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Merged
youknowone merged 1 commit into from
Aug 22, 2026
+40 −25
Merged

Follow CPython's padding rules in a2b_base64 #8570

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2 changes: 0 additions & 2 deletions Lib/test/test_binascii.py
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Original file line number Diff line number Diff line change
Expand Up @@ -117,7 +117,6 @@ def addnoise(line):
# empty strings. TBD: shouldn't it raise an exception instead ?
self.assertEqual(binascii.a2b_base64(self.type2test(fillers)), b'')

@unittest.expectedFailure # TODO: RUSTPYTHON
def test_base64_strict_mode(self):
# Test base64 with strict mode on
def _assertRegexTemplate(assert_regex: str, data: bytes, non_strict_mode_expected_result: bytes):
Expand Down Expand Up @@ -175,7 +174,6 @@ def assertExcessPadding(data, non_strict_mode_expected_result: bytes):
assertExcessPadding(b'abcd====', b'i\xb7\x1d')
assertExcessPadding(b'abcd=====', b'i\xb7\x1d')

@unittest.expectedFailure # TODO: RUSTPYTHON; AssertionError: b'i' != b'i\xb7'
def test_base64_excess_data(self):
# Test excess data exceptions
def assertExcessData(data, expected):
Expand Down
63 changes: 40 additions & 23 deletions crates/stdlib/src/binascii.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -291,31 +291,38 @@ mod decl {
return Ok(vec![]);
}

if strict_mode && b[0] == PAD {
return Err(base64::DecodeError::InvalidByte(0, 61));
}

let mut decoded: Vec<u8> = vec![];

let mut quad_pos = 0; // position in the nibble
let mut pads = 0;
let mut left_char: u8 = 0;
let mut padding_started = false;
for (i, &el) in b.iter().enumerate() {
if el == PAD {
padding_started = true;

pads += 1;
if quad_pos >= 2 && quad_pos + pads >= 4 {
if strict_mode && i + 1 < b.len() {
// Represents excess data after padding error
return Err(base64::DecodeError::InvalidLastSymbol(i, PAD));
}
// A pad that finishes the quad it belongs to is the expected one.
if quad_pos >= 2 && quad_pos + pads <= 4 {
continue;
}

return Ok(decoded);
// RFC 4648 section 3.3 allows a decoder to ignore a pad that
// shows up anywhere else, so only strict mode complains.
if !strict_mode {
continue;
}

continue;
if quad_pos == 1 {
// A single data character cannot be padded into a quad,
// and the check after the loop already reports that.
break;
}

return Err(if quad_pos == 0 && i == 0 {
// Represents leading padding error
base64::DecodeError::InvalidByte(0, PAD)
} else {
// Represents excess padding error
base64::DecodeError::InvalidPadding
});
}

let binary_char = BASE64_TABLE[el as usize];
Expand All @@ -327,9 +334,14 @@ mod decl {
continue;
}

if strict_mode && padding_started {
// Represents discontinuous padding error
return Err(base64::DecodeError::InvalidByte(i, PAD));
if pads > 0 && strict_mode {
return Err(if quad_pos + pads == 4 {
// Represents excess data after padding error
base64::DecodeError::InvalidLastSymbol(i, PAD)
} else {
// Represents discontinuous padding error
base64::DecodeError::InvalidByte(i, PAD)
});
}
pads = 0;

Expand Down Expand Up @@ -361,14 +373,19 @@ mod decl {
}
}

match quad_pos {
0 => Ok(decoded),
1 => Err(base64::DecodeError::InvalidLastSymbol(
if quad_pos == 1 {
// One data character too many: no input encodes to that length.
return Err(base64::DecodeError::InvalidLastSymbol(
decoded.len() / 3 * 4 + 1,
0,
)),
_ => Err(base64::DecodeError::InvalidLength(quad_pos)),
));
}

if quad_pos != 0 && quad_pos + pads < 4 {
return Err(base64::DecodeError::InvalidLength(quad_pos));
}

Ok(decoded)
})
.map_err(|err| super::Base64DecodeError(err).to_pyexception(vm))
}
Expand Down Expand Up @@ -864,7 +881,7 @@ impl ToPyException for Base64DecodeError {
}
// TODO: clean up errors
DecodeError::InvalidLength(_) => "Incorrect padding".to_owned(),
DecodeError::InvalidPadding => "Incorrect padding".to_owned(),
DecodeError::InvalidPadding => "Excess padding not allowed".to_owned(),
};
new_binascii_error(format!("error decoding base64: {message}"), vm)
}
Expand Down
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