Modules/binascii.c (part 6)

Source:

cpython 3.14 @ ab2d84fe1023/Modules/binascii.c ↗

This annotation covers binary-to-ASCII encoding and checksums. See modules_binascii5_detail for b2a_base64, a2b_base64, b2a_uu, and a2b_uu.

Map

Lines	Symbol	Role
1-80	b2a_base64	Encode binary to base64 line
81-160	a2b_base64	Decode base64 to binary
161-240	b2a_hex / a2b_hex	Hex encode/decode
241-320	crc32	CRC-32 checksum (same as zlib.crc32)
321-500	crc_hqx	CRC-CCITT for BinHex 4.0

Reading

b2a_base64

// CPython: Modules/binascii.c:420 binascii_b2a_base64_impl
static PyObject *
binascii_b2a_base64_impl(PyObject *module, Py_buffer *data, int newline)
{
    /* Encode up to 57 bytes (standard base64 line length = 76 chars) */
    const unsigned char *bin_data = data->buf;
    Py_ssize_t bin_len = data->len;
    /* Allocate: (bin_len * 4 / 3) + 2 for padding and newline */
    PyObject *rv = PyBytes_FromStringAndSize(NULL, ...) ;
    unsigned char *ascii_data = (unsigned char *)PyBytes_AS_STRING(rv);
    /* Main loop: encode 3 bytes -> 4 chars */
    while (bin_len >= 3) {
        *ascii_data++ = table_b2a_base64[( bin_data[0] >> 2) & 0x3f];
        *ascii_data++ = table_b2a_base64[((bin_data[0] << 4) | (bin_data[1] >> 4)) & 0x3f];
        *ascii_data++ = table_b2a_base64[((bin_data[1] << 2) | (bin_data[2] >> 6)) & 0x3f];
        *ascii_data++ = table_b2a_base64[  bin_data[2]                               & 0x3f];
        bin_data += 3; bin_len -= 3;
    }
    /* Handle remaining 1 or 2 bytes with padding */
    ...
    if (newline) *ascii_data++ = '\n';
    return rv;
}

Base64 encodes 3 bytes as 4 ASCII characters using a 64-character alphabet. newline=True (default) appends \n, matching the MIME line format.

crc32

// CPython: Modules/binascii.c:760 binascii_crc32_impl
static PyObject *
binascii_crc32_impl(PyObject *module, Py_buffer *data, unsigned int value)
{
    /* CRC-32: IEEE 802.3 polynomial 0xEDB88320 (reflected) */
    Py_ssize_t len = data->len;
    const unsigned char *buf = data->buf;
    for (Py_ssize_t i = 0; i < len; i++) {
        value = (value >> 8) ^ crc_32_tab[(value ^ buf[i]) & 0xff];
    }
    return PyLong_FromUnsignedLong(value & 0xffffffffUL);
}

binascii.crc32 and zlib.crc32 use the same algorithm. The initial value=0 is the standard. Chaining: crc32(data2, crc32(data1)) extends the CRC over concatenated data.

gopy notes

b2a_base64 is module/binascii.B2ABase64 in module/binascii/module.go. It wraps encoding/base64.StdEncoding.EncodeToString. a2b_hex uses hex.DecodeString. crc32 wraps hash/crc32.ChecksumIEEE. crc_hqx uses a separate table for the CCITT polynomial.