security - Very simple password generation scheme; is this secure? -

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edit/clarification: mean password generation in "deterministically generate passwords own use (e.g. sign web services), based on secret , on site-specific data"

i take md5 digest of concatenation of master password , (non-secret) site-specific string. take first 16 digits of hex representation.

the advantages of such simplistic scheme are:

  • usable anywhere md5 available
  • don't have trust firefox extension or whatever generate password you

does have hidden vulnerabilities? obviously, if master compromised, i'm out of luck.

(side note: of course using hex digits suboptimal entropy per character, cares if password longer make it?)

#!/bin/bash  master=mymasterpassword echo "$master$1" | md5sum | head -c16

there systems use this, such supergenpass. in general, assuming hash function secure against preimage attacks (for purpose suggest using other md5), you're okay.

there is, however, better construct purpose: hmac. it's constructed regular hash function, has proofs of security against various attacks, limited preimage attacks. there's section in rfc explicitly dealing using truncated hmac.


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