Module codec 

Hex and Base64 codecs.

These exist so the CLI, MCP server, PEM and ontology exports can move key material and test vectors around without pulling in a dependency. Both directions are constant time in the values they carry, which matters because private keys pass through them: a PKCS#8 key in PEM is Base64, and agents paste hex keys into the CLI.

How, and what went wrong before

No character is classified by a branch and no value indexes a table.

• Encoding computes each character arithmetically: the offset from the value to its character changes at the alphabet’s range boundaries, and each change is a mask made from the sign of a subtraction. A table of 64 or 16 characters indexed by secret bits is a cache-timing channel when it spans two lines, and the encoders used one.
• Decoding computes a value and a validity mask for every character the same way, and accumulates the validity behind core::hint::black_box. The input is rejected once, at the end, if any character was invalid.

The decoders were written branch-free before, but each character’s validity was tested at once with ensure!, an early return, and every compiler this workspace targets – x86-64, both Cortex-M cores, 32-bit RISC-V – turned the OR of the class masks feeding that test into a chain of short-circuit branches: plus? slash? upper case? lower case? digit? Which branch left the chain was the class of a secret character. That is the channel Sieck et al. used against PEM key decoding (“Util::Lookup”, USENIX Security 2021), and it was found here by reading the compiled code.

Functions

b64

Encode input as a padded standard Base64 String.

base64_decode

Decode padded standard Base64, returning the number of bytes written.

base64_encode

Encode input as padded standard Base64 into out.

base64_encoded_len

Length of the standard-alphabet, padded Base64 encoding of n bytes.

hex

Encode input as a lowercase hex String.

hex_decode

Decode a hex string into out, accepting either case.

hex_encode

Encode input as lowercase hex into out.

unb64

Decode a padded standard Base64 string into a freshly allocated Vec.

unhex

Decode a hex string into a freshly allocated Vec.