1005:. Thymolphthalein, which is normally colorless, turns blue in solution with the base. As the base reacts with carbon dioxide (always present in the air), the pH drops below 10.5 and the color disappears. Pens are now also available that can be erased simply by swiping a special pen over the original text. Disappearing inks have been used in gag squirtguns, for limited-time secret messages, for security reasons on non-reusable passes, for fraudulent purposes, and for dress-making and other crafts where measurement marks are required to disappear.
360:
122:
793:
25:
532:
Using either ultraviolet light or an iodine fume cupboard, messages can be quickly screened for invisible ink and also read without first permanently developing the invisible ink. Thus, if a censor uses this method to intercept messages, the letter may then be sent to the intended recipient, who will
455:
undetected screening of millions of electronic communications, than to mass-screen even a small fraction of conventional mail. Apart from in dictatorships where large numbers of personnel are employed to spy on fellow nationals, screening of posted mail is only feasible in particular situations, such
545:
For practical reasons, the inks are listed here according to their method of development. It must be understood however that some inks – particularly those of organic origin or those consisting of a mixture of several chemicals – may be made visible by several methods. For example, invisible writing
519:
Any invisible ink can be made visible by someone who is sufficiently determined, but the limitation is generally time available and the fact that one cannot apply hours of effort to every single piece of paper. Thus successful use of invisible ink depends on not arousing suspicion that invisible ink
554:
Some of these are organic substances that oxidize when heated, which usually turns them brown. For this type of "heat fixed" ink, any acidic fluid will work. The most secure way to use any of the following substances for invisible ink is by dilution, usually with water, close to the point when they
510:
used invisible inks as a back-up method of communication when other, more secure communication techniques were unavailable. The agency was known to supply special inks to its field agents, rather than have them depend upon improvisation from obtainable everyday chemicals. When agents were forced to
528:
of these papers prevents ink from being absorbed deep into the paper and it is easily visible, especially if the paper is examined under glancing light. There are, however, commercially available inks for non-porous surfaces that are only visible under ultraviolet light and are otherwise virtually
523:
Telltale signs of invisible ink, such as pen scratches from a sharp pen, roughness, or changed reflectivity of the paper (either more dull or more shiny, usually from using undiluted ink), can be obvious to a careful observer who simply makes use of strong light, a magnifying glass and their nose.
446:
What an "ideal" invisible ink is depends on its intended use. For example, property marking should ideally be done with ink easily read under ultraviolet light, whereas in espionage such an ink would be considered too easily detectable since a large number of letters may be screened relatively
868:
Fumes created from heating iodine crystals will develop the writing, which will appear brown because the iodine sticks preferentially to the altered areas of the paper. Exposing the paper to strong sunlight will return the writing to its invisible state, as will using a bleach solution.
536:
A "screening station" theoretically could involve visual and olfactory inspection, an examination under ultraviolet light and then the heating of all objects in an oven before finally trying exposure to iodine fumes to produce optimal security in optimal time.
263:
and lemon juice was used by the 'Lemon Juice Spies' (Carl Muller and four other
Germans, who all died for their efforts either by suicide or execution, along with John Hahn, an English baker) during World War I. In World War II, neutral or acidic solutions of
524:
Also, key words in the visible letter, such as "heat" or any other odd code name, in an out of place context may alert a censor to the presence of invisible ink. Invisible ink is not effective with glossy or very smooth paper types, since the
937:(CIA) successfully exempted these documents, arguing that the recipes provided the basis for more advanced formulas still in use at the time. This exemption made the recipes for invisible ink the oldest classified documents held by the
168:, in the 4th century BC. He mentions it in discussing how to survive under siege but does not indicate the type of ink to be used. This was part of his list of the 20 different methods of secret communications in a book called
315:
The ink can be later made visible by different methods according to the type of invisible ink used. The ink may be revealed by heat or by application of an appropriate chemical, or it may be made visible by viewing under
450:
Invisible inks are inherently "insecure" against a determined and well-equipped inspector, which must be balanced against the logistical difficulty in carrying out mass-screening of posted mail. It is easier to perform
1248:
Lukes, Igor (September 2016). "Book
Reviews Prisoners, Lovers, and Spies: The Story of Invisible Ink from Herodotus to al-Qaeda. By Kristie Macrakis. New Haven, CT: Yale University Press, 2014. Pp. xiv+377. $ 27.50".
872:
Slightly dampening paper with a sponge or by steam and then drying it before writing a message will prevent writing from being developed by this method, but overdoing dampening will result in telltale paper cockling.
347:
There are also toy invisible ink pens which have two tips—one tip for invisible ink writing, and another tip for developing the ink. Invisible ink is sometimes used to print parts of pictures or text in
997:
Inks that are visible for a period of time without the intention of being made visible again are called disappearing inks. Disappearing inks typically rely on the chemical reaction between
176:
involved puncturing a tiny hole above or below letters in a document to spell out a secret message. This did not include an invisible ink but the
Germans improved on the method during
427:
mail sorting stations use UV-visible ink to print bar codes on mailed envelopes giving routing information for use by mail handling equipment further down the line before delivery.
949:
Invisible ink is not commonly used in art. Some artists, however, have incorporated invisible ink into their work, either alone or in conjunction with more conventional media.
1649:
908:
information 25 years or older by the year 2000. Six World War I documents referencing the recipes for invisible ink were due to be declassified under this order, including:
423:
light. Typical uses include printing information on business forms for use by the form processor, without cluttering up the visible contents of the form. For example, some
1406:
MacRakis, Kristie; Bell, Elizabeth K.; Perry, Dale L.; Sweeder, Ryan D. (2012). "Invisible Ink
Revealed: Concept, Context, and Chemical Principles of "Cold War" Writing".
865:
This includes virtually all invisible inks, but pure distilled water can also be used in this way. Application of any fluid will alter the paper surface fibers or sizing.
546:
with soap water may be made visible either by heat, reaction with phenolphthalein, viewing under ultraviolet light, or by placing the page inside an iodine fume cupboard.
889:
stated that Pentel
Rolling Writer rollerball pens were extensively used by MI-6 agents to produce secret writing in the form of invisible messages while on missions.
356:" which is used to show the invisible parts of texts or pictures, thus revealing answers to questions printed in regular ink or completing missing parts of pictures.
312:, or even a finger dipped in the liquid. Once dry, the written surface looks as if it were blank, with a similar texture and reflectivity as the surrounding surface.
819:
paper, the inked areas fluoresce less than the surrounding paper area when under an ultraviolet lamp. This is especially a property of inks with a yellow tint.
148:, is a substance used for writing, which is invisible either on application or soon thereafter, and can later be made visible by some means, such as heat or
1630:
1200:
822:
Some UV-visible inks may be detected on a photocopy, due to the relatively strong ultraviolet component in light from the photocopier scanning head.
1558:
941:
until their declassification in 2011. At this time, the CIA no longer considered the documents sensitive due to recent advancements in technology.
1318:
892:
In 2002, a gang was indicted for spreading a riot between federal penitentiaries using coded telephone messages, and messages in invisible ink.
336:
process, or any of hundreds of others. Developer fluids may be applied using a spray bottle, but some developers are in the form of vapor, e.g.
1713:
202:. People soon discovered that they could write invisibly with one of the ingredients and then cause the writing to appear by adding the other.
1309:; Richard Carback; Jeremy Clark; Aleksander Essex; Stefan Popoveniuc; Ronald L. Rivest; Peter Y. A. Ryan; Emily Shen; Alan T. Sherman (2008).
1385:
938:
412:
There are some invisible ink types that can only be invisible when applied to certain types of surfaces, but are still visible on others.
2052:
1145:
1361:
989:
in London. The illustrations, drawn on the walls and floor of the Music
Gallery Performance Space, were only visible under UV light.
382:
with fluorescent ink that glows when illuminated with a UV light is often used to invisibly mark valuable household items in case of
243:(1558, 1589). Since then, a wide variety of invisible inks have been used for all sorts of secretive purposes. A formula similar to
1457:
1646:
1484:
1533:
1583:
506:
agents were trained not to risk their lives through reliance on insecure inks, most of which were from World War I. In general,
2035:
1991:
1507:
89:
1986:
402:
or other UV light source. Security marker pens can be obtained commercially and are widely used as a crime countermeasure.
61:
1741:
431:
1697:
1678:
1139:
963:. In 2018, analysis by an art conservator revealed invisible ink markings on an untitled Basquiat painting from 1981.
1887:
1445:
1343:
1290:
1178:
1107:
1077:
108:
68:
1922:
1847:
1311:"Scantegrity II: End-to-End Verifiability for Optical Scan Election Systems using Invisible Ink Confirmation Codes"
1789:
511:
improvise, they were advised to dilute their invisible ink as much as possible to reduce chances of detection.
75:
46:
42:
463:
training manual used in the Second World War identified the following properties of an "ideal" invisible ink:
224:
1627:
1440:
Tomlinson, Richard: The Big Breach: From Top Secret to
Maximum Security, pg 44. Mainstream Publishing 2001
1212:
507:
503:
460:
424:
57:
1476:
934:
1310:
438:
uses invisible ink to enable the voter to obtain a confirmation code only for the voted selection.
190:
may be the first writer known to describe an invisible ink using a reagent around 217–218 BC, with
1872:
1842:
1734:
1377:
35:
386:. There are specialty security maker pens formulated for writing on non-porous surfaces such as
2147:
727:
717:
693:
599:, which turns blue when heated and becomes invisible again after a while (if not overly heated)
1280:
1067:
808:
lamp. This is a property of many substances, particularly organic substances and body fluids.
2121:
1129:
405:
Some commercially available invisible inks glow very brightly, in a variety of colors, under
272:, were used as invisible ink. It is colorless but turns pink when exposed to alkali such as
2047:
2030:
2003:
1766:
1415:
952:
668:
that affects the pH indicator in red cabbage water. Vinegar may also be developed by heat.
82:
1358:
8:
2109:
2013:
1761:
723:
596:
260:
1419:
978:, by Gianni Motti. It consisted of two drawings created with undeveloped invisible ink.
2142:
1998:
1965:
1939:
1917:
1912:
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325:
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187:
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1783:
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1174:
1135:
1131:
Prisoners, lovers, & spies: the story of invisible ink from
Herodotus to al-Qaeda
1103:
1073:
886:
829:
781:
777:
733:
321:
317:
252:
2079:
1811:
1671:
Prisoners, Lovers, and Spies: The Story of
Invisible Ink from Herodotus to al-Qaeda
1423:
1258:
1040:
1029:
Prisoners, Lovers, and Spies: The Story of
Invisible Ink from Herodotus to al-Qaeda
1002:
905:
713:
709:
699:
685:
655:
636:
1504:
1052:
1044:
959:
London discovered Basquiat's signature painted in invisible ink on the 1982 work,
533:
be unaware that the secret message has already been intercepted by a third party.
2099:
2074:
2008:
1970:
1907:
1653:
1634:
1511:
1365:
1333:
1168:
1097:
998:
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165:
126:
1305:
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light. This makes them suitable for use in readmission such as hand stamping.
359:
2136:
2057:
2025:
1927:
1821:
703:
681:
372:
173:
153:
130:
495:
Easily obtainable and has at least one plausible innocent use by the holder.
2104:
1892:
1877:
1816:
1801:
1794:
956:
901:
844:
816:
801:
580:
329:
297:
293:
288:
Invisible ink can be applied to a writing surface with a specialty purpose
269:
244:
199:
181:
16:
Substance used for writing which is invisible and can later be made visible
895:
210:
gave advice on the use of plant juices and milk to write secret messages.
2094:
1960:
1806:
1306:
805:
665:
473:
Not depositing crystals on paper, i.e. not easily seen in glancing light.
420:
406:
364:
353:
309:
177:
149:
2089:
2069:
2062:
2042:
1949:
1069:
Codes, Ciphers and Spies: Tales of Military Intelligence in World War I
620:
587:
569:
Lemon, apple, orange or onion juice (organic acids and the paper forms
498:
Not a compound of several chemicals, as this would violate No. 7.
399:
368:
1559:"Basquiat used invisible ink to make secret drawings in his paintings"
1427:
227:
is credited with the first recipe for a sympathetic ink, derived from
121:
2084:
1897:
1826:
1750:
982:
792:
379:
333:
301:
248:
191:
1458:"ARYAN PRISON GANG LINKS WITH MAFIA Drugs, money & the Gambinos"
24:
1932:
1262:
608:
486:
Nonreactive with iodine, or with any of the other usual developers.
441:
383:
815:
ultraviolet light but without fluorescing. When these are used on
235:, as well as the first book on secret writing and invisible inks,
2018:
796:
A US$ 20 bill showing a visible stripe under an ultraviolet light
671:
661:
651:
612:
604:
563:
502:
From practical experience "6" and "9" were usually incompatible.
452:
391:
337:
273:
232:
195:
1552:
1550:
631:
In most cases, these substance changes color when mixed with an
1716:- CIA Freedom of Information Act (FOIA) Electronic Reading Room
848:
757:
750:
solution which turns starch dark blue and the paper light blue.
747:
743:
525:
289:
489:
Potential developers for the ink should be as few as possible.
184:. They used invisible ink and microdots instead of pinpricks.
1547:
753:
570:
395:
387:
214:
1719:
1335:
SOE Syllabus: Lessons in Ungentlemanly Warfare, World War II
838:
632:
616:
349:
228:
207:
164:
One of the earliest writers to mention an invisible ink is
1576:
133:
is interspersed with coded communication in Arnold's hand.
882:
456:
as letters to and from a particular suspect or facility.
415:
Some vendors now offer invisible ink for use in computer
1405:
1099:
Codes: The Guide to Secrecy From Ancient to Modern Times
860:
650:
indicator, turns pink in the presence of a base such as
514:
904:
issued an executive order requesting that all agencies
896:
United States declassification of invisible ink recipes
787:
647:
1647:
Dritz Quilting Fine Point Disappearing Ink Marking Pen
626:
129:, originally written in invisible ink. Handwriting by
955:
is known to have worked with invisible ink. In 2012,
1505:
The formula for invisible ink will remain classified
1285:. Clayton, Victoria: CSIRO Publishing. p. 297.
1134:. New Haven: Yale University Press. pp. 11–13.
825:
Examples of inks revealed by ultraviolet light are:
664:, is revealed by red cabbage water Vinegar contains
760:
solution (ink turns white, paper turns light blue).
49:. Unsourced material may be challenged and removed.
1690:Invisible Ink: Spycraft of the American Revolution
1173:. London: Victoria and Albert Museum. p. 98.
2134:
1668:
985:exhibited artwork drawn in invisible ink at the
1584:"Invisible: Art about the Unseen – in pictures"
615:it, using a hair dryer, or by placing it in an
352:for children to play with, always including a "
268:, a chemical compound extracted from pills for
1278:
479:Does not decompose or discolor the paper e.g.
283:
1735:
1237:. University of Michigan. 1957. p. xxvi.
1166:
928:Pamphlet on Invisible Photography and Writing
1687:
1477:"The Very Visible Battle Over Invisible Ink"
1170:Renaissance secrets, recipes & formulas
1072:. New York: Copernicus Books. p. 210.
1027:Dooley, John F. (25 July 2015). "Review of
236:
221:AD, and during the 16th century in Europe.
1742:
1728:
1526:"Spy agency reveals invisible ink formula"
1123:
1121:
1119:
972:Invisible: Art about the Unseen, 1957-2012
811:Other inks work in a near opposite way by
1378:"Cold War Invisible Ink Secrets Unlocked"
1194:
1192:
1190:
549:
213:Lemons were also used as organic inks by
109:Learn how and when to remove this message
1127:
1102:. Boca Raton, FL: CRC Press. p. 9.
791:
562:Honey solution, sugar (sugar turns into
358:
120:
1279:Selinger, Ben; Barrow, Russell (2017).
1198:
1116:
944:
470:Non-volatile, i.e. no pronounced smell.
419:. Such inks are usually visible under
398:, etc. The mark can be read by using a
2135:
1992:Chinese espionage in the United States
1474:
1331:
1187:
1160:
1095:
1065:
1026:
540:
442:Properties of an "ideal" invisible ink
198:. These ingredients were used to make
172:. One of the techniques that involved
1723:
1556:
1247:
1148:from the original on 21 December 2017
1020:
861:Inks which alter the surface of paper
515:Screening letters for secret messages
1987:Chinese intelligence activity abroad
1274:
1272:
1241:
1091:
1089:
992:
788:Inks visible under ultraviolet light
623:is less likely to damage the paper.
152:light. Invisible ink is one form of
47:adding citations to reliable sources
18:
2053:Interpersonal (HUMINT) intelligence
1628:Exhibit A: Schemes and Dreams Recap
1523:
1514:St. Petersburg Times, June 23, 1999
627:Inks developed by chemical reaction
603:The writing is rendered visible by
13:
1662:
1315:Proceedings of USENIX/ACCURATE EVT
1167:Wheeler, Jo; Temple, Katy (2009).
766:(common table salt), developed by
476:Invisible under ultraviolet light.
14:
2159:
1888:Computer and network surveillance
1707:
1269:
1086:
674:, developed by red cabbage water.
1923:Short-range agent communications
1321:from the original on 2008-12-03.
320:. Inks which are developed by a
170:On the Defense of Fortifications
23:
1640:
1621:
1601:
1536:from the original on 2014-01-10
1517:
1498:
1487:from the original on 2010-07-06
1468:
1450:
1434:
1399:
1388:from the original on 2014-03-20
1370:
1352:
1325:
1299:
876:
34:needs additional citations for
1235:Intelligence Digest Supplement
1227:
1059:
1001:and a basic substance such as
492:Should not develop under heat.
1:
1749:
1408:Journal of Chemical Education
1251:The Journal of Modern History
1045:10.1080/01611194.2015.1028684
1008:
555:become difficult to develop.
225:Giovanni Battista della Porta
1282:Chemistry in the Marketplace
579:Bodily fluids such as blood
529:invisible on such surfaces.
425:United States Postal Service
332:), reactions similar to the
7:
1475:Miller, Bill (2001-06-13).
935:Central Intelligence Agency
284:General application and use
10:
2164:
1524:Ure, Laurie (2011-04-19).
1211:(4): 46–47. Archived from
1128:Macrakis, Kristie (2014).
974:, included the 1989 work,
159:
1979:
1948:
1858:
1835:
1757:
1673:. Yale University Press.
1669:Kristie Macrakis (2015).
923:German Secret Ink Formula
576:Milk (lactose dehydrates)
373:messages in invisible ink
1692:. Westholme Publishing.
1096:Mollin, Richard (2005).
1013:
800:Some inks glow faintly (
447:quickly using UV light.
378:Security marker pens or
367:is assemble an eye-safe
1873:Covert listening device
1843:Intelligence assessment
1464:. New York. 2002-11-03.
1199:Reisert, Sarah (2015).
918:Detection of Secret Ink
607:the paper, either on a
1332:Rigden, Denis (2001).
1031:by Kristie Macrakis".
933:In 1999, however, the
797:
728:potassium ferricyanide
718:potassium ferricyanate
694:potassium ferricyanide
550:Inks developed by heat
375:
340:fumes used to develop
237:
134:
2122:Targeted surveillance
1688:John A. Nagy (2009).
1557:Ouellette, Jennifer.
1066:Dooley, John (2016).
795:
646:, commonly used as a
434:voting system called
362:
124:
2048:Industrial espionage
1848:competing hypotheses
1201:"Under Cover of Ink"
961:Orange Sports Figure
953:Jean-Michel Basquiat
945:Invisible ink in art
131:Peggy Shippen Arnold
125:A coded letter from
43:improve this article
1420:2012JChEd..89..529M
834:optical brighteners
724:Cobalt(II) chloride
590:partially oxidizes)
541:Invisible ink types
261:American Revolution
206:and the Roman poet
1999:Cold War espionage
1966:Front organization
1940:Surveillance tools
1918:Phone surveillance
1913:One-way voice link
1868:Concealment device
1652:2016-12-27 at the
1633:2009-01-15 at the
1510:2015-12-22 at the
1364:2009-01-24 at the
830:Laundry detergents
798:
690:ammonium hydroxide
376:
326:acid-base reaction
188:Philo of Byzantium
135:
2130:
2129:
1784:Agent provocateur
1481:Los Angeles Times
1428:10.1021/ed2003252
1359:Red cabbage water
993:Disappearing inks
939:National Archives
902:President Clinton
887:Richard Tomlinson
782:hydrogen peroxide
778:manganese sulfate
734:Iron(III) sulfate
467:Mixes with water.
363:A common task in
324:may depend on an
322:chemical reaction
318:ultraviolet light
253:George Washington
119:
118:
111:
93:
2155:
1744:
1737:
1730:
1721:
1720:
1703:
1684:
1656:
1644:
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1625:
1619:
1618:
1617:. April 1, 2002.
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1222:
1220:
1215:on 24 March 2018
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1125:
1114:
1113:
1093:
1084:
1083:
1063:
1057:
1056:
1024:
1003:sodium hydroxide
804:) when under an
714:sodium carbonate
710:Iron(II) sulfate
700:Lead(II) nitrate
686:sodium carbonate
656:sodium carbonate
593:Wine, or vinegar
520:may be present.
371:torch to reveal
278:bicarbonate soda
242:
220:
140:, also known as
114:
107:
103:
100:
94:
92:
51:
27:
19:
2163:
2162:
2158:
2157:
2156:
2154:
2153:
2152:
2133:
2132:
2131:
2126:
2100:Sting operation
2009:Black operation
1975:
1971:Limited hangout
1953:
1944:
1908:Numbers station
1860:
1854:
1831:
1771:
1753:
1748:
1710:
1700:
1681:
1665:
1663:Further reading
1660:
1659:
1654:Wayback Machine
1645:
1641:
1635:Wayback Machine
1626:
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1607:
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1512:Wayback Machine
1503:
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1366:Wayback Machine
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1151:
1149:
1142:
1126:
1117:
1110:
1094:
1087:
1080:
1064:
1060:
1025:
1021:
1016:
1011:
999:thymolphthalein
995:
987:Horniman Museum
968:Hayward Gallery
947:
898:
879:
863:
790:
764:Sodium chloride
756:, developed by
746:, developed by
736:, developed by
726:, developed by
712:, developed by
702:, developed by
680:, developed by
644:Phenolphthalein
629:
597:Cobalt chloride
586:Soap solution (
566:by dehydration)
552:
543:
517:
444:
417:inkjet printers
342:phenolphthalein
306:calligraphy pen
286:
266:phenolphthalein
257:Culper Spy Ring
247:was created by
239:Magia Naturalis
218:
204:Pliny the Elder
166:Aeneas Tacticus
162:
146:sympathetic ink
127:Benedict Arnold
115:
104:
98:
95:
58:"Invisible ink"
52:
50:
40:
28:
17:
12:
11:
5:
2161:
2151:
2150:
2145:
2128:
2127:
2125:
2124:
2119:
2118:
2117:
2112:
2102:
2097:
2092:
2087:
2082:
2077:
2072:
2067:
2066:
2065:
2060:
2050:
2045:
2040:
2039:
2038:
2033:
2023:
2022:
2021:
2016:
2006:
2001:
1996:
1995:
1994:
1983:
1981:
1977:
1976:
1974:
1973:
1968:
1963:
1957:
1955:
1946:
1945:
1943:
1942:
1937:
1936:
1935:
1925:
1920:
1915:
1910:
1905:
1900:
1895:
1890:
1885:
1880:
1875:
1870:
1864:
1862:
1861:communications
1856:
1855:
1853:
1852:
1851:
1850:
1839:
1837:
1833:
1832:
1830:
1829:
1824:
1819:
1814:
1809:
1804:
1799:
1798:
1797:
1787:
1780:
1778:Agent handling
1774:
1772:
1770:
1769:
1764:
1758:
1755:
1754:
1747:
1746:
1739:
1732:
1724:
1718:
1717:
1714:Secret Writing
1709:
1708:External links
1706:
1705:
1704:
1699:978-1594161414
1698:
1685:
1680:978-0300212600
1679:
1664:
1661:
1658:
1657:
1639:
1620:
1600:
1575:
1546:
1516:
1497:
1467:
1449:
1433:
1414:(4): 529–532.
1398:
1369:
1351:
1344:
1324:
1298:
1291:
1268:
1263:10.1086/687421
1257:(3): 638–640.
1240:
1226:
1186:
1179:
1159:
1141:978-0300179255
1140:
1115:
1108:
1085:
1078:
1058:
1039:(1): 107–112.
1018:
1017:
1015:
1012:
1010:
1007:
994:
991:
946:
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925:
920:
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897:
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878:
875:
862:
859:
858:
857:
854:
851:
841:
836:
814:
789:
786:
785:
784:
774:Cerium oxalate
771:
768:silver nitrate
761:
751:
741:
738:sodium sulfide
731:
721:
707:
697:
678:Copper sulfate
675:
669:
659:
628:
625:
601:
600:
594:
591:
584:
577:
574:
567:
560:
551:
548:
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500:
499:
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487:
484:
481:silver nitrate
477:
474:
471:
468:
443:
440:
436:Scantegrity II
285:
282:
161:
158:
117:
116:
31:
29:
22:
15:
9:
6:
4:
3:
2:
2160:
2149:
2148:Steganography
2146:
2144:
2141:
2140:
2138:
2123:
2120:
2116:
2113:
2111:
2108:
2107:
2106:
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2101:
2098:
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2073:
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2068:
2064:
2061:
2059:
2058:interrogation
2056:
2055:
2054:
2051:
2049:
2046:
2044:
2041:
2037:
2034:
2032:
2029:
2028:
2027:
2026:Eavesdropping
2024:
2020:
2017:
2015:
2012:
2011:
2010:
2007:
2005:
2002:
2000:
1997:
1993:
1990:
1989:
1988:
1985:
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1972:
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1967:
1964:
1962:
1959:
1958:
1956:
1951:
1947:
1941:
1938:
1934:
1931:
1930:
1929:
1928:Steganography
1926:
1924:
1921:
1919:
1916:
1914:
1911:
1909:
1906:
1904:
1903:Invisible ink
1901:
1899:
1896:
1894:
1891:
1889:
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1879:
1876:
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1849:
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1834:
1828:
1825:
1823:
1822:Sleeper agent
1820:
1818:
1815:
1813:
1810:
1808:
1805:
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1800:
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1459:
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1446:1-903813-01-8
1443:
1437:
1429:
1425:
1421:
1417:
1413:
1409:
1402:
1387:
1383:
1379:
1373:
1367:
1363:
1360:
1355:
1347:
1345:9781903365182
1341:
1337:
1336:
1328:
1320:
1316:
1312:
1308:
1302:
1294:
1292:9781486303328
1288:
1284:
1283:
1275:
1273:
1264:
1260:
1256:
1252:
1244:
1236:
1230:
1214:
1210:
1206:
1205:Distillations
1202:
1195:
1193:
1191:
1182:
1180:9781851775774
1176:
1172:
1171:
1163:
1147:
1143:
1137:
1133:
1132:
1124:
1122:
1120:
1111:
1109:9781584884705
1105:
1101:
1100:
1092:
1090:
1081:
1079:9783319294148
1075:
1071:
1070:
1062:
1054:
1050:
1046:
1042:
1038:
1034:
1030:
1023:
1019:
1006:
1004:
1000:
990:
988:
984:
979:
977:
973:
969:
966:In 2012, the
964:
962:
958:
954:
950:
942:
940:
936:
929:
926:
924:
921:
919:
916:
914:
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910:
909:
907:
903:
893:
890:
888:
884:
874:
870:
866:
855:
852:
850:
846:
843:Body fluids,
842:
840:
837:
835:
831:
828:
827:
826:
823:
820:
818:
812:
809:
807:
803:
794:
783:
779:
776:developed by
775:
772:
769:
765:
762:
759:
755:
752:
749:
745:
742:
739:
735:
732:
729:
725:
722:
719:
715:
711:
708:
705:
704:sodium iodide
701:
698:
695:
691:
687:
683:
682:sodium iodide
679:
676:
673:
670:
667:
663:
660:
657:
653:
652:ammonia fumes
649:
645:
642:
641:
640:
638:
634:
624:
622:
619:. A 100-watt
618:
614:
610:
606:
598:
595:
592:
589:
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582:
578:
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572:
568:
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561:
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457:
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448:
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428:
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401:
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385:
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299:
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281:
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230:
226:
222:
216:
211:
209:
205:
201:
197:
193:
189:
185:
183:
179:
175:
174:steganography
171:
167:
157:
155:
154:steganography
151:
147:
143:
139:
138:Invisible ink
132:
128:
123:
113:
110:
102:
91:
88:
84:
81:
77:
74:
70:
67:
63:
60: –
59:
55:
54:Find sources:
48:
44:
38:
37:
32:This article
30:
26:
21:
20:
2105:Surveillance
1902:
1893:Cyber spying
1878:Cryptography
1817:Resident spy
1802:Double agent
1782:
1689:
1670:
1642:
1623:
1614:CFO Magazine
1612:
1603:
1591:. Retrieved
1588:The Guardian
1587:
1578:
1566:. Retrieved
1563:Ars Technica
1562:
1538:. Retrieved
1529:
1519:
1500:
1489:. Retrieved
1480:
1470:
1461:
1452:
1436:
1411:
1407:
1401:
1390:. Retrieved
1382:ScienceDaily
1381:
1372:
1354:
1334:
1327:
1314:
1307:Chaum, David
1301:
1281:
1254:
1250:
1243:
1234:
1229:
1217:. Retrieved
1213:the original
1208:
1204:
1169:
1162:
1150:. Retrieved
1130:
1098:
1068:
1061:
1036:
1032:
1028:
1022:
996:
980:
975:
971:
970:exhibition,
965:
960:
951:
948:
932:
927:
922:
917:
912:
899:
891:
880:
877:Modern usage
871:
867:
864:
824:
821:
810:
799:
630:
602:
553:
544:
535:
531:
522:
518:
501:
459:The British
458:
449:
445:
429:
414:
411:
404:
377:
365:escape rooms
346:
330:litmus paper
314:
298:fountain pen
287:
270:constipation
251:and used by
245:oak gall ink
223:
212:
200:oak gall ink
186:
182:World War II
169:
163:
145:
142:security ink
141:
137:
136:
105:
99:January 2020
96:
86:
79:
72:
65:
53:
41:Please help
36:verification
33:
2095:Stay-behind
2004:Recruitment
1961:Canary trap
1859:Devices and
1807:Field agent
1033:Cryptologia
913:Secret Inks
856:Lemon juice
832:containing
817:fluorescent
806:ultraviolet
754:Lemon juice
666:acetic acid
573:under heat)
453:large-scale
421:ultraviolet
407:ultraviolet
354:decoder pen
310:Cotton swab
259:during the
178:World War I
150:ultraviolet
2137:Categories
2090:Sexpionage
2070:COINTELPRO
2063:safe house
2043:False flag
1980:Operations
1954:techniques
1950:Tradecraft
1609:"Bad Sign"
1540:2011-04-19
1491:2010-05-02
1462:Daily News
1392:2014-06-10
1152:12 January
1009:References
906:declassify
621:light bulb
588:carboxylic
559:Cola drink
400:blacklight
380:UV markers
369:blacklight
217:around 600
69:newspapers
2143:Espionage
2014:black bag
1898:Dead drop
1827:Spymaster
1751:Espionage
983:Aowen Jin
981:In 2015,
976:Magic Ink
957:Sotheby's
900:In 1995,
853:Sunscreen
813:absorbing
802:fluoresce
334:blueprint
302:toothpick
249:James Jay
192:oak galls
2080:SHAMROCK
1933:microdot
1836:Analysis
1650:Archived
1631:Archived
1534:Archived
1508:Archived
1485:Archived
1386:Archived
1362:Archived
1319:Archived
1219:23 March
1146:Archived
609:radiator
384:burglary
255:and the
2075:MINARET
2019:wetwork
1593:17 June
1568:2 March
1416:Bibcode
881:Former
672:Ammonia
662:Vinegar
613:ironing
605:heating
564:caramel
392:plastic
338:ammonia
274:ammonia
233:vinegar
196:vitriol
160:History
83:scholar
2110:Global
2036:MASINT
2031:SIGINT
1883:Cutout
1767:Assets
1762:Agents
1696:
1677:
1637:TV.com
1444:
1342:
1289:
1177:
1138:
1106:
1076:
1053:825438
1051:
885:agent
849:saliva
758:iodine
748:iodine
744:Starch
526:sizing
328:(like
290:stylus
219:
85:
78:
71:
64:
56:
1795:Agent
1790:Cover
1049:S2CID
1014:Notes
845:serum
611:, by
581:serum
571:ester
396:metal
388:glass
350:books
344:ink.
294:stamp
215:Arabs
90:JSTOR
76:books
2115:Mass
2085:FVEY
1812:Mole
1694:ISBN
1675:ISBN
1595:2012
1570:2019
1442:ISBN
1340:ISBN
1287:ISBN
1221:2018
1175:ISBN
1154:2017
1136:ISBN
1104:ISBN
1074:ISBN
839:Soap
780:and
637:base
633:acid
617:oven
276:and
231:and
229:alum
208:Ovid
194:and
180:and
62:news
1952:and
1530:CNN
1424:doi
1259:doi
1041:doi
883:MI6
716:or
692:or
654:or
635:or
508:SOE
504:SOE
461:SOE
432:E2E
430:An
144:or
45:by
2139::
1611:.
1586:.
1561:.
1549:^
1532:.
1528:.
1483:.
1479:.
1460:.
1422:.
1412:89
1410:.
1384:.
1380:.
1338:.
1317:.
1313:.
1271:^
1255:88
1253:.
1207:.
1203:.
1189:^
1144:.
1118:^
1088:^
1047:.
1037:40
1035:.
847:,
688:,
684:,
648:pH
639:.
394:,
390:,
308:,
304:,
300:,
296:,
292:,
280:.
156:.
1743:e
1736:t
1729:v
1702:.
1683:.
1597:.
1572:.
1543:.
1494:.
1430:.
1426::
1418::
1395:.
1348:.
1295:.
1265:.
1261::
1223:.
1209:1
1183:.
1156:.
1112:.
1082:.
1055:.
1043::
770:.
740:.
730:.
720:.
706:.
696:.
658:.
583:.
483:.
112:)
106:(
101:)
97:(
87:·
80:·
73:·
66:·
39:.
Text is available under the Creative Commons Attribution-ShareAlike License. Additional terms may apply.
