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12

If Bob does NOT care to check signatures (as in the question), Eve can send ANY message she wants to Bob pretending to be Alice, including but not limited to messages Eve got from Alice; all Eve needs is Bob's public key (which, as the name implies, is assumed public knowledge thus known to Eve) and straight use of PGP. Therefore the right question is: Can ...


10

In addition to the performance problems poncho already mentioned when using RSA signatures without hashing I just want to add on the security warning of poncho: Reordering If you have a message $m>N$ with $N$ being the RSA modulus, then you have to perform at least 2 RSA signatures as $m$ does not longer fit into $Z_N$. Let us assume that it requires ...


6

Well, one reason to hash the data before signing it is because RSA can handle only so much data; we might want to sign messages longer than that. For example, suppose we are using a 2k RSA key; that means that the RSA operation can handle messages up to 2047 bits; or 255 bytes. We often want to sign messages longer than 255 bytes. By hashing the message ...


5

The attack is even more simple with RSA than with symmetric keys, because the asymmetric encryption key is assumed to be public. Let me tell you a story involving Alice, Bob and Mallory :). Alice wants to send a message to Bob using RSA. Alice encrypts the message using Bob's public key and sends it Mallory performs a Man-In-The-Middle attack, and ...


4

As correctly pointed out in a comment, the authenticated encryption model assumes that the attacker knows the algorithm; the attacker can query the encryption oracle with any plaintext $P$ (and a unique nonce $N$) and get MAC-then-Encrypt ciphertext $C$; the attacker can query the decryption oracle with any string $C$ pretending to be a ciphertext. No ...


1

Given a set of (unhashed) Lamport signatures using the same key, an attacker can trivially forge a signature for any message whose $k$-th bit, for each $k$, is equal to the $k$-th bit of at least one of the signed messages. For example, let's say I know the Lamport signatures for the following 16-bit messages using the same key: $$ m_1 = 0001111101110001 ...



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