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In cryptography and computer security, a man-in-the-middle attack (often abbreviated to MITM, MitM, MIM, MiM or MITMA) is an attack where the attacker secretly relays and possibly alters the communication between two parties who believe they are directly communicating with each other. Man-in-the-middle attacks can be thought about through a chess analogy. Mallory, who barely knows how to play chess, claims that she can play two grandmasters simultaneously and either win one game or draw both. She waits for the first grandmaster to make a move and then makes this same move against the second grandmaster. When the second grandmaster responds, Mallory makes the same play against the first. She plays the entire game this way and cannot lose using this strategy unless she runs into difficulty with time because of the slight delay between relaying moves. A man-in-the-middle attack is a similar strategy and can be used against many cryptographic protocols. One example of man-in-the-middle attacks is active eavesdropping, in which the attacker makes independent connections with the victims and relays messages between them to make them believe they are talking directly to each other over a private connection, when in fact the entire conversation is controlled by the attacker. The attacker must be able to intercept all relevant messages passing between the two victims and inject new ones. This is straightforward in many circumstances; for example, an attacker within reception range of an unencrypted Wi-Fi wireless access point, can insert himself as a man-in-the-middle.〔(【引用サイトリンク】title=How to defend yourself against MITM or Man-in-the-middle attack )〕 As an attack that aims at circumventing mutual authentication, or lack thereof, a man-in-the-middle attack can succeed only when the attacker can impersonate each endpoint to their satisfaction as expected from the legitimate other end. Most cryptographic protocols include some form of endpoint authentication specifically to prevent MITM attacks. For example, TLS can authenticate one or both parties using a mutually trusted certification authority. == Example of an attack == Suppose Alice wishes to communicate with Bob. Meanwhile, Mallory wishes to intercept the conversation to eavesdrop and optionally deliver a false message to Bob. First, Alice asks Bob for his public key. If Bob sends his public key to Alice, but Mallory is able to intercept it, a man-in-the-middle attack can begin. Mallory sends a forged message to Alice that claims to be from Bob, but instead includes Mallory's public key. Alice, believing this public key to be Bob's, encrypts her message with Mallory's key and sends the enciphered message back to Bob. Mallory again intercepts, deciphers the message using her private key, possibly alters it if she wants, and re-enciphers it using the public key Bob originally sent to Alice. When Bob receives the newly enciphered message, he believes it came from Alice. # Alice sends a message to Bob, which is intercepted by Mallory: #: Alice ''"Hi Bob, it's Alice. Give me your key."'' → Mallory Bob # Mallory relays this message to Bob; Bob cannot tell it is not really from Alice: #: Alice Mallory ''"Hi Bob, it's Alice. Give me your key."'' → Bob # Bob responds with his encryption key: #: Alice Mallory ← ''(key )'' Bob # Mallory replaces Bob's key with her own, and relays this to Alice, claiming that it is Bob's key: #: Alice ← ''(key )'' Mallory Bob # Alice encrypts a message with what she believes to be Bob's key, thinking that only Bob can read it: #: Alice ''"Meet me at the bus stop!" (with Mallory's key )'' → Mallory Bob # However, because it was actually encrypted with Mallory's key, Mallory can decrypt it, read it, modify it (if desired), re-encrypt with Bob's key, and forward it to Bob: #: Alice Mallory ''"Meet me in the windowless van on 22nd Ave!" (with Bob's key )'' → Bob # Bob thinks that this message is a secure communication from Alice. This example shows the need for Alice and Bob to have some way to ensure that they are truly using each other's public keys, rather than the public key of an attacker. Otherwise, such attacks are generally possible, in principle, against any message sent using public-key technology. Fortunately, there are a variety of techniques that help defend against MITM attacks. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Man-in-the-middle attack」の詳細全文を読む スポンサード リンク
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