one solution, the better one, IMO is just to use very expensive materials that are too difficult for any but the wealthiest nobles to acquire.

This is similar to the idea of using gold leaf, except that gold is too "common" to create the necessary rarity. Anything with a rare, controllable source would work. Refined aluminum might be one possibility. A custom perfume might be another (especially if commoners have mostly all smelled the "royal scent" at some point, and could thus serve as reasonable judges of a seal), but it would need to have a rare but distinctive ingredient to discourage forgery (like saffron, imported at great cost from the Far East).

Glass

Milling works fine on wood and metal, but not so great on brittle materials like glass, which is why glass is blown, rather than milled. The trick is not to create something trivial, like a stained glass pattern that can be easily copied. Instead, you want to create something distinctive and unique, that only one or two glassblowers in the whole kingdom can produce. Consider a 3-dimensional knot of sorts fashioned from glass of varying colors. Having a micrometer and a CNC machine handy will do you no good in reproducing such a piece. Bonus points if the blowers use refraction to cause their "glass seals" to produce a distinctive, uniquely colored, optical pattern when held up to the light at a proper angle.

Obviously, this would cause each seal to effectively become a royally expensive piece of artwork, but that is sort of the point with a royal seal, is it not? Also, being 3D, it wouldn't work very well to just stamp it onto a letter. Rather, each seal would be housed in a protective box that is associated with the "sealed item". And clearly, the royal house would demand each seal back after use, to prevent hoodlums from absconding with them.

Answer 9

Chemical compounds in the wax/ink

There are many materials we have that when lit up by a certain wavelength, emit another. We use a lot of these for lasers, and some of them are synthetic or have certain mixtures of gas.

However, it can also be biological. Bioluminescence is.. actually quite rare, but animals like fireflies mastered it ages ago. Some of these work on the animals' own biology, but some of them can be stimulated externally. So your country would harvest a certain protein that does this from a top-secret animal, mix it into the ink or wax. Then you can watch it light up when illuminated strongly enough, or by burning a certain metal that gives just the right wavelength of light when burned (or magnesium if you just want a lot of light...).

The major problem with this is that the protein will be hell to harvest, and after use it'll break down over time.

Answer 10

SHORT ANSWER

Forging government documents and seals was a problem throughout history, but probably not a big problem that required advanced technology to defeat.

LONG ANSWER

Four parts explaining why forging of seals was not as big a problem as the OP seems to think, and thus why highly technical methods to prevent such forging have not been necessary.

PART ONE

Here is some information about the use of government seals.

seals of many different types with the names of their owners and/or images of various types have been used since ancient times.

In medieval Europe the standard type of sealing material became wax. The matrix would be used to impress the seal pattern in wax. The usual shape for seals became circular, with the name and titles of the owner being written in a circle around the edge of the seal.

With the development of heraldry in the 12th century in western Europe it became common to depict the coat of arms on the seal. Sometimes the owner would be shown in armor riding a horse with the coat of arms on his shield, and/or surcoat, and/or horse trappings, and/or banner. Sometimes a monarch ruler would be shown seated on a throne with crown and robes and with a shield with the coat of arms floating in space beside the throne. Sometimes the coat of arms or the full achievement of arms would fill the circular area in the center of the seal.

In Britain, the most important documents are sealed with the great seal of the realm.

Edward VIII, who abdicated in order to marry Wallis Simpson only a few months after succeeding to the throne, never selected a design for his own seal and continued to use that of his predecessor, George V. The longer-lived British monarchs have had several Great Seals during their reigns. Only one matrix of the Great Seal exists at a time, and since the wax used for the Great Seal has a high melting point, the silver plates that cast the seal eventually wear out. Queen Victoria had to select four different Great Seal designs during the sixty-three years of her reign.1

The Great Seal is attached to the official documents of state that require the authorisation of the monarch to implement the advice of the Government.

Under today's usage of the Great Seal, seals of dark green wax are affixed to letters patent elevating individuals to the peerage, blue seals authorise actions relating to the Royal family, and scarlet seals appoint bishops and implement various other affairs of state. In some cases the seal is replaced by a wafer version, a smaller representation of the obverse of the Great Seal embossed on coloured paper attached to the document being sealed. This simpler version is used for royal proclamations, letters-patent granting the royal assent, writs of summons to Parliament and for licences for the election of bishops and commissions of the peace. It formerly constituted treason to forge the Great Seal.

The Great Seal of the Realm is in the custody of and administered by the Lord Keeper of the Great Seal. This office has been held jointly with that of Lord Chancellor since 1761. The current Lord Chancellor is Robert Buckland. The Constitutional Reform Act 2005 reiterates that the Lord Chancellor continues to be the custodian of the Great Seal.7

The Clerk of the Crown in Chancery, who is also Permanent Secretary of the Ministry of Justice, heads Her Majesty's Crown Office, and is responsible for the affixing of the Great Seal. He is assisted by the Deputy Clerk of the Crown. Day-to-day custody is entrusted to the Clerk of the Chamber, and subordinate staff include a Sealer, and two Scribes to Her Majesty's Crown Office.

Section 2 of the Great Seal Act 1884 governs the use of the Great Seal of the Realm:

2 - (1) A warrant under Her Majesty’s Royal Sign Manual, countersigned by the Lord Chancellor, or by one of Her Majesty’s Principal Secretaries of State, or by the Lord High Treasurer, or two of the Commissioners of Her Majesty’s Treasury, shall be a necessary and sufficient authority for passing any instrument under the Great Seal of the United Kingdom, according to the tenor of such warrant; Provided that any instrument which may now be passed under the Great Seal by the fiat or under the authority or directions of the Lord Chancellor or otherwise without passing through any other office may continue to be passed as heretofore.

2. The Lord Chancellor may from time to time make, and when made revoke and vary, regulations respecting the passing of instruments under the Great Seal of the United Kingdom, and respecting the warrants for that purpose, and the preparation of such instruments and warrants, and every such warrant shall be prepared by the Clerk of the Crown in Chancery.

(3) No person shall make or prepare any warrant for passing any instrument under the Great Seal of the United Kingdom, or procure any instrument to be passed under that Seal otherwise than in manner provided by this Act or the Crown Office Act 1877; and any person who acts in contravention of this section shall be guilty of a misdemeanour.

https://en.wikipedia.org/wiki/Great_Seal_of_the_Realm[2]

The British monarchy also has a lesser seal:

Privy Seal of England

The Privy Seal of England can be traced back to the reign of King John. It has been suggested that it was originally the seal that accompanied the person of the Sovereign, while the Great Seal was required to remain in the Chancery. Eventually it became a requirement that almost no non-judicial document could pass under the Great Seal without a warrant from the Privy Seal.1 The Barons wrested control of the Privy Seal away from the King by 1312 and it was replaced by the signet as the King's personal seal. The Privy Seal became the heart of a second writing office and clearing house, with warrants being sent to the Chancery and Exchequer under orders made with the Signet.1 (By 1400 the Signet was in the custody of the King's Secretary, and as such it is the precursor to the seals of office held by today's Secretaries of State.)2

The Great Seal Act 1884 effectively ended the use of the Privy Seal by providing that it was no longer necessary for any instrument to be passed under the Privy Seal.

https://en.wikipedia.org/wiki/Privy_seal

You will note that the use of the British royal seals became highly regulated and bureaucratic. This made it difficult for someone to make unauthorized used of the royal seals.

Unauthorized use of the actual royal seals seems to have been the main worry.

The last types of high treason defined by the Treason Act 1351 were the forgery of the Great Seal or Privy Seal, the counterfeiting of English (later British) money and the importing of money known to be counterfeit. These offences, however, were reduced to felonies rather than high treasons in 1861[10] and 1832 respectively.[11]

https://en.wikipedia.org/wiki/High_treason_in_the_United_Kingdom[3]

So medieval and later governments did consider the possibility that forged government seals could be used on forged government documents and did make that a crime of treason.

PART TWO

Medieval forgeries.

But the most common type of forgeries in the middle ages were for the purpose of fraud instead of treason.

As I remember, monasteries usually accumulated various plots of lands (and other rights) which they bought, inherited in people's wills, or were given. And sometimes a monastery might own or claim to own a plot of land (or other right) without having any sort of paperwork to prove it. So monks familiar with the genuine deeds and other papers kept by the monastery might forge fake deeds, wills, and charters granting or confirming the possession of various lands and other rights, including forging the seals attached to those documents.

Diplomatics (in American English, and in most anglophone countries), or diplomatic (in British English),15 is a scholarly discipline centred on the critical analysis of documents: especially, historical documents. It focuses on the conventions, protocols and formulae that have been used by document creators, and uses these to increase understanding of the processes of document creation, of information transmission, and of the relationships between the facts which the documents purport to record and reality.

The discipline originally evolved as a tool for studying and determining the authenticity of the official charters and diplomas issued by royal and papal chanceries. It was subsequently appreciated that many of the same underlying principles could be applied to other types of official document and legal instrument, to non-official documents such as private letters, and, most recently, to the metadata of electronic records.

https://en.wikipedia.org/wiki/Diplomatics[6]

And from my limited knowledge of medieval forgeries I think that most medieval persons wouldn't worry too much about the possibility that somebody would forge a document and their seal decades or centuries after their own death. Most medieval persons wouldn't worry too much about forgeries committed after their own lifetimes that would never affect themselves or their close relatives.

What most medieval persons worried about was someone forging documents in their name during their own lifetimes. Which was apparently rarer than forging documents from long dead people.

The two most famous medieval forgeries are The Donation of Constantine and the Privalegium Maius of Austria.

https://en.wikipedia.org/wiki/Donation_of_Constantine[4]

https://en.wikipedia.org/wiki/Privilegium_Maius[5]

And in those cases the persons whose documents and possibly seals were forged had been dead for several centuries. Though if they cared about what was good for the Roman Empire they would not have been pleased to learn about those future forged documents.

PART THREE

Forging documents of contemporary persons as part of a coup attempt:

What is more relevant to your question is the use of forged documents with contemporary dates, allegedly issued by real contemporary persons, in various plots. For example, forged documents were used the unsuccessful Malet Coup of 1812 against Napoleon, and those documents might have been sealed with fake seals.

https://en.wikipedia.org/wiki/Malet_coup_of_1812[1]

Isaac Doukas Komenos (c.1155-1196/97) Arrrive di Cyprus with falsifed imperial letters naming him the new governor in 1185. He soon made himself the absolute ruler of Cyprus and had himself crowned emperor.

PART FOUR

Publication of government actions.

By the 19th century and the Victorian era (1837-1901) literacy was very widespread and there were many newspapers in circulation. And 19th century governments usually published important government decisions and actions in newspapers and other sources.

There were official notices of government actions and other events as early as, for example, the Roman Empire, in the Acta Diurna or "Daily Acts" posted in the Roman Forum, allegedly begun by Julius Caesar in 59 BC.

https://en.wikipedia.org/wiki/Acta_Diurna[7]

So if someone forged a document saying the emperor abdicated in favor of Senator No Account and forged the imperial seal on it, and Senator No Account showed the document to an official who hadn't already heard that the abdication had been published in the Acta Diurna, the official would be likely to be suspicious of the document no matter how well forged.

In Britain, government actions are publicized in the London Gazette since 1666.

The London Gazette is published each weekday, except for bank holidays. Notices for the following, among others, are published:

Granting of royal assent to bills of the Parliament of the United Kingdom or of the Scottish Parliament

The issuance of writs of election when a vacancy occurs in the House of Commons Appointments to certain public offices

Commissions in the Armed Forces and subsequent promotion of officers Corporate and personal insolvency

Granting of awards of honours and military medals

Changes of names or of coats of arms

Royal Proclamations and other Declarations

And most European culture governments in the 19th century would have used similar methods to publicize their actions.

I think that such methods of publicizing government actions strongly limited what someone could get away with by forging a government seal in a 19th century culture.

So from my knowledge of history it seems that forging of documents and attached seals was sometimes a problem but not very often, and it was much more common to forge the seals and documents of long dead rulers than to forge those of contemporary rulers, so people didn't worry about it as much as the original question supposes.

Answer 11

Keep a copy of the stamped document for comparison.

This is the concept of a receipt, and even some old time carbon copy notebooks had 2 carbon pages where both parties could have a receipt regardless of who receives the official documentation.

You can add to that a OTP (One Time Passcode) such that each time the seal is used a number is associated with that use, and never used again. This will also aid in finding the copy later.

Answer 12

This answer is in part a frame challenge, and in part an answer to a situation where my frame challenge is not applicable.

With the rise of printed and mass-distributed journals (and rapid, reliable transport such as steam ships) such as daily newspapers, advertising royal decrees would serve to act as a cross-check against production of a fraudulent document. If a person produced a sealed decree that appeared genuine, the person examining it could check the various archives of journals recording the issue of royal decrees to verify that this decree was in fact journalled, and the wording of the decree if it was in fact made on the date the document says. Hence the historical reduction in severity of the act of forging a royal seal being reduced from high treason to the lesser change of fraud - the entire document is independently verifiable.

Now, in certain situations - such as where the extent of the realm is such that rapid transport of data does not exist - forged documents and their accompanying verification journals could be presented. In such a situation, another means of verification would be invaluable.

To this end, may I present permanently thermochromic inks. The wax used in the production of official documents would, for example, start out being pale green. When the wax is melted, the permanently thermochromic substances in it would begin to change colour, one from white to red, and the other from green to white. When placed officially, the process would include stirring the molten wax and comparing its colour to the official royal purple colour, and when the colour was right, pouring, stamping and cooling it, fixing its colour. It goes without saying that the thermochromic substances would be a royal secret, and/or very difficult to produce.

If a very recent official document is presented at the fringes of the Empire with its associated attestation journals, it can be verified that the seal is genuine by taking a shaving of the characteristic purple with streaks of red, white and green royal seal, and heating it, and seeing if it changes from largely pale purple with streaks of red, white and green to all pale red or red with white. If it doesn't change, non-thermochromic pigments were used, and the document is verifiably a forgery. If it does change, all is well.

Additionally, since the substances involved are permanently thermochromic, merely obtaining wax from the edges of existing seals will not help a forger - the act of melting the surreptitiously obtained wax will change it from the characteristic purple to something rather redder than is acceptable. Obtaining the right wax would involve either knowing how and being able to produce the necessary thermochromic compounds, or stealing a sample from the heavily guarded stores of official wax, neither of which would be easy for a would-be forger to achieve.

However, since every vessel departing from the centre of the Empire could be expected to be carrying a number of journals for distribution to remote official outposts, and outposts are likely to receive a number of different journals on different vessels that will all agree, the verification process for a recent official document is likely to be longer, and if the content of the document necessitates haste such that follow-up journals cannot be awaited, the procedure should be such that documents of such urgent importance should be accompanied by two official persons known to the verifying authorities at the outpost at which the document might be expected to be presented.

If those persons were also to be subverted, the system has far greater problems than mere forgery.

Answer 13

For centuries (at least since Ben Franklin designed early bank notes), the first defense against any sort of counterfeiting has been to introduce a level of complexity that is difficult to duplicate.

When the concern is mechanical duplication, the original needs to be something that a machine can't readily make: smooth, flowing strokes are outside the usual mien of mechanical measurement and machine cutting. High resolution hatching (like the shading on American currency) or other microscale features can defeat photographic attempts, if they can be made finer than the combined resolution of lens and recording medium.

Likely the hardest to counterfeit while still within Victorian technology, however, would be a seal with the above features, plus a variable magnetic field across its surface, and wax with fine particles of iron oxide or similar magnetic compound -- particles which would, as the melted wax solidifies under the (relatively) cold seal, preserve the magnetic pattern of the original seal. A "reader" glass containing a thin layer of viscous liquid holding similar magnetic particles could then be easily used to verify the seal on a document without need to have access to the original for this step. The magnetic pattern should be finely detailed as well, ideally complex, and of course unique.

Answer 14

The cryptographic solution is indeed powerful, but it's a bit complicated. There's a much easier and cheaper method for validating authenticity.

Think about what a seal is supposed to do. It imprints a reproducible, verifiable, and uncopyable pattern onto the document. You can do the same thing with a simple fingerprint. The recipient would simply need to have a known-valid fingerprint sample to compare it against, and a magnifying glass for checking fine details. A fingerprint is ink on the page, so it can't be transferred to another document. Place the fingerprint across the seal and any tampering will be evident. There's no practical way to open and re-seal the document while keeping all of those tiny details aligned.

Answer 15

You can't eliminate forgery completely. However, in addition to some other answers, you can add a significant barrier by

Using seals on watermarked paper.

After all, it is what we do with banknotes.

Watermarking is an expensive and delicate process in your world. Similarly to ours, watermarking equipment and quality watermarks are near impossible to come by in ordinary life, require several kinds of specialized knowledge, and most skilled artisans who create intricate, recognizable and precise patterns for the cylinder mould. There is a single printing company which is entrusted with producing watermarked paper for all the needs of the royalty and nobility -- the Mint.

Forged papers can only attempt crude, blotchy watermarked paper, which will never pass even cursory examination. The seal adds additional security -- two-factor authentication in fact -- and lets keep up the tradition of seals.

Contrary to some of the other answers, watermarks are easy to check and even a small amount of training can teach people how to evaluate both the quality and the content of the watermark.

It would be easier to steal a proper seal and a sheet of proper watermarked paper rather than attempt to forge both the seal and the paper.

Answer 16

Coloured glass beads in the seal...

...is the key to a One Time Pad

I will make the following assumptions...

• Empress Alice wishes to send a message to her trusted envoy Baron Bob.
• Alice and Bob will meet some time before the message is sent
• Alice wishes the messages to remain secret until accessed by Bob
• Bob wishes to authenticate that Alice sent it
• Bob wishes to ensure the message has not been tampered with
• Bob's loyalty is absolute, he will not attempt to falsely represent Alice, not betray or reveal secrets

Then it is simple...

Bob has a One Time Pad code book

A One Time Pad cipher is pretty much unbreakable in the Victorian era. All Alice needs to do is to tell Bob which page in the code book to use for a particular message.

So, the seal includes 3-6 coloured glass beads (or jewels if you want to get really fancy). These colours represent digits: black is 0, clear is 1, blue is 2... and so on, until you have sufficient number of potential digits.

Reading the stones in the seal, Bob uses the number that the digits forms to pick the correct code from his book of One Time Pads.

Once Bob has the correct page, he can decode the enciphered message.

Strengths

• It ensures secrecy (no-one can read the message without the pad)
• It ensures authenticity (only messages sent by Alice will turn out legible)
• It ensures a tamper proof seal (only messages sent by Alice will turn out legible)

Weaknesses

• It demands distribution of the one-time-pads
• It demands that Bob keeps the pad safe
• It demands that Bob is trusted, when he wants to carry out the orders of the message

Answer 17

Cover the seal in gold leaf and have the amount of wax and gold precisely weighed.

The gold leaf already makes the process more expensive, whereas knowing the density of the entire seal (it would have to be on a ribbon to prevent damaging the document) allows it be easily tested.

However, if the standard amount of gold and wax are leaked, the proportions will have to change.

Answer 18

Holographic stickers

Holograms are widely used today to prevent counterfeiting currency. They are difficult to reproduce. Every owner of a seal would ask a specialised manufacturer to prepare a certain number to holographic stickers and every time a seal is used a sticker is added before the wax solidifies.

BTW the stickers are fragile, once embedded in the wax they cannot be removed without breaking.