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I would like to know how exactly the encryption of private key by passphrase works. I am a software developer with no background in cryptography and I know only some basics. When I open the private key file I see:

    -----BEGIN RSA PRIVATE KEY-----
    Proc-Type: 4,ENCRYPTED
    DEK-Info: AES-128-CBC,225AAFE1E579A5E38EC1C7F479C5F877
......
    IBUX1w9RYjMsgRE6de79iT80A+Er3RYH7tKKcFrjMoIv4z3YxeIgvPtXjmkoXLmv
    -----END RSA PRIVATE KEY-----

As I understand this is private key encrypted using symmetric encryption where passphrase is the key.

The algorithm used is in this case is AES-128-CBCbut I also have seen DES-EDE3-CBC in a different key. What I don't understand is that number/string behind the algorithm type info- 225AAFE1E579A5E38EC1C7F479C5F877. The similar one but shorter was present also in DEK-Info header of the key which had DES.

What are these numbers?

Is the passphrase digested to the key in some standard way? Is it different for every algorithm used?

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According to this: https://stackoverflow.com/questions/1774469/how-does-the-rsa-private-key-passphrase-work-under-the-hood the hex string is the initialization vector used by the encryption algorithm.

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