Both words generally mean sincerity or honesty, however their usage seems to be different, sometimes, but not always.
According to my understanding, you can say, for example:
他对我有诚意。
but can't say:
他对我有诚信。
However, you can say:
他有诚信。
Can anyone please explain the difference? Can someone please translate the above sentences accurately, and perhaps provide a few other useful examples? Thanks.