Since antiquity (and, for ceremonial purposes, up to the present day) physical document have had seals affixed to them. In principle, these seals allow anyone, if he knows what he's looking for, to judge the authenticity of a document without consulting the authority that issued it. (In practice, there are obviously ways of producing convincing forgeries, but leave that to one side for a moment.)

Well, let's not leave that out, because that's exactly where the problem lies: trusting the stamp or, for digital files, signature.

The major difference between the above method and a physical Great Seal, however, is the need to consult someone who has the private key. This would have to be some form of central authority, since making the private key public would allow free reign to any forger.

Sharing a private key is a misunderstanding in itself. You cannot share a private key. You might however share a secret key between a specific set of entities that you trust. This would however degenerate into a secret sharing scheme for a symmetric key to be used with e.g. HMAC. This is not a good idea; you'd want asymmetric cryptography with public and private keys.

How do you detect that a US seal is real? Hopefully it is not just the materials used and the fact that it looks shiny. No, you've either seen it before or you've seen it described.

You can do something similar with digital signing using X.509 certificates. Instead of setting up a PKI you simply share a fingerprint of a public certificate which you then publish. That way you can take the fingerprint of a certificate that was used to sign a document. If the fingerprint validates then the document is valid. You could also use a fingerprint over the public key directly, but a certificate is more like a seal to me.

I don't see any reason to include a time stamp service in this scenario; certainly the US seals don't carry a time stamp after all. If you do it would be for other reasons.

Since antiquity (and, for ceremonial purposes, up to the present day) physical document have had seals affixed to them. In principle, these seals allow anyone, if he knows what he's looking for, to judge the authenticity of a document without consulting the authority that issued it. (In practice, there are obviously ways of producing convincing forgeries, but leave that to one side for a moment.)

Well, let's not leave that out, because that's exactly where the problem lies: trusting the stamp or, for digital files, signature.

The major difference between the above method and a physical Great Seal, however, is the need to consult someone who has the private key. This would have to be some form of central authority, since making the private key public would allow free reign to any forger.

Sharing a private key is a misunderstanding in itself. You cannot share a private key. You might however share a secret key between a specific set of entities that you trust. This would however degenerate into a secret sharing scheme for a symmetric key to be used with e.g. HMAC. This is not a good idea; you'd want asymmetric cryptography with public and private keys.

How do you detect that a US seal is real? Hopefully it is not just the materials used and the fact that it looks shiny. No, you've either seen it before or you've seen it described.

You can do something similar with digital signing using X.509 certificates. Instead of setting up a PKI you simply share a fingerprint of a public certificate which you then publish. That way you can take the fingerprint of a certificate that was used to sign a document. If the fingerprint validates then the document is valid. You could also use a fingerprint over the public key directly, but a certificate is more like a seal to me.

A fingerprint may still be 20-32 bytes in size, or 40 to 64 hexadecimal characters (or any other encoding of the same amount of bytes). If that's too much you can choose 4 locations randomly and then remember just those locations and the values - and keep them secret. An adversary would have to know the exact locations to create a forgery. The chances that 4 bytes are correct by chance is 1 in $2^{32}$, i.e. 1 in ~4 billion.

I don't see any reason to include a time stamp service in this scenario; certainly the US seals don't carry a time stamp after all. If you do it would be for other reasons.

Since antiquity (and, for ceremonial purposes, up to the present day) physical document have had seals affixed to them. In principle, these seals allow anyone, if he knows what he's looking for, to judge the authenticity of a document without consulting the authority that issued it. (In practice, there are obviously ways of producing convincing forgeries, but leave that to one side for a moment.)

Well, let's not leave that out, because that's exactly where the problem lies: trusting the stamp or, for digital files, signature.

The major difference between the above method and a physical Great Seal, however, is the need to consult someone who has the private key. This would have to be some form of central authority, since making the private key public would allow free reign to any forger.

Sharing a private key is a misunderstanding in itself. You cannot share a private key. You might however share a secret key between a specific set of entities that you trust. This would however degenerate into a secret sharing scheme for a symmetric key to be used with e.g. HMAC. This is not a good idea; you'd want asymmetric cryptography with public and private keys.

How do you detect that a US seal is real? Hopefully it is not just the materials used and the fact that it looks shiny. No, you've either seen it before or you've seen it described.

You can do something similar with digital signing using X.509 certificates. Instead of setting up a PKI you simply share a fingerprint of a public certificate which you then publish. That way you can take the fingerprint of a certificate that was used to sign a document. If the fingerprint validates then the document is valid.

I don't see any reason to include a time stamp service in this scenario; certainly the US seals don't carry a time stamp after all. If you do it would be for other reasons.

Since antiquity (and, for ceremonial purposes, up to the present day) physical document have had seals affixed to them. In principle, these seals allow anyone, if he knows what he's looking for, to judge the authenticity of a document without consulting the authority that issued it. (In practice, there are obviously ways of producing convincing forgeries, but leave that to one side for a moment.)

Well, let's not leave that out, because that's exactly where the problem lies: trusting the stamp or, for digital files, signature.

The major difference between the above method and a physical Great Seal, however, is the need to consult someone who has the private key. This would have to be some form of central authority, since making the private key public would allow free reign to any forger.

Sharing a private key is a misunderstanding in itself. You cannot share a private key. You might however share a secret key between a specific set of entities that you trust. This would however degenerate into a secret sharing scheme for a symmetric key to be used with e.g. HMAC. This is not a good idea; you'd want asymmetric cryptography with public and private keys.

How do you detect that a US seal is real? Hopefully it is not just the materials used and the fact that it looks shiny. No, you've either seen it before or you've seen it described.

You can do something similar with digital signing using X.509 certificates. Instead of setting up a PKI you simply share a fingerprint of a public certificate which you then publish. That way you can take the fingerprint of a certificate that was used to sign a document. If the fingerprint validates then the document is valid. You could also use a fingerprint over the public key directly, but a certificate is more like a seal to me.

I don't see any reason to include a time stamp service in this scenario; certainly the US seals don't carry a time stamp after all. If you do it would be for other reasons.