diff --git a/descriptor.md b/descriptor.md
index d101a2313..c42571370 100644
--- a/descriptor.md
+++ b/descriptor.md
@@ -60,7 +60,7 @@ The digest string matches the following grammar:
 
 ```
 digest      := algorithm ":" hex
-algorithm   := /[A-Fa-f0-9_+.-]+/
+algorithm   := /[a-z0-9_+.-]+/
 hex         := /[A-Fa-f0-9]+/
 ```
 
diff --git a/schema/defs-image.json b/schema/defs-image.json
index 3c3e708f0..44553f55e 100644
--- a/schema/defs-image.json
+++ b/schema/defs-image.json
@@ -9,7 +9,7 @@
     "digest": {
       "description": "the cryptographic checksum digest of the object, in the pattern '<hash>:<hexadecimal digest>'",
       "type": "string",
-      "pattern": "^[a-z0-9]+:[a-fA-F0-9]+$"
+      "pattern": "^[a-z0-9_+.-]+:[a-fA-F0-9]+$"
     },
     "manifestDescriptor": {
       "id": "https://opencontainers.org/schema/image/manifestDescriptor",