[Tested on 3.8.2 and 3.9.0, bug may manifest in other versions too]

The IETF sometimes uses the dummy base64 value "base64encodedvalue==" in specifications in lieu of a block of otherwise meaningless b64.  

Even though it is a dummy value, the value should be convertible to binary and back again.  This works using the built-in command `base64` as well as OpenSSL command line, but binascii is unable to do it.  See below:

$ echo "base64encodedvalue==" | base64 | base64 -d
base64encodedvalue==

$ echo "base64encodedvalue==" | openssl enc -base64 -A | openssl enc -d base64 -A
base64encodedvalue==                                                                                                         

$ printf "import binascii\nprint(binascii.b2a_base64(binascii.a2b_base64('base64encodedvalue=='), newline=False).decode('ascii'))" |  python -
base64encodedvaluQ==

After some investigation, it appears that almost any valid base64 matching the pattern "??==" fails.  For instance:

$ printf "import binascii\nprint(binascii.b2a_base64(binascii.a2b_base64('ue=='), newline=False).decode('ascii'))" |  python -                                                           
uQ==

$ printf "import binascii\nprint(binascii.b2a_base64(binascii.a2b_base64('aa=='), newline=False).decode('ascii'))" |  python -                                                           
aQ==

$ printf "import binascii\nprint(binascii.b2a_base64(binascii.a2b_base64('a0=='), newline=False).decode('ascii'))" |  python -                                                           
aw==


Is this a bug?

No activity in 3 weeks.  Selecting a couple components to give it a bump.

It seems to me that your commands are just sequenced wrong, in Python you're performing (examples in parens):

* base64 (ue==) -> decode to binary (0xB9)
* binary (0xB9) -> encode to base64 (uQ==)

whereas in your command line commands you're doing:

* base64 (ue==) -> encode to double base64 (dWU9PQo=)
* double base64 (dWU9PQo=) -> decoded to base64 (ue==)


If we do the same thing on the command line as you're doing in Python, we get:

$ echo "base64encodedvalue==" | base64 -d | base64
base64encodedvaluQ==

$ echo "ue==" | base64 -d | base64
uQ==

$ echo "base64encodedvalue==" | openssl enc -d -base64 -A | openssl enc -base64 -A
base64encodedvaluQ==

I see.  There are two issues:

1) my `base64` and `openssl` CLI commands were flipped, as you point out, giving a false positive - oops ;)

2) more importantly, the base64 value "ue==" is invalid (there is no binary input that could possibly generate it) and none of the implementations issued a warning or error, which is reasonable IMO.

Thank you for your help.  Please close this issue.