TheAlgorithms/Python · error · ValueError

Input must be of length 32

Error message

Input must be of length 32

What it means

Thrown by to_little_endian() in the MD5 implementation when the input is not exactly 32 bytes. The helper reverses four 8-byte groups of a 32-byte (256-bit) chunk to produce little-endian ordering, so any other length has undefined group boundaries and is rejected upfront.

Source

Thrown at hashes/md5.py:38

    Converts the given string to little-endian in groups of 8 chars.

    Arguments:
        string_32 {[string]} -- [32-char string]

    Raises:
        ValueError -- [input is not 32 char]

    Returns:
        32-char little-endian string
    >>> to_little_endian(b'1234567890abcdfghijklmnopqrstuvw')
    b'pqrstuvwhijklmno90abcdfg12345678'
    >>> to_little_endian(b'1234567890')
    Traceback (most recent call last):
    ...
    ValueError: Input must be of length 32
    """
    if len(string_32) != 32:
        raise ValueError("Input must be of length 32")

    little_endian = b""
    for i in [3, 2, 1, 0]:
        little_endian += string_32[8 * i : 8 * i + 8]
    return little_endian


def reformat_hex(i: int) -> bytes:
    """
    Converts the given non-negative integer to hex string.

    Example: Suppose the input is the following:
        i = 1234

        The input is 0x000004d2 in hex, so the little-endian hex string is
        "d2040000".

    Arguments:

View on GitHub (pinned to f5988cc097)

Solutions

1. Do not call to_little_endian directly; run the full md5_me(message) pipeline which slices correctly.
2. If you must call it, pass exactly 32 bytes: slice your buffer into 32-byte chunks first.
3. For arbitrary-width endian swapping use int.from_bytes(..., 'little') / struct instead.

Example fix

# before
le = to_little_endian(raw_bytes)  # raw_bytes has arbitrary length

# after
for pos in range(0, len(raw_bytes), 32):
    le = to_little_endian(raw_bytes[pos:pos + 32])

assert len(chunk) == 32, f"to_little_endian needs 32 bytes, got {len(chunk)}"

def is_32_bytes(buf: bytes) -> bool:
    return isinstance(buf, (bytes, bytearray)) and len(buf) == 32

try:
    le = to_little_endian(chunk)
except ValueError:
    raise ValueError(f"expected 32-byte chunk, got {len(chunk)} bytes") from None

When it happens

Trigger: Calling to_little_endian(b'1234567890') (10 bytes) or with any string whose length != 32. In practice this helper is called internally by get_block_words on 32-bit (32-char) slices, so a direct call with an arbitrary buffer triggers it.

Common situations: Using this internal helper as a generic endian-swap utility on arbitrary-length buffers; passing a hex string or a bit string of the wrong width instead of the 32-char block slice produced by preprocessing.

Related errors

• Input must be non-negative
• Input must have length that's a multiple of 512
• Shift must be non-negative
• number must be positive
• The value of input must be non-negative

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/a12da8a35e25b28c. Report an issue: GitHub.