Cyber Security in the Ancient World

Cyber security in the Ancient World — Part 9

Ancient governments, like modern ones, realized that to keep their borders safe, control their populations and keep abreast of political developments abroad, they needed the means to collect the intelligence which enabled them to make informed decisions.

Martine Diepenbroek, Ph.D.
Oct 12, 2022
9 min read

This article relates to episode 9 in the series ‘Cyber Security in the Ancient World’. There will be two episode on invisible ink. In this episode (number 9) we will look at love letters sent between a male and a female lover to hide secrets from fathers and rivals. In the next episode (number 10) we will look at a special relationship that had to remain a secret: a relationship between two men in the Early Christian world.

In this episode - number 9 - we move from a context of warfare to love letters. According to the Roman authors Ovid and Pliny the Elder, lovers should use invisible ink to write letters to each other to prevent fathers and rivals from understanding the letters.

Ovid was a Roman poet who lived from 43 BCE – 17/18 CE. He was a contemporary of the authors Virgil and Horace, with whom he is often ranked as one of the three canonical poets of Latin literature. The Imperial scholar Quintilian considered Ovid the last of the Latin love elegists. Although Ovid enjoyed enormous popularity during his lifetime, the emperor Augustus banished him to a remote province on the Black Sea, where he remained a decade until his death. Ovid wrote a number of works in which love is a recurring theme. In many of the works of Ovid – especially in the Amores, The Art of Love (Ars Amotoria), and Heroides – we find a range of suggestions for lovers to secretly communicate with each other.                                      

Pliny the Elder was a Roman author, naturalist and natural philosopher, and naval and army commander of the early Roman Empire. He lived from 23/24 CE – 79 CE), and a friend of the emperor Vespasian. Pliny wrote the encyclopaedic work Natural History, which became an editorial model for encyclopaedias. He spent most of his spare time studying, writing, and investigating natural and geographic phenomena in the field.

Significantly, no equivalent references to secret communication by lovers can be found in the earlier Greek context. This suggests that the Greeks and Romans may have had different concerns about and interests in cryptographic and steganographic messaging – in the contexts of both private discourse and official (including military) communication.                              

This apparent Roman interest in steganographic over cryptographic modes of secret communication is illustrated by the variety of stratagems described by Ovid. In Ovid’s work, the use of secret signs between lovers in their efforts to try to communicate with each other, especially at banquets and public gatherings, are a favourite theme with the author (Amores, 1.4.15-28; 1.4.55-58; 1.11; 1.12; 2.2.26; 2.5.15-20.5f.; 3.11.23f.; 3.11.23-24; 3.11a.23-24; Epistles, 16.258; 17.77ff.; Ars Amatoria, 1.91; 1.137f.; 1.341ff.; 1.351-198; 1.487-490; 1.497-502; 1.569-580; 2.131-140; 2.243; 2.246; 2.251; 2.543; 2.549; 3.514; 3.329-332; 3.394; 3.483-498; 3.619-630; 3.633; Remedia Amoris, 751-756;  Heroides, 1.31-36; 17.77; 17.88; Metamorphoses, 3.460ff.; 4.63; Fasti, 1.418; Tristia, 2.453ff). A clear example of Ovid’s steganographic tricks can be found in Ars Amatoria3.621-624, where Ovid suggests that a slave could carry a secret letter under her clothing or concealed in her sandals (3.621-624).

Similar suggestions for lovers to secretly communicate with each other are found in the elegies of Ovid’s near contemporaries, the poets Propertius and Tibullus, indicating that such secret signals between lovers at dinner-parties and other such public occasions formed a stock theme for the Augustan elegists (Propertius, Elegies, 3.8.25-26; Tibullus, Elegies, 1.2.21-22; 1.6.19; 1.8.1f.; 6.19.20).[1] Yet, as in Ovid, none of these cases of private secret communication are examples of cryptography or steganography in the full sense, since no written secret messages are involved. Rather, these are simply suggestions for secret signing. Yet, Ovid does refer to secret letters sent between lovers in some of his works (Amores 2.15.15-18; Ars Amatoria, 2.596; 3.483-398; 3.627-630; Heroides, 4.3-5). In the earliest example, from Amores2.15.15-18, Ovid reminds lovers of the need to seal secret missives (2.15.16-17) written on wax tablets in order to keep their secrets safe (Amores 1.11; 1.12; 3.496; Ars Amatoria, 2.396; 3.621-624).[2] This example tells us little more than the fact that wax tablets were among the material media used by the Romans for their letter writing. Moreover, nothing in this example indicates that Ovid suggested that lovers should write their messages to each other in code.[3]

However, potential references to coded writing can be found in two other Ovidian works. In Ars Amatoria 2.596 Ovid speaks of letters written ‘in a secret hand.’ And in Heroides 4.3-5, Ovid refers to secret or mysterious marks in letters (arcana notis; 4.5). Goold translates the Latin here as ‘secret characters.’[4] However, this does not necessarily indicate coded or encrypted writing. The arcana notis could easily relate to Ovid’s suggestion in the Ars Amatoria 3.485 and 3.493 that lovers let their secret messages be written by someone else (such as a slave or confidante) to disguise their handwriting and thus the identity of the sender. It is also feasible that the ‘secret characters’ referred to here have been written in a kind of ‘invisible ink.’

Remarkably, we do not have any evidence for the use of invisible ink as a form of secret communication in Greek or Roman warfare, while it would have been an easy-to-use method. Moreover, the oldest description of the use of invisible ink is provided by the Greek military engineer Philo of Byzantium in his work 3rd century BCE work Compendium of Mechanics. Philo suggests making invisible ink by mixing crushed gallnuts with water (Compendium of Mechanics, in: Thévenot, Boivin, et al., Veterum Mathematicorum Opera, 102). When someone wrote with this ‘ink’ and then waited for the ink to dry, the text had become invisible. To make the text visible again a sponge soaked in vitriol (an acid which is also known as iron-sulphate) had to be rubbed over the writing. The vitriol that makes the text visible again works as a reagent. A reagent is a substance or compound that is added to a system in order to bring about a chemical reaction or to see if a reaction occurs.[5] In the surviving part of Philo’s work we cannot find where and how vitriol could have been found. Yet, from the Greek physician Dioscorides (first century CE) and Pliny the Elder we know that vitriol could have been found in the vicinity of copper ore deposits in Cyprus. Here vitriol is formed as white dripstones in caves and mine tunnels (Pliny the Elder, Natural History, 34.32; Dioscorides, On Medical Material, 3.36).[6] Whitiak considers that Ovid, and later Pliny the Elder, and Ausonius would have been inspired by Philo.[7]

However, Ovid – followed by Ausonius in the 4th century CE – suggests the use of fresh milk and moistened flax for this purpose, while Pliny the Elder suggests the use of the milky juice of a plant known as the Mediterranean spurge or goat’s lettuce (Ausonius, Epistles, 28.21-22; Ovid, Ars Amatoria, 3.627-628; Pliny, Natural History, 26.39 (62)). Ashes or linseed oil could be used to make the text visible again (Ausonius, Epistles, 28.21-22; Ovid, Ars Amatoria, 3.627-628). There is no mentions of gallnuts, water, or vitriol here. This makes sense since Ovid, Pliny and Ausonius wrote about straightforward ways to contact your lover with simple everyday materials, while Philo wrote a mechanical treatise for a specialised audience. Whitiak further suggests that the Romans might have used various readily available substances including fruit juice (e.g., lemon juice) and urine as their invisible inks and used as suited to various contexts.[8]However, it is not until the 8th century CE that Europe first discovers lemons via merchants from the Middle East.[9] Yet, it has been suggested that the citron could be the fruit described in Pliny’s Natural History (12.7.15) as the malum medicum – the medicinal fruit.[10] Also, depictions of citrus trees appear in later Roman mosaics from North Africa.[11] Yet, although we find that lemon juice in particular has been used on a large scale in steganography in modern warfare up to and including the Second World War, we do not have any extant ancient sources mentioning the use of such substances as invisible inks (in any context) so it is difficult to assume with any confidence that any substances other than the new milk, regular ink, or ‘goat’s lettuce sap’ would have been used by the Romans for secret messaging.[12]

When using any of these substances, Ovid, Pliny and Ausonius clearly had in mind a secret communication through a letter written on a papyrus or paper-like medium – and not a wax tablet. He has adapted his steganographic methodology to suit the medium of the message in each case. Indeed, in his Ars Amatoria Ovid even suggests that lovers can write a secret message on the human body, either in normal ink (3.625-626), or, once again, in invisible ink (3.628). The latter suggestion, that the lovers use a kind of invisible ink to write on a human body, would have made the message doubly secret since it would have been hidden under clothing and written in invisible letters. However, there is no indication in this example or in any of Ovid’s other examples of secret messaging that such ‘secret signs’ are to be written in any kind of code: this is simply another example of hidden steganographic messaging. Despite the ingenuity and variety of Ovid’s descriptions of secret communication in his poetry, all his examples involve steganography and there is nothing at all to indicate any interest in cryptographic methods of messaging. In fact, this pattern of a marked preference for steganographic over cryptographic methods of communication appears to mark a wider trend among our extant Roman sources and discussions of encrypted or encoded communications are comparatively rare.

Invisible ink could have been used in warfare in Greece and Rome. However, we only have evidence for the use of invisible ink in love letters coming from Roman authors. We do not have evidence for invisible ink used in the Greek world, nor in the context of warfare. This is interesting, since the earliest description of invisible ink comes from a Greek military manual. More research is needed to understand why the Greeks never used invisible ink - as far as we know - and why the Romans only used it in love letters - again, as far as we know. In episode 3 we already established the fact that secret messages are - by their very nature 'secret'. That definitely applies to invisible messages. Maybe invisible ink was used in a variety of contexts, but since the messages were invisible, no one has even seen them except the intended recipient. By analysing sources on invisible ink in detail, we may find other references one day. Yet again, we see a continuous development of the use of a method of secret communication: in this case invisible ink. Invisible ink has been used in warfare at a later date, up to the Second World War. And, it is still in use today, even though it is mainly used by kids in games.

[1] Green 1982, 272; McKeown 1989, 85-86.

[2] Brandt 1963, 208; Oliensis 2019, 100-149, especially 145-146; Munari 1959, 137.

[3] Wax tablets seem to have been more commonly used among the Romans for writing (see Adkins & Adkins 2014, 209; Erdkamp 2011, 287; Jeffery 1961, 57; Lewis 2015, xxxix; Sherwood 2006, 536-537.

[4] Goold 1977, 45.

[5] Macrakis 2014, 11; McNaught and Wilkinson 1997, 149.

[6] Karpenko & Norris 2001, 998.

[7] Whitiak 2003, 1-2.

[8] Whitiak 2003, 1-2

[9] Khan 2007; Fiorentino & Zech-Matterne 2018.

[10] Marks 2010, entry: Citrus.

[11] Khan 2007; Fiorentino & Zech-Matterne 2018.

[12] Macrakis 2014, 138.


Adkins, L., & Adkins, R. (2014). Handbook to Life in Ancient Rome. New York: Facts on File.

Brandt, P. (1963). P. Ovidi Nasonis: Amores Libri Tres.Hildesheim: Georg Olms Verlagsbuchhandlung.

Erdkamp, P. (2011). A Companion to the Roman Army. Malden: Wiley-Blackwell.

Fiorentino, G., & Zech-Matterne, V. (2018). Agrumed: Archaeology and history of citrus fruit in the Mediterranean. Naples: Publications du Centre Jean Bérard.

Green, P. (1982). Ovid: The Erotic Poems. London: Penguin Classics.

Jeffery, L. H. (1961). The Local Scripts of Archaic Greece: A Study of the Origin of the Greek Alphabet and its Development from the eighth to the fifth centuries B.C. Oxford: Clarendon Press.

Karpenko, V. & Norris, J.A. 2001, ‘Vitriol in the history of chemistry’. Chemické listy-The official journal of Czech Chemical Society, 96, 997-1005.

Khan, I. A. (2007). Citrus Genetics, Breeding and Biotechnology. Wallingford: CABI.

Lewis, K. M. (2015). How John wrote the Book of Revelation: from concept to publication. Lorton: Kim Mark Lewis.

Macrakis, K. (2014). Prisoners, Lovers, and Spies: The Story of Invisible Ink from Herodotus to Al-Qaeda. New Haven/ London: Yale University Press.

Marks, G. (2010). Encyclopedia of Jewish Food (eBook). Retrieved from: Encyclopedia of Jewish Food (online)

McKeown, J. C. (1989). Ovid: Amores. Text, polegomena and commentary. Leeds: F. Cairns.

McNaught, A. D. and Wilkinson, A. (1997; eds.), IUPAC. Compendium of Chemical Terminology, second editon. (the "Gold Book"). Oxford: Blackwell Scientific Publications.

Munari, F. (1959). Amores Testo, Indotruzione, Traduzione e Note. Firenze: La Nuova Italia.

Oliensis, E. (2019). Loving Writing/ Ovid's Amores. Cambridge/ New York: Cambridge University Press.

Sherwood, A. N. (2006). Papyrus and Parchment. In: Wilson, N. G. (ed.), Encyclopedia of Ancient Greece. New York/ London: Routledge - An Imprint of Taylor and Francis Group, 535-537.

Whitiak, D. A. (2003). The Art of Steganography (Global Information Assurance Certification Paper, SANS Institute 2003 - Part of GIAC practical repository ed.). Retrieved from:

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