Generally when people say “90% of child sexual abusers are known to the victim” they are referring to “contact sex offenders.” All child abusers take advantage of vulnerable children, but children are more likely to be physically vulnerable around a trusted, known adult. In the last decade it has become much more common for children to be psychologically vulnerable to online predators as many more children, disproportionately those who are vulnerable for other reasons, have private access to the internet via smartphone 24 hours a day. Some predators use the internet to groom children and then commit contact offences against them. Others manipulate children into creating more child pornography.
In 2014, Aslan and Edelmann [1] undertook “a comparison of sex offenders convicted of possessing indecent images of children, committing contact sex offences or both offences” and, while expressing caution about the “contradictory findings” of previous studies, examined a data set of “230 offenders who had been convicted either of possessing indecent images (Internet offenders n = 74) or committing actual direct abuse of children (contact offenders n = 118) or committing both offences (Internet-contact offenders n = 38).” They found:
> There were significant differences between the three groups of offenders in the way the victim was found. Internet-contact offenders (45%) were more likely to target their victims online and use downloaded indecent images to help recruit their victims … Only 15% of Internet offenders initiated online contact, grooming their victims then requesting indecent images without physically coming into contact with the victim. The majority of contact sex offenders (87%) were known to their victims … Internet-contact offenders were more likely to target stranger victims than contact offenders.
[1] https://dx.doi.org/10.1080/14789949.2014.884618
This data reflects the offences that are detected and prosecuted, so you could read it as suggesting that law enforcement (in London) is focusing on internet offending at the expense of contact offending. It’s hard to say. The data also says nothing about whether anti-encryption laws are needed. However, it does indicate that there is a substantial amount of internet-enabled child sexual abuse and that law enforcement bodies should use some of their finite resources to address it.
What is proportionate is certainly debatable. There is often a fundamental difference of values between civil liberties advocates on the one hand, and victims’ advocates and law enforcement on the other, with respect to the seriousness of internet-based non-contact offences, including the possession of child pornography. When these offenders are counted among child sexual abusers, the proportion who are known to their victims is much less than 90%.