Flawed Encryption Logic

[edgio]

Flawed Encryption Logic

[jcline]

Hi!

I think I understand how this could be confusing.

There's a couple parts to this which I will attempt to illustrate. I've also created a new test which validates the behavior is as I describe below.

base64 encoders work by taking ever 6 bits of an input and mapping those to an ASCII character. If the number of bits in the input is not evenly divisible by 6, then the encoder must pad the resulting output with some number of unused bits.

Example:

$echo 'a' | base64
YQ=

ASCII a is 0x61, or 01100001 and requires 8 bits to represent, so we have to use two 6 bit base64 digits to represent it.

ASCII:   |    a    |
binary:  |011000|010000|
base 10: |  24  |  16  |
base 64: |  Y   |   Q  |

The encoder will normally then pad the number of base64 digits out to a multiple of 4 with =s, but ectoken strips those.

That those last 4 bits (in this example) are discarded means that I can do this echo 'YZ==' | base64 -d and still get a.

ASCII:   |    a    |
binary:  |011000|011001|
base 10: |  24  |  25  |
base 64: |  Y   |   Z  |

If you try changing the first character in the token it should fail completely.

I'm going to post my new tests soon, they run very slowly because they basically brute force changing base64 encoded bytes in the token and validate that the decryption fails properly unless the changed bits are in the discarded padding. I'll leave this open until that's merged.

[hattan]

Thanks, @AbhishekTripathi . We'll investigate this issue and provide an update shortly.