In the context of attack, there are two types of preimage resistance:
- preimage resistance: for essentially all pre-specified outputs, it is computationally infeasible to find any input which hashes to that output, i.e., it is difficult to find any preimage x given a "y" such that h(x) = y. 
- second-preimage resistance: it is computationally infeasible to find any second input which has the same output as a specified input, i.e., given x, it is difficult to find a second preimage x' ≠ x such that h(x) = h(x′).
Applied preimage attacks
By definition, an ideal hash function is such that the fastest way to compute a first or second preimage is through a brute force attack. For an n-bit hash, this attack has a time complexity 2n, which is considered too high for a typical output size of n = 128 bits. If such complexity is the best that can be achieved by an adversary, then the hash function is considered preimage-resistant.
Faster preimage attacks can be found by cryptanalysing certain hash functions, and are specific to that function. Some significant preimage attacks have already been discovered, but they are not yet practical. If a practical preimage attack is discovered, it would drastically affect many Internet protocols. In this case, "practical" means that it could be executed by an attacker with a reasonable amount of resources. For example, a preimaging attack that costs trillions of dollars and takes decades to preimage one desired hash value or one message is not practical; one that costs a few thousand dollars and takes a few weeks might be very practical.
All currently known practical or almost-practical attacks on MD5 and SHA-1 are collision attacks. In general, a collision attack is easier to mount than a preimage attack, as it is not restricted by any set value (any two values can be used to collide). The time complexity of the collision attack, in contrast, is .
- IETF RFC 4270: Attacks on Cryptographic Hashes in Internet Protocols
- Rogaway, P.; Shrimpton, T. "Cryptographic Hash-Function Basics: Definitions, Implications, and Separations for Preimage Resistance, Second-Preimage Resistance, and Collision Resistance" (PDF). Fast Software Encryption (2004) (Springer-Verlag). Retrieved 17 November 2012.
- Bruce Morton, Clayton Smith (30 January 2014). "Why We Need to Move to SHA-2". CA Security Council.
- "MD5 and Perspectives". 1 January 2009.
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