Molecular tagging is an approach to labeling physical objects using DNA or other molecules that can be used in cases where methods like RFID tags and QR codes are not suitable. No molecular tagging method exists that is inexpensive, fast and reliable to decode, and usable outside a lab setting to create or read tags. To address this, we present Porcupine, an end-user molecular tagging system that features DNA-based tags readable within seconds using a portable nanopore device. Porcupine's digital bits are represented by the presence or absence of distinct, nanopore-orthogonal DNA strands, which we call molecular bits (molbits). We classify molbits directly from the raw nanopore signal, avoiding basecalling. To extend the tag's shelf life, decrease readout time, and make tags robust to environmental contamination, molbits are prepared for readout during tag assembly and can be stabilized by dehydration. The result is an extensible, real time, high accuracy tagging system that includes a novel approach to developing nanopore-orthogonal barcodes.