Do not try to write --keyspace yourself

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Here's more crazy stuff. The mask itself can have an influence even if the length does not change:

$ ./hashcat --keyspace -a 3 ?d?d?d?d?d?d?d?d
100000
$ ./hashcat --keyspace -a 3 ?d?dx?d?d?d?d?d
10000

But not always:

$ ./hashcat --keyspace -a 3 ?d?d?d?d?d?d?d?d
100000
$ ./hashcat --keyspace -a 3 ?a?d?d?d?d?d?d?d
100000