The Part 97 rules do not specifically authorize or prohibit encryption in Amateur Radio. In fact, the word "encryption" does not appear anywhere in the Part 97 rules. Arguments (often heated) center around interpretations of the following words or phrases that do appear in the Part 97 rules.
I would be very interested to hear a valid argument (either way) stating the paragraph that either specifically authorities or specifically prohibits the use of encryption over an AREDN mesh network that incorporates RF emissions in the Part 97 regulated spectrum. If you think you have found that magic paragraph to prove your case once and for all, please check the §97.3 Definitions section to make sure you are using the Part 97 definition of the words in that paragraph to ensure that you are interpreting the paragraph correctly.
Most of the following searches lead to paragraphs that specifically do not relate to the type of RF emanations used from one AREDN node to another AREDN node.
You can search for the following words/phrases:
1. Encryption (found 0 times)
2. for the purpose of obscuring (found 3 times)
3. Codes (found 7 times)
4. Ciphers (found 2 times)
5. Data emission (found 15 times)
6. Telemetry (found 8 times)
7. Automatically (found 14 times)
8. Repeater (found 21 times)
I have included links to the full text of the Part 97 rules here.
Part 97 Rules on arrl.org (.pdf)