Yuluoxk What I saw was a coil with a value of 0.8, plus two sheets of cardboard. Since printing is relatively expensive, I estimated the cost to be $2 per sheet, totaling $2.8. This avoids any limitations imposed by JLC's free credit.
Additionally, because it uses the standard Mifare M1 protocol, I can use my phone to write to the card, saving on the cost of the PN532 module.
In fact, a small suggestion is to directly write the text data within the card and add a digital signature, which can be referenced from PGP. Since the decrypted data itself has no confidentiality, improving data accessibility is a reasonable approach. If confidentiality is considered, it requires knowing the recipient's private key, which is not feasible. Adding a digital signature in advance to ensure that everyone can decrypt the data is better for verifying authenticity. Also, PGP can be used for other signing tasks (the most important application should be signing emails and Git commits? I previously sent emails to others, and they were curious why my emails always had a strange attachment...). PGP also has encryption modes, but obtaining the recipient's public key reliably is a big problem (the current solution is to use Keyserver as a PKI to provide a weaker level of security), which can be learned.
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