Use Pohlig-Hellman to solve discrete log

Question

We have $$7^x = 166 \pmod{433}$$

I need to find $x$ using the Pohlig-Hellman algorithm.

Answer 1

I will assume you have access to a better description of the algorithm than that web site. If not, refer to:

• A Course in Number Theory and Cryptography, 2nd Ed., N. Koblitz
• An Introduction to Cryptography, R. A. Mollin
• An Introduction to Mathematical Cryptography, J. Hoffstein, J. Pipher, J. H. Silverman

We are asked to use the Pohlig-Hellman algorithm to solve a Discrete Log Problem and find $x$ for:

$$7^x = 166 \pmod{433}$$

Using the notation:

$$g^x = h \pmod p$$

We have:

$$g = 7, h = 166, p = 433, N = p - 1 = \prod q_i^{e_i} = q_1^{e_1} \cdot q_2^{e_2} = 432 = 2^4 \cdot 3^3$$

We can summarize the necessary algorithm calculations in a handy table as:

$$\begin{array}{|c|c|c|c|c|} \hline \large q & \large e & \large g^{(p-1)/q^e} & \large h^{(p-1)/q^e} & \mbox{Solve}~ \large \left(g^{(p-1)/q^e} \right)^x = ~ \large h^{(p-1)/q^e}~ \mbox{for} ~ \large x \\ \hline 2 & 4 & 265 & 250 & \mbox{Calculation I = ?}\\ \hline 3 & 3 & 374 & 335 & \mbox{Calculation II = ?}\\ \hline \end{array}$$

Calculation I:

We want to solve:

$$x \equiv x_0 + x_1q + \ldots + x_{e-1}q^{e−1} \pmod {2^4} \equiv x_0 + 2x_1 + 4x_2 + 8x_3 \pmod {2^4}$$

• Solve $(265)^x = 250 \pmod {433}$ for $x_0, x_1, x_2, x_3$.
• $x_0: (265^{2^3})^{x_0} = 250^{2^3} \pmod {433} \implies (432)^{x_0} = 432 \implies x_0 = 1$
• $x_1: (265^{2^3})^{x_1} = (250 \times 265^{-x_0})^{2^2} \pmod {433} = (250 \times 265^{-1})^{2^2} \pmod {433} = (250 \times 250)^{2^2} \pmod {433} \implies (432)^{x_1} = 432 \implies x_1 = 1$
• $x_2: (265^{2^3})^{x_2} = (250 \times 265^{-x_0-2x_1})^{2^1} \pmod {433} = (250 \times 265^{-3})^{2^2} \pmod {433} = (250 \times 195)^{2^1} \pmod {433} \implies (432)^{x_2} = 432 \implies x_2 = 1$
• $x_3: (265^{2^3})^{x_3} = (250 \times 265^{-x_0-2x_1-4x_2})^{2^0} \pmod {433} = (250 \times 265^{-7})^{2^0} \pmod {433} = (250 \times 168)^{2^0} \pmod {433} \implies (432)^{x_3} = 432 \implies x_3 = 1$

Thus, our first result is:

$$x \equiv x_0 + 2x_1 + 4x_2 + 8x_3 \pmod {2^4} \equiv 1 + 2 + 4 + 8 \pmod {2^4} \equiv 15 \pmod {2^4}$$

Calculation II:

We want to solve:

$$x \equiv x_0 + x_1q + \ldots + x_{e-1}q^{e−1} \pmod {3^3} \equiv x_0 + 3x_1 + 9x_2 \pmod {3^3}$$

• Solve $(374)^x = 335 \pmod {433}$ for $x_0, x_1, x_2$.
• $x_0: (374^{3^2})^{x_0} = 335^{3^2} \pmod {433} \implies (234)^{x_0} = 198 \implies x_0 = 2$. Note: you only needed to test $x_0 = 0, 1, 2$, so it is clear which one $x_0$ is.
• $x_1: (374^{3^2})^{x_1} = (335 \times 374^{-x_0})^{3^1} \pmod {433} = (335 \times 374^{-2})^{3^1} \pmod {433} = (335 \times 51)^{3^1} \pmod {433} = 1 \pmod{433} \implies (234)^{x_1} = 1 \pmod {433} \implies x_1 = 0$
• $x_2: (374^{3^2})^{x_2} = (335 \times 374^{-x_0-3x_1})^{3^0} \pmod {433} = (335 \times 374^{-2})^{3^0} \pmod {433} = (335 \times 51)^{3^0} \pmod {433} = 198 \pmod{433} \implies (234)^{x_2} = 198 \pmod {433} \implies x_2 = 2$. Note: you only needed to test $x_2 = 0, 1, 2$, so it is clear which one $x_2$ is.

Thus, our second result is:

$$x \equiv x_0 + 3x_1 + 9x_2 \pmod {3^3} \equiv 2 + 0 + 9 \times 2 \pmod {3^3} \equiv 20 \pmod {3^3}$$

Next, we have to use the Chinese Remainder Theorem to solve the simultaneous congruences:

$$x \equiv 15 \pmod {2^4} , ~~ x \equiv 20\pmod {3^3}$$

This yields:

$$x = 47$$

We check our answer by computing:

$$7^{47} = 166 \pmod {433} ~~ \checkmark$$