Is there some obvious way to encode text?

I am currently in a hotel which uses a Mifare NFC tag for entry authentication. The tag is in the factory default state - they are using only the serial number of the tag and the rest is unused. I would like to write some "I was here" message onto the tag. Is there a way to do so that when someone else reads the tag they will notice the data is a text and not some random authentication key?

Since you chose to NOT supply a link to the device, I used Google and discovered there are quite a few product lines. Which one are you referring to?

Paul

Don’t fiddle with other people’s property.

In short the tag has 16 sectors and each sector has 3 16-byte data blocks (and one control block). The first data block is read-only unique ID, the rest may be read and written by anyone with the right hardware. Such as any smartphone with suitable app installed. Unless lock bits are programmed no measurable harm is done to the owner.

"no measurable harm is done to the owner" ... well, assuming the TO is right in his assumption that only the ID is used.

Smajdalf:
when someone else reads the tag they will notice the data is a text and not some random authentication key?

Do you want to prevent the person from reading the text?

If not then you have to consider the possibility that the person will download the encoded text onto a powerful computer and take the necessary time to decode it.

...R

@Robin2:
No! I want to make it obvious the data contain a text and make it easy to decode! I.e. use some header meaning "ASCII encoded text follows". But AFAIK no such commonly used marker exists.

If you write ASCII to the tag it will be apparent to the user - why do you need a "commonly used marker"?

Either way, you as the project owner gets to decide what is written out to the tag and how to decipher it. As long as applications that try to read from from the tags follow your rules it will work no matter what. Does that make sense?

I wonder if my English is so poor or people are not reading what I write. Last try with other words:
When someone else someday later reads the tag they will see a block of raw numbers. The tag is a raw memory with no indication what data it contains. If the reader is clever they may try to interpret the numbers as ASCII and they will see the text. I want to help them realize they should try to do so.

Pretty much any hex dump facility which shows hex values, also shows the corresponding ASCII characters - but will substitute a dot for non-printable values, possibly inverse video if that is available for "hi-bit" characters.

Download nfc apps and see how they display stuff.
A hex-dump style app may also show ascii, but a more specific app will likely ignore unexpected fields.
Are you hoping to be noticed by hotel staff, or by other hackers who go “I wonder if I can read this card with my nfc reader hack?”

westfw:
Download nfc apps and see how they display stuff.
A hex-dump style app may also show ascii, but a more specific app will likely ignore unexpected fields.
Are you hoping to be noticed by hotel staff, or by other hackers who go “I wonder if I can read this card with my nfc reader hack?”

I have two NFC apps. Both show only raw data as hex values; only one of them has an option to show it as ASCII. (I guess due to small screen optimization.)
I want other hackers to notice.

Do you know for a fact that when the key is reassigned all data fields are not overwritten? Your work may be for nought.

"I would like to write some "I was here" message onto the tag."

Use some UV ink or similar, so when they are checking the room out with a UV light, they will know.

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