Question

I would like to sign a device, and I have 64 bits to store my signature in the device. This device has a MAC address and some other details (about 30 bytes worth) I can mangle to create my signature.

If possible, I would like the method to be one-way, so that I can verify that the signature is valid without knowing how to create a valid signature. Most public-private keys have this feature but they generate signatures that are 48 bytes long (I only have 8 bytes).

Implementation in Python is a plus.

Thanks

EDIT: Thanks for the advice everyone. It sounds like there is no secure way to do this, only a way that is moderately inconvenient to attackers. I’ll probably use a cryptographic hash combined with secret bit-shuffling. This will be as secure as any other link in my (very weak) ‘security’.

Answer 1

Hash functions and digital signatures are very different things.

The size of a digital signature depends on the underlying hash function and the key length. So in theory, you can create an RSA implementation that generates 64-bit signatures, but that’d be an extremely weak signature.

For smaller key lengths, you might want to look at elliptic curve cryptography.

EDIT: Yes, I’m a cryptographer.

EDIT 2: Yet if you only need a hash function, you can look at elf64 or RIPEMD-64 as Fernando Miguélez suggested.

EDIT 3: Doing the math, you’d need to use 16-bit keys in ECC to generate 64-bit signatures, which is very weak. For ECC, anything less than 128 bits can be considered weak. For RSA this is 1024 bits.