# RSA with small decryption exponent

I've heard that textbook RSA is insecure when decryption exponent $d$ is smaller than $N^{1/4}$ where $N$ is the public modulus. Why is it the case and what would be a simple explanation of the attack ?

The original attack on plain old RSA using such small exponents was due to Wiener. There has been further work showing that an even larger $d$ than $d>N^{1/4}$ is required, such as work by Maitra and Sarkar.