Forging government documents and seals was a problem throughout history, but probably not a big problem that required advanced technology to defeat.
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.
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.
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.
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 and 1832 respectively.
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.
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.
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.
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.
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.
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.
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.