http://vojgy5ep4wffmtpjmpnbpa4gq64bgn4yicuw6qmhbm6nqa2ysrva.b32.i2p/spec.html
The $numBytes body is an encrypted zip archive, though the encryption method
depends upon the type line. For posts and metadata messages, the data is
AES/256/CBC encrypted (with a 16 byte IV at the beginning). For private
messages, the first 512 bytes are ElGamal/2048 encrypted to the channel's
encryption key, which has the AES/256 session key and IV within it, and the
remaining bytes are AES/256/CBC encrypted.