Provides the implementation of SSLv3 PRF function:

     SSLv3-PRF(secret, seed) =
        MD5(secret || SHA-1("A" || secret || seed)) ||
        MD5(secret || SHA-1("BB" || secret || seed)) ||
        MD5(secret || SHA-1("CCC" || secret || seed)) || ...

    req_len should not be more than  26 x 16 = 416.

def _ssl_PRF(secret, seed, req_len):
    """
    Provides the implementation of SSLv3 PRF function:

     SSLv3-PRF(secret, seed) =
        MD5(secret || SHA-1("A" || secret || seed)) ||
        MD5(secret || SHA-1("BB" || secret || seed)) ||
        MD5(secret || SHA-1("CCC" || secret || seed)) || ...

    req_len should not be more than  26 x 16 = 416.
    """
    if req_len > 416:
        warning("_ssl_PRF() is not expected to provide more than 416 bytes")
        return ""

    d = [b"A", b"B", b"C", b"D", b"E", b"F", b"G", b"H", b"I", b"J", b"K", b"L",  # noqa: E501
         b"M", b"N", b"O", b"P", b"Q", b"R", b"S", b"T", b"U", b"V", b"W", b"X",  # noqa: E501
         b"Y", b"Z"]
    res = b""
    hash_sha1 = _tls_hash_algs["SHA"]()
    hash_md5 = _tls_hash_algs["MD5"]()
    rounds = (req_len + hash_md5.hash_len - 1) // hash_md5.hash_len

    for i in range(rounds):
        label = d[i] * (i + 1)
        tmp = hash_sha1.digest(label + secret + seed)
        res += hash_md5.digest(secret + tmp)

    return res[:req_len]