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DER ASCII is a small human-editable language to emit DER (Distinguished Encoding Rules) or BER (Basic Encoding Rules) encodings of ASN.1 structures and malformed variants of them.

It provides two tools, ascii2der and der2ascii, to convert DER ASCII to a byte string and vice versa. To install them, run:

go get

These tools may be used to create test inputs by taking an existing DER or BER structure, disassembling it with der2ascii into DER ASCII, making adjustments, and assembling back into binary with ascii2der. This avoids having to manually fix up all the length prefixes. As a bonus, it acts as a human-readable view for DER structures.

For the language specification and basic examples, see language.txt. The samples directory includes more complex examples from real inputs.

Backwards compatibility

The DER ASCII language itself may be extended over time, but the intention is for extensions to be backwards-compatible. Specifically:

  • The command-line interface to ascii2der and der2ascii will remain compatible, though new options may be added in the future.

  • Previously valid inputs to ascii2der will remain valid and produce the same output. In particular, checking in test data as ascii2der inputs should be future-proof, though it is recommended to check in the generated version as well in case of mistakes.

  • Previously invalid inputs to ascii2der may become valid in the future if the language is extended.

  • der2ascii is necessarily a heuristic, so its output may change over time. For example, later revisions may recognize new OIDs, tweak the formatting, or disassemble a malformed DER input in a (hopefully) more useful form.


This is not an official Google project.


A reversible DER and BER pretty-printer.




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