I found a small cut-n-paste error in nettle.texinfo.
SHA256_DIGEST_SIZE should be 32, not 20. This agrees with the text a
few lines above the patch.
/ceder
--- nettle.texinfo~ 2007-03-03 10:16:46.000000000 +0100
+++ nettle.texinfo 2007-03-03 10:17:23.000000000 +0100
@@ -510,7 +510,7 @@
@end deftp
@defvr Constant SHA256_DIGEST_SIZE
-The size of an SHA256 digest, i.e. 20.
+The size of an SHA256 digest, i.e. 32.
@end defvr
@defvr Constant SHA256_DATA_SIZE