The keypair was created this way:

$ openssl genrsa -out ./cache-key.sec 2048
$ openssl rsa -in ./cache-key.sec -pubout > ./cache-key.pub

All the tests pass.  What’s wrong?

$ guix authenticate rsautl -sign -inkey cache-key.sec -in 3r2n9zm3kxb28klpzgarjxj8x3y3y6wf.narinfo.hash
Backtrace:
In ice-9/boot-9.scm:
 157: 11 [catch #t #<catch-closure 8ed5f30> ...]
In unknown file:
   ?: 10 [apply-smob/1 #<catch-closure 8ed5f30>]
In ice-9/boot-9.scm:
  63: 9 [call-with-prompt prompt0 ...]
In ice-9/eval.scm:
 432: 8 [eval # #]
In ice-9/boot-9.scm:
2320: 7 [save-module-excursion #<procedure 8e46e60 at ice-9/boot-9.scm:3961:3 ()>]
3966: 6 [#<procedure 8e46e60 at ice-9/boot-9.scm:3961:3 ()>]
1645: 5 [%start-stack load-stack ...]
1650: 4 [#<procedure 8ed6960 ()>]
In unknown file:
   ?: 3 [primitive-load "/usr/local/bin/guix"]
In guix/ui.scm:
 593: 2 [run-guix-command authenticate "rsautl" ...]
In guix/scripts/authenticate.scm:
  60: 1 [guix-authenticate "rsautl" "-sign" ...]
In ice-9/boot-9.scm:
 793: 0 [call-with-input-file "cache-key.sec" ...]

ice-9/boot-9.scm:793:17: In procedure call-with-input-file:
ice-9/boot-9.scm:793:17: Throw to key `gcry-error' with args `(536871113)'.