--- distribution/reference
+++ opencontainers/image-spec
 digest               ::= algorithm ":" encoded
 algorithm            ::= component (separator component)*
-component            ::= [A-Za-z][A-Za-z0-9]*
+component            ::= [a-z0-9]+
 separator            ::= [+._-]
-encoded              ::= [a-fA-F0-9]{32,} /* At least 128 bit digest value */
+encoded              ::= [a-zA-Z0-9=_-]+