However, there is one major, interesting exploit that makes a tethered downgrade possible: (the bootrom exploit).
Here is an outline and summary for a paper titled: Core Idea of the Paper The iPod Touch 5 uses the 32-bit A5 chip (same as iPhone 4S, iPad 2). The Limera1n bootrom exploit (discovered by geohot) is unpatchable on this device because it’s a hardware-level vulnerability. This allows an attacker to bypass code signing at boot time, enabling a tethered downgrade to any iOS version, including iOS 6. downgrade ipod touch 5 to ios 6
This is a fascinating and technically deep topic. The originally shipped with iOS 6.0 , but Apple no longer signs those old versions. Downgrading without a valid SHSH blob (signature ticket saved when iOS 6 was being signed ) is extremely difficult due to cryptographic checks. However, there is one major, interesting exploit that