CODEWITHSUNDEEP
cryptographyshanumber-theorycryptographic-hash-function
alias_neo92
alias_neo92

Reputation: 75

Whether cracking SHA1 will be easier if I know part of the input?

Assume that I know 80% of SHA1 input. Whether cracking remaining 20% from the SHA1 hash value is easier than cracking the whole input? If it is so, by what percentage?

Eg: I know the x's in the input SHA1(xxxxxxxxyy)=hash value

Upvotes: 1

Views: 194

Answers (1)

David Schwartz
David Schwartz

Reputation: 182789

Assume there are 10 bytes in the input. To crack the whole input, we'd have to try 2^(10*8) inputs. With 80% given, we only have to try 2^(2*8) inputs. That's about a quintillion times fewer. If the input size goes up, the ratio gets even larger.

SHA1 is irreversible today with about 100 unknown bits (12 bytes) in the input. With only 20% of the input unknown, that means the input size would need to be about 500 bits to be secure, or about 62 bytes.

It actually matters whether the unknown part is at the beginning or the end. Each 32 bits of known data at the beginning reduces the number of needed operations by a bit more than you might expect because some of the calculations can be re-used.

Upvotes: 3

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