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Multi Precision
Each integer in the circuit has a certain bit-width, which is determined by the inputset. These bit-widths can be observed when graphs are printed:
%0 = x # EncryptedScalar<uint3> ∈ [0, 7]
%1 = y # EncryptedScalar<uint4> ∈ [0, 15]
%2 = add(%0, %1) # EncryptedScalar<uint5> ∈ [2, 22]
return %2 ^ these are ^^^^^^^
the assigned based on
bit-widths these bounds
However, it's not possible to add 3-bit and 4-bit numbers together because their encoding is different:
D: data
N: noise
3-bit number
------------
D2 D1 D0 0 0 0 ... 0 0 0 N N N N
4-bit number
------------
D3 D2 D1 D0 0 0 0 ... 0 0 0 N N N N
The result of such an addition is a 5-bit number, which also has a different encoding:
5-bit number
------------
D4 D3 D2 D1 D0 0 0 0 ... 0 0 0 N N N N
Because of these encoding differences, we perform a graph processing step called bit-width assignment, which takes the graph and updates the bit-widths to be compatible with FHE.
After this graph processing step, the graph would look like:
%0 = x # EncryptedScalar<uint5>
%1 = y # EncryptedScalar<uint5>
%2 = add(%0, %1) # EncryptedScalar<uint5>
return %2
Most operations cannot change the encoding, which means that the input and output bit-widths need to be the same. However, there is an operation which can change the encoding: the table lookup operation.
Let's say you have this graph:
%0 = x # EncryptedScalar<uint2> ∈ [0, 3]
%1 = y # EncryptedScalar<uint5> ∈ [0, 31]
%2 = 2 # ClearScalar<uint2> ∈ [2, 2]
%3 = power(%0, %2) # EncryptedScalar<uint4> ∈ [0, 9]
%4 = add(%3, %1) # EncryptedScalar<uint6> ∈ [1, 39]
return %4
This is the graph for
(x**2) + y where x is 2-bits and y is 5-bits. If the table lookup operation wasn't able to change the encoding, we'd need to make everything 6-bits. However, since the encoding can be changed, the bit-widths can be assigned like so:%0 = x # EncryptedScalar<uint2> ∈ [0, 3]
%1 = y # EncryptedScalar<uint6> ∈ [0, 31]
%2 = 2 # ClearScalar<uint2> ∈ [2, 2]
%3 = power(%0, %2) # EncryptedScalar<uint6> ∈ [0, 9]
%4 = add(%3, %1) # EncryptedScalar<uint6> ∈ [1, 39]
return %4
In this case, we kept
x as 2-bits, but set the table lookup result and y to be 6-bits, so that the addition can be performed.This style of bit-width assignment is called multi-precision, and it is enabled by default. To disable it and use a single precision across the circuit, you can use the
single_precision=True configuration option.Last modified 1mo ago