Module hctr2fp 

HCTR2+FP (Format-Preserving) variant of HCTR2.

While standard HCTR2 operates on arbitrary bytes, HCTR2+FP ensures that ciphertext consists only of digits in a specified radix (e.g., decimal digits 0-9 for radix-10).

Use cases:

• Encrypting credit card numbers (decimal)
• Encrypting alphanumeric identifiers (hexadecimal or custom radix)
• Systems requiring format-preserving encryption

Security properties:

• Ciphertext length equals plaintext length
• All ciphertext digits are in range [0, radix)
• Requires unique (key, tweak) pairs for security
• No authentication - consider AEAD if integrity protection is needed
• Minimum message length depends on radix (e.g., 39 digits for decimal)

Structs

Hctr2Fp

Generic HCTR2+FP cipher.

Functions

bits_per_digit

Compute bits per digit for power-of-2 radix.

decode_base_radix

Decode base-radix digits (little-endian) to a 128-bit value.

encode_base_radix

Encode a 128-bit value as base-radix digits (little-endian).

first_block_length

Compute the minimum number of base-radix digits needed to represent 128 bits. This is: ceil(128 / log2(radix)) = smallest k where radix^k >= 2^128

is_power_of_two

Check if a number is a power of two.

Type Aliases

Hctr2FpErrorDeprecated

Hctr2Fp_128_Base64

HCTR2+FP with AES-128 and base-64 (radix-64) format preservation.

Hctr2Fp_128_Decimal

HCTR2+FP with AES-128 and decimal (radix-10) format preservation.

Hctr2Fp_128_Hex

HCTR2+FP with AES-128 and hexadecimal (radix-16) format preservation.

Hctr2Fp_256_Base64

HCTR2+FP with AES-256 and base-64 (radix-64) format preservation.

Hctr2Fp_256_Decimal

HCTR2+FP with AES-256 and decimal (radix-10) format preservation.

Hctr2Fp_256_Hex

HCTR2+FP with AES-256 and hexadecimal (radix-16) format preservation.