I am given a private RSA key, which header indicates that it is AES-128-CBC encypted. Note that the last ten lines of the key are missing. (This is part of an educational task.)
-----BEGIN RSA PRIVATE KEY----- Proc-Type: 4,ENCRYPTED DEK-Info: AES-128-CBC,8B7CE35DB731727CEABA217FAE404DB8 ... 37 and a half lines with 64 characters each
The task is to find out the passphrase of the key (is known to be a four digit number). I already achieved the general cracking mechanism by using John The Ripper and added the obligatory
-----END RSA PRIVATE KEY----- to the end of the partial key, but so far, the key could not be decrypted, which might be related to the fact that the last few lines of the key are missing (because CBC involves all blocks of a file). I already tried to cut the key content length to a multiple of 128bit and decrypt that, but that did not change anything. To find a proper way to still be able to decrypt the key, I need to furtherly understand the decryption process.
How does the process of decryption work, and how is the passphrase of the key involved in this? I already looked up some webpages containing wikipedia on CBC encryption in general, but I was unable to understand on how the actual decryption process does take place involving the key's passphrase.
Thanks in advance for all explanations or hints on how to approach this task.