ASN.1 is indeed a spec. You'd need to find a library to use it, and there are more in C than in whatever fancy modern language you're probably using. But, there are lots to choose from.
There's multiple canonical wire encodings. XER is the "XML encoding rules" if you want something human readable. "BER" is the binary format, although it has some ambiguities so there's "DER", the Distinguished Encoding Rules, which resolves a lot of that by using essentially a subset of BER and specifying how you should behave in various corner cases. In practice, you want to write your messages as DER, but accept messages from other parties using the full BER.
It's an older standard, but it's used heavily in telecom. To pick an example, if you make a cell phone call, especially out in the sticks somewhere, there's probably going to be a media gateway controller that figures out how to route your call without decompressing and recompressing it a bunch of times, and it talks to the various devices routing your call over H.248, which is specified entirely in ASN.1.
There's multiple canonical wire encodings. XER is the "XML encoding rules" if you want something human readable. "BER" is the binary format, although it has some ambiguities so there's "DER", the Distinguished Encoding Rules, which resolves a lot of that by using essentially a subset of BER and specifying how you should behave in various corner cases. In practice, you want to write your messages as DER, but accept messages from other parties using the full BER.
It's an older standard, but it's used heavily in telecom. To pick an example, if you make a cell phone call, especially out in the sticks somewhere, there's probably going to be a media gateway controller that figures out how to route your call without decompressing and recompressing it a bunch of times, and it talks to the various devices routing your call over H.248, which is specified entirely in ASN.1.