Apache NiFi Security Reference
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Salt and IV Encoding

Initially, the EncryptContent processor had a single method of deriving the encryption key from a user-provided password. This is now referred to as NiFiLegacy mode, effectively MD5 digest, 1000 iterations. In v0.4.0, another method of deriving the key, OpenSSL PKCS#5 v1.5 EVP_BytesToKey was added for compatibility with content encrypted outside of NiFi using the openssl command-line tool. Both of these Key Derivation Functions (KDF) had hard-coded digest functions and iteration counts, and the salt format was also hard-coded. With v0.5.0, additional KDFs are introduced with variable iteration counts, work factors, and salt formats. In addition, raw keyed encryption was also introduced. This required the capacity to encode arbitrary salts and Initialization Vectors (IV) into the cipher stream in order to be recovered by NiFi or a follow-on system to decrypt these messages.

For the existing KDFs, the salt format has not changed.