py_ecc/bls/utils.py
def compress_G2(pt: G2Uncompressed) -> G2Compressed:
    """
    The compressed point (z1, z2) has the bit order:
    z1: (c_flag1, b_flag1, a_flag1, x1)
    z2: (c_flag2, b_flag2, a_flag2, x2)
    where
    - c_flag1 is always set to 1
    - b_flag1 indicates infinity when set to 1
    - a_flag1 helps determine the y-coordinate when decompressing,
    - a_flag2, b_flag2, and c_flag2 are always set to 0
    """
    if not is_on_curve(pt, b2):
        raise ValueError(
            "The given point is not on the twisted curve over FQ**2"
        )
    if is_inf(pt):
        return G2Compressed((POW_2_383 + POW_2_382, 0))
    x, y = normalize(pt)
    x_re, x_im = x.coeffs
    y_re, y_im = y.coeffs
    # Record the leftmost bit of y_im to the a_flag1
    # If y_im happens to be zero, then use the bit of y_re
    a_flag1 = (y_im * 2) // q if y_im > 0 else (y_re * 2) // q

    # Imaginary part of x goes to z1, real part goes to z2
    # c_flag1 = 1, b_flag1 = 0
    z1 = x_im + a_flag1 * POW_2_381 + POW_2_383
    # a_flag2 = b_flag2 = c_flag2 = 0
    z2 = x_re
    return G2Compressed((z1, z2))
def compress_G2(pt: G2Uncompressed) -> G2Compressed:
    """
    The compressed point (z1, z2) has the bit order:
    z1: (c_flag1, b_flag1, a_flag1, x1)
    z2: (c_flag2, b_flag2, a_flag2, x2)
    where
    - c_flag1 is always set to 1
    - b_flag1 indicates infinity when set to 1
    - a_flag1 helps determine the y-coordinate when decompressing,
    - a_flag2, b_flag2, and c_flag2 are always set to 0
    """
    if not is_on_curve(pt, b2):
        raise ValueError(
            "The given point is not on the twisted curve over FQ**2"
        )
    if is_inf(pt):
        return G2Compressed((POW_2_383 + POW_2_382, 0))
    x, y = normalize(pt)
    x_re, x_im = x.coeffs
    y_re, y_im = y.coeffs
    # Record the leftmost bit of y_im to the a_flag1
    # If y_im happens to be zero, then use the bit of y_re
    a_flag1 = (y_im * 2) // q if y_im > 0 else (y_re * 2) // q

    # Imaginary part of x goes to z1, real part goes to z2
    # c_flag1 = 1, b_flag1 = 0
    z1 = x_im + a_flag1 * POW_2_381 + POW_2_383
    # a_flag2 = b_flag2 = c_flag2 = 0
    z2 = x_re
    return G2Compressed((z1, z2))
The compressed point (z1, z2) has the bit order:
    z1: (c_flag1, b_flag1, a_flag1, x1)
    z2: (c_flag2, b_flag2, a_flag2, x2)
    where
    - c_flag1 is always set to 1
    - b_flag1 indicates infinity when set to 1
    - a_flag1 helps determine the y-coordinate when decompressing,
    - a_flag2, b_flag2, and c_flag2 are always set to 0