Struct Base64UrlSafe 

pub struct Base64UrlSafe {
    pub decoded: Vec<Wire>,
    pub encoded: Vec<Wire>,
    pub len_bytes: Wire,
}

Base64 encoding (URL-safe, without trailing padding characters) encoding verification.

Verifies that encoded data is a valid base64 URL-safe encoding (without trailing padding characters) of decoded data.

This encoding is defined in the JSON Web Signature (JWS) spec: https://datatracker.ietf.org/doc/html/rfc7515#section-2 (Base64url Encoding)

Base64 URL-Safe Alphabet (RFC 4648 §5)

• Characters 0-61: Same as standard base64 (A-Z, a-z, 0-9)
• Character 62: ‘-’ (minus) instead of ‘+’
• Character 63: ‘_’ (underscore) instead of ‘/’

Circuit Behavior

The circuit performs the following validations:

• encoded is valid base64 URL-safe encoding of decoded
• len_decoded is the actual length of data in decoded (in bytes)
• len_decoded ≤ max_len_decoded (compile-time maximum)

Input Packing

• decoded: Pack 8 bytes per 64-bit word in little-endian format
• encoded: Pack 8 base64 characters per 64-bit word in little-endian format
• len_decoded: Single 64-bit word containing byte count

Fields

decoded: Vec<Wire>

Decoded data array (packed 8 bytes per word).

encoded: Vec<Wire>

Encoded base64 array (packed 8 chars per word).

len_bytes: Wire

Actual length of decoded data in bytes.

Implementations

impl Base64UrlSafe

pub fn new( builder: &CircuitBuilder, decoded: Vec<Wire>, encoded: Vec<Wire>, len_bytes: Wire, ) -> Self

Creates a new Base64UrlSafe verifier.

Arguments

• builder - Circuit builder for constructing constraints
• decoded - raw byte array wires
• encoded - Base64 encoded array wires
• len_bytes - Wire containing actual length of raw data in bytes

Panics

• If decoded.len() is not a multiple of 3

Implementation Notes

The requirement that decoded.len() be a multiple of 3 ensures:

• Word alignment: divisible by 8 for packing bytes into 64-bit words
• Base64 group alignment: divisible by 3 for processing complete groups
• Exact array sizing with no rounding needed

pub fn populate_len_bytes(&self, w: &mut WitnessFiller<'_>, len_bytes: usize)

Populates the length wire with the actual decoded data length.

Arguments

• w - Witness filler to populate
• length - Actual length of decoded data in bytes

pub fn populate_decoded(&self, w: &mut WitnessFiller<'_>, data: &[u8])

Populates the decoded data array from a byte slice.

Arguments

• w - Witness filler to populate
• data - Decoded bytes

Panics

Panics if data.len() exceeds the maximum size specified during construction.

pub fn populate_encoded(&self, w: &mut WitnessFiller<'_>, data: &[u8])

Populates the encoded base64 array from a byte slice.

Arguments

• w - Witness filler to populate
• data - Base64-encoded bytes

Panics

Panics if data.len() exceeds the maximum size specified during construction.