otp_crypto/utils library

OTP Crypto – Low-level utilities

Collection of byte helpers used across the crypto stack:

• Base64 (standard) encode/decode with padding normalization
• u64 big-endian encoding (for window field)
• Constant-time byte equality
• Byte concatenation / zeroization

SECURITY NOTES:

• constantTimeEquals prevents timing leaks when comparing MACs.
• Use secureZero on temporary sensitive buffers you control.
• fromBase64Strict rejects invalid Base64 (after trimming whitespace).