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After spending a good hour searching I came across an article that explained that you can't reverse a hash value. How true is this? I need to reverse a base64 encoded Sha-1 email address but I can't seem to find a way without knowing what function was used to hash. Or is there an easier way of going about this?

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    $\begingroup$ This question is a bit confusing. You say you don't know what function was used to hash, but you say it's a SHA-1 hash...so you know SHA-1 was used to hash. $\endgroup$ – user47922 Jun 8 '17 at 17:11
  • $\begingroup$ My apologies for the confusion. Any thoughts on reversing the hash? $\endgroup$ – Inaloz_H Jun 8 '17 at 17:16
  • $\begingroup$ I suggest reading cryptographic hash function. How true is it for SHA-1 if you know nothing about the input? Unlikely that you can reverse it before our sun burns out - even if you had all the computers in the world. SHA-1 is inescure and there were interesting results recently - but breaking preimage resistance is on an entirely different level. $\endgroup$ – tylo Jun 9 '17 at 10:02
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  • You can very easily Base64-decode a string.
  • SHA-1 was designed to prevent people from inverting it. It approximates a one-way function.

See here for more. If it could be simply inverted, it would provide no information security.

Your only feasible option is brute-force search.

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    $\begingroup$ And don't forget to google the hash (as hex preferably), sometimes some DBs will have the string... $\endgroup$ – SEJPM Jun 8 '17 at 17:51

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