If classical ciphers are used with compressed plaintext, how much does it make frequency analysis attack harder? Classical ciphers, such as the Vigenère cipher, are weak and no longer used. They can be broken by using frequency analysis, which is a well-known fact.
However, frequency analysis often depends on the number of captured ciphertexts and/or their duplication. What if the ciphertext was compressed by an algorithm such as Huffman encoding，zlib, or lzma before encrypting the plaintext? For better security, assume there is no constant header, magic number, or any identifier in the plaintext.
How much more difficult does compression make attack by frequency analysis?
Compression works by approximating an optimal code—one where if the probability of a given message is $p$, its encoded length is $-log(p)$. This means that the lengths of the encryptions of compressed messages potentially leak information about the plaintexts. 

Also, the way a scheme like Huffman coding works is by outputting shorter code words for more frequent source symbols than for less frequent ones. This means that the relative frequencies of Huffman code words will be the same as that of the frequencies of the source symbols. It does nothing to disguise those frequencies.

A good real-life example to consider is attacks tht break the confidentiality of some encrypted voice-over-IP codecs that use variable bit rate encoding (which is a form of compression):


  Despite the rapid adoption of Voice over IP (VoIP), its security implications are not yet fully understood. Since VoIP calls may traverse untrusted networks, packets should be encrypted to ensure confidentiality. However, we show that when the audio is encoded using variable bit rate codecs, the lengths of encrypted VoIP packets can be used to identify the phrases spoken within a call. Our results indicate that a passive observer can identify phrases from a standard speech corpus within encrypted calls with an average accuracy of 50%, and with accuracy greater than 90% for some phrases.