Chromiumでmultipartのboundaryを生成していると思われる箇所

以下が、該当箇所だと思われる。

code:third_party/blink/renderer/platform/network/form_data_encoder.cc

...

Vector<char> FormDataEncoder::GenerateUniqueBoundaryString() {

Vector<char> boundary;

// TODO(rsleevi): crbug.com/575779: Follow the spec or fix the spec.

// The RFC 2046 spec says the alphanumeric characters plus the

// following characters are legal for boundaries: '()+_,-./:=?

// However the following characters, though legal, cause some sites

// to fail: (),./:=+

//

// Note that our algorithm makes it twice as much likely for 'A' or 'B'

// to appear in the boundary string, because 0x41 and 0x42 are present in

// the below array twice.

0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49, 0x4A, 0x4B,

0x4C, 0x4D, 0x4E, 0x4F, 0x50, 0x51, 0x52, 0x53, 0x54, 0x55, 0x56,

0x57, 0x58, 0x59, 0x5A, 0x61, 0x62, 0x63, 0x64, 0x65, 0x66, 0x67,

0x68, 0x69, 0x6A, 0x6B, 0x6C, 0x6D, 0x6E, 0x6F, 0x70, 0x71, 0x72,

0x73, 0x74, 0x75, 0x76, 0x77, 0x78, 0x79, 0x7A, 0x30, 0x31, 0x32,

0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x41, 0x42};

// Start with an informative prefix.

Append(boundary, "----WebKitFormBoundary");

// Append 16 random 7bit ascii AlphaNumeric characters.

base::RandBytes(random_bytes, sizeof(random_bytes));

for (char& c : random_bytes)

boundary.Append(random_bytes, sizeof(random_bytes));

boundary.push_back(

0); // Add a 0 at the end so we can use this as a C-style string.

return boundary;

}

...

一見、大文字小文字アルファベットと数字の乱数のように見えるが、

上記のコード内のコメント欄で触れられているように、"A"と"B"の出現確率が2倍になっている。

// Note that our algorithm makes it twice as much likely for 'A' or 'B'

// to appear in the boundary string, because 0x41 and 0x42 are present in

// the below array twice.

なぜ、2倍の確率なのかは、どこかのissueなどを探せば見つかるかも。