Output should be zero-padded

[William Swanson]

When initializing RFC4226 with the base-32 encoded string AAAAAAAA (i.e. 5 bytes of 0's), the HTOP for counter=2 should be "073348", and the HOTP for counter=9 should be "003773". The following code does not appear to produce the necessary leading 0's:

return codeStr.substr(codeStr.length - length);

[Keyang Xiang]

Interesting. I never had this issue for the past few years. I will take a look.

[William Swanson]

Ah, my test vector does not actually trigger the issue. The odds of triggering this issue are about 1/20000, since final hash output needs to be less than 99999 (out of ~2 billion possibilities). I will attempt to find a test vector that does trigger this problem.

[William Swanson]

Try it with the base-32 encoded secret AAAAAAAA and counter=41952. The output will be 5 digits long, not 6 digits long like it should be.

[William Swanson]

Try it with the base-32 encoded secret AAAAAAAA and counter=41952. The output will be 5 digits long, not 6 digits long like it should be.

The following code should solve the issue:

const padding = Array(length).join('0')
return (padding + codeStr).slice(-length)

This tacks a bunch of 0's in front of the number, and then trims it down to length characters.