4 ms·

Banksy uses a similar system to authenticate his work: https://boingboing.net/2019/03/31/di-faced-note.html https://boingboing.net/2019/03/31/di-faced-note.htm

Banksy uses a similar system to authenticate his work: https://boingboing.net/2019/03/31/di-faced-note.html https://boingboing.net/2019/03/31/di-faced-note.html

gruez 7y ago
>this is a great piece of cryptographic engineering: faking a Banksy cert involves matching the tear precisely, and also requires that a would-be counterfeiter know what was written on the other half of the note, which is stored at Pest Control and is not made available. 1. Is a piece of paper torn in half really cryptography? Wikipedia says: >Cryptography or cryptology is the practice and study of techniques for secure communication in the presence of third parties called adversaries I don't see how this is any different than any other anti-counterfeiting schemes that uses a hard to duplicate physical object. 2. I get how the tear might be used for authentication purposes (hard to duplicate), but what's the point of validating the contents on the other half of the note? If you matched the public half the note exactly, it's not like the private half will help you authenticate it.
- nsajko 7y ago
  > 1. Is a piece of paper torn in half really cryptography? Cryptography is not just about confidentiality, ensuring authenticity is also cryptography. > If you matched the public half the note exactly The point is that it is hard to duplicate even the "public half" of the note.
  - gruez 7y ago
    >Cryptography is not just about confidentiality, ensuring authenticity is also cryptography. So by that logic, a wax seal is also cryptography?
    - crankylinuxuser 7y ago
      I'd argue that a wax seal was more a crypto signature (md5, sha, etc) than it was a cipher of the text enclosed. Of course, reading by breaking means armed thugs killed you. But we'll leave that analogy aside!
    - dredmorbius 7y ago
      See: Dagger-Trap letterlocking https://youtube.com/watch?v=16GAIaYN_Gk https://youtube.com/watch?v=16GAIaYN_Gk
- gumby 7y ago
  > 1. Is a piece of paper torn in half really cryptography? The "crypt" in "cryptography" means "hidden" (e.g. we hide dead bodies away in a "crypt"). Certainly if you don't know what's on the other side it s literally hidden from you. The tear itself is a key. Depending on your level of technology you could make a matching part by inspection of the part you have, but consider the tearing a somewhat random process given the influence of the medium (paper money will be more random than polymer). It's a quick way to exclude candidates for the match. The actual authentication then is through the completion of the text which is presumably a harder problem. You could make some text that matches, but it's presumably hard -- not perhaps has hard as finding a hash collision.
ChinoD 7y ago
The concept of tearing a document in half is an age old authentication mechanism. This is where we get the lega terminology "party of the first part" and "party of the second part": a deed to land would be written out by a scrivener in duplicate on parchment, and then the parchment would be cut in half by hand using a scalloped cut. Matching up the halves would authenticate the title of the grantee.
- nsajko 7y ago
  https://en.wikipedia.org/wiki/Indenture https://en.wikipedia.org/wiki/Indenture > The term comes from the medieval English "indenture of retainer"[1] — a legal contract written in duplicate on the same sheet, with the copies separated by cutting along a jagged (toothed, hence the term "indenture") line so that the teeth of the two parts could later be refitted to confirm authenticity https://en.wikipedia.org/wiki/Chirograph https://en.wikipedia.org/wiki/Chirograph > A chirograph is a medieval document, which has been written in duplicate, triplicate or very occasionally quadruplicate (four copies) on a single piece of parchment, with the Latin word chirographum (occasionally replaced by some other term) written across the middle, and then cut through to separate the parts.
- gist 7y ago
  I am curious if that is something that is taught at law school or just something that is picked up over time (like anything else related or unrelated to a person's career)? [1] [1] I am thinking that the vast majority of info that I know about business was not learned in business school.
- jbverschoor 7y ago
  Or you have a stock market. Where you break a stick (Dutch: stok) in half and match pairs
  - dredmorbius 7y ago
    Similarly, talley sticks. Responsible for the 1834 burning of Parliament: https://addiator.blogspot.com/2011/05/tally-sticks-and-burning-of-parliament.html https://addiator.blogspot.com/2011/05/tally-sticks-and-burni...

Banksy uses a similar system to authenticate his work: https://boingboing.net/2019/03/31/di-faced-note.html https://boingboing.net/2019/03/31/di-faced-note.html

gruez 7y ago

>this is a great piece of cryptographic engineering: faking a Banksy cert involves matching the tear precisely, and also requires that a would-be counterfeiter know what was written on the other half of the note, which is stored at Pest Control and is not made available. 1. Is a piece of paper torn in half really cryptography? Wikipedia says: >Cryptography or cryptology is the practice and study of techniques for secure communication in the presence of third parties called adversaries I don't see how this is any different than any other anti-counterfeiting schemes that uses a hard to duplicate physical object. 2. I get how the tear might be used for authentication purposes (hard to duplicate), but what's the point of validating the contents on the other half of the note? If you matched the public half the note exactly, it's not like the private half will help you authenticate it.

nsajko 7y ago
> 1. Is a piece of paper torn in half really cryptography? Cryptography is not just about confidentiality, ensuring authenticity is also cryptography. > If you matched the public half the note exactly The point is that it is hard to duplicate even the "public half" of the note.
- gruez 7y ago
  >Cryptography is not just about confidentiality, ensuring authenticity is also cryptography. So by that logic, a wax seal is also cryptography?
  - crankylinuxuser 7y ago
    I'd argue that a wax seal was more a crypto signature (md5, sha, etc) than it was a cipher of the text enclosed. Of course, reading by breaking means armed thugs killed you. But we'll leave that analogy aside!
  - dredmorbius 7y ago
    See: Dagger-Trap letterlocking https://youtube.com/watch?v=16GAIaYN_Gk https://youtube.com/watch?v=16GAIaYN_Gk
gumby 7y ago
> 1. Is a piece of paper torn in half really cryptography? The "crypt" in "cryptography" means "hidden" (e.g. we hide dead bodies away in a "crypt"). Certainly if you don't know what's on the other side it s literally hidden from you. The tear itself is a key. Depending on your level of technology you could make a matching part by inspection of the part you have, but consider the tearing a somewhat random process given the influence of the medium (paper money will be more random than polymer). It's a quick way to exclude candidates for the match. The actual authentication then is through the completion of the text which is presumably a harder problem. You could make some text that matches, but it's presumably hard -- not perhaps has hard as finding a hash collision.

ChinoD 7y ago

The concept of tearing a document in half is an age old authentication mechanism. This is where we get the lega terminology "party of the first part" and "party of the second part": a deed to land would be written out by a scrivener in duplicate on parchment, and then the parchment would be cut in half by hand using a scalloped cut. Matching up the halves would authenticate the title of the grantee.

nsajko 7y ago
https://en.wikipedia.org/wiki/Indenture https://en.wikipedia.org/wiki/Indenture > The term comes from the medieval English "indenture of retainer"[1] — a legal contract written in duplicate on the same sheet, with the copies separated by cutting along a jagged (toothed, hence the term "indenture") line so that the teeth of the two parts could later be refitted to confirm authenticity https://en.wikipedia.org/wiki/Chirograph https://en.wikipedia.org/wiki/Chirograph > A chirograph is a medieval document, which has been written in duplicate, triplicate or very occasionally quadruplicate (four copies) on a single piece of parchment, with the Latin word chirographum (occasionally replaced by some other term) written across the middle, and then cut through to separate the parts.
gist 7y ago
I am curious if that is something that is taught at law school or just something that is picked up over time (like anything else related or unrelated to a person's career)? [1] [1] I am thinking that the vast majority of info that I know about business was not learned in business school.
jbverschoor 7y ago
Or you have a stock market. Where you break a stick (Dutch: stok) in half and match pairs
- dredmorbius 7y ago
  Similarly, talley sticks. Responsible for the 1834 burning of Parliament: https://addiator.blogspot.com/2011/05/tally-sticks-and-burning-of-parliament.html https://addiator.blogspot.com/2011/05/tally-sticks-and-burni...