How is the Unix / PostgreSQL crypt function a trapdoor function?

ahron

1/16/24, 7:58 AM

I am looking at this in the context of password hashing in PostgreSQL, specifically, the crypt function of the pgcrypto extension.

The PostgreSQL documentation refers to the Unix man page of the crypt function. The man page (man 3 crypt) of the crypt function states that it performs Trapdoor encryption. Now a trapdoor function should

Be easy to compute in one direction - this I understand.
Be hard to compute in the other direction - this I also understand
Be easy to compute in the other direction if one knows the 'trapdoor' value - I don't understand if/how one might be able to get this value.

I know how to generate a hash, but how might I be able to get the trapdoor value to compute back the original password? Or are the algorithms written in a way so as to discard the trapdoor value?

It did not make sense to ask this in the dba.stackexchange community, as that's primarily about database design and related topics.

Edit/update - Only the BSD man page for crypt states it is a Trapdoor function. The Linux man page states no such thing.

181

1 + 0

trapdoor

password-hashing

Score:2

Crypto

fgrieu

1/16/24, 8:32 AM

TLDR: crypt is not a trapdoor function.

crypt does not meet the question's condition 3. It does not apply a trapdoor function, which the question defines correctly at some level of detail. Whether crypt "performs Trapdoor encryption" depends on the definition given to this. I would not say that.

The cryptographic functions used by crypt nowadays are called [key-stretching | purposely slow] [password-based] hash. On top of this it's also memory-hard when it uses scrypt or Argon2.

+ 1

ahron

1/16/24, 10:34 AM

Thanks. The [BSD man page for `crypt`](https://www.freebsd.org/cgi/man.cgi?crypt(3)) says Trapdoor in the description. On Ubuntu Linux, it only says passphrase hashing.

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