base64.decode on whitespace prefixed string can lead to index defect

[miki725]

Nim Version

2.2.6

Description

(example using fish shell)

➜ nim c -r (echo '
  import base64
  echo decode("   a")
  ' | psub -s '.nim')
/tmp/.psub.PVFMMD/psub.nim(3) psub
/home/root/.choosenim/toolchains/nim-2.2.6/lib/pure/base64.nim(230) decode
/home/root/.choosenim/toolchains/nim-2.2.6/lib/system/fatal.nim(53) sysFatal
Error: unhandled exception: index 4 not in 0 .. 3 [IndexDefect]

if a string to be decoded is prefixed with whitespace due to:

Nim/lib/pure/base64.nim

Lines 252 to 259 in ab00c56

	inputEnds = inputLen - 4
	while inputIndex <= inputEnds:
	while s[inputIndex] in {'\n', '\r', ' '}:
	inc inputIndex
	inputChar(a)
	inputChar(b)
	inputChar(c)
	inputChar(d)

as it increments inputIndex it can increment it outside of the outer loop bound inputLen - 4 hence inputChar blows up with out of bounds error

Current Output

`IndexDefect` is raised

Expected Output

base64 should decode as normal

Known Workarounds

call strip before using decode:

decode(s.strip())

Additional Information

No response

[miki725]

did a quick proposed PR #25531 as POC fix