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it was reported a couple of times that M2Crypto tends to be a component
that can be difficult to install, particularly on OS X.
Hence in commit e3366d884  I’ve just replaced M2Crypto with rsa 
and PyCrypto  modules.
Why two of them?
I like the rsa’s API for encrypting and decrypting but it can only read
private keys stored in PKCS#1 whereas keys Zato’s development CA
produces are PKCS#8 hence PyCrypto is used to convert PKCS#8 to PKCS#1
which is then fed to rsa.
Both  and  are pure Python so installation will pose no issues.