using encrypted connection string

You don't need to decrypt it in order to read/use. If you want to change then you have to decrypt it.

ASP.NET 2.0 introduced Protected Configuration model that allows to encrypt data using two Protected Configuration Providers. They are:
1. RSAProtectedConfigurationProvider: This is the default provider and uses the RSA Public Key Encryption algorithm to encrypt and decrypt data.
2. DataProtectionConfigurationProvider: This provider uses Windows Data Protection Application Programming Interface (DPAPI) to encrypt and decrypt data

MSDN Reads on them:
1. DPAPI[^]
2. RSA[^]

Explaination with Sample here: Edit and Encrypt Web.Config sections using C# 2.0[^]